RSA encryption in Java

Question

I am trying to write an encryption algorithm using RSA in Java, I am getting a "javax.crypto.BadPaddingException: Data must start with zero"; I do not know what is this exception for. This is the example I used here

and Here is my code; please help.

public byte[] getEncryptedValue(byte[] bytes, PublicKey key) {
    try {
        cipher.init(Cipher.ENCRYPT_MODE, key);
        return blockCipher(bytes, Cipher.ENCRYPT_MODE);
    } catch (Exception ex) {
        Logger.getLogger(SecurityUtil.class.getName()).log(Level.SEVERE, null, ex);
    }
    return null;
}

public byte[] getDecryptedValue(byte[] bytes, PrivateKey key) {
    try {
        cipher.init(Cipher.DECRYPT_MODE, key);
        return blockCipher(bytes, Cipher.DECRYPT_MODE);
    } catch (Exception ex) {
        Logger.getLogger(SecurityUtil.class.getName()).log(Level.SEVERE, null, ex);
    }
    return null;
}

private byte[] append(byte[] prefix, byte[] suffix) {
    byte[] toReturn = new byte[prefix.length + suffix.length];
    System.arraycopy(prefix, 0, toReturn, 0, prefix.length);
    System.arraycopy(suffix, 0, toReturn, prefix.length, suffix.length);
    return toReturn;
}

private byte[] blockCipher(byte[] bytes, int mode) throws IllegalBlockSizeException, BadPaddingException {
    byte[] scrambled = new byte[0];
    byte[] toReturn = new byte[0];blocks (because of RSA)
    int length = (mode == Cipher.ENCRYPT_MODE) ? 100 : 128;
    int n = 0;
    byte[] buffer = new byte[length];

    for (int i = 0; i < bytes.length; i++) {
        if ((i > 0) && (i % length == 0)) {
            n = 0;
            scrambled = cipher.doFinal(buffer);
            toReturn = append(toReturn, scrambled);
        }
        buffer[i % length] = bytes[i];
        n++;
    }
    ***scrambled = cipher.doFinal(buffer, 0, n);*** <-- the exception is caught here
    toReturn = append(toReturn, scrambled);
    return toReturn;
}

When are you getting the exception - while encryption or decryption? — sinha, May 19 '11 at 05:50
Did you search on javax.crypto.BadPaddingException before posting your question? This looks like a possible duplicate of [this](http://stackoverflow.com/questions/4895773) and [this](http://stackoverflow.com/questions/4580982). Which answer: you are not converting the hex string back to a byte array correctly for decryption — this.josh, May 20 '11 at 21:28
Call me stupid, but how is the variable 'cipher' initialized and what's the content of it? There seems to be no declaration or is it global? — bl4ckb0l7, May 23 '11 at 13:49

score 2 · Answer 1 · answered May 23 '11 at 13:25

The problem could be the data sent over the network using sockets may be corrupted due to some encoding problems. I had the same problem while developing a simple client/server chat program that encrypts/decrypts using asymmetric key the messages between the server and client and vise versa, instead of sending the message as a string, I sent it as a byte array which is the encrypted message.

score 0 · Answer 2 · answered Jul 10 '11 at 16:39

0

check if keys are matching
check if data returned by getEncryptedValue are the same that you pass to getDecryptedValue
check corectness of loop in blockCipher method

answered Jul 10 '11 at 16:39

zacheusz

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RSA encryption in Java

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