Encrypting a message digest

Question

Hi I'm trying to encrypt a message digest using RSA Encryption. For some reason when the encrypted message digest is decrypted it does not match the original. If this is hard to follow the following code illustrates this point:
String input = "Testing message";
MessageDigest hash = MessageDigest.getInstance("SHA1");
hash.update( input.getBytes() );
generator.initialize(512, random);
KeyPair pair = generator.generateKeyPair();
Key pubKey = pair.getPublic();
Key privKey = pair.getPrivate();
cipher.init(Cipher.ENCRYPT_MODE, privKey); // encrypt
byte[] cipherText = cipher.doFinal( hash.digest() );
// now decrypt
cipher.init(Cipher.DECRYPT_MODE, pubKey);
byte[] plainText = cipher.doFinal(cipherText);Here the byte array plainText does not match the original message digest from hash.digest() Any help on how to correct this problem would be great.
thanks
-B
Edited by: BenWhethers on Dec 13, 2007 12:49 PM
Edited by: BenWhethers on Dec 13, 2007 12:50 PM

Answer 1

You don't provide testable code so I have made a guess as to the missing code and for me the decrypted digest is the same are the original.
        Cipher cipher = Cipher.getInstance("RSA");
        SecureRandom random = new SecureRandom();
        KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
        String input = "Testing message";
        MessageDigest hash = MessageDigest.getInstance("SHA1");
        hash.update( input.getBytes() );
        generator.initialize(512, random);
        KeyPair pair = generator.generateKeyPair();
        Key pubKey = pair.getPublic();
        Key privKey = pair.getPrivate();
        cipher.init(Cipher.ENCRYPT_MODE, privKey); // encrypt
        byte[] digest = hash.digest();
        byte[] cipherText = cipher.doFinal( digest );
        // now decrypt
        cipher.init(Cipher.DECRYPT_MODE, pubKey);
        byte[] plainText = cipher.doFinal(cipherText);
        System.out.println(Arrays.equals(plainText,digest ));