Encrypt and decrypt a string in C

Question

How can I encrypt and decrypt a string in C

User · Answer

Here is the sample how AES-GCM encryption/decryption can be done using Bouncy castle package.

I've found that sample when googled for possibility to decrypt data from GOlang crypto/aes api:

const (
    gcmBlockSize         = 16 // this is key size
    gcmTagSize           = 16 // this is mac
    gcmStandardNonceSize = 12 // this is nonce
)

func encrypt(data []byte, passphrase string) []byte {
    block, _ := aes.NewCipher([]byte(createHash(passphrase)))
    gcm, err := cipher.NewGCM(block)
    if err != nil {
        panic(err.Error())
    }
    nonce := make([]byte, gcm.NonceSize())
    if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
        panic(err.Error())
    }
    ciphertext := gcm.Seal(nonce, nonce, data, nil)
    return ciphertext
}

.Net sample works like a charm with key (256 bit), mac (128bit) and nonce (96 bit).

User · Answer

Here is a simple example of encrypting strings in C  using AES CBC mode with random IV and HMAC and password-derived keys  to show the basic moving parts   private byte   EncryptBytes byte   key  byte   plaintext        using  var cipher   new RijndaelManaged   Key   key                  using  var encryptor   cipher CreateEncryptor                          var ciphertext   encryptor TransformFinalBlock plaintext  0  plaintext Length                   IV is prepended to ciphertext             return cipher IV Concat ciphertext  ToArray                       private byte   DecryptBytes byte   key  byte   packed        using  var cipher   new RijndaelManaged   Key   key                  int ivSize   cipher BlockSize   8           cipher IV   packed Take ivSize  ToArray             using  var encryptor   cipher CreateDecryptor                          return encryptor TransformFinalBlock packed  ivSize  packed Length - ivSize                      private byte   AddMac byte   key  byte   data        using  var hmac   new HMACSHA256 key                 var macBytes   hmac ComputeHash data               HMAC is appended to data         return data Concat macBytes  ToArray             private bool BadMac byte   found  byte   computed        int mismatch   0          Aim for consistent timing regardless of inputs     for  int i   0  i  lt  found Length  i                  mismatch    found i     computed i    0   1             return mismatch    0     private byte   RemoveMac byte   key  byte   data        using  var hmac   new HMACSHA256 key                 int macSize   hmac HashSize   8           var packed   data Take data Length - macSize  ToArray             var foundMac   data Skip packed Length  ToArray             var computedMac   hmac ComputeHash packed            if  this BadMac foundMac  computedMac                         throw new Exception  Bad MAC                       return packed                       private List lt byte   gt  DeriveTwoKeys string password        var salt   new byte     1  2  3  4  5  6  7  8         var kdf   new Rfc2898DeriveBytes password  salt  10000        var bytes   kdf GetBytes 32      Two keys 128 bits each      return new List lt byte   gt    bytes Take 16  ToArray    bytes Skip 16  ToArray         public byte   EncryptString string password  String message        var keys   this DeriveTwoKeys password        var plaintext   Encoding UTF8 GetBytes message        var packed   this EncryptBytes keys 0   plaintext        return this AddMac keys 1   packed      public String DecryptString string password  byte   secret        var keys   this DeriveTwoKeys password        var packed   this RemoveMac keys 1   secret        var plaintext   this DecryptBytes keys 0   packed        return Encoding UTF8 GetString plaintext      public void Example         var password    correcthorsebatterystaple        var secret   this EncryptString password   Hello World         Console WriteLine  secret      BitConverter ToString secret         var recovered   this DecryptString password  secret        Console WriteLine recovered

User · Answer

using System  using System Data  using System Configuration  using System Text  using System Security Cryptography   namespace Encription       class CryptorEngine               public static string Encrypt string ToEncrypt  bool useHasing                        byte   keyArray              byte   toEncryptArray   UTF8Encoding UTF8 GetBytes ToEncrypt                 System Configuration AppSettingsReader settingsReader   new     AppSettingsReader               string Key    Bhagwati               if  useHasing                                MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                    keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes Key                    hashmd5 Clear                                else                               keyArray   UTF8Encoding UTF8 GetBytes Key                             TripleDESCryptoServiceProvider tDes   new TripleDESCryptoServiceProvider                tDes Key   keyArray              tDes Mode   CipherMode ECB              tDes Padding   PaddingMode PKCS7              ICryptoTransform cTransform   tDes CreateEncryptor                byte   resultArray   cTransform TransformFinalBlock toEncryptArray  0      toEncryptArray Length               tDes Clear                return Convert ToBase64String resultArray  0  resultArray Length                     public static string Decrypt string cypherString  bool useHasing                        byte   keyArray              byte   toDecryptArray   Convert FromBase64String cypherString                 byte   toEncryptArray   Convert FromBase64String cypherString                 System Configuration AppSettingsReader settingReader   new     AppSettingsReader                string key    Bhagwati               if  useHasing                                MD5CryptoServiceProvider hashmd   new MD5CryptoServiceProvider                    keyArray   hashmd ComputeHash UTF8Encoding UTF8 GetBytes key                    hashmd Clear                              else                               keyArray   UTF8Encoding UTF8 GetBytes key                             TripleDESCryptoServiceProvider tDes   new TripleDESCryptoServiceProvider                tDes Key   keyArray              tDes Mode   CipherMode ECB              tDes Padding   PaddingMode PKCS7              ICryptoTransform cTransform   tDes CreateDecryptor                try                               byte   resultArray   cTransform TransformFinalBlock toDecryptArray  0          toDecryptArray Length                    tDes Clear                    return UTF8Encoding UTF8 GetString resultArray 0 resultArray Length                             catch  Exception ex                                throw ex

User · Answer

using System              using System Collections Generic              using System Text              using System Text RegularExpressions      This is for password validation             using System Security Cryptography              using System Configuration      This is where the hash functions reside              namespace BullyTracker Common                               public class HashEncryption                                         public string GenerateHashvalue string thisPassword                                                    MD5CryptoServiceProvider md5   new MD5CryptoServiceProvider                              byte   tmpSource                            byte   tmpHash                             tmpSource   ASCIIEncoding ASCII GetBytes thisPassword      Turn password into byte array                           tmpHash   md5 ComputeHash tmpSource                              StringBuilder sOutput   new StringBuilder tmpHash Length                             for  int i   0  i  lt  tmpHash Length  i                                                              sOutput Append tmpHash i  ToString  X2         X2 formats to hexadecimal                                                       return sOutput ToString                                                  public Boolean VerifyHashPassword string thisPassword  string thisHash                                                    Boolean IsValid   false                            string tmpHash   GenerateHashvalue thisPassword      Call the routine on user input                           if  tmpHash    thisHash  IsValid   true      Compare to previously generated hash                           return IsValid                                              public string GenerateHashvalue string toEncrypt  bool useHashing                                                byte   keyArray                          byte   toEncryptArray   UTF8Encoding UTF8 GetBytes toEncrypt                            System Configuration AppSettingsReader settingsReader   new AppSettingsReader                               Get the key from config file                         string key    string settingsReader GetValue  SecurityKey   typeof String                              System Windows Forms MessageBox Show key                           if  useHashing                                                        MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                                keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes key                                hashmd5 Clear                                                      else                             keyArray   UTF8Encoding UTF8 GetBytes key                            TripleDESCryptoServiceProvider tdes   new TripleDESCryptoServiceProvider                            tdes Key   keyArray                          tdes Mode   CipherMode ECB                          tdes Padding   PaddingMode PKCS7                           ICryptoTransform cTransform   tdes CreateEncryptor                            byte   resultArray   cTransform TransformFinalBlock toEncryptArray  0  toEncryptArray Length                           tdes Clear                            return Convert ToBase64String resultArray  0  resultArray Length                                                  lt summary gt                          DeCrypt a string using dual encryption method  Return a DeCrypted clear string                          lt  summary gt                           lt param name  cipherString  gt encrypted string lt  param gt                           lt param name  useHashing  gt Did you use hashing to encrypt this data  pass true is yes lt  param gt                           lt returns gt  lt  returns gt                      public string Decrypt string cipherString  bool useHashing                                                byte   keyArray                          byte   toEncryptArray   Convert FromBase64String cipherString                            System Configuration AppSettingsReader settingsReader   new AppSettingsReader                              Get your key from config file to open the lock                          string key    string settingsReader GetValue  SecurityKey   typeof String                             if  useHashing                                                        MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                                keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes key                                hashmd5 Clear                                                      else                             keyArray   UTF8Encoding UTF8 GetBytes key                            TripleDESCryptoServiceProvider tdes   new TripleDESCryptoServiceProvider                            tdes Key   keyArray                          tdes Mode   CipherMode ECB                          tdes Padding   PaddingMode PKCS7                           ICryptoTransform cTransform   tdes CreateDecryptor                            byte   resultArray   cTransform TransformFinalBlock toEncryptArray  0  toEncryptArray Length                            tdes Clear                            return UTF8Encoding UTF8 GetString resultArray

User · Answer

To support mattmanser answer  Here s an example using MachineKey class to encrypt decrypt URL safe values    Something to bear in mind  as mentioned before  this will use Machine config settings  https   msdn microsoft com en-us library ff649308 aspx   You can set encryption and decryption key algorithm manually  you might need this specially if your site is running on multiple servers  in web config file  You can generate keys from IIS  see here  https   blogs msdn microsoft com vijaysk 2009 05 13 iis-7-tip-10-you-can-generate-machine-keys-from-the-iis-manager   or can use an online machine key generator like  http   www developerfusion com tools generatemachinekey       private static readonly UTF8Encoding Encoder   new UTF8Encoding         public static string Encrypt string unencrypted                if  string IsNullOrEmpty unencrypted                return string Empty           try                       var encryptedBytes   MachineKey Protect Encoder GetBytes unencrypted                 if  encryptedBytes    null  amp  amp  encryptedBytes Length  gt  0                  return HttpServerUtility UrlTokenEncode encryptedBytes                         catch  Exception                        return string Empty                     return string Empty             public static string Decrypt string encrypted                if  string IsNullOrEmpty encrypted                return string Empty           try                       var bytes   HttpServerUtility UrlTokenDecode encrypted               if  bytes    null  amp  amp  bytes Length  gt  0                                var decryptedBytes   MachineKey Unprotect bytes                   if decryptedBytes    null  amp  amp  decryptedBytes Length  gt  0                      return Encoder GetString decryptedBytes                                    catch  Exception                        return string Empty                     return string Empty

User · Answer

AES Algorithm   public static class CryptographyProvider               public static string EncryptString string plainText  out string Key                        if  plainText    null    plainText Length  lt   0                  throw new ArgumentNullException  plainText                 using  Aes  aesAlg   Aes Create                                  Key   Convert ToBase64String  aesAlg Key                   ICryptoTransform  encryptor    aesAlg CreateEncryptor  aesAlg Key   aesAlg IV                    using  MemoryStream  memoryStream   new MemoryStream                                           memoryStream Write  aesAlg IV  0  16                       using  CryptoStream  cryptoStream   new CryptoStream  memoryStream   encryptor  CryptoStreamMode Write                                                 using  StreamWriter  streamWriter   new StreamWriter  cryptoStream                                                          streamWriter Write plainText                                                     return Convert ToBase64String  memoryStream ToArray                                                                             public static string DecryptString string cipherText  string Key                         if  string IsNullOrEmpty cipherText                   throw new ArgumentNullException  cipherText                if  string IsNullOrEmpty Key                   throw new ArgumentNullException  Key                 string plaintext   null               byte    initialVector   new byte 16               byte    Key   Convert FromBase64String Key               byte    cipherTextBytesArray   Convert FromBase64String cipherText               byte    originalString   new byte  cipherTextBytesArray Length - 16                Array Copy  cipherTextBytesArray  0   initialVector  0   initialVector Length               Array Copy  cipherTextBytesArray  16   originalString  0   cipherTextBytesArray Length - 16                using  Aes  aesAlg   Aes Create                                   aesAlg Key    Key                   aesAlg IV    initialVector                  ICryptoTransform decryptor    aesAlg CreateDecryptor  aesAlg Key   aesAlg IV                    using  MemoryStream  memoryStream   new MemoryStream  originalString                                         using  CryptoStream  cryptoStream   new CryptoStream  memoryStream  decryptor  CryptoStreamMode Read                                                 using  StreamReader  streamReader   new StreamReader  cryptoStream                                                         plaintext    streamReader ReadToEnd                                                                                                return plaintext

User · Answer

Here is the sample how AES-GCM encryption/decryption can be done using Bouncy castle package.

I've found that sample when googled for possibility to decrypt data from GOlang crypto/aes api:

const (
    gcmBlockSize         = 16 // this is key size
    gcmTagSize           = 16 // this is mac
    gcmStandardNonceSize = 12 // this is nonce
)

func encrypt(data []byte, passphrase string) []byte {
    block, _ := aes.NewCipher([]byte(createHash(passphrase)))
    gcm, err := cipher.NewGCM(block)
    if err != nil {
        panic(err.Error())
    }
    nonce := make([]byte, gcm.NonceSize())
    if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
        panic(err.Error())
    }
    ciphertext := gcm.Seal(nonce, nonce, data, nil)
    return ciphertext
}

.Net sample works like a charm with key (256 bit), mac (128bit) and nonce (96 bit).

User · Answer

Here is an example using RSA    Important  There is a limit to the size of data you can encrypt with the RSA encryption  KeySize - MinimumPadding  e g  256 bytes  assuming 2048 bit key  - 42 bytes  min OEAP padding    214 bytes  max plaintext size   Replace your rsa key with your RSA key   var provider   new System Security Cryptography RSACryptoServiceProvider    provider ImportParameters your rsa key    var encryptedBytes   provider Encrypt      System Text Encoding UTF8 GetBytes  Hello World     true    string decryptedTest   System Text Encoding UTF8 GetString      provider Decrypt encryptedBytes  true      For more info  visit MSDN - RSACryptoServiceProvider

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using System  using System Data  using System Configuration  using System Text  using System Security Cryptography   namespace Encription       class CryptorEngine               public static string Encrypt string ToEncrypt  bool useHasing                        byte   keyArray              byte   toEncryptArray   UTF8Encoding UTF8 GetBytes ToEncrypt                 System Configuration AppSettingsReader settingsReader   new     AppSettingsReader               string Key    Bhagwati               if  useHasing                                MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                    keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes Key                    hashmd5 Clear                                else                               keyArray   UTF8Encoding UTF8 GetBytes Key                             TripleDESCryptoServiceProvider tDes   new TripleDESCryptoServiceProvider                tDes Key   keyArray              tDes Mode   CipherMode ECB              tDes Padding   PaddingMode PKCS7              ICryptoTransform cTransform   tDes CreateEncryptor                byte   resultArray   cTransform TransformFinalBlock toEncryptArray  0      toEncryptArray Length               tDes Clear                return Convert ToBase64String resultArray  0  resultArray Length                     public static string Decrypt string cypherString  bool useHasing                        byte   keyArray              byte   toDecryptArray   Convert FromBase64String cypherString                 byte   toEncryptArray   Convert FromBase64String cypherString                 System Configuration AppSettingsReader settingReader   new     AppSettingsReader                string key    Bhagwati               if  useHasing                                MD5CryptoServiceProvider hashmd   new MD5CryptoServiceProvider                    keyArray   hashmd ComputeHash UTF8Encoding UTF8 GetBytes key                    hashmd Clear                              else                               keyArray   UTF8Encoding UTF8 GetBytes key                             TripleDESCryptoServiceProvider tDes   new TripleDESCryptoServiceProvider                tDes Key   keyArray              tDes Mode   CipherMode ECB              tDes Padding   PaddingMode PKCS7              ICryptoTransform cTransform   tDes CreateDecryptor                try                               byte   resultArray   cTransform TransformFinalBlock toDecryptArray  0          toDecryptArray Length                    tDes Clear                    return UTF8Encoding UTF8 GetString resultArray 0 resultArray Length                             catch  Exception ex                                throw ex

User · Answer

AES Algorithm   public static class CryptographyProvider               public static string EncryptString string plainText  out string Key                        if  plainText    null    plainText Length  lt   0                  throw new ArgumentNullException  plainText                 using  Aes  aesAlg   Aes Create                                  Key   Convert ToBase64String  aesAlg Key                   ICryptoTransform  encryptor    aesAlg CreateEncryptor  aesAlg Key   aesAlg IV                    using  MemoryStream  memoryStream   new MemoryStream                                           memoryStream Write  aesAlg IV  0  16                       using  CryptoStream  cryptoStream   new CryptoStream  memoryStream   encryptor  CryptoStreamMode Write                                                 using  StreamWriter  streamWriter   new StreamWriter  cryptoStream                                                          streamWriter Write plainText                                                     return Convert ToBase64String  memoryStream ToArray                                                                             public static string DecryptString string cipherText  string Key                         if  string IsNullOrEmpty cipherText                   throw new ArgumentNullException  cipherText                if  string IsNullOrEmpty Key                   throw new ArgumentNullException  Key                 string plaintext   null               byte    initialVector   new byte 16               byte    Key   Convert FromBase64String Key               byte    cipherTextBytesArray   Convert FromBase64String cipherText               byte    originalString   new byte  cipherTextBytesArray Length - 16                Array Copy  cipherTextBytesArray  0   initialVector  0   initialVector Length               Array Copy  cipherTextBytesArray  16   originalString  0   cipherTextBytesArray Length - 16                using  Aes  aesAlg   Aes Create                                   aesAlg Key    Key                   aesAlg IV    initialVector                  ICryptoTransform decryptor    aesAlg CreateDecryptor  aesAlg Key   aesAlg IV                    using  MemoryStream  memoryStream   new MemoryStream  originalString                                         using  CryptoStream  cryptoStream   new CryptoStream  memoryStream  decryptor  CryptoStreamMode Read                                                 using  StreamReader  streamReader   new StreamReader  cryptoStream                                                         plaintext    streamReader ReadToEnd                                                                                                return plaintext

User · Answer

To support mattmanser answer  Here s an example using MachineKey class to encrypt decrypt URL safe values    Something to bear in mind  as mentioned before  this will use Machine config settings  https   msdn microsoft com en-us library ff649308 aspx   You can set encryption and decryption key algorithm manually  you might need this specially if your site is running on multiple servers  in web config file  You can generate keys from IIS  see here  https   blogs msdn microsoft com vijaysk 2009 05 13 iis-7-tip-10-you-can-generate-machine-keys-from-the-iis-manager   or can use an online machine key generator like  http   www developerfusion com tools generatemachinekey       private static readonly UTF8Encoding Encoder   new UTF8Encoding         public static string Encrypt string unencrypted                if  string IsNullOrEmpty unencrypted                return string Empty           try                       var encryptedBytes   MachineKey Protect Encoder GetBytes unencrypted                 if  encryptedBytes    null  amp  amp  encryptedBytes Length  gt  0                  return HttpServerUtility UrlTokenEncode encryptedBytes                         catch  Exception                        return string Empty                     return string Empty             public static string Decrypt string encrypted                if  string IsNullOrEmpty encrypted                return string Empty           try                       var bytes   HttpServerUtility UrlTokenDecode encrypted               if  bytes    null  amp  amp  bytes Length  gt  0                                var decryptedBytes   MachineKey Unprotect bytes                   if decryptedBytes    null  amp  amp  decryptedBytes Length  gt  0                      return Encoder GetString decryptedBytes                                    catch  Exception                        return string Empty                     return string Empty

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I want to give you my contribute  with my code for AES Rfc2898DeriveBytes  here the documentation  algorhytm  written in C    NET framework 4  and fully working also for limited platforms  as  NET Compact Framework for Windows Phone 7 0   not all platforms support every criptographic method of the  NET framework     I hope this can help anyone   using System  using System IO  using System Security Cryptography  using System Text   public static class Crypto       private static readonly byte   IVa   new byte     0x0b  0x0c  0x0d  0x0e  0x0f  0x11  0x11  0x12  0x13  0x14  0x0e  0x16  0x17          public static string Encrypt this string text  string salt                try                       using  Aes aes   new AesManaged                                  Rfc2898DeriveBytes deriveBytes   new Rfc2898DeriveBytes Encoding UTF8 GetString IVa  0  IVa Length   Encoding UTF8 GetBytes salt                    aes Key   deriveBytes GetBytes 128   8                   aes IV   aes Key                  using  MemoryStream encryptionStream   new MemoryStream                                          using  CryptoStream encrypt   new CryptoStream encryptionStream  aes CreateEncryptor    CryptoStreamMode Write                                                 byte   cleanText   Encoding UTF8 GetBytes text                           encrypt Write cleanText  0  cleanText Length                           encrypt FlushFinalBlock                                               byte   encryptedData   encryptionStream ToArray                        string encryptedText   Convert ToBase64String encryptedData                         return encryptedText                                                    catch                       return String Empty                       public static string Decrypt this string text  string salt                try                       using  Aes aes   new AesManaged                                  Rfc2898DeriveBytes deriveBytes   new Rfc2898DeriveBytes Encoding UTF8 GetString IVa  0  IVa Length   Encoding UTF8 GetBytes salt                    aes Key   deriveBytes GetBytes 128   8                   aes IV   aes Key                   using  MemoryStream decryptionStream   new MemoryStream                                          using  CryptoStream decrypt   new CryptoStream decryptionStream  aes CreateDecryptor    CryptoStreamMode Write                                                 byte   encryptedData   Convert FromBase64String text                             decrypt Write encryptedData  0  encryptedData Length                           decrypt Flush                                               byte   decryptedData   decryptionStream ToArray                        string decryptedText   Encoding UTF8 GetString decryptedData  0  decryptedData Length                         return decryptedText                                                    catch                       return String Empty

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This is the class that was placed here by Brett  However I made a slight edit since I was receiving the error  Invalid length for a Base-64 char array  when using it for URL strings to encrypt and decrypt   public class CryptoURL       private static byte    salt   Encoding ASCII GetBytes  Catto Salt Enter Any Value99              lt summary gt          Encrypt the given string using AES   The string can be decrypted using          DecryptStringAES     The sharedSecret parameters must match           The SharedSecret for the Password Reset that is used is in the next line          string sharedSecret    OneUpSharedSecret9            lt  summary gt           lt param name  plainText  gt The text to encrypt  lt  param gt           lt param name  sharedSecret  gt A password used to generate a key for encryption  lt  param gt      public static string EncryptString string plainText  string sharedSecret                if  string IsNullOrEmpty plainText               throw new ArgumentNullException  plainText            if  string IsNullOrEmpty sharedSecret               throw new ArgumentNullException  sharedSecret             string outStr   null                           Encrypted string to return         RijndaelManaged aesAlg   null                  RijndaelManaged object used to encrypt the data           try                          generate the key from the shared secret and the salt             Rfc2898DeriveBytes key   new Rfc2898DeriveBytes sharedSecret   salt                   Create a RijndaelManaged object             aesAlg   new RijndaelManaged                aesAlg Key   key GetBytes aesAlg KeySize   8                   Create a decryptor to perform the stream transform              ICryptoTransform encryptor   aesAlg CreateEncryptor aesAlg Key  aesAlg IV                   Create the streams used for encryption              using  MemoryStream msEncrypt   new MemoryStream                                     prepend the IV                 msEncrypt Write BitConverter GetBytes aesAlg IV Length   0  sizeof int                    msEncrypt Write aesAlg IV  0  aesAlg IV Length                   using  CryptoStream csEncrypt   new CryptoStream msEncrypt  encryptor  CryptoStreamMode Write                                         using  StreamWriter swEncrypt   new StreamWriter csEncrypt                                                   Write all data to the stream                          swEncrypt Write plainText                                                            outStr   HttpServerUtility UrlTokenEncode msEncrypt ToArray                       outStr   Convert ToBase64String msEncrypt ToArray                        you may need to add a reference  right click reference in solution explorer   gt   add Reference    gt   NET tab   gt  select  System Web                                  finally                          Clear the RijndaelManaged object              if  aesAlg    null                  aesAlg Clear                          Return the encrypted bytes from the memory stream          return outStr                  lt summary gt          Decrypt the given string   Assumes the string was encrypted using          EncryptStringAES    using an identical sharedSecret           lt  summary gt           lt param name  cipherText  gt The text to decrypt  lt  param gt           lt param name  sharedSecret  gt A password used to generate a key for decryption  lt  param gt      public static string DecryptString string cipherText  string sharedSecret                if  string IsNullOrEmpty cipherText               throw new ArgumentNullException  cipherText            if  string IsNullOrEmpty sharedSecret               throw new ArgumentNullException  sharedSecret                Declare the RijndaelManaged object            used to decrypt the data          RijndaelManaged aesAlg   null              Declare the string used to hold            the decrypted text          string plaintext   null           byte   inputByteArray           try                          generate the key from the shared secret and the salt             Rfc2898DeriveBytes key   new Rfc2898DeriveBytes sharedSecret   salt                   Create the streams used for decryption                                byte   bytes   Convert FromBase64String cipherText               inputByteArray   HttpServerUtility UrlTokenDecode cipherText                using  MemoryStream msDecrypt   new MemoryStream inputByteArray                                    Create a RijndaelManaged object                    with the specified key and IV                  aesAlg   new RijndaelManaged                    aesAlg Key   key GetBytes aesAlg KeySize   8                      Get the initialization vector from the encrypted stream                 aesAlg IV   ReadByteArray msDecrypt                      Create a decrytor to perform the stream transform                  ICryptoTransform decryptor   aesAlg CreateDecryptor aesAlg Key  aesAlg IV                   using  CryptoStream csDecrypt   new CryptoStream msDecrypt  decryptor  CryptoStreamMode Read                                         using  StreamReader srDecrypt   new StreamReader csDecrypt                               Read the decrypted bytes from the decrypting stream                            and place them in a string                          plaintext   srDecrypt ReadToEnd                                                      catch  System Exception ex                        return  ERROR                 throw ex                     finally                          Clear the RijndaelManaged object              if  aesAlg    null                  aesAlg Clear                       return plaintext             static string ConvertStringArrayToString string   array                              Concatenate all the elements into a StringBuilder                     StringBuilder builder   new StringBuilder            foreach  string value in array                        builder Append value               builder Append                         return builder ToString               private static byte   ReadByteArray Stream s                byte   rawLength   new byte sizeof int            if  s Read rawLength  0  rawLength Length     rawLength Length                        throw new SystemException  Stream did not contain properly formatted byte array                       byte   buffer   new byte BitConverter ToInt32 rawLength  0            if  s Read buffer  0  buffer Length     buffer Length                        throw new SystemException  Did not read byte array properly                       return buffer

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If you are using ASP Net you can now use built in functionality in  Net 4 0 onwards   System Web Security MachineKey   Net 4 5 has MachineKey Protect   and MachineKey Unprotect      Net 4 0 has MachineKey Encode   and MachineKey Decode    You should just set the MachineKeyProtection to  All    Outside of ASP Net this class seems to generate a new key with every app restart so doesn t work  With a quick peek in ILSpy it looks to me like it generates its own defaults if the appropriate app settings are missing  So you may actually be able to set it up outside ASP Net   I haven t been able to find a non-ASP Net equivalent outside the System Web namespace

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Encryption  public string EncryptString string inputString        MemoryStream memStream   null      try               byte   key                byte   IV     12  21  43  17  57  35  67  27            string encryptKey    aXb2uy4z      MUST be 8 characters         key   Encoding UTF8 GetBytes encryptKey           byte   byteInput   Encoding UTF8 GetBytes inputString           DESCryptoServiceProvider provider   new DESCryptoServiceProvider            memStream   new MemoryStream            ICryptoTransform transform   provider CreateEncryptor key  IV           CryptoStream cryptoStream   new CryptoStream memStream  transform  CryptoStreamMode Write           cryptoStream Write byteInput  0  byteInput Length           cryptoStream FlushFinalBlock              catch  Exception ex                Response Write ex Message             return Convert ToBase64String memStream ToArray         Decryption       public string DecryptString string inputString        MemoryStream memStream   null      try               byte   key                byte   IV     12  21  43  17  57  35  67  27            string encryptKey    aXb2uy4z      MUST be 8 characters         key   Encoding UTF8 GetBytes encryptKey           byte   byteInput   new byte inputString Length           byteInput   Convert FromBase64String inputString           DESCryptoServiceProvider provider   new DESCryptoServiceProvider            memStream   new MemoryStream            ICryptoTransform transform   provider CreateDecryptor key  IV           CryptoStream cryptoStream   new CryptoStream memStream  transform  CryptoStreamMode Write           cryptoStream Write byteInput  0  byteInput Length           cryptoStream FlushFinalBlock              catch  Exception ex                Response Write ex Message              Encoding encoding1   Encoding UTF8      return encoding1 GetString memStream ToArray

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Disclaimer  This solution should only be used for data at rest that is not exposed to the public  for example - a configuration file or DB   Only in this scenario  the quick-and-dirty solution can be considered better than  jbtule s solution  due to lower maintanance   Original post  I found jbtule s answer a bit complicated for a quick and dirty secured AES string encryption and Brett s answer had a bug with the Initialization Vector being a fixed value making it vulnerable to padding attacks  so I fixed Brett s code and added a random IV that is added to the chipered string  creating a different encrypted value each and every encryption of the same value   Encryption   public static string Encrypt string clearText                    byte   clearBytes   Encoding Unicode GetBytes clearText       using  Aes encryptor   Aes Create                  byte   IV   new byte 15           rand NextBytes IV           Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  IV           encryptor Key   pdb GetBytes 32           encryptor IV   pdb GetBytes 16           using  MemoryStream ms   new MemoryStream                          using  CryptoStream cs   new CryptoStream ms  encryptor CreateEncryptor    CryptoStreamMode Write                                 cs Write clearBytes  0  clearBytes Length                   cs Close                              clearText   Convert ToBase64String IV    Convert ToBase64String ms ToArray                         return clearText      Decryption   public static string Decrypt string cipherText        byte   IV   Convert FromBase64String cipherText Substring 0  20        cipherText   cipherText Substring 20  Replace                byte   cipherBytes   Convert FromBase64String cipherText       using  Aes encryptor   Aes Create                  Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  IV           encryptor Key   pdb GetBytes 32           encryptor IV   pdb GetBytes 16           using  MemoryStream ms   new MemoryStream                          using  CryptoStream cs   new CryptoStream ms  encryptor CreateDecryptor    CryptoStreamMode Write                                 cs Write cipherBytes  0  cipherBytes Length                   cs Close                              cipherText   Encoding Unicode GetString ms ToArray                         return cipherText      Replace EncryptionKey with your key  In my implementation  the key is being saved in the configuration file  web config app config  as you shouldn t save it hard coded  The configuration file should be also encrypted so the key won t be saved as clear text in it   protected static string  Key       protected static string EncryptionKey       get               if  String IsNullOrEmpty  Key                          Key   ConfigurationManager AppSettings  AESKey   ToString                       return  Key

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using System  using System Collections Generic  using System Linq  using System Web  using System Security Cryptography  using System IO  using System Text          lt summary gt      Summary description for Encryption      lt  summary gt  public class Encryption       public TripleDES CreateDES string key                MD5 md5   new MD5CryptoServiceProvider            TripleDES des   new TripleDESCryptoServiceProvider            des Key   md5 ComputeHash Encoding Unicode GetBytes key            des IV   new byte des BlockSize   8           return des            public  byte   Encryptiondata string PlainText                TripleDES des   CreateDES  DreamMLMKey            ICryptoTransform ct   des CreateEncryptor            byte   input   Encoding Unicode GetBytes PlainText           return ct TransformFinalBlock input  0  input Length              public string Decryptiondata string CypherText                string stringToDecrypt   CypherText Replace                    int len   stringToDecrypt Length          byte   inputByteArray   Convert FromBase64String stringToDecrypt             byte   b   Convert FromBase64String CypherText           TripleDES des   CreateDES  DreamMLMKey            ICryptoTransform ct   des CreateDecryptor            byte   output   ct TransformFinalBlock b  0  b Length           return Encoding Unicode GetString output             public string Decryptiondataurl string CypherText                string newcyperttext CypherText Replace                    byte   b   Convert FromBase64String newcyperttext           TripleDES des   CreateDES  DreamMLMKey            ICryptoTransform ct   des CreateDecryptor            byte   output   ct TransformFinalBlock b  0  b Length           return Encoding Unicode GetString output                region  encryption  amp  Decription     public  string Encrypt string input  string key                byte   inputArray   UTF8Encoding UTF8 GetBytes input           TripleDESCryptoServiceProvider tripleDES   new TripleDESCryptoServiceProvider            tripleDES Key   UTF8Encoding UTF8 GetBytes key           tripleDES Mode   CipherMode ECB          tripleDES Padding   PaddingMode PKCS7          ICryptoTransform cTransform   tripleDES CreateEncryptor            byte   resultArray   cTransform TransformFinalBlock inputArray  0  inputArray Length           tripleDES Clear            return Convert ToBase64String resultArray  0  resultArray Length             public  string Decrypt string input  string key                byte   inputArray   Convert FromBase64String input           TripleDESCryptoServiceProvider tripleDES   new TripleDESCryptoServiceProvider            tripleDES Key   UTF8Encoding UTF8 GetBytes key           tripleDES Mode   CipherMode ECB          tripleDES Padding   PaddingMode PKCS7          ICryptoTransform cTransform   tripleDES CreateDecryptor            byte   resultArray   cTransform TransformFinalBlock inputArray  0  inputArray Length           tripleDES Clear            return UTF8Encoding UTF8 GetString resultArray              public string encrypt string encryptString                string EncryptionKey    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ           byte   clearBytes   Encoding Unicode GetBytes encryptString           using  Aes encryptor   Aes Create                          Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  new byte                     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76                             encryptor Key   pdb GetBytes 32               encryptor IV   pdb GetBytes 16               using  MemoryStream ms   new MemoryStream                                  using  CryptoStream cs   new CryptoStream ms  encryptor CreateEncryptor    CryptoStreamMode Write                                         cs Write clearBytes  0  clearBytes Length                       cs Close                                      encryptString   Convert ToBase64String ms ToArray                                     return encryptString             public string Decrypt string cipherText                string EncryptionKey    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ           cipherText   cipherText Replace                    byte   cipherBytes   Convert FromBase64String cipherText           using  Aes encryptor   Aes Create                          Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  new byte                     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76                             encryptor Key   pdb GetBytes 32               encryptor IV   pdb GetBytes 16               using  MemoryStream ms   new MemoryStream                                  using  CryptoStream cs   new CryptoStream ms  encryptor CreateDecryptor    CryptoStreamMode Write                                         cs Write cipherBytes  0  cipherBytes Length                       cs Close                                      cipherText   Encoding Unicode GetString ms ToArray                                     return cipherText              endregion

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Modern Examples of Symmetric Authenticated Encryption of a string    The general best practice for symmetric encryption is to use Authenticated Encryption with Associated Data  AEAD   however this isn t a part of the standard  net crypto libraries  So the first example uses AES256 and then HMAC256  a two step Encrypt then MAC  which requires more overhead and more keys   The second example uses the simpler practice of AES256-GCM using the open source Bouncy Castle  via nuget    Both examples have a main function that takes secret message string  key s  and an optional non-secret payload and return and authenticated encrypted string optionally prepended with the non-secret data  Ideally you would use these with 256bit key s  randomly generated see NewKey     Both examples also have a helper methods that use a string password to generate the keys  These helper methods are provided as a convenience to match up with other examples  however they are far less secure because the strength of the password is going to be far weaker than a 256 bit key   Update  Added byte   overloads  and only the Gist has the full formatting with 4 spaces indent and api docs due to StackOverflow answer limits      NET Built-in Encrypt AES -Then-MAC HMAC   Gist         This work  Modern Encryption of a String C   by James Tuley       identified by James Tuley  is free of known copyright restrictions     https   gist github com 4336842    http   creativecommons org publicdomain mark 1 0        using System  using System IO  using System Security Cryptography  using System Text   namespace Encryption     public static class AESThenHMAC         private static readonly RandomNumberGenerator Random   RandomNumberGenerator Create           Preconfigured Encryption Parameters     public static readonly int BlockBitSize   128      public static readonly int KeyBitSize   256         Preconfigured Password Key Derivation Parameters     public static readonly int SaltBitSize   64      public static readonly int Iterations   10000      public static readonly int MinPasswordLength   12            lt summary gt          Helper that generates a random key on each call           lt  summary gt           lt returns gt  lt  returns gt      public static byte   NewKey               var key   new byte KeyBitSize   8         Random GetBytes key         return key                  lt summary gt          Simple Encryption  AES  then Authentication  HMAC  for a UTF8 Message           lt  summary gt           lt param name  secretMessage  gt The secret message  lt  param gt           lt param name  cryptKey  gt The crypt key  lt  param gt           lt param name  authKey  gt The auth key  lt  param gt           lt param name  nonSecretPayload  gt  Optional  Non-Secret Payload  lt  param gt           lt returns gt          Encrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Secret Message Required  secretMessage lt  exception gt           lt remarks gt          Adds overhead of  Optional-Payload   BlockSize 16    Message-Padded-To-Blocksize    HMac-Tag 32     1 33 Base64          lt  remarks gt      public static string SimpleEncrypt string secretMessage  byte   cryptKey  byte   authKey                         byte   nonSecretPayload   null              if  string IsNullOrEmpty secretMessage           throw new ArgumentException  Secret Message Required     secretMessage           var plainText   Encoding UTF8 GetBytes secretMessage         var cipherText   SimpleEncrypt plainText  cryptKey  authKey  nonSecretPayload         return Convert ToBase64String cipherText                   lt summary gt          Simple Authentication  HMAC  then Decryption  AES  for a secrets UTF8 Message           lt  summary gt           lt param name  encryptedMessage  gt The encrypted message  lt  param gt           lt param name  cryptKey  gt The crypt key  lt  param gt           lt param name  authKey  gt The auth key  lt  param gt           lt param name  nonSecretPayloadLength  gt Length of the non secret payload  lt  param gt           lt returns gt          Decrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Encrypted Message Required  encryptedMessage lt  exception gt      public static string SimpleDecrypt string encryptedMessage  byte   cryptKey  byte   authKey                         int nonSecretPayloadLength   0              if  string IsNullOrWhiteSpace encryptedMessage           throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cipherText   Convert FromBase64String encryptedMessage         var plainText   SimpleDecrypt cipherText  cryptKey  authKey  nonSecretPayloadLength         return plainText    null   null   Encoding UTF8 GetString plainText                   lt summary gt          Simple Encryption  AES  then Authentication  HMAC  of a UTF8 message         using Keys derived from a Password  PBKDF2            lt  summary gt           lt param name  secretMessage  gt The secret message  lt  param gt           lt param name  password  gt The password  lt  param gt           lt param name  nonSecretPayload  gt The non secret payload  lt  param gt           lt returns gt          Encrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt password lt  exception gt           lt remarks gt          Significantly less secure than using random binary keys          Adds additional non secret payload for key generation parameters           lt  remarks gt      public static string SimpleEncryptWithPassword string secretMessage  string password                               byte   nonSecretPayload   null              if  string IsNullOrEmpty secretMessage           throw new ArgumentException  Secret Message Required     secretMessage           var plainText   Encoding UTF8 GetBytes secretMessage         var cipherText   SimpleEncryptWithPassword plainText  password  nonSecretPayload         return Convert ToBase64String cipherText                   lt summary gt          Simple Authentication  HMAC  and then Descryption  AES  of a UTF8 Message         using keys derived from a password  PBKDF2             lt  summary gt           lt param name  encryptedMessage  gt The encrypted message  lt  param gt           lt param name  password  gt The password  lt  param gt           lt param name  nonSecretPayloadLength  gt Length of the non secret payload  lt  param gt           lt returns gt          Decrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Encrypted Message Required  encryptedMessage lt  exception gt           lt remarks gt          Significantly less secure than using random binary keys           lt  remarks gt      public static string SimpleDecryptWithPassword string encryptedMessage  string password                               int nonSecretPayloadLength   0              if  string IsNullOrWhiteSpace encryptedMessage           throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cipherText   Convert FromBase64String encryptedMessage         var plainText   SimpleDecryptWithPassword cipherText  password  nonSecretPayloadLength         return plainText    null   null   Encoding UTF8 GetString plainText              public static byte   SimpleEncrypt byte   secretMessage  byte   cryptKey  byte   authKey  byte   nonSecretPayload   null                User Error Checks       if  cryptKey    null    cryptKey Length    KeyBitSize   8          throw new ArgumentException String Format  Key needs to be  0  bit    KeyBitSize    cryptKey           if  authKey    null    authKey Length    KeyBitSize   8          throw new ArgumentException String Format  Key needs to be  0  bit    KeyBitSize    authKey           if  secretMessage    null    secretMessage Length  lt  1          throw new ArgumentException  Secret Message Required     secretMessage             non-secret payload optional       nonSecretPayload   nonSecretPayload    new byte               byte   cipherText        byte   iv         using  var aes   new AesManaged                 KeySize   KeyBitSize          BlockSize   BlockBitSize          Mode   CipherMode CBC          Padding   PaddingMode PKCS7                             Use random IV         aes GenerateIV            iv   aes IV           using  var encrypter   aes CreateEncryptor cryptKey  iv           using  var cipherStream   new MemoryStream                        using  var cryptoStream   new CryptoStream cipherStream  encrypter  CryptoStreamMode Write             using  var binaryWriter   new BinaryWriter cryptoStream                             Encrypt Data             binaryWriter Write secretMessage                          cipherText   cipherStream ToArray                                Assemble encrypted message and add authentication       using  var hmac   new HMACSHA256 authKey         using  var encryptedStream   new MemoryStream                    using  var binaryWriter   new BinaryWriter encryptedStream                         Prepend non-secret payload if any           binaryWriter Write nonSecretPayload               Prepend IV           binaryWriter Write iv               Write Ciphertext           binaryWriter Write cipherText             binaryWriter Flush                 Authenticate all data           var tag   hmac ComputeHash encryptedStream ToArray                 Postpend tag           binaryWriter Write tag                     return encryptedStream ToArray                        public static byte   SimpleDecrypt byte   encryptedMessage  byte   cryptKey  byte   authKey  int nonSecretPayloadLength   0                 Basic Usage Error Checks       if  cryptKey    null    cryptKey Length    KeyBitSize   8          throw new ArgumentException String Format  CryptKey needs to be  0  bit    KeyBitSize    cryptKey           if  authKey    null    authKey Length    KeyBitSize   8          throw new ArgumentException String Format  AuthKey needs to be  0  bit    KeyBitSize    authKey           if  encryptedMessage    null    encryptedMessage Length    0          throw new ArgumentException  Encrypted Message Required     encryptedMessage           using  var hmac   new HMACSHA256 authKey                   var sentTag   new byte hmac HashSize   8             Calculate Tag         var calcTag   hmac ComputeHash encryptedMessage  0  encryptedMessage Length - sentTag Length           var ivLength    BlockBitSize   8              if message length is to small just return null         if  encryptedMessage Length  lt  sentTag Length   nonSecretPayloadLength   ivLength            return null             Grab Sent Tag         Array Copy encryptedMessage  encryptedMessage Length - sentTag Length  sentTag  0  sentTag Length              Compare Tag with constant time comparison         var compare   0          for  var i   0  i  lt  sentTag Length  i              compare    sentTag i    calcTag i               if message doesn t authenticate return null         if  compare    0            return null           using  var aes   new AesManaged                     KeySize   KeyBitSize            BlockSize   BlockBitSize            Mode   CipherMode CBC            Padding   PaddingMode PKCS7                                   Grab IV from message           var iv   new byte ivLength             Array Copy encryptedMessage  nonSecretPayloadLength  iv  0  iv Length              using  var decrypter   aes CreateDecryptor cryptKey  iv             using  var plainTextStream   new MemoryStream                            using  var decrypterStream   new CryptoStream plainTextStream  decrypter  CryptoStreamMode Write               using  var binaryWriter   new BinaryWriter decrypterStream                                 Decrypt Cipher Text from Message               binaryWriter Write                  encryptedMessage                  nonSecretPayloadLength   iv Length                  encryptedMessage Length - nonSecretPayloadLength - iv Length - sentTag Length                                              Return Plain Text             return plainTextStream ToArray                                             public static byte   SimpleEncryptWithPassword byte   secretMessage  string password  byte   nonSecretPayload   null              nonSecretPayload   nonSecretPayload    new byte                User Error Checks       if  string IsNullOrWhiteSpace password     password Length  lt  MinPasswordLength          throw new ArgumentException String Format  Must have a password of at least  0  characters    MinPasswordLength    password           if  secretMessage    null    secretMessage Length   0          throw new ArgumentException  Secret Message Required     secretMessage           var payload   new byte   SaltBitSize   8    2    nonSecretPayload Length          Array Copy nonSecretPayload  payload  nonSecretPayload Length         int payloadIndex   nonSecretPayload Length         byte   cryptKey        byte   authKey          Use Random Salt to prevent pre-generated weak password attacks        using  var generator   new Rfc2898DeriveBytes password  SaltBitSize   8  Iterations                   var salt   generator Salt             Generate Keys         cryptKey   generator GetBytes KeyBitSize   8              Create Non Secret Payload         Array Copy salt  0  payload  payloadIndex  salt Length           payloadIndex    salt Length                   Deriving separate key  might be less efficient than using HKDF           but now compatible with RNEncryptor which had a very similar wireformat and requires less code than HKDF        using  var generator   new Rfc2898DeriveBytes password  SaltBitSize   8  Iterations                   var salt   generator Salt             Generate Keys         authKey   generator GetBytes KeyBitSize   8              Create Rest of Non Secret Payload         Array Copy salt  0  payload  payloadIndex  salt Length                  return SimpleEncrypt secretMessage  cryptKey  authKey  payload              public static byte   SimpleDecryptWithPassword byte   encryptedMessage  string password  int nonSecretPayloadLength   0                User Error Checks       if  string IsNullOrWhiteSpace password     password Length  lt  MinPasswordLength          throw new ArgumentException String Format  Must have a password of at least  0  characters    MinPasswordLength    password           if  encryptedMessage    null    encryptedMessage Length    0          throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cryptSalt   new byte SaltBitSize   8         var authSalt   new byte SaltBitSize   8            Grab Salt from Non-Secret Payload       Array Copy encryptedMessage  nonSecretPayloadLength  cryptSalt  0  cryptSalt Length         Array Copy encryptedMessage  nonSecretPayloadLength   cryptSalt Length  authSalt  0  authSalt Length          byte   cryptKey        byte   authKey           Generate crypt key       using  var generator   new Rfc2898DeriveBytes password  cryptSalt  Iterations                   cryptKey   generator GetBytes KeyBitSize   8                   Generate auth key       using  var generator   new Rfc2898DeriveBytes password  authSalt  Iterations                   authKey   generator GetBytes KeyBitSize   8                  return SimpleDecrypt encryptedMessage  cryptKey  authKey  cryptSalt Length   authSalt Length   nonSecretPayloadLength                   Bouncy Castle AES-GCM  Gist         This work  Modern Encryption of a String C   by James Tuley       identified by James Tuley  is free of known copyright restrictions     https   gist github com 4336842    http   creativecommons org publicdomain mark 1 0        using System  using System IO  using System Text  using Org BouncyCastle Crypto  using Org BouncyCastle Crypto Engines  using Org BouncyCastle Crypto Generators  using Org BouncyCastle Crypto Modes  using Org BouncyCastle Crypto Parameters  using Org BouncyCastle Security  namespace Encryption      public static class AESGCM         private static readonly SecureRandom Random   new SecureRandom           Preconfigured Encryption Parameters     public static readonly int NonceBitSize   128      public static readonly int MacBitSize   128      public static readonly int KeyBitSize   256         Preconfigured Password Key Derivation Parameters     public static readonly int SaltBitSize   128      public static readonly int Iterations   10000      public static readonly int MinPasswordLength   12             lt summary gt          Helper that generates a random new key on each call           lt  summary gt           lt returns gt  lt  returns gt      public static byte   NewKey               var key   new byte KeyBitSize   8         Random NextBytes key         return key                  lt summary gt          Simple Encryption And Authentication  AES-GCM  of a UTF8 string           lt  summary gt           lt param name  secretMessage  gt The secret message  lt  param gt           lt param name  key  gt The key  lt  param gt           lt param name  nonSecretPayload  gt Optional non-secret payload  lt  param gt           lt returns gt          Encrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Secret Message Required  secretMessage lt  exception gt           lt remarks gt          Adds overhead of  Optional-Payload   BlockSize 16    Message    HMac-Tag 16     1 33 Base64          lt  remarks gt      public static string SimpleEncrypt string secretMessage  byte   key  byte   nonSecretPayload   null              if  string IsNullOrEmpty secretMessage           throw new ArgumentException  Secret Message Required     secretMessage           var plainText   Encoding UTF8 GetBytes secretMessage         var cipherText   SimpleEncrypt plainText  key  nonSecretPayload         return Convert ToBase64String cipherText                    lt summary gt          Simple Decryption  amp  Authentication  AES-GCM  of a UTF8 Message          lt  summary gt           lt param name  encryptedMessage  gt The encrypted message  lt  param gt           lt param name  key  gt The key  lt  param gt           lt param name  nonSecretPayloadLength  gt Length of the optional non-secret payload  lt  param gt           lt returns gt Decrypted Message lt  returns gt      public static string SimpleDecrypt string encryptedMessage  byte   key  int nonSecretPayloadLength   0              if  string IsNullOrEmpty encryptedMessage           throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cipherText   Convert FromBase64String encryptedMessage         var plainText   SimpleDecrypt cipherText  key  nonSecretPayloadLength         return plainText    null   null   Encoding UTF8 GetString plainText                   lt summary gt          Simple Encryption And Authentication  AES-GCM  of a UTF8 String         using key derived from a password  PBKDF2            lt  summary gt           lt param name  secretMessage  gt The secret message  lt  param gt           lt param name  password  gt The password  lt  param gt           lt param name  nonSecretPayload  gt The non secret payload  lt  param gt           lt returns gt          Encrypted Message          lt  returns gt           lt remarks gt          Significantly less secure than using random binary keys          Adds additional non secret payload for key generation parameters           lt  remarks gt      public static string SimpleEncryptWithPassword string secretMessage  string password                               byte   nonSecretPayload   null              if  string IsNullOrEmpty secretMessage           throw new ArgumentException  Secret Message Required     secretMessage           var plainText   Encoding UTF8 GetBytes secretMessage         var cipherText   SimpleEncryptWithPassword plainText  password  nonSecretPayload         return Convert ToBase64String cipherText                    lt summary gt          Simple Decryption and Authentication  AES-GCM  of a UTF8 message         using a key derived from a password  PBKDF2           lt  summary gt           lt param name  encryptedMessage  gt The encrypted message  lt  param gt           lt param name  password  gt The password  lt  param gt           lt param name  nonSecretPayloadLength  gt Length of the non secret payload  lt  param gt           lt returns gt          Decrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Encrypted Message Required  encryptedMessage lt  exception gt           lt remarks gt          Significantly less secure than using random binary keys           lt  remarks gt      public static string SimpleDecryptWithPassword string encryptedMessage  string password                               int nonSecretPayloadLength   0              if  string IsNullOrWhiteSpace encryptedMessage           throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cipherText   Convert FromBase64String encryptedMessage         var plainText   SimpleDecryptWithPassword cipherText  password  nonSecretPayloadLength         return plainText    null   null   Encoding UTF8 GetString plainText              public static byte   SimpleEncrypt byte   secretMessage  byte   key  byte   nonSecretPayload   null                User Error Checks       if  key    null    key Length    KeyBitSize   8          throw new ArgumentException String Format  Key needs to be  0  bit    KeyBitSize    key           if  secretMessage    null    secretMessage Length    0          throw new ArgumentException  Secret Message Required     secretMessage             Non-secret Payload Optional       nonSecretPayload   nonSecretPayload    new byte                 Using random nonce large enough not to repeat       var nonce   new byte NonceBitSize   8         Random NextBytes nonce  0  nonce Length          var cipher   new GcmBlockCipher new AesFastEngine           var parameters   new AeadParameters new KeyParameter key   MacBitSize  nonce  nonSecretPayload         cipher Init true  parameters            Generate Cipher Text With Auth Tag       var cipherText   new byte cipher GetOutputSize secretMessage Length          var len   cipher ProcessBytes secretMessage  0  secretMessage Length  cipherText  0         cipher DoFinal cipherText  len            Assemble Message       using  var combinedStream   new MemoryStream                    using  var binaryWriter   new BinaryWriter combinedStream                         Prepend Authenticated Payload           binaryWriter Write nonSecretPayload               Prepend Nonce           binaryWriter Write nonce               Write Cipher Text           binaryWriter Write cipherText                     return combinedStream ToArray                       public static byte   SimpleDecrypt byte   encryptedMessage  byte   key  int nonSecretPayloadLength   0                User Error Checks       if  key    null    key Length    KeyBitSize   8          throw new ArgumentException String Format  Key needs to be  0  bit    KeyBitSize    key           if  encryptedMessage    null    encryptedMessage Length    0          throw new ArgumentException  Encrypted Message Required     encryptedMessage           using  var cipherStream   new MemoryStream encryptedMessage         using  var cipherReader   new BinaryReader cipherStream                     Grab Payload         var nonSecretPayload   cipherReader ReadBytes nonSecretPayloadLength              Grab Nonce         var nonce   cipherReader ReadBytes NonceBitSize   8            var cipher   new GcmBlockCipher new AesFastEngine             var parameters   new AeadParameters new KeyParameter key   MacBitSize  nonce  nonSecretPayload           cipher Init false  parameters              Decrypt Cipher Text         var cipherText   cipherReader ReadBytes encryptedMessage Length - nonSecretPayloadLength - nonce Length           var plainText   new byte cipher GetOutputSize cipherText Length               try                     var len   cipher ProcessBytes cipherText  0  cipherText Length  plainText  0             cipher DoFinal plainText  len                      catch  InvalidCipherTextException                        Return null if it doesn t authenticate           return null                     return plainText                      public static byte   SimpleEncryptWithPassword byte   secretMessage  string password  byte   nonSecretPayload   null              nonSecretPayload   nonSecretPayload    new byte                User Error Checks       if  string IsNullOrWhiteSpace password     password Length  lt  MinPasswordLength          throw new ArgumentException String Format  Must have a password of at least  0  characters    MinPasswordLength    password           if  secretMessage    null    secretMessage Length    0          throw new ArgumentException  Secret Message Required     secretMessage           var generator   new Pkcs5S2ParametersGenerator             Use Random Salt to minimize pre-generated weak password attacks        var salt   new byte SaltBitSize   8         Random NextBytes salt          generator Init          PbeParametersGenerator Pkcs5PasswordToBytes password ToCharArray             salt          Iterations            Generate Key       var key    KeyParameter generator GenerateDerivedMacParameters KeyBitSize            Create Full Non Secret Payload       var payload   new byte salt Length   nonSecretPayload Length         Array Copy nonSecretPayload  payload  nonSecretPayload Length         Array Copy salt 0  payload nonSecretPayload Length  salt Length          return SimpleEncrypt secretMessage  key GetKey    payload              public static byte   SimpleDecryptWithPassword byte   encryptedMessage  string password  int nonSecretPayloadLength   0                User Error Checks       if  string IsNullOrWhiteSpace password     password Length  lt  MinPasswordLength          throw new ArgumentException String Format  Must have a password of at least  0  characters    MinPasswordLength    password           if  encryptedMessage    null    encryptedMessage Length    0          throw new ArgumentException  Encrypted Message Required     encryptedMessage           var generator   new Pkcs5S2ParametersGenerator             Grab Salt from Payload       var salt   new byte SaltBitSize   8         Array Copy encryptedMessage  nonSecretPayloadLength  salt  0  salt Length          generator Init          PbeParametersGenerator Pkcs5PasswordToBytes password ToCharArray             salt          Iterations            Generate Key       var key    KeyParameter generator GenerateDerivedMacParameters KeyBitSize          return SimpleDecrypt encryptedMessage  key GetKey    salt Length   nonSecretPayloadLength

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for simplicity i made for myself this function that i use for non crypto purposes   replace  yourpassphrase  with your password      using System  using System Collections Generic  using System Linq  using System Text  using System Security Cryptography  using System IO    namespace My       public class strCrypto                  This constant string is used as a  salt  value for the PasswordDeriveBytes function calls         This size of the IV  in bytes  must    keysize   8    Default keysize is 256  so the IV must be        32 bytes long   Using a 16 character string here gives us 32 bytes when converted to a byte array      private const string initVector    r5dm5fgm24mfhfku       private const string passPhrase    yourpassphrase      email password encryption password         This constant is used to determine the keysize of the encryption algorithm      private const int keysize   256       public static string encryptString string plainText                  if the plaintext  is empty or null string just return an empty string         if  plainText          plainText    null                         return                        byte   initVectorBytes   Encoding UTF8 GetBytes initVector           byte   plainTextBytes   Encoding UTF8 GetBytes plainText           PasswordDeriveBytes password   new PasswordDeriveBytes passPhrase  null           byte   keyBytes   password GetBytes keysize   8           RijndaelManaged symmetricKey   new RijndaelManaged            symmetricKey Mode   CipherMode CBC          ICryptoTransform encryptor   symmetricKey CreateEncryptor keyBytes  initVectorBytes           MemoryStream memoryStream   new MemoryStream            CryptoStream cryptoStream   new CryptoStream memoryStream  encryptor  CryptoStreamMode Write           cryptoStream Write plainTextBytes  0  plainTextBytes Length           cryptoStream FlushFinalBlock            byte   cipherTextBytes   memoryStream ToArray            memoryStream Close            cryptoStream Close            return Convert ToBase64String cipherTextBytes              public static string decryptString string cipherText                  if the ciphertext is empty or null string just return an empty string         if  cipherText          cipherText    null                         return                        byte   initVectorBytes   Encoding ASCII GetBytes initVector           byte   cipherTextBytes   Convert FromBase64String cipherText           PasswordDeriveBytes password   new PasswordDeriveBytes passPhrase  null           byte   keyBytes   password GetBytes keysize   8           RijndaelManaged symmetricKey   new RijndaelManaged            symmetricKey Mode   CipherMode CBC          ICryptoTransform decryptor   symmetricKey CreateDecryptor keyBytes  initVectorBytes           MemoryStream memoryStream   new MemoryStream cipherTextBytes           CryptoStream cryptoStream   new CryptoStream memoryStream  decryptor  CryptoStreamMode Read           byte   plainTextBytes   new byte cipherTextBytes Length           int decryptedByteCount   cryptoStream Read plainTextBytes  0  plainTextBytes Length           memoryStream Close            cryptoStream Close            return Encoding UTF8 GetString plainTextBytes  0  decryptedByteCount

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With the reference of Encrypt and Decrypt a String in c   I found one of good solution    static readonly string PasswordHash    P  Sw0rd   static readonly string SaltKey    S LT amp KEY   static readonly string VIKey     1B2c3D4e5F6g7H8     For Encrypt  public static string Encrypt string plainText        byte   plainTextBytes   Encoding UTF8 GetBytes plainText        byte   keyBytes   new Rfc2898DeriveBytes PasswordHash  Encoding ASCII GetBytes SaltKey   GetBytes 256   8       var symmetricKey   new RijndaelManaged     Mode   CipherMode CBC  Padding   PaddingMode Zeros        var encryptor   symmetricKey CreateEncryptor keyBytes  Encoding ASCII GetBytes VIKey         byte   cipherTextBytes       using  var memoryStream   new MemoryStream                  using  var cryptoStream   new CryptoStream memoryStream  encryptor  CryptoStreamMode Write                         cryptoStream Write plainTextBytes  0  plainTextBytes Length               cryptoStream FlushFinalBlock                cipherTextBytes   memoryStream ToArray                cryptoStream Close                      memoryStream Close              return Convert ToBase64String cipherTextBytes       For Decrypt  public static string Decrypt string encryptedText        byte   cipherTextBytes   Convert FromBase64String encryptedText       byte   keyBytes   new Rfc2898DeriveBytes PasswordHash  Encoding ASCII GetBytes SaltKey   GetBytes 256   8       var symmetricKey   new RijndaelManaged     Mode   CipherMode CBC  Padding   PaddingMode None         var decryptor   symmetricKey CreateDecryptor keyBytes  Encoding ASCII GetBytes VIKey        var memoryStream   new MemoryStream cipherTextBytes       var cryptoStream   new CryptoStream memoryStream  decryptor  CryptoStreamMode Read       byte   plainTextBytes   new byte cipherTextBytes Length        int decryptedByteCount   cryptoStream Read plainTextBytes  0  plainTextBytes Length       memoryStream Close        cryptoStream Close        return Encoding UTF8 GetString plainTextBytes  0  decryptedByteCount  TrimEnd   0  ToCharArray

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I have an open source project called X509Crypto that leverages certificates to encrypt and decrypt strings  It s very easy to use  Here is an example for how to use it  1  Use the X509Crypto Commandline Interface  CLI  to generate a new encryption certificate and key pair  gt x509crypto exe X509Crypto gt  makecert -context user -keysize medium -alias myvault  Certificate with thumbprint B31FE7E7AE5229F8186782742CF579197FA859FD was added to X509Alias  quot myvault quot  in the user X509Context  X509Crypto gt   2  Use the Encrypt CLI command to add a secret to your new X509Alias X509Crypto gt  encrypt -text -alias myvault -context user -secret apikey -in  quot 80EAF03248965AC2B78090 quot   Secret apikey has been added to X509Alias myvault in the user X509Context  X509Crypto gt   3  Reference the secret in your program Once you have an X509Alias established with your secret s  added  it is trivial to retreive them in your program with the Org X509Crypto nuget package installed  using Org X509Crypto   namespace SampleApp       class Program               static void Main string   args                        var Alias   new X509Alias   quot myvault quot   X509Context UserReadOnly               var apiKey   Alias RecoverSecret   quot apikey quot

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Encryption  public string EncryptString string inputString        MemoryStream memStream   null      try               byte   key                byte   IV     12  21  43  17  57  35  67  27            string encryptKey    aXb2uy4z      MUST be 8 characters         key   Encoding UTF8 GetBytes encryptKey           byte   byteInput   Encoding UTF8 GetBytes inputString           DESCryptoServiceProvider provider   new DESCryptoServiceProvider            memStream   new MemoryStream            ICryptoTransform transform   provider CreateEncryptor key  IV           CryptoStream cryptoStream   new CryptoStream memStream  transform  CryptoStreamMode Write           cryptoStream Write byteInput  0  byteInput Length           cryptoStream FlushFinalBlock              catch  Exception ex                Response Write ex Message             return Convert ToBase64String memStream ToArray         Decryption       public string DecryptString string inputString        MemoryStream memStream   null      try               byte   key                byte   IV     12  21  43  17  57  35  67  27            string encryptKey    aXb2uy4z      MUST be 8 characters         key   Encoding UTF8 GetBytes encryptKey           byte   byteInput   new byte inputString Length           byteInput   Convert FromBase64String inputString           DESCryptoServiceProvider provider   new DESCryptoServiceProvider            memStream   new MemoryStream            ICryptoTransform transform   provider CreateDecryptor key  IV           CryptoStream cryptoStream   new CryptoStream memStream  transform  CryptoStreamMode Write           cryptoStream Write byteInput  0  byteInput Length           cryptoStream FlushFinalBlock              catch  Exception ex                Response Write ex Message              Encoding encoding1   Encoding UTF8      return encoding1 GetString memStream ToArray

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Good example how to do this using PGPCore with BouncyCastle  very simple solution   https   blog bitscry com 2018 07 05 pgp-encryption-and-decryption-in-c   I tried different solutions but this works best for me  some have bugs but this is perfect for me   using  PGP pgp   new PGP         Generate keys pgp GenerateKey   C  TEMP keys public asc     C  TEMP keys private asc    email email com    password       Encrypt file pgp EncryptFile   C  TEMP keys content txt     C  TEMP keys content  encrypted pgp     C  TEMP keys public asc   true  true      Encrypt and sign file pgp EncryptFileAndSign   C  TEMP keys content txt     C  TEMP keys content  encrypted signed pgp     C  TEMP keys public asc     C  TEMP keys private asc    password   true  true      Decrypt file pgp DecryptFile   C  TEMP keys content  encrypted pgp     C  TEMP keys content  decrypted txt     C  TEMP keys private asc    password       Decrypt signed file pgp DecryptFile   C  TEMP keys content  encrypted signed pgp     C  TEMP keys content  decrypted signed txt     C  TEMP keys private asc    password        Encrypt stream using  FileStream inputFileStream   new FileStream   C  TEMP keys content txt   FileMode Open   using  Stream outputFileStream   File Create   C  TEMP keys content  encrypted2 pgp    using  Stream publicKeyStream   new FileStream   C  TEMP keys public asc   FileMode Open       pgp EncryptStream inputFileStream  outputFileStream  publicKeyStream  true  true       Decrypt stream using  FileStream inputFileStream   new FileStream   C  TEMP keys content  encrypted2 pgp   FileMode Open   using  Stream outputFileStream   File Create   C  TEMP keys content  decrypted2 txt    using  Stream privateKeyStream   new FileStream   C  TEMP keys private asc   FileMode Open       pgp DecryptStream inputFileStream  outputFileStream  privateKeyStream   password

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using System  using System IO  using System Security Cryptography  using System Text   public class Program       public static void Main                 var key   Encoding UTF8 GetBytes  SUkbqO2ycDo7QwpR25kfgmC7f8CoyrZy            var data   Encoding UTF8 GetBytes  testData               Encrypt data         var encrypted   CryptoHelper EncryptData data key              Decrypt data         var decrypted   CryptoHelper DecryptData encrypted key              Display result         Console WriteLine Encoding UTF8 GetString decrypted             public static class CryptoHelper       public static byte   EncryptData byte   data  byte   key                using  var aesAlg   Aes Create                          aesAlg Mode   CipherMode CBC              using  var encryptor   aesAlg CreateEncryptor key  aesAlg IV                                 using  var msEncrypt   new MemoryStream                                          msEncrypt Write aesAlg IV  0  aesAlg IV Length                        using  var csEncrypt   new CryptoStream msEncrypt  encryptor  CryptoStreamMode Write                           csEncrypt Write data  0  data Length                        return msEncrypt ToArray                                                          public static byte   DecryptData byte   encrypted  byte   key                var iv   new byte 16           Buffer BlockCopy encrypted  0  iv  0  iv Length           using  var aesAlg   Aes Create                          aesAlg Mode   CipherMode CBC              using  var decryptor   aesAlg CreateDecryptor key  iv                                 using  var msDecrypt   new MemoryStream encrypted  iv Length  encrypted Length - iv Length                                         using  var csDecrypt   new CryptoStream msDecrypt  decryptor  CryptoStreamMode Read                                                 using  var resultStream   new MemoryStream                                                          csDecrypt CopyTo resultStream                               return resultStream ToArray

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Copied in my answer here from a similar question  Simple two-way encryption for C    Based on multiple answers and comments    Random initialization vector prepended to crypto text   jbtule  Use TransformFinalBlock   instead of MemoryStream   RenniePet  No pre-filled keys to avoid anyone copy  amp  pasting a disaster Proper dispose and using patterns   Code        lt summary gt      Simple encryption decryption using a random initialization vector     and prepending it to the crypto text       lt  summary gt       lt remarks gt Based on multiple answers in https   stackoverflow com questions 165808 simple-two-way-encryption-for-c-sharp  lt  remarks gt  public class SimpleAes   IDisposable            lt summary gt              Initialization vector length in bytes           lt  summary gt      private const int IvBytes   16            lt summary gt              Must be exactly 16  24 or 32 characters long           lt  summary gt      private static readonly byte   Key   Convert FromBase64String  FILL ME WITH 16  24 OR 32 CHARS         private readonly UTF8Encoding  encoder      private readonly ICryptoTransform  encryptor      private readonly RijndaelManaged  rijndael       public SimpleAes                  rijndael   new RijndaelManaged  Key   Key            rijndael GenerateIV             encryptor    rijndael CreateEncryptor             encoder   new UTF8Encoding               public string Decrypt string encrypted                return  encoder GetString Decrypt Convert FromBase64String encrypted                public void Dispose                  rijndael Dispose             encryptor Dispose               public string Encrypt string unencrypted                return Convert ToBase64String Encrypt  encoder GetBytes unencrypted                private byte   Decrypt byte   buffer                   IV is prepended to cryptotext         byte   iv   buffer Take IvBytes  ToArray            using  ICryptoTransform decryptor    rijndael CreateDecryptor  rijndael Key  iv                         return decryptor TransformFinalBlock buffer  IvBytes  buffer Length - IvBytes                        private byte   Encrypt byte   buffer                   Prepend cryptotext with IV         byte   inputBuffer    rijndael IV Concat buffer  ToArray            return  encryptor TransformFinalBlock inputBuffer  IvBytes  buffer Length

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using System  using System Collections Generic  using System Linq  using System Web  using System Security Cryptography  using System IO  using System Text          lt summary gt      Summary description for Encryption      lt  summary gt  public class Encryption       public TripleDES CreateDES string key                MD5 md5   new MD5CryptoServiceProvider            TripleDES des   new TripleDESCryptoServiceProvider            des Key   md5 ComputeHash Encoding Unicode GetBytes key            des IV   new byte des BlockSize   8           return des            public  byte   Encryptiondata string PlainText                TripleDES des   CreateDES  DreamMLMKey            ICryptoTransform ct   des CreateEncryptor            byte   input   Encoding Unicode GetBytes PlainText           return ct TransformFinalBlock input  0  input Length              public string Decryptiondata string CypherText                string stringToDecrypt   CypherText Replace                    int len   stringToDecrypt Length          byte   inputByteArray   Convert FromBase64String stringToDecrypt             byte   b   Convert FromBase64String CypherText           TripleDES des   CreateDES  DreamMLMKey            ICryptoTransform ct   des CreateDecryptor            byte   output   ct TransformFinalBlock b  0  b Length           return Encoding Unicode GetString output             public string Decryptiondataurl string CypherText                string newcyperttext CypherText Replace                    byte   b   Convert FromBase64String newcyperttext           TripleDES des   CreateDES  DreamMLMKey            ICryptoTransform ct   des CreateDecryptor            byte   output   ct TransformFinalBlock b  0  b Length           return Encoding Unicode GetString output                region  encryption  amp  Decription     public  string Encrypt string input  string key                byte   inputArray   UTF8Encoding UTF8 GetBytes input           TripleDESCryptoServiceProvider tripleDES   new TripleDESCryptoServiceProvider            tripleDES Key   UTF8Encoding UTF8 GetBytes key           tripleDES Mode   CipherMode ECB          tripleDES Padding   PaddingMode PKCS7          ICryptoTransform cTransform   tripleDES CreateEncryptor            byte   resultArray   cTransform TransformFinalBlock inputArray  0  inputArray Length           tripleDES Clear            return Convert ToBase64String resultArray  0  resultArray Length             public  string Decrypt string input  string key                byte   inputArray   Convert FromBase64String input           TripleDESCryptoServiceProvider tripleDES   new TripleDESCryptoServiceProvider            tripleDES Key   UTF8Encoding UTF8 GetBytes key           tripleDES Mode   CipherMode ECB          tripleDES Padding   PaddingMode PKCS7          ICryptoTransform cTransform   tripleDES CreateDecryptor            byte   resultArray   cTransform TransformFinalBlock inputArray  0  inputArray Length           tripleDES Clear            return UTF8Encoding UTF8 GetString resultArray              public string encrypt string encryptString                string EncryptionKey    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ           byte   clearBytes   Encoding Unicode GetBytes encryptString           using  Aes encryptor   Aes Create                          Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  new byte                     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76                             encryptor Key   pdb GetBytes 32               encryptor IV   pdb GetBytes 16               using  MemoryStream ms   new MemoryStream                                  using  CryptoStream cs   new CryptoStream ms  encryptor CreateEncryptor    CryptoStreamMode Write                                         cs Write clearBytes  0  clearBytes Length                       cs Close                                      encryptString   Convert ToBase64String ms ToArray                                     return encryptString             public string Decrypt string cipherText                string EncryptionKey    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ           cipherText   cipherText Replace                    byte   cipherBytes   Convert FromBase64String cipherText           using  Aes encryptor   Aes Create                          Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  new byte                     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76                             encryptor Key   pdb GetBytes 32               encryptor IV   pdb GetBytes 16               using  MemoryStream ms   new MemoryStream                                  using  CryptoStream cs   new CryptoStream ms  encryptor CreateDecryptor    CryptoStreamMode Write                                         cs Write cipherBytes  0  cipherBytes Length                       cs Close                                      cipherText   Encoding Unicode GetString ms ToArray                                     return cipherText              endregion

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This is the class that was placed here by Brett  However I made a slight edit since I was receiving the error  Invalid length for a Base-64 char array  when using it for URL strings to encrypt and decrypt   public class CryptoURL       private static byte    salt   Encoding ASCII GetBytes  Catto Salt Enter Any Value99              lt summary gt          Encrypt the given string using AES   The string can be decrypted using          DecryptStringAES     The sharedSecret parameters must match           The SharedSecret for the Password Reset that is used is in the next line          string sharedSecret    OneUpSharedSecret9            lt  summary gt           lt param name  plainText  gt The text to encrypt  lt  param gt           lt param name  sharedSecret  gt A password used to generate a key for encryption  lt  param gt      public static string EncryptString string plainText  string sharedSecret                if  string IsNullOrEmpty plainText               throw new ArgumentNullException  plainText            if  string IsNullOrEmpty sharedSecret               throw new ArgumentNullException  sharedSecret             string outStr   null                           Encrypted string to return         RijndaelManaged aesAlg   null                  RijndaelManaged object used to encrypt the data           try                          generate the key from the shared secret and the salt             Rfc2898DeriveBytes key   new Rfc2898DeriveBytes sharedSecret   salt                   Create a RijndaelManaged object             aesAlg   new RijndaelManaged                aesAlg Key   key GetBytes aesAlg KeySize   8                   Create a decryptor to perform the stream transform              ICryptoTransform encryptor   aesAlg CreateEncryptor aesAlg Key  aesAlg IV                   Create the streams used for encryption              using  MemoryStream msEncrypt   new MemoryStream                                     prepend the IV                 msEncrypt Write BitConverter GetBytes aesAlg IV Length   0  sizeof int                    msEncrypt Write aesAlg IV  0  aesAlg IV Length                   using  CryptoStream csEncrypt   new CryptoStream msEncrypt  encryptor  CryptoStreamMode Write                                         using  StreamWriter swEncrypt   new StreamWriter csEncrypt                                                   Write all data to the stream                          swEncrypt Write plainText                                                            outStr   HttpServerUtility UrlTokenEncode msEncrypt ToArray                       outStr   Convert ToBase64String msEncrypt ToArray                        you may need to add a reference  right click reference in solution explorer   gt   add Reference    gt   NET tab   gt  select  System Web                                  finally                          Clear the RijndaelManaged object              if  aesAlg    null                  aesAlg Clear                          Return the encrypted bytes from the memory stream          return outStr                  lt summary gt          Decrypt the given string   Assumes the string was encrypted using          EncryptStringAES    using an identical sharedSecret           lt  summary gt           lt param name  cipherText  gt The text to decrypt  lt  param gt           lt param name  sharedSecret  gt A password used to generate a key for decryption  lt  param gt      public static string DecryptString string cipherText  string sharedSecret                if  string IsNullOrEmpty cipherText               throw new ArgumentNullException  cipherText            if  string IsNullOrEmpty sharedSecret               throw new ArgumentNullException  sharedSecret                Declare the RijndaelManaged object            used to decrypt the data          RijndaelManaged aesAlg   null              Declare the string used to hold            the decrypted text          string plaintext   null           byte   inputByteArray           try                          generate the key from the shared secret and the salt             Rfc2898DeriveBytes key   new Rfc2898DeriveBytes sharedSecret   salt                   Create the streams used for decryption                                byte   bytes   Convert FromBase64String cipherText               inputByteArray   HttpServerUtility UrlTokenDecode cipherText                using  MemoryStream msDecrypt   new MemoryStream inputByteArray                                    Create a RijndaelManaged object                    with the specified key and IV                  aesAlg   new RijndaelManaged                    aesAlg Key   key GetBytes aesAlg KeySize   8                      Get the initialization vector from the encrypted stream                 aesAlg IV   ReadByteArray msDecrypt                      Create a decrytor to perform the stream transform                  ICryptoTransform decryptor   aesAlg CreateDecryptor aesAlg Key  aesAlg IV                   using  CryptoStream csDecrypt   new CryptoStream msDecrypt  decryptor  CryptoStreamMode Read                                         using  StreamReader srDecrypt   new StreamReader csDecrypt                               Read the decrypted bytes from the decrypting stream                            and place them in a string                          plaintext   srDecrypt ReadToEnd                                                      catch  System Exception ex                        return  ERROR                 throw ex                     finally                          Clear the RijndaelManaged object              if  aesAlg    null                  aesAlg Clear                       return plaintext             static string ConvertStringArrayToString string   array                              Concatenate all the elements into a StringBuilder                     StringBuilder builder   new StringBuilder            foreach  string value in array                        builder Append value               builder Append                         return builder ToString               private static byte   ReadByteArray Stream s                byte   rawLength   new byte sizeof int            if  s Read rawLength  0  rawLength Length     rawLength Length                        throw new SystemException  Stream did not contain properly formatted byte array                       byte   buffer   new byte BitConverter ToInt32 rawLength  0            if  s Read buffer  0  buffer Length     buffer Length                        throw new SystemException  Did not read byte array properly                       return buffer

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The following code is an improved version of Ghazal s answer to a similar question   public class EncryptionHelper       private Aes aesEncryptor       public EncryptionHelper                    private void BuildAesEncryptor string key                aesEncryptor   Aes Create            var pdb   new Rfc2898DeriveBytes key  new byte     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76             aesEncryptor Key   pdb GetBytes 32           aesEncryptor IV   pdb GetBytes 16              public string EncryptString string clearText  string key                BuildAesEncryptor key           var clearBytes   Encoding Unicode GetBytes clearText           using  var ms   new MemoryStream                          using  var cs   new CryptoStream ms  aesEncryptor CreateEncryptor    CryptoStreamMode Write                                 cs Write clearBytes  0  clearBytes Length                             var encryptedText   Convert ToBase64String ms ToArray                 return encryptedText                       public string DecryptString string cipherText  string key                BuildAesEncryptor key           cipherText   cipherText Replace                    var cipherBytes   Convert FromBase64String cipherText           using  var ms   new MemoryStream                          using  var cs   new CryptoStream ms  aesEncryptor CreateDecryptor    CryptoStreamMode Write                                 cs Write cipherBytes  0  cipherBytes Length                             var clearText   Encoding Unicode GetString ms ToArray                 return clearText

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An alternative to BouncyCastle for AES-GCM encryption is libsodium-net  It wraps the libsodium C library  One nice advantage is that it uses the AES-NI extension in CPUs for very fast encryption  The down side is that it won t work at all if the CPU doesn t have the extension  There s no software fall back

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With the reference of Encrypt and Decrypt a String in c   I found one of good solution    static readonly string PasswordHash    P  Sw0rd   static readonly string SaltKey    S LT amp KEY   static readonly string VIKey     1B2c3D4e5F6g7H8     For Encrypt  public static string Encrypt string plainText        byte   plainTextBytes   Encoding UTF8 GetBytes plainText        byte   keyBytes   new Rfc2898DeriveBytes PasswordHash  Encoding ASCII GetBytes SaltKey   GetBytes 256   8       var symmetricKey   new RijndaelManaged     Mode   CipherMode CBC  Padding   PaddingMode Zeros        var encryptor   symmetricKey CreateEncryptor keyBytes  Encoding ASCII GetBytes VIKey         byte   cipherTextBytes       using  var memoryStream   new MemoryStream                  using  var cryptoStream   new CryptoStream memoryStream  encryptor  CryptoStreamMode Write                         cryptoStream Write plainTextBytes  0  plainTextBytes Length               cryptoStream FlushFinalBlock                cipherTextBytes   memoryStream ToArray                cryptoStream Close                      memoryStream Close              return Convert ToBase64String cipherTextBytes       For Decrypt  public static string Decrypt string encryptedText        byte   cipherTextBytes   Convert FromBase64String encryptedText       byte   keyBytes   new Rfc2898DeriveBytes PasswordHash  Encoding ASCII GetBytes SaltKey   GetBytes 256   8       var symmetricKey   new RijndaelManaged     Mode   CipherMode CBC  Padding   PaddingMode None         var decryptor   symmetricKey CreateDecryptor keyBytes  Encoding ASCII GetBytes VIKey        var memoryStream   new MemoryStream cipherTextBytes       var cryptoStream   new CryptoStream memoryStream  decryptor  CryptoStreamMode Read       byte   plainTextBytes   new byte cipherTextBytes Length        int decryptedByteCount   cryptoStream Read plainTextBytes  0  plainTextBytes Length       memoryStream Close        cryptoStream Close        return Encoding UTF8 GetString plainTextBytes  0  decryptedByteCount  TrimEnd   0  ToCharArray

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EDIT 2013-Oct  Although I ve edited this answer over time to address shortcomings  please see jbtule s answer for a more robust  informed solution   https   stackoverflow com a 10366194 188474  Original Answer   Here s a working example derived from the  RijndaelManaged Class  documentation and the MCTS Training Kit     EDIT 2012-April  This answer was edited to pre-pend the IV per jbtule s suggestion and as illustrated here   http   msdn microsoft com en-us library system security cryptography aesmanaged 28v vs 95 29 aspx  Good luck   public class Crypto          While an app specific salt is not the best practice for       password based encryption  it s probably safe enough as long as       it is truly uncommon  Also too much work to alter this answer otherwise      private static byte    salt     To Do    Add a app specific salt here              lt summary gt          Encrypt the given string using AES   The string can be decrypted using          DecryptStringAES     The sharedSecret parameters must match           lt  summary gt           lt param name  plainText  gt The text to encrypt  lt  param gt           lt param name  sharedSecret  gt A password used to generate a key for encryption  lt  param gt      public static string EncryptStringAES string plainText  string sharedSecret                if  string IsNullOrEmpty plainText               throw new ArgumentNullException  plainText            if  string IsNullOrEmpty sharedSecret               throw new ArgumentNullException  sharedSecret             string outStr   null                           Encrypted string to return         RijndaelManaged aesAlg   null                  RijndaelManaged object used to encrypt the data           try                          generate the key from the shared secret and the salt             Rfc2898DeriveBytes key   new Rfc2898DeriveBytes sharedSecret   salt                   Create a RijndaelManaged object             aesAlg   new RijndaelManaged                aesAlg Key   key GetBytes aesAlg KeySize   8                   Create a decryptor to perform the stream transform              ICryptoTransform encryptor   aesAlg CreateEncryptor aesAlg Key  aesAlg IV                   Create the streams used for encryption              using  MemoryStream msEncrypt   new MemoryStream                                     prepend the IV                 msEncrypt Write BitConverter GetBytes aesAlg IV Length   0  sizeof int                    msEncrypt Write aesAlg IV  0  aesAlg IV Length                   using  CryptoStream csEncrypt   new CryptoStream msEncrypt  encryptor  CryptoStreamMode Write                                         using  StreamWriter swEncrypt   new StreamWriter csEncrypt                                                   Write all data to the stream                          swEncrypt Write plainText                                                           outStr   Convert ToBase64String msEncrypt ToArray                                     finally                          Clear the RijndaelManaged object              if  aesAlg    null                  aesAlg Clear                          Return the encrypted bytes from the memory stream          return outStr                  lt summary gt          Decrypt the given string   Assumes the string was encrypted using          EncryptStringAES    using an identical sharedSecret           lt  summary gt           lt param name  cipherText  gt The text to decrypt  lt  param gt           lt param name  sharedSecret  gt A password used to generate a key for decryption  lt  param gt      public static string DecryptStringAES string cipherText  string sharedSecret                if  string IsNullOrEmpty cipherText               throw new ArgumentNullException  cipherText            if  string IsNullOrEmpty sharedSecret               throw new ArgumentNullException  sharedSecret                Declare the RijndaelManaged object            used to decrypt the data          RijndaelManaged aesAlg   null              Declare the string used to hold            the decrypted text          string plaintext   null           try                          generate the key from the shared secret and the salt             Rfc2898DeriveBytes key   new Rfc2898DeriveBytes sharedSecret   salt                   Create the streams used for decryption                              byte   bytes   Convert FromBase64String cipherText               using  MemoryStream msDecrypt   new MemoryStream bytes                                    Create a RijndaelManaged object                    with the specified key and IV                  aesAlg   new RijndaelManaged                    aesAlg Key   key GetBytes aesAlg KeySize   8                      Get the initialization vector from the encrypted stream                 aesAlg IV   ReadByteArray msDecrypt                      Create a decrytor to perform the stream transform                  ICryptoTransform decryptor   aesAlg CreateDecryptor aesAlg Key  aesAlg IV                   using  CryptoStream csDecrypt   new CryptoStream msDecrypt  decryptor  CryptoStreamMode Read                                         using  StreamReader srDecrypt   new StreamReader csDecrypt                               Read the decrypted bytes from the decrypting stream                            and place them in a string                          plaintext   srDecrypt ReadToEnd                                                      finally                          Clear the RijndaelManaged object              if  aesAlg    null                  aesAlg Clear                       return plaintext             private static byte   ReadByteArray Stream s                byte   rawLength   new byte sizeof int            if  s Read rawLength  0  rawLength Length     rawLength Length                        throw new SystemException  Stream did not contain properly formatted byte array                       byte   buffer   new byte BitConverter ToInt32 rawLength  0            if  s Read buffer  0  buffer Length     buffer Length                        throw new SystemException  Did not read byte array properly                       return buffer

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The following code is an improved version of Ghazal s answer to a similar question   public class EncryptionHelper       private Aes aesEncryptor       public EncryptionHelper                    private void BuildAesEncryptor string key                aesEncryptor   Aes Create            var pdb   new Rfc2898DeriveBytes key  new byte     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76             aesEncryptor Key   pdb GetBytes 32           aesEncryptor IV   pdb GetBytes 16              public string EncryptString string clearText  string key                BuildAesEncryptor key           var clearBytes   Encoding Unicode GetBytes clearText           using  var ms   new MemoryStream                          using  var cs   new CryptoStream ms  aesEncryptor CreateEncryptor    CryptoStreamMode Write                                 cs Write clearBytes  0  clearBytes Length                             var encryptedText   Convert ToBase64String ms ToArray                 return encryptedText                       public string DecryptString string cipherText  string key                BuildAesEncryptor key           cipherText   cipherText Replace                    var cipherBytes   Convert FromBase64String cipherText           using  var ms   new MemoryStream                          using  var cs   new CryptoStream ms  aesEncryptor CreateDecryptor    CryptoStreamMode Write                                 cs Write cipherBytes  0  cipherBytes Length                             var clearText   Encoding Unicode GetString ms ToArray                 return clearText

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Encryption is a very common matter in programming  I think it is better to install a package to do the task for you  Maybe a simple open source Nuget project like  Simple Aes Encryption  The key is in the config file and therefore it is easy to change in the production environment  and I don t see any drawbacks   lt MessageEncryption gt     lt EncryptionKey KeySize  256  Key  3q2 796tvu erb7v3q2 796tvu erb7v3q2 796tvu8    gt   lt  MessageEncryption gt

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BouncyCastle is a great Crypto library for  NET  it s available as a Nuget package for install into your projects   I like it a lot more than what s currently available in the System Security Cryptography library   It gives you a lot more options in terms of available algorithms  and provides more modes for those algorithms   This is an example of an implementation of TwoFish  which was written by Bruce Schneier  hero to all us paranoid people out there     It s a symmetric algorithm like the Rijndael  aka AES    It was one of the three finalists for the AES standard and sibling to another famous algorithm written by Bruce Schneier called BlowFish   First thing with bouncycastle is to create an encryptor class  this will make it easier to implement other block ciphers within the library   The following encryptor class takes in a generic argument T where T implements IBlockCipher and has a default constructor   UPDATE   Due to popular demand I have decided to implement generating a random IV as well as include an HMAC into this class   Although from a style perspective this goes against the SOLID principle of single responsibility  because of the nature of what this class does I reniged   This class will now take two generic parameters  one for the cipher and one for the digest   It automatically generates the IV using RNGCryptoServiceProvider to provide good RNG entropy  and allows you to use whatever digest algorithm you want from BouncyCastle to generate the MAC   using System  using System Security Cryptography  using System Text  using Org BouncyCastle Crypto  using Org BouncyCastle Crypto Macs  using Org BouncyCastle Crypto Modes  using Org BouncyCastle Crypto Paddings  using Org BouncyCastle Crypto Parameters   public sealed class Encryptor lt TBlockCipher  TDigest gt      where TBlockCipher   IBlockCipher  new       where TDigest   IDigest  new         private Encoding encoding       private IBlockCipher blockCipher       private BufferedBlockCipher cipher       private HMac mac       private byte   key       public Encryptor Encoding encoding  byte   key  byte   macKey                this encoding   encoding          this key   key          this Init key  macKey  new Pkcs7Padding                public Encryptor Encoding encoding  byte   key  byte   macKey  IBlockCipherPadding padding                this encoding   encoding          this key   key          this Init key  macKey  padding              private void Init byte   key  byte   macKey  IBlockCipherPadding padding                this blockCipher   new CbcBlockCipher new TBlockCipher             this cipher   new PaddedBufferedBlockCipher this blockCipher  padding           this mac   new HMac new TDigest             this mac Init new KeyParameter macKey               public string Encrypt string plain                return Convert ToBase64String EncryptBytes plain               public byte   EncryptBytes string plain                byte   input   this encoding GetBytes plain            var iv   this GenerateIV             var cipher   this BouncyCastleCrypto true  input  new ParametersWithIV new KeyParameter key   iv            byte   message   CombineArrays iv  cipher            this mac Reset            this mac BlockUpdate message  0  message Length           byte   digest   new byte this mac GetUnderlyingDigest   GetDigestSize             this mac DoFinal digest  0            var result   CombineArrays digest  message           return result             public byte   DecryptBytes byte   bytes                   split the digest into component parts         var digest   new byte this mac GetUnderlyingDigest   GetDigestSize             var message   new byte bytes Length - digest Length           var iv   new byte this blockCipher GetBlockSize             var cipher   new byte message Length - iv Length            Buffer BlockCopy bytes  0  digest  0  digest Length           Buffer BlockCopy bytes  digest Length  message  0  message Length           if   IsValidHMac digest  message                         throw new CryptoException                       Buffer BlockCopy message  0  iv  0  iv Length           Buffer BlockCopy message  iv Length  cipher  0  cipher Length            byte   result   this BouncyCastleCrypto false  cipher  new ParametersWithIV new KeyParameter key   iv            return result             public string Decrypt byte   bytes                return this encoding GetString DecryptBytes bytes               public string Decrypt string cipher                return this Decrypt Convert FromBase64String cipher               private bool IsValidHMac byte   digest  byte   message                this mac Reset            this mac BlockUpdate message  0  message Length           byte   computed   new byte this mac GetUnderlyingDigest   GetDigestSize             this mac DoFinal computed  0            return AreEqual digest computed              private static bool AreEqual byte    digest  byte   computed                if digest Length    computed Length                        return false                     int result   0          for  int i   0  i  lt  digest Length  i                             compute equality of all bytes before returning                   helps prevent timing attacks                    https   codahale com a-lesson-in-timing-attacks              result    digest i    computed i                      return result    0             private byte   BouncyCastleCrypto bool forEncrypt  byte   input  ICipherParameters parameters                try                       cipher Init forEncrypt  parameters                return this cipher DoFinal input                     catch  CryptoException                        throw                       private byte   GenerateIV                 using  var provider   new RNGCryptoServiceProvider                             1st block             byte   result   new byte this blockCipher GetBlockSize                 provider GetBytes result                return result                       private static byte   CombineArrays byte   source1  byte   source2                byte   result   new byte source1 Length   source2 Length           Buffer BlockCopy source1  0  result  0  source1 Length           Buffer BlockCopy source2  0  result  source1 Length  source2 Length            return result            Next just call the encrypt and decrypt methods on the new class  here s the example using twofish   var encrypt   new Encryptor lt TwofishEngine  Sha1Digest gt  Encoding UTF8  key  hmacKey    string cipher   encrypt Encrypt  TEST       string plainText   encrypt Decrypt cipher     It s just as easy to substitute another block cipher like TripleDES   var des   new Encryptor lt DesEdeEngine  Sha1Digest gt  Encoding UTF8  key  hmacKey    string cipher   des Encrypt  TEST    string plainText   des Decrypt cipher     Finally if you want to use AES with SHA256 HMAC you can do the following   var aes   new Encryptor lt AesEngine  Sha256Digest gt  Encoding UTF8  key  hmacKey    cipher   aes Encrypt  TEST    plainText   aes Decrypt cipher     The hardest part about encryption actually deals with the keys and not the algorithms   You ll have to think about where you store your keys  and if you have to  how you exchange them   These algorithms have all withstood the test of time  and are extremely hard to break  Someone who wants to steal information from you isn t going to spend eternity doing cryptanalysis on your messages  they re going to try to figure out what or where your key is   So  1 choose your keys wisely   2 store them in a safe place  if you use a web config and IIS then you can encrypt parts of the the web config  and finally if you have to exchange keys make sure that your protocol for exchanging the key is secure   Update 2 Changed compare method to mitigate against timing attacks   See more info here http   codahale com a-lesson-in-timing-attacks     Also updated to default to PKCS7 padding and added new constructor to allow end user the ability to choose which padding they would like to use   Thanks  CodesInChaos for the suggestions

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Good example how to do this using PGPCore with BouncyCastle  very simple solution   https   blog bitscry com 2018 07 05 pgp-encryption-and-decryption-in-c   I tried different solutions but this works best for me  some have bugs but this is perfect for me   using  PGP pgp   new PGP         Generate keys pgp GenerateKey   C  TEMP keys public asc     C  TEMP keys private asc    email email com    password       Encrypt file pgp EncryptFile   C  TEMP keys content txt     C  TEMP keys content  encrypted pgp     C  TEMP keys public asc   true  true      Encrypt and sign file pgp EncryptFileAndSign   C  TEMP keys content txt     C  TEMP keys content  encrypted signed pgp     C  TEMP keys public asc     C  TEMP keys private asc    password   true  true      Decrypt file pgp DecryptFile   C  TEMP keys content  encrypted pgp     C  TEMP keys content  decrypted txt     C  TEMP keys private asc    password       Decrypt signed file pgp DecryptFile   C  TEMP keys content  encrypted signed pgp     C  TEMP keys content  decrypted signed txt     C  TEMP keys private asc    password        Encrypt stream using  FileStream inputFileStream   new FileStream   C  TEMP keys content txt   FileMode Open   using  Stream outputFileStream   File Create   C  TEMP keys content  encrypted2 pgp    using  Stream publicKeyStream   new FileStream   C  TEMP keys public asc   FileMode Open       pgp EncryptStream inputFileStream  outputFileStream  publicKeyStream  true  true       Decrypt stream using  FileStream inputFileStream   new FileStream   C  TEMP keys content  encrypted2 pgp   FileMode Open   using  Stream outputFileStream   File Create   C  TEMP keys content  decrypted2 txt    using  Stream privateKeyStream   new FileStream   C  TEMP keys private asc   FileMode Open       pgp DecryptStream inputFileStream  outputFileStream  privateKeyStream   password

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If you are using ASP Net you can now use built in functionality in  Net 4 0 onwards   System Web Security MachineKey   Net 4 5 has MachineKey Protect   and MachineKey Unprotect      Net 4 0 has MachineKey Encode   and MachineKey Decode    You should just set the MachineKeyProtection to  All    Outside of ASP Net this class seems to generate a new key with every app restart so doesn t work  With a quick peek in ILSpy it looks to me like it generates its own defaults if the appropriate app settings are missing  So you may actually be able to set it up outside ASP Net   I haven t been able to find a non-ASP Net equivalent outside the System Web namespace

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Here is an example using RSA    Important  There is a limit to the size of data you can encrypt with the RSA encryption  KeySize - MinimumPadding  e g  256 bytes  assuming 2048 bit key  - 42 bytes  min OEAP padding    214 bytes  max plaintext size   Replace your rsa key with your RSA key   var provider   new System Security Cryptography RSACryptoServiceProvider    provider ImportParameters your rsa key    var encryptedBytes   provider Encrypt      System Text Encoding UTF8 GetBytes  Hello World     true    string decryptedTest   System Text Encoding UTF8 GetString      provider Decrypt encryptedBytes  true      For more info  visit MSDN - RSACryptoServiceProvider

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A good algorithm to securely hash data is BCrypt      Besides incorporating a salt to protect against rainbow table attacks    bcrypt is an adaptive function  over time  the iteration count can be   increased to make it slower  so it remains resistant to brute-force   search attacks even with increasing computation power    There s a nice  NET implementation of BCrypt that is available also as a NuGet package

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I want to give you my contribute  with my code for AES Rfc2898DeriveBytes  here the documentation  algorhytm  written in C    NET framework 4  and fully working also for limited platforms  as  NET Compact Framework for Windows Phone 7 0   not all platforms support every criptographic method of the  NET framework     I hope this can help anyone   using System  using System IO  using System Security Cryptography  using System Text   public static class Crypto       private static readonly byte   IVa   new byte     0x0b  0x0c  0x0d  0x0e  0x0f  0x11  0x11  0x12  0x13  0x14  0x0e  0x16  0x17          public static string Encrypt this string text  string salt                try                       using  Aes aes   new AesManaged                                  Rfc2898DeriveBytes deriveBytes   new Rfc2898DeriveBytes Encoding UTF8 GetString IVa  0  IVa Length   Encoding UTF8 GetBytes salt                    aes Key   deriveBytes GetBytes 128   8                   aes IV   aes Key                  using  MemoryStream encryptionStream   new MemoryStream                                          using  CryptoStream encrypt   new CryptoStream encryptionStream  aes CreateEncryptor    CryptoStreamMode Write                                                 byte   cleanText   Encoding UTF8 GetBytes text                           encrypt Write cleanText  0  cleanText Length                           encrypt FlushFinalBlock                                               byte   encryptedData   encryptionStream ToArray                        string encryptedText   Convert ToBase64String encryptedData                         return encryptedText                                                    catch                       return String Empty                       public static string Decrypt this string text  string salt                try                       using  Aes aes   new AesManaged                                  Rfc2898DeriveBytes deriveBytes   new Rfc2898DeriveBytes Encoding UTF8 GetString IVa  0  IVa Length   Encoding UTF8 GetBytes salt                    aes Key   deriveBytes GetBytes 128   8                   aes IV   aes Key                   using  MemoryStream decryptionStream   new MemoryStream                                          using  CryptoStream decrypt   new CryptoStream decryptionStream  aes CreateDecryptor    CryptoStreamMode Write                                                 byte   encryptedData   Convert FromBase64String text                             decrypt Write encryptedData  0  encryptedData Length                           decrypt Flush                                               byte   decryptedData   decryptionStream ToArray                        string decryptedText   Encoding UTF8 GetString decryptedData  0  decryptedData Length                         return decryptedText                                                    catch                       return String Empty

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EDIT 2013-Oct  Although I ve edited this answer over time to address shortcomings  please see jbtule s answer for a more robust  informed solution   https   stackoverflow com a 10366194 188474  Original Answer   Here s a working example derived from the  RijndaelManaged Class  documentation and the MCTS Training Kit     EDIT 2012-April  This answer was edited to pre-pend the IV per jbtule s suggestion and as illustrated here   http   msdn microsoft com en-us library system security cryptography aesmanaged 28v vs 95 29 aspx  Good luck   public class Crypto          While an app specific salt is not the best practice for       password based encryption  it s probably safe enough as long as       it is truly uncommon  Also too much work to alter this answer otherwise      private static byte    salt     To Do    Add a app specific salt here              lt summary gt          Encrypt the given string using AES   The string can be decrypted using          DecryptStringAES     The sharedSecret parameters must match           lt  summary gt           lt param name  plainText  gt The text to encrypt  lt  param gt           lt param name  sharedSecret  gt A password used to generate a key for encryption  lt  param gt      public static string EncryptStringAES string plainText  string sharedSecret                if  string IsNullOrEmpty plainText               throw new ArgumentNullException  plainText            if  string IsNullOrEmpty sharedSecret               throw new ArgumentNullException  sharedSecret             string outStr   null                           Encrypted string to return         RijndaelManaged aesAlg   null                  RijndaelManaged object used to encrypt the data           try                          generate the key from the shared secret and the salt             Rfc2898DeriveBytes key   new Rfc2898DeriveBytes sharedSecret   salt                   Create a RijndaelManaged object             aesAlg   new RijndaelManaged                aesAlg Key   key GetBytes aesAlg KeySize   8                   Create a decryptor to perform the stream transform              ICryptoTransform encryptor   aesAlg CreateEncryptor aesAlg Key  aesAlg IV                   Create the streams used for encryption              using  MemoryStream msEncrypt   new MemoryStream                                     prepend the IV                 msEncrypt Write BitConverter GetBytes aesAlg IV Length   0  sizeof int                    msEncrypt Write aesAlg IV  0  aesAlg IV Length                   using  CryptoStream csEncrypt   new CryptoStream msEncrypt  encryptor  CryptoStreamMode Write                                         using  StreamWriter swEncrypt   new StreamWriter csEncrypt                                                   Write all data to the stream                          swEncrypt Write plainText                                                           outStr   Convert ToBase64String msEncrypt ToArray                                     finally                          Clear the RijndaelManaged object              if  aesAlg    null                  aesAlg Clear                          Return the encrypted bytes from the memory stream          return outStr                  lt summary gt          Decrypt the given string   Assumes the string was encrypted using          EncryptStringAES    using an identical sharedSecret           lt  summary gt           lt param name  cipherText  gt The text to decrypt  lt  param gt           lt param name  sharedSecret  gt A password used to generate a key for decryption  lt  param gt      public static string DecryptStringAES string cipherText  string sharedSecret                if  string IsNullOrEmpty cipherText               throw new ArgumentNullException  cipherText            if  string IsNullOrEmpty sharedSecret               throw new ArgumentNullException  sharedSecret                Declare the RijndaelManaged object            used to decrypt the data          RijndaelManaged aesAlg   null              Declare the string used to hold            the decrypted text          string plaintext   null           try                          generate the key from the shared secret and the salt             Rfc2898DeriveBytes key   new Rfc2898DeriveBytes sharedSecret   salt                   Create the streams used for decryption                              byte   bytes   Convert FromBase64String cipherText               using  MemoryStream msDecrypt   new MemoryStream bytes                                    Create a RijndaelManaged object                    with the specified key and IV                  aesAlg   new RijndaelManaged                    aesAlg Key   key GetBytes aesAlg KeySize   8                      Get the initialization vector from the encrypted stream                 aesAlg IV   ReadByteArray msDecrypt                      Create a decrytor to perform the stream transform                  ICryptoTransform decryptor   aesAlg CreateDecryptor aesAlg Key  aesAlg IV                   using  CryptoStream csDecrypt   new CryptoStream msDecrypt  decryptor  CryptoStreamMode Read                                         using  StreamReader srDecrypt   new StreamReader csDecrypt                               Read the decrypted bytes from the decrypting stream                            and place them in a string                          plaintext   srDecrypt ReadToEnd                                                      finally                          Clear the RijndaelManaged object              if  aesAlg    null                  aesAlg Clear                       return plaintext             private static byte   ReadByteArray Stream s                byte   rawLength   new byte sizeof int            if  s Read rawLength  0  rawLength Length     rawLength Length                        throw new SystemException  Stream did not contain properly formatted byte array                       byte   buffer   new byte BitConverter ToInt32 rawLength  0            if  s Read buffer  0  buffer Length     buffer Length                        throw new SystemException  Did not read byte array properly                       return buffer

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You have to use the namespace using System Security Cryptography  and useHashing is a bool type either true or false  String variable  key  should be same for Encryption and for Decryption    Encryption public string EncryptText string toEncrypt  bool useHashing                try                       byte   keyArray              byte   toEncryptArray   UTF8Encoding UTF8 GetBytes toEncrypt                string key    String Key Value     Based on this key stirng is encrypting               System Windows Forms MessageBox Show key                 If hashing use get hashcode regards to your key             if  useHashing                                MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                    keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes key                      Always release the resources and flush data                   of the Cryptographic service provide  Best Practice                  hashmd5 Clear                              else                 keyArray   UTF8Encoding UTF8 GetBytes key                TripleDESCryptoServiceProvider tdes   new TripleDESCryptoServiceProvider                  set the secret key for the tripleDES algorithm             tdes Key   keyArray                mode of operation  there are other 4 modes  We choose ECB Electronic code Book              tdes Mode   CipherMode ECB                padding mode if any extra byte added              tdes Padding   PaddingMode PKCS7               ICryptoTransform cTransform   tdes CreateEncryptor                  transform the specified region of bytes array to resultArray             byte   resultArray   cTransform TransformFinalBlock toEncryptArray  0           toEncryptArray Length                 Release resources held by TripleDes Encryptor             tdes Clear                  Return the encrypted data into unreadable string format             return Convert ToBase64String resultArray  0  resultArray Length                     catch  Exception e                        throw e                         Decryption     public string DecryptText string cipherString  bool useHashing                 try                       byte   keyArray                get the byte code of the string              byte   toEncryptArray   Convert FromBase64String cipherString                string key    String Key Value     Based on this key string is decrypted              if  useHashing                                  if hashing was used get the hash code with regards to your key                 MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                    keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes key                      release any resource held by the MD5CryptoServiceProvider                  hashmd5 Clear                              else                                 if hashing was not implemented get the byte code of the key                 keyArray   UTF8Encoding UTF8 GetBytes key                              TripleDESCryptoServiceProvider tdes   new TripleDESCryptoServiceProvider                  set the secret key for the tripleDES algorithm             tdes Key   keyArray                mode of operation  there are other 4 modes                We choose ECB Electronic code Book               tdes Mode   CipherMode ECB                padding mode if any extra byte added              tdes Padding   PaddingMode PKCS7               ICryptoTransform cTransform   tdes CreateDecryptor                byte   resultArray   cTransform TransformFinalBlock                      toEncryptArray  0  toEncryptArray Length                 Release resources held by TripleDes Encryptor             tdes Clear                  return the Clear decrypted TEXT             return UTF8Encoding UTF8 GetString resultArray                     catch  Exception ex                        throw ex

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Modern Examples of Symmetric Authenticated Encryption of a string    The general best practice for symmetric encryption is to use Authenticated Encryption with Associated Data  AEAD   however this isn t a part of the standard  net crypto libraries  So the first example uses AES256 and then HMAC256  a two step Encrypt then MAC  which requires more overhead and more keys   The second example uses the simpler practice of AES256-GCM using the open source Bouncy Castle  via nuget    Both examples have a main function that takes secret message string  key s  and an optional non-secret payload and return and authenticated encrypted string optionally prepended with the non-secret data  Ideally you would use these with 256bit key s  randomly generated see NewKey     Both examples also have a helper methods that use a string password to generate the keys  These helper methods are provided as a convenience to match up with other examples  however they are far less secure because the strength of the password is going to be far weaker than a 256 bit key   Update  Added byte   overloads  and only the Gist has the full formatting with 4 spaces indent and api docs due to StackOverflow answer limits      NET Built-in Encrypt AES -Then-MAC HMAC   Gist         This work  Modern Encryption of a String C   by James Tuley       identified by James Tuley  is free of known copyright restrictions     https   gist github com 4336842    http   creativecommons org publicdomain mark 1 0        using System  using System IO  using System Security Cryptography  using System Text   namespace Encryption     public static class AESThenHMAC         private static readonly RandomNumberGenerator Random   RandomNumberGenerator Create           Preconfigured Encryption Parameters     public static readonly int BlockBitSize   128      public static readonly int KeyBitSize   256         Preconfigured Password Key Derivation Parameters     public static readonly int SaltBitSize   64      public static readonly int Iterations   10000      public static readonly int MinPasswordLength   12            lt summary gt          Helper that generates a random key on each call           lt  summary gt           lt returns gt  lt  returns gt      public static byte   NewKey               var key   new byte KeyBitSize   8         Random GetBytes key         return key                  lt summary gt          Simple Encryption  AES  then Authentication  HMAC  for a UTF8 Message           lt  summary gt           lt param name  secretMessage  gt The secret message  lt  param gt           lt param name  cryptKey  gt The crypt key  lt  param gt           lt param name  authKey  gt The auth key  lt  param gt           lt param name  nonSecretPayload  gt  Optional  Non-Secret Payload  lt  param gt           lt returns gt          Encrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Secret Message Required  secretMessage lt  exception gt           lt remarks gt          Adds overhead of  Optional-Payload   BlockSize 16    Message-Padded-To-Blocksize    HMac-Tag 32     1 33 Base64          lt  remarks gt      public static string SimpleEncrypt string secretMessage  byte   cryptKey  byte   authKey                         byte   nonSecretPayload   null              if  string IsNullOrEmpty secretMessage           throw new ArgumentException  Secret Message Required     secretMessage           var plainText   Encoding UTF8 GetBytes secretMessage         var cipherText   SimpleEncrypt plainText  cryptKey  authKey  nonSecretPayload         return Convert ToBase64String cipherText                   lt summary gt          Simple Authentication  HMAC  then Decryption  AES  for a secrets UTF8 Message           lt  summary gt           lt param name  encryptedMessage  gt The encrypted message  lt  param gt           lt param name  cryptKey  gt The crypt key  lt  param gt           lt param name  authKey  gt The auth key  lt  param gt           lt param name  nonSecretPayloadLength  gt Length of the non secret payload  lt  param gt           lt returns gt          Decrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Encrypted Message Required  encryptedMessage lt  exception gt      public static string SimpleDecrypt string encryptedMessage  byte   cryptKey  byte   authKey                         int nonSecretPayloadLength   0              if  string IsNullOrWhiteSpace encryptedMessage           throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cipherText   Convert FromBase64String encryptedMessage         var plainText   SimpleDecrypt cipherText  cryptKey  authKey  nonSecretPayloadLength         return plainText    null   null   Encoding UTF8 GetString plainText                   lt summary gt          Simple Encryption  AES  then Authentication  HMAC  of a UTF8 message         using Keys derived from a Password  PBKDF2            lt  summary gt           lt param name  secretMessage  gt The secret message  lt  param gt           lt param name  password  gt The password  lt  param gt           lt param name  nonSecretPayload  gt The non secret payload  lt  param gt           lt returns gt          Encrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt password lt  exception gt           lt remarks gt          Significantly less secure than using random binary keys          Adds additional non secret payload for key generation parameters           lt  remarks gt      public static string SimpleEncryptWithPassword string secretMessage  string password                               byte   nonSecretPayload   null              if  string IsNullOrEmpty secretMessage           throw new ArgumentException  Secret Message Required     secretMessage           var plainText   Encoding UTF8 GetBytes secretMessage         var cipherText   SimpleEncryptWithPassword plainText  password  nonSecretPayload         return Convert ToBase64String cipherText                   lt summary gt          Simple Authentication  HMAC  and then Descryption  AES  of a UTF8 Message         using keys derived from a password  PBKDF2             lt  summary gt           lt param name  encryptedMessage  gt The encrypted message  lt  param gt           lt param name  password  gt The password  lt  param gt           lt param name  nonSecretPayloadLength  gt Length of the non secret payload  lt  param gt           lt returns gt          Decrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Encrypted Message Required  encryptedMessage lt  exception gt           lt remarks gt          Significantly less secure than using random binary keys           lt  remarks gt      public static string SimpleDecryptWithPassword string encryptedMessage  string password                               int nonSecretPayloadLength   0              if  string IsNullOrWhiteSpace encryptedMessage           throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cipherText   Convert FromBase64String encryptedMessage         var plainText   SimpleDecryptWithPassword cipherText  password  nonSecretPayloadLength         return plainText    null   null   Encoding UTF8 GetString plainText              public static byte   SimpleEncrypt byte   secretMessage  byte   cryptKey  byte   authKey  byte   nonSecretPayload   null                User Error Checks       if  cryptKey    null    cryptKey Length    KeyBitSize   8          throw new ArgumentException String Format  Key needs to be  0  bit    KeyBitSize    cryptKey           if  authKey    null    authKey Length    KeyBitSize   8          throw new ArgumentException String Format  Key needs to be  0  bit    KeyBitSize    authKey           if  secretMessage    null    secretMessage Length  lt  1          throw new ArgumentException  Secret Message Required     secretMessage             non-secret payload optional       nonSecretPayload   nonSecretPayload    new byte               byte   cipherText        byte   iv         using  var aes   new AesManaged                 KeySize   KeyBitSize          BlockSize   BlockBitSize          Mode   CipherMode CBC          Padding   PaddingMode PKCS7                             Use random IV         aes GenerateIV            iv   aes IV           using  var encrypter   aes CreateEncryptor cryptKey  iv           using  var cipherStream   new MemoryStream                        using  var cryptoStream   new CryptoStream cipherStream  encrypter  CryptoStreamMode Write             using  var binaryWriter   new BinaryWriter cryptoStream                             Encrypt Data             binaryWriter Write secretMessage                          cipherText   cipherStream ToArray                                Assemble encrypted message and add authentication       using  var hmac   new HMACSHA256 authKey         using  var encryptedStream   new MemoryStream                    using  var binaryWriter   new BinaryWriter encryptedStream                         Prepend non-secret payload if any           binaryWriter Write nonSecretPayload               Prepend IV           binaryWriter Write iv               Write Ciphertext           binaryWriter Write cipherText             binaryWriter Flush                 Authenticate all data           var tag   hmac ComputeHash encryptedStream ToArray                 Postpend tag           binaryWriter Write tag                     return encryptedStream ToArray                        public static byte   SimpleDecrypt byte   encryptedMessage  byte   cryptKey  byte   authKey  int nonSecretPayloadLength   0                 Basic Usage Error Checks       if  cryptKey    null    cryptKey Length    KeyBitSize   8          throw new ArgumentException String Format  CryptKey needs to be  0  bit    KeyBitSize    cryptKey           if  authKey    null    authKey Length    KeyBitSize   8          throw new ArgumentException String Format  AuthKey needs to be  0  bit    KeyBitSize    authKey           if  encryptedMessage    null    encryptedMessage Length    0          throw new ArgumentException  Encrypted Message Required     encryptedMessage           using  var hmac   new HMACSHA256 authKey                   var sentTag   new byte hmac HashSize   8             Calculate Tag         var calcTag   hmac ComputeHash encryptedMessage  0  encryptedMessage Length - sentTag Length           var ivLength    BlockBitSize   8              if message length is to small just return null         if  encryptedMessage Length  lt  sentTag Length   nonSecretPayloadLength   ivLength            return null             Grab Sent Tag         Array Copy encryptedMessage  encryptedMessage Length - sentTag Length  sentTag  0  sentTag Length              Compare Tag with constant time comparison         var compare   0          for  var i   0  i  lt  sentTag Length  i              compare    sentTag i    calcTag i               if message doesn t authenticate return null         if  compare    0            return null           using  var aes   new AesManaged                     KeySize   KeyBitSize            BlockSize   BlockBitSize            Mode   CipherMode CBC            Padding   PaddingMode PKCS7                                   Grab IV from message           var iv   new byte ivLength             Array Copy encryptedMessage  nonSecretPayloadLength  iv  0  iv Length              using  var decrypter   aes CreateDecryptor cryptKey  iv             using  var plainTextStream   new MemoryStream                            using  var decrypterStream   new CryptoStream plainTextStream  decrypter  CryptoStreamMode Write               using  var binaryWriter   new BinaryWriter decrypterStream                                 Decrypt Cipher Text from Message               binaryWriter Write                  encryptedMessage                  nonSecretPayloadLength   iv Length                  encryptedMessage Length - nonSecretPayloadLength - iv Length - sentTag Length                                              Return Plain Text             return plainTextStream ToArray                                             public static byte   SimpleEncryptWithPassword byte   secretMessage  string password  byte   nonSecretPayload   null              nonSecretPayload   nonSecretPayload    new byte                User Error Checks       if  string IsNullOrWhiteSpace password     password Length  lt  MinPasswordLength          throw new ArgumentException String Format  Must have a password of at least  0  characters    MinPasswordLength    password           if  secretMessage    null    secretMessage Length   0          throw new ArgumentException  Secret Message Required     secretMessage           var payload   new byte   SaltBitSize   8    2    nonSecretPayload Length          Array Copy nonSecretPayload  payload  nonSecretPayload Length         int payloadIndex   nonSecretPayload Length         byte   cryptKey        byte   authKey          Use Random Salt to prevent pre-generated weak password attacks        using  var generator   new Rfc2898DeriveBytes password  SaltBitSize   8  Iterations                   var salt   generator Salt             Generate Keys         cryptKey   generator GetBytes KeyBitSize   8              Create Non Secret Payload         Array Copy salt  0  payload  payloadIndex  salt Length           payloadIndex    salt Length                   Deriving separate key  might be less efficient than using HKDF           but now compatible with RNEncryptor which had a very similar wireformat and requires less code than HKDF        using  var generator   new Rfc2898DeriveBytes password  SaltBitSize   8  Iterations                   var salt   generator Salt             Generate Keys         authKey   generator GetBytes KeyBitSize   8              Create Rest of Non Secret Payload         Array Copy salt  0  payload  payloadIndex  salt Length                  return SimpleEncrypt secretMessage  cryptKey  authKey  payload              public static byte   SimpleDecryptWithPassword byte   encryptedMessage  string password  int nonSecretPayloadLength   0                User Error Checks       if  string IsNullOrWhiteSpace password     password Length  lt  MinPasswordLength          throw new ArgumentException String Format  Must have a password of at least  0  characters    MinPasswordLength    password           if  encryptedMessage    null    encryptedMessage Length    0          throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cryptSalt   new byte SaltBitSize   8         var authSalt   new byte SaltBitSize   8            Grab Salt from Non-Secret Payload       Array Copy encryptedMessage  nonSecretPayloadLength  cryptSalt  0  cryptSalt Length         Array Copy encryptedMessage  nonSecretPayloadLength   cryptSalt Length  authSalt  0  authSalt Length          byte   cryptKey        byte   authKey           Generate crypt key       using  var generator   new Rfc2898DeriveBytes password  cryptSalt  Iterations                   cryptKey   generator GetBytes KeyBitSize   8                   Generate auth key       using  var generator   new Rfc2898DeriveBytes password  authSalt  Iterations                   authKey   generator GetBytes KeyBitSize   8                  return SimpleDecrypt encryptedMessage  cryptKey  authKey  cryptSalt Length   authSalt Length   nonSecretPayloadLength                   Bouncy Castle AES-GCM  Gist         This work  Modern Encryption of a String C   by James Tuley       identified by James Tuley  is free of known copyright restrictions     https   gist github com 4336842    http   creativecommons org publicdomain mark 1 0        using System  using System IO  using System Text  using Org BouncyCastle Crypto  using Org BouncyCastle Crypto Engines  using Org BouncyCastle Crypto Generators  using Org BouncyCastle Crypto Modes  using Org BouncyCastle Crypto Parameters  using Org BouncyCastle Security  namespace Encryption      public static class AESGCM         private static readonly SecureRandom Random   new SecureRandom           Preconfigured Encryption Parameters     public static readonly int NonceBitSize   128      public static readonly int MacBitSize   128      public static readonly int KeyBitSize   256         Preconfigured Password Key Derivation Parameters     public static readonly int SaltBitSize   128      public static readonly int Iterations   10000      public static readonly int MinPasswordLength   12             lt summary gt          Helper that generates a random new key on each call           lt  summary gt           lt returns gt  lt  returns gt      public static byte   NewKey               var key   new byte KeyBitSize   8         Random NextBytes key         return key                  lt summary gt          Simple Encryption And Authentication  AES-GCM  of a UTF8 string           lt  summary gt           lt param name  secretMessage  gt The secret message  lt  param gt           lt param name  key  gt The key  lt  param gt           lt param name  nonSecretPayload  gt Optional non-secret payload  lt  param gt           lt returns gt          Encrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Secret Message Required  secretMessage lt  exception gt           lt remarks gt          Adds overhead of  Optional-Payload   BlockSize 16    Message    HMac-Tag 16     1 33 Base64          lt  remarks gt      public static string SimpleEncrypt string secretMessage  byte   key  byte   nonSecretPayload   null              if  string IsNullOrEmpty secretMessage           throw new ArgumentException  Secret Message Required     secretMessage           var plainText   Encoding UTF8 GetBytes secretMessage         var cipherText   SimpleEncrypt plainText  key  nonSecretPayload         return Convert ToBase64String cipherText                    lt summary gt          Simple Decryption  amp  Authentication  AES-GCM  of a UTF8 Message          lt  summary gt           lt param name  encryptedMessage  gt The encrypted message  lt  param gt           lt param name  key  gt The key  lt  param gt           lt param name  nonSecretPayloadLength  gt Length of the optional non-secret payload  lt  param gt           lt returns gt Decrypted Message lt  returns gt      public static string SimpleDecrypt string encryptedMessage  byte   key  int nonSecretPayloadLength   0              if  string IsNullOrEmpty encryptedMessage           throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cipherText   Convert FromBase64String encryptedMessage         var plainText   SimpleDecrypt cipherText  key  nonSecretPayloadLength         return plainText    null   null   Encoding UTF8 GetString plainText                   lt summary gt          Simple Encryption And Authentication  AES-GCM  of a UTF8 String         using key derived from a password  PBKDF2            lt  summary gt           lt param name  secretMessage  gt The secret message  lt  param gt           lt param name  password  gt The password  lt  param gt           lt param name  nonSecretPayload  gt The non secret payload  lt  param gt           lt returns gt          Encrypted Message          lt  returns gt           lt remarks gt          Significantly less secure than using random binary keys          Adds additional non secret payload for key generation parameters           lt  remarks gt      public static string SimpleEncryptWithPassword string secretMessage  string password                               byte   nonSecretPayload   null              if  string IsNullOrEmpty secretMessage           throw new ArgumentException  Secret Message Required     secretMessage           var plainText   Encoding UTF8 GetBytes secretMessage         var cipherText   SimpleEncryptWithPassword plainText  password  nonSecretPayload         return Convert ToBase64String cipherText                    lt summary gt          Simple Decryption and Authentication  AES-GCM  of a UTF8 message         using a key derived from a password  PBKDF2           lt  summary gt           lt param name  encryptedMessage  gt The encrypted message  lt  param gt           lt param name  password  gt The password  lt  param gt           lt param name  nonSecretPayloadLength  gt Length of the non secret payload  lt  param gt           lt returns gt          Decrypted Message          lt  returns gt           lt exception cref  System ArgumentException  gt Encrypted Message Required  encryptedMessage lt  exception gt           lt remarks gt          Significantly less secure than using random binary keys           lt  remarks gt      public static string SimpleDecryptWithPassword string encryptedMessage  string password                               int nonSecretPayloadLength   0              if  string IsNullOrWhiteSpace encryptedMessage           throw new ArgumentException  Encrypted Message Required     encryptedMessage           var cipherText   Convert FromBase64String encryptedMessage         var plainText   SimpleDecryptWithPassword cipherText  password  nonSecretPayloadLength         return plainText    null   null   Encoding UTF8 GetString plainText              public static byte   SimpleEncrypt byte   secretMessage  byte   key  byte   nonSecretPayload   null                User Error Checks       if  key    null    key Length    KeyBitSize   8          throw new ArgumentException String Format  Key needs to be  0  bit    KeyBitSize    key           if  secretMessage    null    secretMessage Length    0          throw new ArgumentException  Secret Message Required     secretMessage             Non-secret Payload Optional       nonSecretPayload   nonSecretPayload    new byte                 Using random nonce large enough not to repeat       var nonce   new byte NonceBitSize   8         Random NextBytes nonce  0  nonce Length          var cipher   new GcmBlockCipher new AesFastEngine           var parameters   new AeadParameters new KeyParameter key   MacBitSize  nonce  nonSecretPayload         cipher Init true  parameters            Generate Cipher Text With Auth Tag       var cipherText   new byte cipher GetOutputSize secretMessage Length          var len   cipher ProcessBytes secretMessage  0  secretMessage Length  cipherText  0         cipher DoFinal cipherText  len            Assemble Message       using  var combinedStream   new MemoryStream                    using  var binaryWriter   new BinaryWriter combinedStream                         Prepend Authenticated Payload           binaryWriter Write nonSecretPayload               Prepend Nonce           binaryWriter Write nonce               Write Cipher Text           binaryWriter Write cipherText                     return combinedStream ToArray                       public static byte   SimpleDecrypt byte   encryptedMessage  byte   key  int nonSecretPayloadLength   0                User Error Checks       if  key    null    key Length    KeyBitSize   8          throw new ArgumentException String Format  Key needs to be  0  bit    KeyBitSize    key           if  encryptedMessage    null    encryptedMessage Length    0          throw new ArgumentException  Encrypted Message Required     encryptedMessage           using  var cipherStream   new MemoryStream encryptedMessage         using  var cipherReader   new BinaryReader cipherStream                     Grab Payload         var nonSecretPayload   cipherReader ReadBytes nonSecretPayloadLength              Grab Nonce         var nonce   cipherReader ReadBytes NonceBitSize   8            var cipher   new GcmBlockCipher new AesFastEngine             var parameters   new AeadParameters new KeyParameter key   MacBitSize  nonce  nonSecretPayload           cipher Init false  parameters              Decrypt Cipher Text         var cipherText   cipherReader ReadBytes encryptedMessage Length - nonSecretPayloadLength - nonce Length           var plainText   new byte cipher GetOutputSize cipherText Length               try                     var len   cipher ProcessBytes cipherText  0  cipherText Length  plainText  0             cipher DoFinal plainText  len                      catch  InvalidCipherTextException                        Return null if it doesn t authenticate           return null                     return plainText                      public static byte   SimpleEncryptWithPassword byte   secretMessage  string password  byte   nonSecretPayload   null              nonSecretPayload   nonSecretPayload    new byte                User Error Checks       if  string IsNullOrWhiteSpace password     password Length  lt  MinPasswordLength          throw new ArgumentException String Format  Must have a password of at least  0  characters    MinPasswordLength    password           if  secretMessage    null    secretMessage Length    0          throw new ArgumentException  Secret Message Required     secretMessage           var generator   new Pkcs5S2ParametersGenerator             Use Random Salt to minimize pre-generated weak password attacks        var salt   new byte SaltBitSize   8         Random NextBytes salt          generator Init          PbeParametersGenerator Pkcs5PasswordToBytes password ToCharArray             salt          Iterations            Generate Key       var key    KeyParameter generator GenerateDerivedMacParameters KeyBitSize            Create Full Non Secret Payload       var payload   new byte salt Length   nonSecretPayload Length         Array Copy nonSecretPayload  payload  nonSecretPayload Length         Array Copy salt 0  payload nonSecretPayload Length  salt Length          return SimpleEncrypt secretMessage  key GetKey    payload              public static byte   SimpleDecryptWithPassword byte   encryptedMessage  string password  int nonSecretPayloadLength   0                User Error Checks       if  string IsNullOrWhiteSpace password     password Length  lt  MinPasswordLength          throw new ArgumentException String Format  Must have a password of at least  0  characters    MinPasswordLength    password           if  encryptedMessage    null    encryptedMessage Length    0          throw new ArgumentException  Encrypted Message Required     encryptedMessage           var generator   new Pkcs5S2ParametersGenerator             Grab Salt from Payload       var salt   new byte SaltBitSize   8         Array Copy encryptedMessage  nonSecretPayloadLength  salt  0  salt Length          generator Init          PbeParametersGenerator Pkcs5PasswordToBytes password ToCharArray             salt          Iterations            Generate Key       var key    KeyParameter generator GenerateDerivedMacParameters KeyBitSize          return SimpleDecrypt encryptedMessage  key GetKey    salt Length   nonSecretPayloadLength

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An alternative to BouncyCastle for AES-GCM encryption is libsodium-net  It wraps the libsodium C library  One nice advantage is that it uses the AES-NI extension in CPUs for very fast encryption  The down side is that it won t work at all if the CPU doesn t have the extension  There s no software fall back

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for simplicity i made for myself this function that i use for non crypto purposes   replace  yourpassphrase  with your password      using System  using System Collections Generic  using System Linq  using System Text  using System Security Cryptography  using System IO    namespace My       public class strCrypto                  This constant string is used as a  salt  value for the PasswordDeriveBytes function calls         This size of the IV  in bytes  must    keysize   8    Default keysize is 256  so the IV must be        32 bytes long   Using a 16 character string here gives us 32 bytes when converted to a byte array      private const string initVector    r5dm5fgm24mfhfku       private const string passPhrase    yourpassphrase      email password encryption password         This constant is used to determine the keysize of the encryption algorithm      private const int keysize   256       public static string encryptString string plainText                  if the plaintext  is empty or null string just return an empty string         if  plainText          plainText    null                         return                        byte   initVectorBytes   Encoding UTF8 GetBytes initVector           byte   plainTextBytes   Encoding UTF8 GetBytes plainText           PasswordDeriveBytes password   new PasswordDeriveBytes passPhrase  null           byte   keyBytes   password GetBytes keysize   8           RijndaelManaged symmetricKey   new RijndaelManaged            symmetricKey Mode   CipherMode CBC          ICryptoTransform encryptor   symmetricKey CreateEncryptor keyBytes  initVectorBytes           MemoryStream memoryStream   new MemoryStream            CryptoStream cryptoStream   new CryptoStream memoryStream  encryptor  CryptoStreamMode Write           cryptoStream Write plainTextBytes  0  plainTextBytes Length           cryptoStream FlushFinalBlock            byte   cipherTextBytes   memoryStream ToArray            memoryStream Close            cryptoStream Close            return Convert ToBase64String cipherTextBytes              public static string decryptString string cipherText                  if the ciphertext is empty or null string just return an empty string         if  cipherText          cipherText    null                         return                        byte   initVectorBytes   Encoding ASCII GetBytes initVector           byte   cipherTextBytes   Convert FromBase64String cipherText           PasswordDeriveBytes password   new PasswordDeriveBytes passPhrase  null           byte   keyBytes   password GetBytes keysize   8           RijndaelManaged symmetricKey   new RijndaelManaged            symmetricKey Mode   CipherMode CBC          ICryptoTransform decryptor   symmetricKey CreateDecryptor keyBytes  initVectorBytes           MemoryStream memoryStream   new MemoryStream cipherTextBytes           CryptoStream cryptoStream   new CryptoStream memoryStream  decryptor  CryptoStreamMode Read           byte   plainTextBytes   new byte cipherTextBytes Length           int decryptedByteCount   cryptoStream Read plainTextBytes  0  plainTextBytes Length           memoryStream Close            cryptoStream Close            return Encoding UTF8 GetString plainTextBytes  0  decryptedByteCount

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Here is an example using RSA    Important  There is a limit to the size of data you can encrypt with the RSA encryption  KeySize - MinimumPadding  e g  256 bytes  assuming 2048 bit key  - 42 bytes  min OEAP padding    214 bytes  max plaintext size   Replace your rsa key with your RSA key   var provider   new System Security Cryptography RSACryptoServiceProvider    provider ImportParameters your rsa key    var encryptedBytes   provider Encrypt      System Text Encoding UTF8 GetBytes  Hello World     true    string decryptedTest   System Text Encoding UTF8 GetString      provider Decrypt encryptedBytes  true      For more info  visit MSDN - RSACryptoServiceProvider

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I have an open source project called X509Crypto that leverages certificates to encrypt and decrypt strings  It s very easy to use  Here is an example for how to use it  1  Use the X509Crypto Commandline Interface  CLI  to generate a new encryption certificate and key pair  gt x509crypto exe X509Crypto gt  makecert -context user -keysize medium -alias myvault  Certificate with thumbprint B31FE7E7AE5229F8186782742CF579197FA859FD was added to X509Alias  quot myvault quot  in the user X509Context  X509Crypto gt   2  Use the Encrypt CLI command to add a secret to your new X509Alias X509Crypto gt  encrypt -text -alias myvault -context user -secret apikey -in  quot 80EAF03248965AC2B78090 quot   Secret apikey has been added to X509Alias myvault in the user X509Context  X509Crypto gt   3  Reference the secret in your program Once you have an X509Alias established with your secret s  added  it is trivial to retreive them in your program with the Org X509Crypto nuget package installed  using Org X509Crypto   namespace SampleApp       class Program               static void Main string   args                        var Alias   new X509Alias   quot myvault quot   X509Context UserReadOnly               var apiKey   Alias RecoverSecret   quot apikey quot

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BouncyCastle is a great Crypto library for  NET  it s available as a Nuget package for install into your projects   I like it a lot more than what s currently available in the System Security Cryptography library   It gives you a lot more options in terms of available algorithms  and provides more modes for those algorithms   This is an example of an implementation of TwoFish  which was written by Bruce Schneier  hero to all us paranoid people out there     It s a symmetric algorithm like the Rijndael  aka AES    It was one of the three finalists for the AES standard and sibling to another famous algorithm written by Bruce Schneier called BlowFish   First thing with bouncycastle is to create an encryptor class  this will make it easier to implement other block ciphers within the library   The following encryptor class takes in a generic argument T where T implements IBlockCipher and has a default constructor   UPDATE   Due to popular demand I have decided to implement generating a random IV as well as include an HMAC into this class   Although from a style perspective this goes against the SOLID principle of single responsibility  because of the nature of what this class does I reniged   This class will now take two generic parameters  one for the cipher and one for the digest   It automatically generates the IV using RNGCryptoServiceProvider to provide good RNG entropy  and allows you to use whatever digest algorithm you want from BouncyCastle to generate the MAC   using System  using System Security Cryptography  using System Text  using Org BouncyCastle Crypto  using Org BouncyCastle Crypto Macs  using Org BouncyCastle Crypto Modes  using Org BouncyCastle Crypto Paddings  using Org BouncyCastle Crypto Parameters   public sealed class Encryptor lt TBlockCipher  TDigest gt      where TBlockCipher   IBlockCipher  new       where TDigest   IDigest  new         private Encoding encoding       private IBlockCipher blockCipher       private BufferedBlockCipher cipher       private HMac mac       private byte   key       public Encryptor Encoding encoding  byte   key  byte   macKey                this encoding   encoding          this key   key          this Init key  macKey  new Pkcs7Padding                public Encryptor Encoding encoding  byte   key  byte   macKey  IBlockCipherPadding padding                this encoding   encoding          this key   key          this Init key  macKey  padding              private void Init byte   key  byte   macKey  IBlockCipherPadding padding                this blockCipher   new CbcBlockCipher new TBlockCipher             this cipher   new PaddedBufferedBlockCipher this blockCipher  padding           this mac   new HMac new TDigest             this mac Init new KeyParameter macKey               public string Encrypt string plain                return Convert ToBase64String EncryptBytes plain               public byte   EncryptBytes string plain                byte   input   this encoding GetBytes plain            var iv   this GenerateIV             var cipher   this BouncyCastleCrypto true  input  new ParametersWithIV new KeyParameter key   iv            byte   message   CombineArrays iv  cipher            this mac Reset            this mac BlockUpdate message  0  message Length           byte   digest   new byte this mac GetUnderlyingDigest   GetDigestSize             this mac DoFinal digest  0            var result   CombineArrays digest  message           return result             public byte   DecryptBytes byte   bytes                   split the digest into component parts         var digest   new byte this mac GetUnderlyingDigest   GetDigestSize             var message   new byte bytes Length - digest Length           var iv   new byte this blockCipher GetBlockSize             var cipher   new byte message Length - iv Length            Buffer BlockCopy bytes  0  digest  0  digest Length           Buffer BlockCopy bytes  digest Length  message  0  message Length           if   IsValidHMac digest  message                         throw new CryptoException                       Buffer BlockCopy message  0  iv  0  iv Length           Buffer BlockCopy message  iv Length  cipher  0  cipher Length            byte   result   this BouncyCastleCrypto false  cipher  new ParametersWithIV new KeyParameter key   iv            return result             public string Decrypt byte   bytes                return this encoding GetString DecryptBytes bytes               public string Decrypt string cipher                return this Decrypt Convert FromBase64String cipher               private bool IsValidHMac byte   digest  byte   message                this mac Reset            this mac BlockUpdate message  0  message Length           byte   computed   new byte this mac GetUnderlyingDigest   GetDigestSize             this mac DoFinal computed  0            return AreEqual digest computed              private static bool AreEqual byte    digest  byte   computed                if digest Length    computed Length                        return false                     int result   0          for  int i   0  i  lt  digest Length  i                             compute equality of all bytes before returning                   helps prevent timing attacks                    https   codahale com a-lesson-in-timing-attacks              result    digest i    computed i                      return result    0             private byte   BouncyCastleCrypto bool forEncrypt  byte   input  ICipherParameters parameters                try                       cipher Init forEncrypt  parameters                return this cipher DoFinal input                     catch  CryptoException                        throw                       private byte   GenerateIV                 using  var provider   new RNGCryptoServiceProvider                             1st block             byte   result   new byte this blockCipher GetBlockSize                 provider GetBytes result                return result                       private static byte   CombineArrays byte   source1  byte   source2                byte   result   new byte source1 Length   source2 Length           Buffer BlockCopy source1  0  result  0  source1 Length           Buffer BlockCopy source2  0  result  source1 Length  source2 Length            return result            Next just call the encrypt and decrypt methods on the new class  here s the example using twofish   var encrypt   new Encryptor lt TwofishEngine  Sha1Digest gt  Encoding UTF8  key  hmacKey    string cipher   encrypt Encrypt  TEST       string plainText   encrypt Decrypt cipher     It s just as easy to substitute another block cipher like TripleDES   var des   new Encryptor lt DesEdeEngine  Sha1Digest gt  Encoding UTF8  key  hmacKey    string cipher   des Encrypt  TEST    string plainText   des Decrypt cipher     Finally if you want to use AES with SHA256 HMAC you can do the following   var aes   new Encryptor lt AesEngine  Sha256Digest gt  Encoding UTF8  key  hmacKey    cipher   aes Encrypt  TEST    plainText   aes Decrypt cipher     The hardest part about encryption actually deals with the keys and not the algorithms   You ll have to think about where you store your keys  and if you have to  how you exchange them   These algorithms have all withstood the test of time  and are extremely hard to break  Someone who wants to steal information from you isn t going to spend eternity doing cryptanalysis on your messages  they re going to try to figure out what or where your key is   So  1 choose your keys wisely   2 store them in a safe place  if you use a web config and IIS then you can encrypt parts of the the web config  and finally if you have to exchange keys make sure that your protocol for exchanging the key is secure   Update 2 Changed compare method to mitigate against timing attacks   See more info here http   codahale com a-lesson-in-timing-attacks     Also updated to default to PKCS7 padding and added new constructor to allow end user the ability to choose which padding they would like to use   Thanks  CodesInChaos for the suggestions

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A good algorithm to securely hash data is BCrypt      Besides incorporating a salt to protect against rainbow table attacks    bcrypt is an adaptive function  over time  the iteration count can be   increased to make it slower  so it remains resistant to brute-force   search attacks even with increasing computation power    There s a nice  NET implementation of BCrypt that is available also as a NuGet package

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using System              using System Collections Generic              using System Text              using System Text RegularExpressions      This is for password validation             using System Security Cryptography              using System Configuration      This is where the hash functions reside              namespace BullyTracker Common                               public class HashEncryption                                         public string GenerateHashvalue string thisPassword                                                    MD5CryptoServiceProvider md5   new MD5CryptoServiceProvider                              byte   tmpSource                            byte   tmpHash                             tmpSource   ASCIIEncoding ASCII GetBytes thisPassword      Turn password into byte array                           tmpHash   md5 ComputeHash tmpSource                              StringBuilder sOutput   new StringBuilder tmpHash Length                             for  int i   0  i  lt  tmpHash Length  i                                                              sOutput Append tmpHash i  ToString  X2         X2 formats to hexadecimal                                                       return sOutput ToString                                                  public Boolean VerifyHashPassword string thisPassword  string thisHash                                                    Boolean IsValid   false                            string tmpHash   GenerateHashvalue thisPassword      Call the routine on user input                           if  tmpHash    thisHash  IsValid   true      Compare to previously generated hash                           return IsValid                                              public string GenerateHashvalue string toEncrypt  bool useHashing                                                byte   keyArray                          byte   toEncryptArray   UTF8Encoding UTF8 GetBytes toEncrypt                            System Configuration AppSettingsReader settingsReader   new AppSettingsReader                               Get the key from config file                         string key    string settingsReader GetValue  SecurityKey   typeof String                              System Windows Forms MessageBox Show key                           if  useHashing                                                        MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                                keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes key                                hashmd5 Clear                                                      else                             keyArray   UTF8Encoding UTF8 GetBytes key                            TripleDESCryptoServiceProvider tdes   new TripleDESCryptoServiceProvider                            tdes Key   keyArray                          tdes Mode   CipherMode ECB                          tdes Padding   PaddingMode PKCS7                           ICryptoTransform cTransform   tdes CreateEncryptor                            byte   resultArray   cTransform TransformFinalBlock toEncryptArray  0  toEncryptArray Length                           tdes Clear                            return Convert ToBase64String resultArray  0  resultArray Length                                                  lt summary gt                          DeCrypt a string using dual encryption method  Return a DeCrypted clear string                          lt  summary gt                           lt param name  cipherString  gt encrypted string lt  param gt                           lt param name  useHashing  gt Did you use hashing to encrypt this data  pass true is yes lt  param gt                           lt returns gt  lt  returns gt                      public string Decrypt string cipherString  bool useHashing                                                byte   keyArray                          byte   toEncryptArray   Convert FromBase64String cipherString                            System Configuration AppSettingsReader settingsReader   new AppSettingsReader                              Get your key from config file to open the lock                          string key    string settingsReader GetValue  SecurityKey   typeof String                             if  useHashing                                                        MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                                keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes key                                hashmd5 Clear                                                      else                             keyArray   UTF8Encoding UTF8 GetBytes key                            TripleDESCryptoServiceProvider tdes   new TripleDESCryptoServiceProvider                            tdes Key   keyArray                          tdes Mode   CipherMode ECB                          tdes Padding   PaddingMode PKCS7                           ICryptoTransform cTransform   tdes CreateDecryptor                            byte   resultArray   cTransform TransformFinalBlock toEncryptArray  0  toEncryptArray Length                            tdes Clear                            return UTF8Encoding UTF8 GetString resultArray

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Here is simple Snippet originally by ASP Snippets     using System Text  using System Security Cryptography  using System IO     private string Encrypt string clearText                string EncryptionKey    yourkey           byte   clearBytes   Encoding Unicode GetBytes clearText           using  Aes encryptor   Aes Create                          Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  new byte     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76                 encryptor Key   pdb GetBytes 32               encryptor IV   pdb GetBytes 16               using  MemoryStream ms   new MemoryStream                                  using  CryptoStream cs   new CryptoStream ms  encryptor CreateEncryptor    CryptoStreamMode Write                                         cs Write clearBytes  0  clearBytes Length                       cs Close                                      clearText   Convert ToBase64String ms ToArray                                     return clearText          private string Decrypt string cipherText                string EncryptionKey    yourkey           cipherText   cipherText Replace                    byte   cipherBytes   Convert FromBase64String cipherText           using  Aes encryptor   Aes Create                          Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  new byte     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76                 encryptor Key   pdb GetBytes 32               encryptor IV   pdb GetBytes 16               using  MemoryStream ms   new MemoryStream                                  using  CryptoStream cs   new CryptoStream ms  encryptor CreateDecryptor    CryptoStreamMode Write                                         cs Write cipherBytes  0  cipherBytes Length                       cs Close                                      cipherText   Encoding Unicode GetString ms ToArray                                     return cipherText

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Here is simple Snippet originally by ASP Snippets     using System Text  using System Security Cryptography  using System IO     private string Encrypt string clearText                string EncryptionKey    yourkey           byte   clearBytes   Encoding Unicode GetBytes clearText           using  Aes encryptor   Aes Create                          Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  new byte     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76                 encryptor Key   pdb GetBytes 32               encryptor IV   pdb GetBytes 16               using  MemoryStream ms   new MemoryStream                                  using  CryptoStream cs   new CryptoStream ms  encryptor CreateEncryptor    CryptoStreamMode Write                                         cs Write clearBytes  0  clearBytes Length                       cs Close                                      clearText   Convert ToBase64String ms ToArray                                     return clearText          private string Decrypt string cipherText                string EncryptionKey    yourkey           cipherText   cipherText Replace                    byte   cipherBytes   Convert FromBase64String cipherText           using  Aes encryptor   Aes Create                          Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  new byte     0x49  0x76  0x61  0x6e  0x20  0x4d  0x65  0x64  0x76  0x65  0x64  0x65  0x76                 encryptor Key   pdb GetBytes 32               encryptor IV   pdb GetBytes 16               using  MemoryStream ms   new MemoryStream                                  using  CryptoStream cs   new CryptoStream ms  encryptor CreateDecryptor    CryptoStreamMode Write                                         cs Write cipherBytes  0  cipherBytes Length                       cs Close                                      cipherText   Encoding Unicode GetString ms ToArray                                     return cipherText

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Disclaimer  This solution should only be used for data at rest that is not exposed to the public  for example - a configuration file or DB   Only in this scenario  the quick-and-dirty solution can be considered better than  jbtule s solution  due to lower maintanance   Original post  I found jbtule s answer a bit complicated for a quick and dirty secured AES string encryption and Brett s answer had a bug with the Initialization Vector being a fixed value making it vulnerable to padding attacks  so I fixed Brett s code and added a random IV that is added to the chipered string  creating a different encrypted value each and every encryption of the same value   Encryption   public static string Encrypt string clearText                    byte   clearBytes   Encoding Unicode GetBytes clearText       using  Aes encryptor   Aes Create                  byte   IV   new byte 15           rand NextBytes IV           Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  IV           encryptor Key   pdb GetBytes 32           encryptor IV   pdb GetBytes 16           using  MemoryStream ms   new MemoryStream                          using  CryptoStream cs   new CryptoStream ms  encryptor CreateEncryptor    CryptoStreamMode Write                                 cs Write clearBytes  0  clearBytes Length                   cs Close                              clearText   Convert ToBase64String IV    Convert ToBase64String ms ToArray                         return clearText      Decryption   public static string Decrypt string cipherText        byte   IV   Convert FromBase64String cipherText Substring 0  20        cipherText   cipherText Substring 20  Replace                byte   cipherBytes   Convert FromBase64String cipherText       using  Aes encryptor   Aes Create                  Rfc2898DeriveBytes pdb   new Rfc2898DeriveBytes EncryptionKey  IV           encryptor Key   pdb GetBytes 32           encryptor IV   pdb GetBytes 16           using  MemoryStream ms   new MemoryStream                          using  CryptoStream cs   new CryptoStream ms  encryptor CreateDecryptor    CryptoStreamMode Write                                 cs Write cipherBytes  0  cipherBytes Length                   cs Close                              cipherText   Encoding Unicode GetString ms ToArray                         return cipherText      Replace EncryptionKey with your key  In my implementation  the key is being saved in the configuration file  web config app config  as you shouldn t save it hard coded  The configuration file should be also encrypted so the key won t be saved as clear text in it   protected static string  Key       protected static string EncryptionKey       get               if  String IsNullOrEmpty  Key                          Key   ConfigurationManager AppSettings  AESKey   ToString                       return  Key

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Here is a simple example of encrypting strings in C  using AES CBC mode with random IV and HMAC and password-derived keys  to show the basic moving parts   private byte   EncryptBytes byte   key  byte   plaintext        using  var cipher   new RijndaelManaged   Key   key                  using  var encryptor   cipher CreateEncryptor                          var ciphertext   encryptor TransformFinalBlock plaintext  0  plaintext Length                   IV is prepended to ciphertext             return cipher IV Concat ciphertext  ToArray                       private byte   DecryptBytes byte   key  byte   packed        using  var cipher   new RijndaelManaged   Key   key                  int ivSize   cipher BlockSize   8           cipher IV   packed Take ivSize  ToArray             using  var encryptor   cipher CreateDecryptor                          return encryptor TransformFinalBlock packed  ivSize  packed Length - ivSize                      private byte   AddMac byte   key  byte   data        using  var hmac   new HMACSHA256 key                 var macBytes   hmac ComputeHash data               HMAC is appended to data         return data Concat macBytes  ToArray             private bool BadMac byte   found  byte   computed        int mismatch   0          Aim for consistent timing regardless of inputs     for  int i   0  i  lt  found Length  i                  mismatch    found i     computed i    0   1             return mismatch    0     private byte   RemoveMac byte   key  byte   data        using  var hmac   new HMACSHA256 key                 int macSize   hmac HashSize   8           var packed   data Take data Length - macSize  ToArray             var foundMac   data Skip packed Length  ToArray             var computedMac   hmac ComputeHash packed            if  this BadMac foundMac  computedMac                         throw new Exception  Bad MAC                       return packed                       private List lt byte   gt  DeriveTwoKeys string password        var salt   new byte     1  2  3  4  5  6  7  8         var kdf   new Rfc2898DeriveBytes password  salt  10000        var bytes   kdf GetBytes 32      Two keys 128 bits each      return new List lt byte   gt    bytes Take 16  ToArray    bytes Skip 16  ToArray         public byte   EncryptString string password  String message        var keys   this DeriveTwoKeys password        var plaintext   Encoding UTF8 GetBytes message        var packed   this EncryptBytes keys 0   plaintext        return this AddMac keys 1   packed      public String DecryptString string password  byte   secret        var keys   this DeriveTwoKeys password        var packed   this RemoveMac keys 1   secret        var plaintext   this DecryptBytes keys 0   packed        return Encoding UTF8 GetString plaintext      public void Example         var password    correcthorsebatterystaple        var secret   this EncryptString password   Hello World         Console WriteLine  secret      BitConverter ToString secret         var recovered   this DecryptString password  secret        Console WriteLine recovered

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The following example demonstrates how to encrypt and decrypt sample data          This constant is used to determine the keysize of the encryption algorithm in bits         We divide this by 8 within the code below to get the equivalent number of bytes      private const int Keysize   128          This constant determines the number of iterations for the password bytes generation function      private const int DerivationIterations   1000       public static string Encrypt string plainText  string passPhrase                   Salt and IV is randomly generated each time  but is preprended to encrypted cipher text            so that the same Salt and IV values can be used when decrypting            var saltStringBytes   GenerateBitsOfRandomEntropy 16           var ivStringBytes   GenerateBitsOfRandomEntropy 16           var plainTextBytes   Encoding UTF8 GetBytes plainText           using  var password   new Rfc2898DeriveBytes passPhrase  saltStringBytes  DerivationIterations                         var keyBytes   password GetBytes Keysize   8               using  var symmetricKey   new RijndaelManaged                                  symmetricKey BlockSize   128                  symmetricKey Mode   CipherMode CBC                  symmetricKey Padding   PaddingMode PKCS7                  using  var encryptor   symmetricKey CreateEncryptor keyBytes  ivStringBytes                                         using  var memoryStream   new MemoryStream                                                  using  var cryptoStream   new CryptoStream memoryStream  encryptor  CryptoStreamMode Write                                                         cryptoStream Write plainTextBytes  0  plainTextBytes Length                               cryptoStream FlushFinalBlock                                   Create the final bytes as a concatenation of the random salt bytes  the random iv bytes and the cipher bytes                              var cipherTextBytes   saltStringBytes                              cipherTextBytes   cipherTextBytes Concat ivStringBytes  ToArray                                cipherTextBytes   cipherTextBytes Concat memoryStream ToArray    ToArray                                memoryStream Close                                cryptoStream Close                                return Convert ToBase64String cipherTextBytes                                                                                                        public static string Decrypt string cipherText  string passPhrase                   Get the complete stream of bytes that represent              32 bytes of Salt     32 bytes of IV     n bytes of CipherText          var cipherTextBytesWithSaltAndIv   Convert FromBase64String cipherText              Get the saltbytes by extracting the first 32 bytes from the supplied cipherText bytes          var saltStringBytes   cipherTextBytesWithSaltAndIv Take Keysize   8  ToArray               Get the IV bytes by extracting the next 32 bytes from the supplied cipherText bytes          var ivStringBytes   cipherTextBytesWithSaltAndIv Skip Keysize   8  Take Keysize   8  ToArray               Get the actual cipher text bytes by removing the first 64 bytes from the cipherText string          var cipherTextBytes   cipherTextBytesWithSaltAndIv Skip  Keysize   8    2  Take cipherTextBytesWithSaltAndIv Length -   Keysize   8    2   ToArray             using  var password   new Rfc2898DeriveBytes passPhrase  saltStringBytes  DerivationIterations                         var keyBytes   password GetBytes Keysize   8               using  var symmetricKey   new RijndaelManaged                                  symmetricKey BlockSize   128                  symmetricKey Mode   CipherMode CBC                  symmetricKey Padding   PaddingMode PKCS7                  using  var decryptor   symmetricKey CreateDecryptor keyBytes  ivStringBytes                                         using  var memoryStream   new MemoryStream cipherTextBytes                                                 using  var cryptoStream   new CryptoStream memoryStream  decryptor  CryptoStreamMode Read                                                         var plainTextBytes   new byte cipherTextBytes Length                               var decryptedByteCount   cryptoStream Read plainTextBytes  0  plainTextBytes Length                               memoryStream Close                                cryptoStream Close                                return Encoding UTF8 GetString plainTextBytes  0  decryptedByteCount                                                                                                        private static byte   GenerateBitsOfRandomEntropy int size                   32 Bytes will give us 256 bits             16 Bytes will give us 128 bits          var randomBytes   new byte size            using  var rngCsp   new RNGCryptoServiceProvider                             Fill the array with cryptographically secure random bytes              rngCsp GetBytes randomBytes                     return randomBytes

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Copied in my answer here from a similar question  Simple two-way encryption for C    Based on multiple answers and comments    Random initialization vector prepended to crypto text   jbtule  Use TransformFinalBlock   instead of MemoryStream   RenniePet  No pre-filled keys to avoid anyone copy  amp  pasting a disaster Proper dispose and using patterns   Code        lt summary gt      Simple encryption decryption using a random initialization vector     and prepending it to the crypto text       lt  summary gt       lt remarks gt Based on multiple answers in https   stackoverflow com questions 165808 simple-two-way-encryption-for-c-sharp  lt  remarks gt  public class SimpleAes   IDisposable            lt summary gt              Initialization vector length in bytes           lt  summary gt      private const int IvBytes   16            lt summary gt              Must be exactly 16  24 or 32 characters long           lt  summary gt      private static readonly byte   Key   Convert FromBase64String  FILL ME WITH 16  24 OR 32 CHARS         private readonly UTF8Encoding  encoder      private readonly ICryptoTransform  encryptor      private readonly RijndaelManaged  rijndael       public SimpleAes                  rijndael   new RijndaelManaged  Key   Key            rijndael GenerateIV             encryptor    rijndael CreateEncryptor             encoder   new UTF8Encoding               public string Decrypt string encrypted                return  encoder GetString Decrypt Convert FromBase64String encrypted                public void Dispose                  rijndael Dispose             encryptor Dispose               public string Encrypt string unencrypted                return Convert ToBase64String Encrypt  encoder GetBytes unencrypted                private byte   Decrypt byte   buffer                   IV is prepended to cryptotext         byte   iv   buffer Take IvBytes  ToArray            using  ICryptoTransform decryptor    rijndael CreateDecryptor  rijndael Key  iv                         return decryptor TransformFinalBlock buffer  IvBytes  buffer Length - IvBytes                        private byte   Encrypt byte   buffer                   Prepend cryptotext with IV         byte   inputBuffer    rijndael IV Concat buffer  ToArray            return  encryptor TransformFinalBlock inputBuffer  IvBytes  buffer Length

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Here is an example using RSA    Important  There is a limit to the size of data you can encrypt with the RSA encryption  KeySize - MinimumPadding  e g  256 bytes  assuming 2048 bit key  - 42 bytes  min OEAP padding    214 bytes  max plaintext size   Replace your rsa key with your RSA key   var provider   new System Security Cryptography RSACryptoServiceProvider    provider ImportParameters your rsa key    var encryptedBytes   provider Encrypt      System Text Encoding UTF8 GetBytes  Hello World     true    string decryptedTest   System Text Encoding UTF8 GetString      provider Decrypt encryptedBytes  true      For more info  visit MSDN - RSACryptoServiceProvider

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Encryption is a very common matter in programming  I think it is better to install a package to do the task for you  Maybe a simple open source Nuget project like  Simple Aes Encryption  The key is in the config file and therefore it is easy to change in the production environment  and I don t see any drawbacks   lt MessageEncryption gt     lt EncryptionKey KeySize  256  Key  3q2 796tvu erb7v3q2 796tvu erb7v3q2 796tvu8    gt   lt  MessageEncryption gt

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using System  using System IO  using System Security Cryptography  using System Text   public class Program       public static void Main                 var key   Encoding UTF8 GetBytes  SUkbqO2ycDo7QwpR25kfgmC7f8CoyrZy            var data   Encoding UTF8 GetBytes  testData               Encrypt data         var encrypted   CryptoHelper EncryptData data key              Decrypt data         var decrypted   CryptoHelper DecryptData encrypted key              Display result         Console WriteLine Encoding UTF8 GetString decrypted             public static class CryptoHelper       public static byte   EncryptData byte   data  byte   key                using  var aesAlg   Aes Create                          aesAlg Mode   CipherMode CBC              using  var encryptor   aesAlg CreateEncryptor key  aesAlg IV                                 using  var msEncrypt   new MemoryStream                                          msEncrypt Write aesAlg IV  0  aesAlg IV Length                        using  var csEncrypt   new CryptoStream msEncrypt  encryptor  CryptoStreamMode Write                           csEncrypt Write data  0  data Length                        return msEncrypt ToArray                                                          public static byte   DecryptData byte   encrypted  byte   key                var iv   new byte 16           Buffer BlockCopy encrypted  0  iv  0  iv Length           using  var aesAlg   Aes Create                          aesAlg Mode   CipherMode CBC              using  var decryptor   aesAlg CreateDecryptor key  iv                                 using  var msDecrypt   new MemoryStream encrypted  iv Length  encrypted Length - iv Length                                         using  var csDecrypt   new CryptoStream msDecrypt  decryptor  CryptoStreamMode Read                                                 using  var resultStream   new MemoryStream                                                          csDecrypt CopyTo resultStream                               return resultStream ToArray

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The following example demonstrates how to encrypt and decrypt sample data          This constant is used to determine the keysize of the encryption algorithm in bits         We divide this by 8 within the code below to get the equivalent number of bytes      private const int Keysize   128          This constant determines the number of iterations for the password bytes generation function      private const int DerivationIterations   1000       public static string Encrypt string plainText  string passPhrase                   Salt and IV is randomly generated each time  but is preprended to encrypted cipher text            so that the same Salt and IV values can be used when decrypting            var saltStringBytes   GenerateBitsOfRandomEntropy 16           var ivStringBytes   GenerateBitsOfRandomEntropy 16           var plainTextBytes   Encoding UTF8 GetBytes plainText           using  var password   new Rfc2898DeriveBytes passPhrase  saltStringBytes  DerivationIterations                         var keyBytes   password GetBytes Keysize   8               using  var symmetricKey   new RijndaelManaged                                  symmetricKey BlockSize   128                  symmetricKey Mode   CipherMode CBC                  symmetricKey Padding   PaddingMode PKCS7                  using  var encryptor   symmetricKey CreateEncryptor keyBytes  ivStringBytes                                         using  var memoryStream   new MemoryStream                                                  using  var cryptoStream   new CryptoStream memoryStream  encryptor  CryptoStreamMode Write                                                         cryptoStream Write plainTextBytes  0  plainTextBytes Length                               cryptoStream FlushFinalBlock                                   Create the final bytes as a concatenation of the random salt bytes  the random iv bytes and the cipher bytes                              var cipherTextBytes   saltStringBytes                              cipherTextBytes   cipherTextBytes Concat ivStringBytes  ToArray                                cipherTextBytes   cipherTextBytes Concat memoryStream ToArray    ToArray                                memoryStream Close                                cryptoStream Close                                return Convert ToBase64String cipherTextBytes                                                                                                        public static string Decrypt string cipherText  string passPhrase                   Get the complete stream of bytes that represent              32 bytes of Salt     32 bytes of IV     n bytes of CipherText          var cipherTextBytesWithSaltAndIv   Convert FromBase64String cipherText              Get the saltbytes by extracting the first 32 bytes from the supplied cipherText bytes          var saltStringBytes   cipherTextBytesWithSaltAndIv Take Keysize   8  ToArray               Get the IV bytes by extracting the next 32 bytes from the supplied cipherText bytes          var ivStringBytes   cipherTextBytesWithSaltAndIv Skip Keysize   8  Take Keysize   8  ToArray               Get the actual cipher text bytes by removing the first 64 bytes from the cipherText string          var cipherTextBytes   cipherTextBytesWithSaltAndIv Skip  Keysize   8    2  Take cipherTextBytesWithSaltAndIv Length -   Keysize   8    2   ToArray             using  var password   new Rfc2898DeriveBytes passPhrase  saltStringBytes  DerivationIterations                         var keyBytes   password GetBytes Keysize   8               using  var symmetricKey   new RijndaelManaged                                  symmetricKey BlockSize   128                  symmetricKey Mode   CipherMode CBC                  symmetricKey Padding   PaddingMode PKCS7                  using  var decryptor   symmetricKey CreateDecryptor keyBytes  ivStringBytes                                         using  var memoryStream   new MemoryStream cipherTextBytes                                                 using  var cryptoStream   new CryptoStream memoryStream  decryptor  CryptoStreamMode Read                                                         var plainTextBytes   new byte cipherTextBytes Length                               var decryptedByteCount   cryptoStream Read plainTextBytes  0  plainTextBytes Length                               memoryStream Close                                cryptoStream Close                                return Encoding UTF8 GetString plainTextBytes  0  decryptedByteCount                                                                                                        private static byte   GenerateBitsOfRandomEntropy int size                   32 Bytes will give us 256 bits             16 Bytes will give us 128 bits          var randomBytes   new byte size            using  var rngCsp   new RNGCryptoServiceProvider                             Fill the array with cryptographically secure random bytes              rngCsp GetBytes randomBytes                     return randomBytes

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You have to use the namespace using System Security Cryptography  and useHashing is a bool type either true or false  String variable  key  should be same for Encryption and for Decryption    Encryption public string EncryptText string toEncrypt  bool useHashing                try                       byte   keyArray              byte   toEncryptArray   UTF8Encoding UTF8 GetBytes toEncrypt                string key    String Key Value     Based on this key stirng is encrypting               System Windows Forms MessageBox Show key                 If hashing use get hashcode regards to your key             if  useHashing                                MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                    keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes key                      Always release the resources and flush data                   of the Cryptographic service provide  Best Practice                  hashmd5 Clear                              else                 keyArray   UTF8Encoding UTF8 GetBytes key                TripleDESCryptoServiceProvider tdes   new TripleDESCryptoServiceProvider                  set the secret key for the tripleDES algorithm             tdes Key   keyArray                mode of operation  there are other 4 modes  We choose ECB Electronic code Book              tdes Mode   CipherMode ECB                padding mode if any extra byte added              tdes Padding   PaddingMode PKCS7               ICryptoTransform cTransform   tdes CreateEncryptor                  transform the specified region of bytes array to resultArray             byte   resultArray   cTransform TransformFinalBlock toEncryptArray  0           toEncryptArray Length                 Release resources held by TripleDes Encryptor             tdes Clear                  Return the encrypted data into unreadable string format             return Convert ToBase64String resultArray  0  resultArray Length                     catch  Exception e                        throw e                         Decryption     public string DecryptText string cipherString  bool useHashing                 try                       byte   keyArray                get the byte code of the string              byte   toEncryptArray   Convert FromBase64String cipherString                string key    String Key Value     Based on this key string is decrypted              if  useHashing                                  if hashing was used get the hash code with regards to your key                 MD5CryptoServiceProvider hashmd5   new MD5CryptoServiceProvider                    keyArray   hashmd5 ComputeHash UTF8Encoding UTF8 GetBytes key                      release any resource held by the MD5CryptoServiceProvider                  hashmd5 Clear                              else                                 if hashing was not implemented get the byte code of the key                 keyArray   UTF8Encoding UTF8 GetBytes key                              TripleDESCryptoServiceProvider tdes   new TripleDESCryptoServiceProvider                  set the secret key for the tripleDES algorithm             tdes Key   keyArray                mode of operation  there are other 4 modes                We choose ECB Electronic code Book               tdes Mode   CipherMode ECB                padding mode if any extra byte added              tdes Padding   PaddingMode PKCS7               ICryptoTransform cTransform   tdes CreateDecryptor                byte   resultArray   cTransform TransformFinalBlock                      toEncryptArray  0  toEncryptArray Length                 Release resources held by TripleDes Encryptor             tdes Clear                  return the Clear decrypted TEXT             return UTF8Encoding UTF8 GetString resultArray                     catch  Exception ex                        throw ex

[c#] Encrypt and decrypt a string in C#?

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