Question

I am trying to encode a simple String “test” back and forth.

public static String encode(Key publicKey, String data) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException {

    byte[] byteData = data.getBytes(); // convert string to byte array

    Cipher cipher = Cipher.getInstance(ALGORITHM); // create conversion processing object
    cipher.init(Cipher.ENCRYPT_MODE, publicKey); // initialize object's mode and key

    byte[] encryptedByteData = cipher.doFinal(byteData); // use object for encryption

    return new String(encryptedByteData); // convert encrypted byte array to string and return it

}

public static String decode(Key privateKey, String data) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException {

    byte[] byteData = data.getBytes(); // convert string to byte array

    Cipher cipher = Cipher.getInstance(ALGORITHM); // create conversion processing object
    cipher.init(Cipher.DECRYPT_MODE, privateKey); // initialize object's mode and key

    System.out.println(byteData.length);

    byte[] decryptedByteData = cipher.doFinal(byteData); // use object for decryption

    return new String(decryptedByteData); // convert decrypted byte array to string and return it

}

However, although the encryption works just fine (ALGORITHM is “RSA”), when trying to decrypt the string I have just gotten from encrypting “test”, I get following exception:

javax.crypto.IllegalBlockSizeException: Data must not be longer than 256 bytes

Should I split the encrypted bytes in chunks of 256 in order to be able to decrypt it?

Answer 1

You can’t reliably convert random bytes to a String. The results will depend on what your default character encoding is on the machine where you run this. With many encodings, the cipher text will be corrupted, and information will be lost.

Modify your code to use a byte[] instead (the result of the ‘doFinal()` method.

If you need to convert the byte[] to a character string, use an encoding like Base-64.