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

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  1. POISO 9797-1 Algorithm 1 [CBC-MAC] in C#
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    <p>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].</p> <p>I got an excellent start from <a href="http://bytes.com/topic/net/answers/654069-iso-iec-9797-1-cbc-mac-using-vb-net" rel="nofollow noreferrer">here</a>.</p> <p>And I coded the algorithm as below:</p> <pre><code>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&lt;byte[], int&gt; prepArray = (bArr, offset) =&gt; { bArr[offset] = 0; //80 for (var i = offset + 1; i &lt; bArr.Length; i++) bArr[i] = 0; }; var length = message.Length; var mod = length &gt; 8? length % 8: length - 8; var newLength = length + ((mod &lt; 0) ? -mod : (mod &gt; 0) ? 8 - mod : 0); //var newLength = length + ((mod &lt; 0) ? -mod : (mod &gt; 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; } </code></pre> <p>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:</p> <pre><code> Action&lt;byte[], int&gt; prepArray = (bArr, offset) =&gt; { bArr[offset] = 0; ... } </code></pre> <p>No additional block (if mod 0 [divisible by 8] do not change array length)</p> <pre><code>var newLength = length + ((mod &lt; 0) ? -mod : (mod &gt; 0) ? 8 - mod : 0); </code></pre> <p>The original suggestion wanted me to decrypt at the second step... but Valery <a href="http://www.derkeiler.com/Newsgroups/microsoft.public.dotnet.security/2005-04/0180.html" rel="nofollow noreferrer">here</a> suggests that it's encrypt all the way. So I changed Decrypt to Encrypt. But still I'm unable to get the requisite MAC...</p> <p>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]</p> <p>Can someone help? Please?</p> <hr> <p>Since Pinpad requires that the first character in message is a 0x1a...</p> <pre><code>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); } </code></pre> <p>I'm calling the code above with this input:</p> <pre><code>var result = pad.CalculateAugmentedMAC("MENTERODOMETER", new byte[] { 106, 194, 146, 250, 161, 49, 91, 77, 130, 52, 179, 163, 215, 213, 147, 58 }); </code></pre>
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    1. COCurious. Why would you need to get involved in en/decrypting PINPad data? The PINPad workflow is designed to send encrypted data directly to the merchant's bank using the DUKPT keys generated by the bank and installed in the PINPad by the PINPad Vendor. Even getting to the point where we were writing software to directly control various PINPads' UI and other behavior, we were never able to, nor did we need to encrypt/decrypt the encrypted PIN data envelope provided as part of the process.
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    2. COBill, you're right... But if you were to write a loyalty management application that allows patrons a PIN, you need to use it without these pre-programmed keys.... PINPad 1000SE's earlier model - can't remember on top of my head - did allow you to get a PIN and send that as clear text. The later models encrypt the PIN and we can't get at it...
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