Encryption in Android equivalent to php's MCRYPT_RIJNDAEL_256

android, encryption, php

Solution

I've created a `main` method in Java using Bouncy Castle to show the inner workings of `mcrypt_encrypt()` used in your code sample.

This is mainly to show other developers that PHP's `mcrypt_encrypt()` is a very dangerous method to use. It won't fail much, but that is because it rather continues where it should have stopped long ago. For instance, it adds or removes values from the key. It emits a warning when it does do this, but it won't directly show in the code.

public static void main(String[] args) throws DataLengthException, IllegalStateException, InvalidCipherTextException {

    // just some constants
    boolean ENCRYPT = true;
    boolean DECRYPT = false;

    // the key is either in binary in PHP or a string (dynamic isn't it?), lets assume ASCII
    byte[] givenKey = args[0].getBytes(Charset.forName("ASCII"));

    // determine the key size dynamically, somebody thought this was a good idea...
    // NOTE: PHP will emit a warning if the key size is larger, but will simply use the
    // largest key size otherwise
    final int keysize;
    if (givenKey.length <= 128 / Byte.SIZE) {
        keysize = 128;
    } else if (givenKey.length <= 192 / Byte.SIZE) {
        keysize = 192;
    } else {
        keysize = 256;
    }

    // create a 256 bit key by adding zero bytes to the decoded key
    byte[] keyData = new byte[keysize / Byte.SIZE];
    System.arraycopy(givenKey, 0, keyData, 0, Math.min(givenKey.length, keyData.length));
    KeyParameter key = new KeyParameter(keyData);

    // create a Rijndael cipher with 256 bit block size, this is not AES
    BlockCipher rijndael = new RijndaelEngine(256);

    // use a padding method that only works on data that cannot end with zero valued bytes
    ZeroBytePadding c = new ZeroBytePadding();

    // use ECB mode encryption, which should never be used
    PaddedBufferedBlockCipher pbbc = new PaddedBufferedBlockCipher(rijndael, c);

    // initialize the cipher using the key (no need for an IV, this is ECB)
    pbbc.init(ENCRYPT, key);

    // create a plain text byte array
    byte[] plaintext = args[1].getBytes(Charset.forName("UTF8"));

    // create a buffer for the ciphertext
    byte[] ciphertext = new byte[pbbc.getOutputSize(plaintext.length)];

    int offset = 0;
    offset += pbbc.processBytes(plaintext, 0, plaintext.length, ciphertext, offset);
    offset += pbbc.doFinal(ciphertext, offset);

    // show the ciphertext
    System.out.println(new String(Hex.encode(ciphertext), Charset.forName("ASCII")));

    // reverse the encryption
    pbbc.init(DECRYPT, key);
    byte[] decrypted = new byte[pbbc.getOutputSize(ciphertext.length)];
    offset = 0;
    offset += pbbc.processBytes(ciphertext, 0, ciphertext.length, decrypted, offset);
    offset += pbbc.doFinal(decrypted, offset);

    // this will probably print out correctly, but it isn't actually correct
    System.out.println(new String(decrypted, Charset.forName("UTF8")));

    // check out the zero's at the end
    System.out.println(new String(Hex.encode(decrypted), Charset.forName("UTF8")));

    // so lets make it a bit shorter... the PHP way
    // note that in PHP, the string may *not* contain a null terminator
    // add it yourself before printing the string
    System.out.println(new String(decrypted, Charset.forName("UTF8")).replaceAll("\\x00+$", ""));
}

Warning: the above code contains `ZeroBytePadding`. I later discovered that there is a difference between Bouncy Castle and PHP in this respect: Bouncy Castle expects that you always have to pad, while PHP doesn't. So Bouncy adds 1..n bytes while PHP adds 0..(n-1) bytes, where n is the block size (32 bytes for Rijndael-256/256). So you may have to do the padding/unpadding yourself; be sure to test the edge cases!

Problem

Original source

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