Generate a keyed hash value using the HMAC method with Google Apps Script

Question

Is there a way to create a hash value in Google Apps Script? Google Apps Script will run server side code in the .gs code file. The .gs file is written in JavaScript. Because JavaScript is mostly a client side language, and encrypting anything client side isn't secure, maybe something like HMAC for Javascript isn't available? When I do a web search on hmac in javascript the first thing I get is crypto-js. But it looks like I need to link to some services in <script> tags:

<script src="http://crypto-js.googlecode.com/svn/tags/3.1.2/build/rollups/hmac-md5.js"></script>
<script src="http://crypto-js.googlecode.com/svn/tags/3.1.2/build/rollups/hmac-sha1.js"></script>
<script src="http://crypto-js.googlecode.com/svn/tags/3.1.2/build/rollups/hmac-sha256.js"></script>
<script src="http://crypto-js.googlecode.com/svn/tags/3.1.2/build/rollups/hmac-sha512.js"></script>
<script>
    var hash = CryptoJS.HmacMD5("Message", "Secret Passphrase");
    var hash = CryptoJS.HmacSHA1("Message", "Secret Passphrase");
    var hash = CryptoJS.HmacSHA256("Message", "Secret Passphrase");
    var hash = CryptoJS.HmacSHA512("Message", "Secret Passphrase");
</script>

The Secret Passphrase would be in your client side HTML. That doesn't make any sense! Oh! I just found some pseudocode in wikipedia.

Wikipedia HMAC pseudo Code

Here is my attempt at refactoring it:

//blocksize is the size in bytes and is set to 64 bytes.
//byte size of any UTF-8 string

function byteCount(s) {
  return encodeURI(s).split(/%..|./).length - 1;
};

function hmac(key, message) {
    var blocksize = 64;
    var keyLngth = byteCount(key);

    if (keyLngth > blocksize) {
        key = hash(key); // keys longer than blocksize are shortened
    }
    else if (keyLngth < blocksize) {
        key = key + [0x00 * (blocksize - keyLngth)]; // keys shorter than blocksize are zero-padded 
    };

    var o_key_pad = [0x5c * blocksize] ⊕ key; // Where blocksize is that of the underlying hash function
    var i_key_pad = [0x36 * blocksize] ⊕ key; // Where ⊕ is exclusive or (XOR)

    return hash(o_key_pad + hash(i_key_pad + message));
};

I guess wherever the pseudo Code states: hash(key) one of the following hash functions: SHA-1, MD5, RIPEMD-128/160 needs to be used.

So I did a search on SHA-1 in JavaScript and found this:

http://www.movable-type.co.uk

Any info on how to create a HMAC value using Javascript would be greatly appreciated. I'll probably keep working on it, in the meantime. Even though it's Javascript, it's a Google .gs Apps Script code file, which runs on the server.

Answer 1

Apps Script has a built in Class Utility for creating HMAC Sha256 signature or token:

Official Apps Script Documentation HMAC Sha256 signature