CODEWITHSUNDEEP
javascriptnode.jscbc-modecmac
Zen Zac
Zen Zac

Reputation: 136

aes-cmac in javascript not giving the same results as in python

I'm writing code to read/write to NTAG424 DNA NFC tags. I'm doing this completely in javascript, because I want to be able to use it in a react native app.

In NTAG 424 DNA and NTAG 424 DNA TagTamper features and hints they show the results you should be getting for every step. But they use a python solution.

The input message is A55A0001008013C56268A548D8FBBF237CCCAA20EC7E6E48C3DEF9A4C675360F and the output (according to the manual) is 1309C877509E5A215007FF0ED19CA564. Whereas I get 7CCEF6FEB32F34CA48CB685ECAA0F32C.

Because I need to be able to use this code commercially, I cannot just use any library.

function generateSubkeys(key) {
  const cipher = crypto.createCipheriv("aes-128-ecb", key, "");

  const cryptedKey = cipher.update(iv);

  let subkey1 = bt.bitShiftLeft(cryptedKey);
  if (msb(cryptedKey[0]) & 0x80) {
    subkey1 = xor(subkey1, 0x87);
  }

  let subkey2 = bt.bitShiftLeft(subkey1);
  if (msb(subkey1[0]) & 0x80) {
    subkey2 = xor(subkey2, 0x87);
  }
  return { subkey1: subkey1, subkey2: subkey2 };
}

function msb(bytes) {
  return bytes >>> 31;
}
function aes(key, message) {
  const cipher = crypto.createCipheriv(
    "aes-" + key.length * 8 + "-cbc",
    key,
    iv
  );

  var result = cipher.update(message);
  cipher.final();

  return result;
}

function aesCmac(key, message) {
  const { subkey1, subkey2 } = generateSubkeys(Buffer.from(key, "hex"));
  let numBlocks = Math.ceil(message.length / blockSize);
  var lastBlockRemainder = message.length % blockSize;

  if (numBlocks === 0) {
    numBlocks = 1;
  }

  var messageArray = getMessageArray(message, numBlocks, lastBlockRemainder);
  if (lastBlockRemainder === 0) {
    messageArray[numBlocks - 1] = xor(messageArray[numBlocks - 1], subkey1);
  } else {
    messageArray[numBlocks - 1] = xor(messageArray[numBlocks - 1], subkey2);
  }

  var c = aes(
    key,
    Buffer.concat(messageArray.slice(0, messageArray.length - 1))
  );
  let c_xor_m = xor(c, messageArray[messageArray.length - 1]);
  c = aes(key, c_xor_m);

  return c;
}

function getMessageArray(message, numBlocks, lastBlockRemainder) {
  var index = 0;
  var messageArray = [];
  if (lastBlockRemainder !== 0) {
    let padding = "80" + "00".repeat(16 - lastBlockRemainder - 1);
    let appendToMessage = Buffer.from(padding, "hex");
    message = Buffer.concat([message, appendToMessage]);
  }
  for (index = 0; index < numBlocks; index++) {
    let messageBlock = message.slice(
      index * blockSize,
      (index + 1) * blockSize
    );
    messageArray.push(messageBlock);
  }

  return messageArray;
}

I already tried the one mentioned here AES-CMAC module for Node.js? and completely rewriting the code to my own version of an AES-CMAC algorithm. In both the one I tried and the one I made (with the help of NIST Special Publication 800-38B), I get the same results.

Now I'm stuck between thinking either my code is wrong, or the python crypto library (where I don't completely understand the code) is wrong.

Can anyone help me figure out which of the two is true? And in case my code is wrong, help me fix it.

Upvotes: 2

Views: 530

Answers (1)

Zen Zac
Zen Zac

Reputation: 136

I found the answer: The Crypto library in javascript has an aes-cbc cipher, that says (and it does) accepts buffers and arrays. But the outcomes of both are different.

When I used a UInt8Array I got the right outcome. I completely rewrote an aes-cmac algorithm, just to figure out this what all I needed.

Upvotes: 0

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