Question

I have written a BlackBerry app that uses AES encryption. I am trying to decrypt this using the AesCryptoServiceProvider in C#.

The BlackBerry code doesn’t seem to use an IV which means I have nothing to pass to the AesCryptoServiceProvider.

Is it possible for me to decrypt AES without an IV, if so, how?

Answer 1

Reading the blackberry java crypto documentation, it appears you are not supposed to directly use AESEncryptionEngine. If you use it straight, you end up with ( I assume) ECB mode which results in the following encryption of an image of a penguin. don’t do it.

Rather it appears that to use some secure mode of operation, you need to actually use a wrapper around the basic AESEncrypt / Decrypt Engine. You want to use the CBCEncryptionEngine to do this. Here is some sample code from here. Note that the IV is randomized on creation, so you don’t need to set it or worry about reuse. Just replace DES with AES here.

// sampleDESCBCEncryption
private static int sampleDESCBCEncryption( 
    byte[] secretKey, byte[] initVector, byte[] plainText, byte[] cipherText, int
    dataLength ) 
    throws CryptoException, IOException
{
    // Create a new DES key based on the 8 bytes in the secretKey array
    DESKey key = new DESKey( secretKey );

    // Create a new initialization vector using the 8 bytes in initVector
    InitializationVector iv = new InitializationVector( initVector );

    // Create a new byte array output stream for use in encryption
    NoCopyByteArrayOutputStream out = new NoCopyByteArrayOutputStream();

    // Create a new instance of a BlockEncryptor passing in an instance of a CBC encryptor engine
    // (containing an instance of a DES encryptor engine), the initialization vector, and the
    // output stream
    BlockEncryptor cryptoStream = new BlockEncryptor( 
        new CBCEncryptorEngine( new DESEncryptorEngine( key ), iv ), out );

    // Write dataLength bytes from plainText to the CFB encryptor stream
    cryptoStream.write( plainText, 0, dataLength );
    cryptoStream.close();

    // Now copy the encrypted bytes from out into cipherText and return the length
    int finalLength = out.size();
    System.arraycopy( out.getByteArray(), 0, cipherText, 0, finalLength );
    return finalLength;
}