Unique random string generation

Question

I d like to generate random unique strings like the ones being generated by MSDN library  Error Object   for example  A string like  t9zk6eay  should be generated

User · Answer

If you want an alphanumeric strings with lowercase and uppercase characters ([a-zA-Z0-9]), you can use Convert.ToBase64String() for a fast and simple solution.

As for uniqueness, check out the birthday problem to calculate how likely a collission is given (A) the length of the strings generated and (B) the number of strings generated.

Random random = new Random();

int outputLength = 10;
int byteLength = (int)Math.Ceiling(3f / 4f * outputLength); // Base64 uses 4 characters for every 3 bytes of data; so in random bytes we need only 3/4 of the desired length
byte[] randomBytes = new byte[byteLength];
string output;
do
{
    random.NextBytes(randomBytes); // Fill bytes with random data
    output = Convert.ToBase64String(randomBytes); // Convert to base64
    output = output.Substring(0, outputLength); // Truncate any superfluous characters and/or padding
} while (output.Contains('/') || output.Contains('+')); // Repeat if we contain non-alphanumeric characters (~25% chance if length=10; ~50% chance if length=20; ~35% chance if length=32)

User · Answer

Try combination between Guid and Time Ticks   var randomNumber   Convert ToBase64String Guid NewGuid   ToByteArray      DateTime Now Ticks       randomNumber   System Text RegularExpressions Regex Replace randomNumber     0-9a-zA-Z

User · Answer

Get Unique Key using GUID Hash code  public static string GetUniqueKey int length        string guidResult   string Empty       while  guidResult Length  lt  length                   Get the GUID          guidResult    Guid NewGuid   ToString   GetHashCode   ToString  x                  Make sure length is valid      if  length  lt   0    length  gt  guidResult Length          throw new ArgumentException  Length must be between 1 and     guidResult Length           Return the first length bytes      return guidResult Substring 0  length

User · Answer

This works perfect for me       private string GeneratePasswordResetToken                 string token   Guid NewGuid   ToString            var plainTextBytes   System Text Encoding UTF8 GetBytes token           return Convert ToBase64String plainTextBytes

User · Answer

I am surprised why there is not a CrytpoGraphic solution in place  GUID is unique but not cryptographically safe  See this Dotnet Fiddle   var bytes   new byte 40      byte size using  var crypto   new RNGCryptoServiceProvider      crypto GetBytes bytes    var base64   Convert ToBase64String bytes   Console WriteLine base64     In case you want to Prepend with a Guid   var result   Guid NewGuid   ToString  N     base64  Console WriteLine result     A cleaner alphanumeric string   result   Regex Replace result    A-Za-z0-9        Console WriteLine result

User · Answer

This has been asked for various languages  Here s one question about passwords which should be applicable here as well    If you want to use the strings for URL shortening  you ll also need a Dictionary lt   or database check to see whether a generated ID has already been used

User · Answer

I don t think that they really are random  but my guess is those are some hashes   Whenever I need some random identifier  I usually use a GUID and convert it to its  naked   representation   Guid NewGuid   ToString  n

User · Answer

Update 2016 1 23  If you find this answer useful  you may be interested in a simple   500 SLOC  password generation library I published   Install-Package MlkPwgen   Then you can generate random strings just like in the answer below   var str   PasswordGenerator Generate length  10  allowed  Sets Alphanumerics     One advantage of the library is that the code is better factored out so you can use secure randomness for more than generating strings  Check out the project site for more details   Original Answer  Since no one has provided secure code yet  I post the following in case anyone finds it useful   string RandomString int length  string allowedChars    abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789         if  length  lt  0  throw new ArgumentOutOfRangeException  length    length cannot be less than zero         if  string IsNullOrEmpty allowedChars   throw new ArgumentException  allowedChars may not be empty          const int byteSize   0x100      var allowedCharSet   new HashSet lt char gt  allowedChars  ToArray        if  byteSize  lt  allowedCharSet Length  throw new ArgumentException String Format  allowedChars may contain no more than  0  characters    byteSize            Guid NewGuid and System Random are not particularly random  By using a        cryptographically-secure random number generator  the caller is always        protected  regardless of use      using  var rng   System Security Cryptography RandomNumberGenerator Create              var result   new StringBuilder            var buf   new byte 128           while  result Length  lt  length                rng GetBytes buf               for  var i   0  i  lt  buf Length  amp  amp  result Length  lt  length    i                       Divide the byte into allowedCharSet-sized groups  If the                    random value falls into the last group and the last group is                    too small to choose from the entire allowedCharSet  ignore                    the value in order to avoid biasing the result                  var outOfRangeStart   byteSize -  byteSize   allowedCharSet Length                   if  outOfRangeStart  lt   buf i   continue                  result Append allowedCharSet buf i    allowedCharSet Length                                    return result ToString              Thanks to Ahmad for pointing out how to get the code working on  NET Core

User · Answer

My one stop solution for Linux commands on windows is scoop  Install scoop from scoop sh scoop install openssl openssl rand -base64 32 Dca3c3pptVkcb8fx243wN 3f rQxx rWYL8y7rZrGrA

User · Answer

I simplified  Michael Kropats solution and made a LINQ-esque version   string RandomString int length  string alphabet    abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789                var outOfRange   byte MaxValue   1 -  byte MaxValue   1    alphabet Length       return string Concat          Enumerable              Repeat 0  int MaxValue               Select e   gt  RandomByte                 Where randomByte   gt  randomByte  lt  outOfRange               Take length               Select randomByte   gt  alphabet randomByte   alphabet Length             byte RandomByte         using  var randomizationProvider   new RNGCryptoServiceProvider                  var randomBytes   new byte 1           randomizationProvider GetBytes randomBytes           return randomBytes Single

User · Answer

Michael Kropats solution in VB net  Private Function RandomString ByVal length As Integer  Optional ByVal allowedChars As String    abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789   As String     If length  lt  0 Then Throw New ArgumentOutOfRangeException  length    length cannot be less than zero        If String IsNullOrEmpty allowedChars  Then Throw New ArgumentException  allowedChars may not be empty          Dim byteSize As Integer   256     Dim hash As HashSet Of Char    New HashSet Of Char  allowedChars       Dim hash As HashSet Of String    New HashSet Of String  allowedChars      Dim allowedCharSet     hash ToArray      If byteSize  lt  allowedCharSet Length Then Throw New ArgumentException String Format  allowedChars may contain no more than  0  characters    byteSize           Guid NewGuid and System Random are not particularly random  By using a       cryptographically-secure random number generator  the caller is always       protected  regardless of use      Dim rng   New System Security Cryptography RNGCryptoServiceProvider       Dim result   New System Text StringBuilder       Dim buf   New Byte 128         While result Length  lt  length         rng GetBytes buf          Dim i         For i   0 To buf Length - 1 Step  1             If result Length  gt   length Then Exit For               Divide the byte into allowedCharSet-sized groups  If the               random value falls into the last group and the last group is               too small to choose from the entire allowedCharSet  ignore               the value in order to avoid biasing the result              Dim outOfRangeStart   byteSize -  byteSize Mod allowedCharSet Length              If outOfRangeStart  lt   buf i  Then                 Continue For             End If             result Append allowedCharSet buf i  Mod allowedCharSet Length           Next     End While     Return result ToString   End Function

User · Answer

Using Guid would be a pretty good way  but to get something looking like your example  you probably want to convert it to a Base64 string       Guid g   Guid NewGuid        string GuidString   Convert ToBase64String g ToByteArray         GuidString   GuidString Replace              GuidString   GuidString Replace            I get rid of     and     to get a little closer to your example  otherwise you get      at the end of your string and a     in the middle   Here s an example output string    OZVV5TpP4U6wJthaCORZEQ

User · Answer

not sure Microsoft s link are randomly generated have a look to new Guid   ToString

User · Answer

I would caution that GUIDs are not random numbers  They should not be used as the basis to generate anything that you expect to be totally random  see http   en wikipedia org wiki Globally Unique Identifier    Cryptanalysis of the WinAPI GUID generator shows that  since the sequence of V4 GUIDs is pseudo-random  given the initial state one can predict up to next 250 000 GUIDs returned by the function UuidCreate  This is why GUIDs should not be used in cryptography  e  g   as random keys   Instead  just use the C  Random method  Something like this  code found here   private string RandomString int size      StringBuilder builder   new StringBuilder      Random random   new Random      char ch     for int i 0  i lt size  i            ch   Convert ToChar Convert ToInt32 Math Floor 26   random NextDouble     65          builder Append ch         return builder ToString       GUIDs are fine if you want something unique  like a unique filename or key in a database   but they are not good for something you want to be random  like a password or encryption key   So it depends on your application  Edit  Microsoft says that Random is not that great either  http   msdn microsoft com en-us library system random VS 71  aspx    To generate a cryptographically secure random number suitable for creating a random   password  for example  use a class derived from System Security Cryptography RandomNumberGenerator such as System Security Cryptography RNGCryptoServiceProvider

[c#] Unique random string generation

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