ISO 9797-1 Algorithm 1 [CBC-MAC] in C#

Question

It seems that there're 6 variations to CBC-MAC algorithm. I've been trying to match the MAC algorithm on the PINPad 1000SE [which per manual is ISO 9797-1 Algorithm 1].

I got an excellent start from here.

And I coded the algorithm as below:

public static byte[] CalculateMAC(this IPinPad pinpad, byte[] message, byte[] key)
{
    //Divide the key with Key1[ first 64 bits] and key2 [last 64 bits]
    var key1 = new byte[8];
    Array.Copy(key, 0, key1, 0, 8);

    var key2 = new byte[8];
    Array.Copy(key, 8, key2, 0, 8); //64 bits

    //divide the message into 8 bytes blocks
    //pad the last block with "80" and "00","00","00" until it reaches 8 bytes
    //if the message already can be divided by 8, then add 
    //another block "80 00 00 00 00 00 00 00"
    Action<byte[], int> prepArray = (bArr, offset) =>
                                     {
                                         bArr[offset] = 0; //80
                                         for (var i = offset + 1; i < bArr.Length; i++)
                                             bArr[i] = 0;
                                     };
    var length = message.Length;
    var mod = length > 8? length % 8: length - 8;

    var newLength = length + ((mod < 0) ? -mod : (mod > 0) ? 8 - mod : 0);
    //var newLength = length + ((mod < 0) ? -mod : (mod > 0) ? 8 - mod : 8);
    Debug.Assert(newLength % 8 == 0);

    var arr = new byte[newLength];
    Array.Copy(message, 0, arr, 0, length);
    //Encoding.ASCII.GetBytes(message, 0, length, arr, 0);
    prepArray(arr, length);
    //use initial vector {0,0,0,0,0,0,0,0} 
    var vector = new byte[] { 0, 0, 0, 0, 0, 0, 0, 0 };

    //encrypt by DES CBC algorith with the first key KEY 1
    var des = new DESCryptoServiceProvider { Mode = CipherMode.CBC };
    var cryptor = des.CreateEncryptor(key1, vector);
    var outputBuffer = new byte[arr.Length];
    cryptor.TransformBlock(arr, 0, arr.Length, outputBuffer, 0);

    //Decrypt the result by DES ECB with the second key KEY2 [Original suggestion]
    //Now I'm Encrypting
    var decOutputBuffer = new byte[outputBuffer.Length];
    des.Mode = CipherMode.ECB;
    var decryptor = des.CreateEncryptor(key2, vector);
    //var decryptor = des.CreateDecryptor(key2, vector);
    decryptor.TransformBlock(outputBuffer, 0, outputBuffer.Length, decOutputBuffer, 0);

    //Encrypt the result by DES ECB with the first key KEY1
    var finalOutputBuffer = new byte[decOutputBuffer.Length];
    var cryptor2 = des.CreateEncryptor(key1, vector);
    cryptor2.TransformBlock(decOutputBuffer, 0, decOutputBuffer.Length, finalOutputBuffer, 0);

    //take the first 4 bytes as the MAC
    var rval = new byte[4];
    Array.Copy(finalOutputBuffer, 0, rval, 0, 4);
    return rval;
}

Then I discovered there're 3 padding schemes and the one that gave me a start may not necessarily be right. The manual came to my rescue again. It seems the device only pads with 0s. Additional block is also nowhere mentioned so I made the below changes:

    Action<byte[], int> prepArray = (bArr, offset) =>
                                     {
                                         bArr[offset] = 0; ... }

No additional block (if mod 0 [divisible by 8] do not change array length)

var newLength = length + ((mod < 0) ? -mod : (mod > 0) ? 8 - mod : 0);

The original suggestion wanted me to decrypt at the second step... but Valery here suggests that it's encrypt all the way. So I changed Decrypt to Encrypt. But still I'm unable to get the requisite MAC...

Manual says for key "6AC292FAA1315B4D8234B3A3D7D5933A" [since the key should be 16 bytes, I figured the key here's hex string so I took byte values of 6A, C2, 92, FA... new byte[] { 106, 194, 146, ...] the MAC should be 7B,40,BA,95 [4 bytes] if the message is [0x1a + byte array of MENTERODOMETER]

Can someone help? Please?

---

Since Pinpad requires that the first character in message is a 0x1a...

public static byte[] CalculateAugmentedMAC(this IPinPad pinpad, string message, byte[] key)
{
    var arr = new byte[message.Length + 1];
    var source = Encoding.ASCII.GetBytes(message);
    arr[0] = 0x1a; //ClearScreenIndicator
    Array.Copy(source, 0, arr, 1, source.Length);
    return CalculateMAC(pinpad, arr, key);
}

I'm calling the code above with this input:

var result = pad.CalculateAugmentedMAC("MENTERODOMETER", new byte[] { 106, 194, 146, 250, 161, 49, 91, 77, 130, 52, 179, 163, 215, 213, 147, 58 });

Answer 1

Can't answer to your specific terminal, but I use this to test MACs.

public static byte[] GenerateMAC(byte[] key, byte[] data)
{
    using (MACTripleDES mac = new MACTripleDES(key))
        return mac.ComputeHash(data);
}

Answer 2

Most CBC MAC algorithms are implemented in BouncyCastle's JCE provider.

Look at: BouncyCastleProvider.java

You're probably looking for DESEDEISO9797ALG1MACWITHISO7816-4PADDING, which is an alias for DESEDEMAC64WITHISO7816-4PADDING, implemented here (well, it's a specific configuration of CBCBlockCipherMac using the DESedeEngine and ISO7816d4Padding, you'll have to jump between some classes to get the full picture): JCEMac.java

Also, have a look at jPos:

JCESecurityModule.java

and their contributed retail MAC algorithm implementation:

retail-mac-contributed-by-vsalaman.zip

Answer 3

I am pretty sure (IIRC) that you need to call TransformFinalBlock at the end (per encryptor).