A few friends and I are building a system for home automation. Specifically, it is an application that opens and closes a garage door. One of the design decisions was to write the server side in node.js but to use Azure when it made sense. One of the Azure features we are using is the Access Control Service. When a client presents a token, you need to make sure that the signature on that token is valid. That turns out to be fairly interesting if you are new to node.js and have never used it before. I fit that model well. After a lot of tinkering and learning, I was able to write a function that validated a wrap_access_token using node.js and some associated, standard libraries. Here is the code, in its entirety. I include some ‘test’ code as well to allow others to verify results. I’ve already rotated the ACS signing key so that I don’t breach security too badly. This whole thing works surprisingly well.

In case you can’t read the code too well, here is what it does:

1. Parse the token into it’s constituent parts.

2. Pass the wrap_access_token to the function, along with the associated key.

Within the function:

1. Remove the signature part from the token since we need to verify that we generate the same signature. Since the signature is generated based on the bytes that precede it, the signature can’t be part of itself (this part is obvious when you think of it; the hard part is remembering to think of it!)

2. Unescape the signature and remember the base64 version of the signature, which is really just a byte array.

3. Generate the SHA256 HMAC signature using the shared secret/key.

4. Verify that the base64 encoding of the digest that we generated matches the one that was sent it.

5. If the signature passed in matches the one we generated, then the other entity knows the secret and can be trusted to have signed the tokens.

6. Party on, because the claims are valid.

The code would next need to split out the claims. The claims are just form-encoded key value pairs within the wrap_access_token. That step is left as an exercise for the reader.

var crypto = require(‘crypto’);
var util = require(‘util’);
var querystring = require(‘querystring’);
var buffer = require(‘buffer’);

function ValidateToken(token, key){
var hmacToken = “&HMACSHA256=”;
var indexOfToken = token.indexOf(hmacToken) + hmacToken.length;
var swtSignature = querystring.unescape(token.substr(indexOfToken, token.length – indexOfToken));
var signedPiece = token.substr(0, indexOfToken – hmacToken.length);
var buffer = new Buffer(key, encoding=”base64″);
var hmac = crypto.createHmac(“sha256″, buffer);
hmac.update(signedPiece);
var digest = hmac.digest(encoding=”base64”);
return digest == swtSignature;
}

var theToken = “wrap_access_token=developersmackdown.garage%3dOpenClose%26http%253a%252f%252fschemas.microsoft.com%252faccesscontrolservice%252f2010%252f07%252fclaims%252fidentityprovider%3dhttps%253a%252f%252ffriseton.accesscontrol.windows.net%252f%26Audience%3dhttp%253a%252f%252fcontosocontacts%252f%26ExpiresOn%3d1325287532%26Issuer%3dhttps%253a%252f%252ffriseton.accesscontrol.windows.net%252f%26HMACSHA256%3dSrgqXJv9pkxjweT2Lr%252bIV%252fGAncqIc34SnbrHdbr3VOQ%253d&wrap_access_token_expires_in=5999”;
var theData = querystring.parse(theToken, sep=’&’, eq=’=’);
var theKey = “Bn7TfLML5wK+R5TAa2VrO/9JANwuk3lzt/ykc4no+h0=”;

util.puts(ValidateToken(theData.wrap_access_token, theKey));