AES CBC Not Producing Correct Vectors

Question

I have two methods for encryption and decryption in CBC mode. I have all ready verified my ECB methods and they function properly. My issue is that when I test encryption in CBC with test vectors it passes them. And when I test CBC decryption with those same vectors to make sure it can go the other way, it passes those tests as well. But when I pass from encryption to decryption, I don't get the same vector back. This really doesn't make any sense to me.

public byte[,] Encrypt(byte[,] dataToEncrypt, byte[] givenKey, byte[] initializationVector)
    {
        //XOR the data with the IV
        for (int row = 0; row < 4; row++)
        {
            dataToEncrypt[row,0] ^= initializationVector[4*row];
            dataToEncrypt[row, 1] ^= initializationVector[(4 * row) + 1];
            dataToEncrypt[row, 2] ^= initializationVector[(4 * row) + 2];
            dataToEncrypt[row, 3] ^= initializationVector[(4 * row) + 3];
        }

        ECB encryptor = new ECB();
        return encryptor.Encrypt(dataToEncrypt, givenKey);
    }

    public byte[,] Decrypt(byte[,] dataToDecrypt, byte[] givenKey, byte[] initializationVector)
    {
        ECB encryptor = new ECB();

        byte[,] plainText = encryptor.Decrypt(dataToDecrypt, givenKey);

        for (int row = 0; row < 4; row++)
        {
            plainText[row, 0] ^= initializationVector[4 * row];
            plainText[row, 1] ^= initializationVector[(4 * row) + 1];
            plainText[row, 2] ^= initializationVector[(4 * row) + 2];
            plainText[row, 3] ^= initializationVector[(4 * row) + 3];
        }

        return plainText;
    }

Encryptor is just my class for the AES in ECB form. So all these methods are supposed to do is take in the extra XOR's for CBC. Can anyone tell me if this seems like something is wrong?

This is for a single block of data for testing, not for large amounts of data.

I seem to remember somewhere somebody telling me not to roll my own crypto. — ose
Sorry about that, thank you for adding the language in there. And ose, I don't suppose you remember why they told you not to do that? From what I've seen of how the process works, it wouldn't make any sense to write it all up again as it's the same process....but I've been wrong before. — Repareman
Please note that you haven't actually programmed CBC here. You have only introduced an IV to your first block, further blocks have been left untouched. As you will notice, the encryption and decryption perform perfectly, but you are still doing things wrong. This is just one reason why you should not trust your own routines. At the very minimum test them with other libs and NIST test vectors. But you are better off using a library and learning how to do that correctly, that's hard enough as it is. — Maarten Bodewes
I'm more confused by why everyone's comments on a forum to help people is, "Don't even try". I'm aware that it was not taking into consideration further blocks. If you look at the last line of my post it says that that was for a single block of data -For Testing-. I wanted to make sure that the IV was being introduced correctly, before moving onto larger amounts of data. I have since moved onto more data, and I'm still testing quite a bit to make sure that each part is functioning correctly. I will look at the NIST test vectors, I haven't heard of those before, thank you for that. — Repareman

Repareman Repareman · Accepted Answer · 2013-05-22T05:58:17

I fixed the issue. It was a problem with the fact that the encryption up there has a reference to the object data that I was encrypting rather then just equaling the values. I apologize for posting the question for a stupid error that I was making. Thanks for the comments though. And here is the new encryption method, in case anyone cares:

public byte[,] Encrypt(byte[,] dataToEncrypt, byte[] givenKey, byte[] initializationVector)
    {
        //Setup
        ECB encryptor = new ECB();
        byte[,] cypherText = new byte[4,4];

        //XOR the data with the IV
        for (int row = 0; row < 4; row++)
        {
            cypherText[row, 0] = (byte)(dataToEncrypt[row, 0] ^ initializationVector[4 * row]);
            cypherText[row, 1] = (byte)(dataToEncrypt[row, 1] ^ initializationVector[(4 * row) + 1]);
            cypherText[row, 2] = (byte)(dataToEncrypt[row, 2] ^ initializationVector[(4 * row) + 2]);
            cypherText[row, 3] = (byte)(dataToEncrypt[row, 3] ^ initializationVector[(4 * row) + 3]);
        }


        return encryptor.Encrypt(cypherText, givenKey);
    }

AES CBC Not Producing Correct Vectors

1 Answers