Using SHA1 and RSA with java security Signature vs MessageDigest and Cipher

Question

I m trying to understand what the Java java security Signature class does  If I compute an SHA1 message digest  and then encrypt that digest using RSA  I get a different result to asking the Signature class to sign the same thing      Generate new key KeyPair keyPair   KeyPairGenerator getInstance  RSA   generateKeyPair    PrivateKey privateKey   keyPair getPrivate    String plaintext    This is the message being signed       Compute signature Signature instance   Signature getInstance  SHA1withRSA    instance initSign privateKey   instance update  plaintext  getBytes     byte   signature   instance sign        Compute digest MessageDigest sha1   MessageDigest getInstance  SHA1    byte   digest   sha1 digest  plaintext  getBytes         Encrypt digest Cipher cipher   Cipher getInstance  RSA    cipher init Cipher ENCRYPT MODE  privateKey   byte   cipherText   cipher doFinal digest       Display results System out println  Input data      plaintext   System out println  Digest      bytes2String digest    System out println  Cipher text      bytes2String cipherText    System out println  Signature      bytes2String signature      Results in  for example       Input data  This is the message being signed   Digest  62b0a9ef15461c82766fb5bdaae9edbe4ac2e067   Cipher text  057dc0d2f7f54acc95d3cf5cba9f944619394711003bdd12      Signature  7177c74bbbb871cc0af92e30d2808ebae146f25d3fd8ba1622      I must have a fundamental misunderstanding of what Signature is doing - I ve traced through it  and it appears to be calling update on a MessageDigest object  with the algorithm set to SHA1 as I would expect  then getting the digest  then doing the encryption  What s making the results differ   EDIT   Leonidas made me check whether the signature scheme is supposed to do what I think it does  There are two types of signature defined in the RFC    RSASSA-PKCS1-v1 5 RSASSA-PSS   The first of these  PKCS1  is the one I describe above  It uses a hash function to create a digest  and then encrypts the result with a private key   The second algorithm uses a random salt value  and is more secure but non-deterministic  The signature produced from the code above does not change if the same key is used repeatedly  so I don t think it can be PSS   EDIT   Here s the bytes2string method I was using   private static String bytes2String byte   bytes        StringBuilder string   new StringBuilder        for  byte b   bytes            String hexString   Integer toHexString 0x00FF  amp  b           string append hexString length      1    0    hexString   hexString             return string toString

User · Answer

To produce the same results   MessageDigest sha1   MessageDigest getInstance  SHA1   BOUNCY CASTLE PROVIDER   byte   digest   sha1 digest content   DERObjectIdentifier sha1oid    new DERObjectIdentifier  1 3 14 3 2 26     AlgorithmIdentifier sha1aid    new AlgorithmIdentifier sha1oid   null   DigestInfo di   new DigestInfo sha1aid   digest    byte   plainSig   di getDEREncoded    Cipher cipher   Cipher getInstance  RSA ECB PKCS1Padding   BOUNCY CASTLE PROVIDER   cipher init Cipher ENCRYPT MODE  privateKey   byte   signature   cipher doFinal plainSig

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I have a similar problem  I tested adding code and found some interesting results  With this code I add  I can deduce that depending on the  provider  to use  the firm can be different   because the data included in the encryption is not always equal in all providers    Results of my test   Conclusion -  Signature Decipher      trash    DigestInfo    if we know the value of  trash   the digital signatures will be equal   IDE Eclipse OUTPUT     Input data  This is the message being signed  Digest  62b0a9ef15461c82766fb5bdaae9edbe4ac2e067  DigestInfo  3021300906052b0e03021a0500041462b0a9ef15461c82766fb5bdaae9edbe4ac2e067  Signature Decipher  1ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff003021300906052b0e03021a0500041462b0a9ef15461c82766fb5bdaae9edbe4ac2e067  CODE  import java security InvalidKeyException  import java security KeyPair  import java security KeyPairGenerator  import java security MessageDigest  import java security NoSuchAlgorithmException  import java security NoSuchProviderException  import java security PrivateKey  import java security PublicKey  import java security Signature  import java security SignatureException  import javax crypto BadPaddingException  import javax crypto Cipher  import javax crypto IllegalBlockSizeException  import javax crypto NoSuchPaddingException  import org bouncycastle asn1 x509 DigestInfo  import org bouncycastle asn1 DERObjectIdentifier  import org bouncycastle asn1 x509 AlgorithmIdentifier  public class prueba          param args    throws NoSuchProviderException     throws NoSuchAlgorithmException     throws InvalidKeyException     throws SignatureException     throws NoSuchPaddingException     throws BadPaddingException     throws IllegalBlockSizeException       public static void main String   args  throws NoSuchAlgorithmException  NoSuchProviderException  InvalidKeyException  SignatureException  NoSuchPaddingException  IllegalBlockSizeException  BadPaddingException      TODO Auto-generated method stub KeyPair keyPair   KeyPairGenerator getInstance  RSA   BC   generateKeyPair    PrivateKey privateKey   keyPair getPrivate    PublicKey puKey   keyPair getPublic    String plaintext    This is the message being signed      Hacer la firma Signature instance   Signature getInstance  SHA1withRSA   BC    instance initSign privateKey   instance update  plaintext  getBytes     byte   signature   instance sign       En dos partes primero hago un Hash MessageDigest digest   MessageDigest getInstance  SHA1    BC    byte   hash   digest digest  plaintext  getBytes        El digest es identico a  openssl dgst -sha1 texto txt   MessageDigest sha1   MessageDigest getInstance  SHA1   BC      byte   digest   sha1 digest  plaintext  getBytes     AlgorithmIdentifier digestAlgorithm   new AlgorithmIdentifier new DERObjectIdentifier  1 3 14 3 2 26    null      create the digest info DigestInfo di   new DigestInfo digestAlgorithm  hash   byte   digestInfo   di getDEREncoded      Luego cifro el hash Cipher cipher   Cipher getInstance  RSA   BC    cipher init Cipher ENCRYPT MODE  privateKey   byte   cipherText   cipher doFinal digestInfo     byte   cipherText   cipher doFinal digest2   Cipher cipher2   Cipher getInstance  RSA   BC    cipher2 init Cipher DECRYPT MODE  puKey   byte   cipherText2   cipher2 doFinal signature   System out println  Input data      plaintext   System out println  Digest      bytes2String hash    System out println  Signature      bytes2String signature    System out println  Signature2      bytes2String cipherText    System out println  DigestInfo      bytes2String digestInfo    System out println  Signature Decipher      bytes2String cipherText2

User · Answer

A slightly more efficient version of the bytes2String method is  private static final char   hex     0    1    2    3    4    5    6    7    8    9    a    b    c    d    e    f    private static String byteArray2Hex byte   bytes        StringBuilder sb   new StringBuilder bytes length   2       for  final byte b   bytes            sb append hex  b  amp  0xF0   gt  gt  4            sb append hex b  amp  0x0F              return sb toString

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Code below  taken from my blog article - http   todayguesswhat blogspot com 2021 01 manually-verifying-rsa-sha-signature-in html   is hopefully helpful in understanding what is present in a standard SHA with RSA signature   This should work in standard Oracle JDK and does not require Bouncy Castle libraries   It is using the sun security classes to process the decrypted signature contents - you could just as easily manually parse  In the example below  the message digest algorithm is SHA-512 which produces a 64 byte  512-bit  checksum  SHA-1 would be pretty similar - but producing a 20-byte  160-bit  checksum  import java security KeyPair  import java security KeyPairGenerator  import java security MessageDigest  import java security PrivateKey  import java security PublicKey  import java security Signature   import java util Arrays   import javax crypto Cipher   import sun security util DerInputStream  import sun security util DerValue   public class RSASignatureVerification       public static void main String   args  throws Exception               KeyPairGenerator generator   KeyPairGenerator getInstance  quot RSA quot            generator initialize 2048            KeyPair keyPair   generator generateKeyPair            PrivateKey privateKey   keyPair getPrivate            PublicKey publicKey   keyPair getPublic             String data    quot hello oracle quot           byte   dataBytes   data getBytes  quot UTF8 quot             Signature signer   Signature getInstance  quot SHA512withRSA quot            signer initSign privateKey            signer update dataBytes            byte   signature   signer sign       signature bytes of the signing operation s result           Signature verifier   Signature getInstance  quot SHA512withRSA quot            verifier initVerify publicKey           verifier update dataBytes            boolean verified   verifier verify signature           if  verified                        System out println  quot Signature verified  quot                      The statement that describes signing to be equivalent to RSA encrypting the     hash of the message using the private key is a greatly simplified view     The decrypted signatures bytes likely convey a structure  ASN 1  encoded     using DER with the hash just one component of the structure                 lets try decrypt signature and see what is in it             Cipher cipher   Cipher getInstance  quot RSA quot            cipher init Cipher DECRYPT MODE  publicKey            byte   decryptedSignatureBytes   cipher doFinal signature           sample value of decrypted signature which was 83 bytes long      30 51 30 0D 06 09 60 86 48 01 65 03 04 02 03 05     00 04 40 51 00 41 75 CA 3B 2B 6B C0 0A 3F 99 E3     6B 7A 01 DC F2 9B 36 E6 0D D4 31 89 53 A3 D9 80     6D AE DD 45 7E 55 45 01 FC C8 73 D2 DD 8D E5 B9     E0 71 57 13 41 D0 CD FF CA 58 01 03 A3 DD 95 A1     C1 EE C8      Taking above sample bytes         0x30 means A SEQUENCE - which contains an ordered field of one or more types      It is encoded into a TLV triplet that begins with a Tag byte of 0x30      DER uses T L V  tag bytes  length bytes  value bytes  format      0x51 is the length   81 decimal  13 bytes       the 0x30  48 decimal  that follows begins a second sequence      https   tools ietf org html rfc3447 page-43     the DER encoding T of the DigestInfo value is equal to the following for SHA-512     0D 06 09 60 86 48 01 65 03 04 02 03 05 00 04 40    H     where    is concatenation and H is the hash value       0x0D is the length   13 decimal  13 bytes       0x06 means an OBJECT ID tag     0x09 means the object id is 9 bytes          https   docs microsoft com en-au windows win32 seccertenroll about-object-identifier redirectedfrom MSDN      taking 2 16 840 1 101 3 4 2 3  object id for SHA512 Hash Algorithm       The first two nodes of the OID are encoded onto a single byte      The first node is multiplied by the decimal 40 and the result is added to the value of the second node     2   40   16   96 decimal   60 hex     Node values less than or equal to 127 are encoded on one byte      1 101 3 4 2 3 corresponds to in hex 01 65 03 04 02 03     Node values greater than or equal to 128 are encoded on multiple bytes      Bit 7 of the leftmost byte is set to one  Bits 0 through 6 of each byte contains the encoded value      840 decimal   348 hex     - gt  0000 0011 0100 1000     set bit 7 of the left most byte to 1  ignore bit 7 of the right most byte      shifting right nibble of leftmost byte to the left by 1 bit     - gt  1000 0110 X100 1000 in hex 86 48      05 00            NULL  0 Bytes       04 40            OCTET STRING  0x40 Bytes   64 bytes     SHA512 produces a 512-bit  64-byte  hash value      51 00 41     C1 EE C8 is the 64 byte hash value                parse DER encoded data         DerInputStream derReader   new DerInputStream decryptedSignatureBytes            byte   hashValueFromSignature   null              obtain sequence of entities         DerValue   seq   derReader getSequence 0           for  DerValue v   seq                        if  v getTag      4                                hashValueFromSignature   v getOctetString       SHA-512 checksum extracted from decrypted signature bytes                                  MessageDigest md   MessageDigest getInstance  quot SHA-512 quot            md update dataBytes            byte   hashValueCalculated   md digest             boolean manuallyVerified   Arrays equals hashValueFromSignature  hashValueCalculated           if  manuallyVerified                        System out println  quot Signature manually verified  quot                      else                       System out println  quot Signature could NOT be manually verified  quot

User · Answer

Taking  Mike Houston s answer as pointer  here is a complete sample code that does Signature and Hash and encryption           param args     public static void main String   args        try               boolean useBouncyCastleProvider   false           Provider provider   null          if  useBouncyCastleProvider                        provider   new BouncyCastleProvider                Security addProvider provider                      String plainText    This is a plain text                 KeyPair         KeyPairGenerator keyPairGenerator   null          if  null    provider              keyPairGenerator   KeyPairGenerator getInstance  RSA   provider           else             keyPairGenerator   KeyPairGenerator getInstance  RSA            keyPairGenerator initialize 2048            KeyPair keyPair   keyPairGenerator generateKeyPair                Signature         Signature signatureProvider   null          if  null    provider              signatureProvider   Signature getInstance  SHA256WithRSA   provider           else             signatureProvider   Signature getInstance  SHA256WithRSA            signatureProvider initSign keyPair getPrivate              signatureProvider update plainText getBytes             byte   signature   signatureProvider sign             System out println  Signature Output               System out println   t    new String Base64 encode signature                 Message Digest         String hashingAlgorithm    SHA-256           MessageDigest messageDigestProvider   null          if  null    provider              messageDigestProvider   MessageDigest getInstance hashingAlgorithm  provider           else             messageDigestProvider   MessageDigest getInstance hashingAlgorithm           messageDigestProvider update plainText getBytes              byte   hash   messageDigestProvider digest             DigestAlgorithmIdentifierFinder hashAlgorithmFinder   new DefaultDigestAlgorithmIdentifierFinder            AlgorithmIdentifier hashingAlgorithmIdentifier   hashAlgorithmFinder find hashingAlgorithm            DigestInfo digestInfo   new DigestInfo hashingAlgorithmIdentifier  hash           byte   hashToEncrypt   digestInfo getEncoded                Crypto            You could also use  RSA ECB PKCS1Padding  for both the BC and SUN Providers          Cipher encCipher   null          if  null    provider              encCipher   Cipher getInstance  RSA NONE PKCS1Padding   provider           else             encCipher   Cipher getInstance  RSA            encCipher init Cipher ENCRYPT MODE  keyPair getPrivate              byte   encrypted   encCipher doFinal hashToEncrypt            System out println  Hash and Encryption Output               System out println   t    new String Base64 encode encrypted               catch  Throwable e                e printStackTrace              You can use BouncyCastle Provider or default Sun Provider

User · Answer

OK  I ve worked out what s going on  Leonidas is right  it s not just the hash that gets encrypted  in the case of the Cipher class method   it s the ID of the hash algorithm concatenated with the digest    DigestInfo     SEQUENCE         digestAlgorithm AlgorithmIdentifier        digest OCTET STRING      Which is why the encryption by the Cipher and Signature are different

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Erm  after understanding your question  are you sure that the signature-method only creates a SHA1 and encrypts it  GPG et al offer to compress clear sign the data  Maybe this java-signature-alg also creates a detachable attachable signature

[java] Using SHA1 and RSA with java.security.Signature vs. MessageDigest and Cipher

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