Question

I’m trying to parse and validate a JWT token in node.js based on this sample (authored in .NET): https://github.com/liveservices/LiveSDK/blob/master/Samples/Asp.net/AuthenticationTokenSample/JsonWebToken.cs

Here is my node js javascript that validates the token:

var validateSignature = function(key, claims, envelope, signature) {
    var hasher = crypto.createHash('sha256');
    hasher.update(key + "JWTSig");
    var key = hasher.digest('binary');
    var hmac = crypto.createHmac('sha256', key);
    hmac.update(envelope + '.' + claims);
    var out = hmac.digest('base64');
    console.log(out);
    console.log(signature);
    console.log(out === signature);
}

Now, the very weird thing is – it almost works. Here’s the output of the three console.log statements:

pEwNPJ+LUHBdvNx631UzdyVhPFUOvFY8jG3x/cP81FE=
pEwNPJ-LUHBdvNx631UzdyVhPFUOvFY8jG3x_cP81FE
false

It seems suspicious to me that the hashes are both the same except for the +-/_=

Anybody spot my mistake? Something to do with my base64 encoding.

UPDATE

I played some more and there seems to be something funky going on with base64 encoding here. The following code in node js:

console.log(signature);
var b = new Buffer(signature, 'base64');
console.log(b.toString('base64'));

yields:

pEwNPJ-LUHBdvNx631UzdyVhPFUOvFY8jG3x_cP81FE
pEwNPJLUHBdvNx631UzdyVhPFUOvFY8jG3xcP81F

Which seems very odd, right?

Answer 1

Thanks to Timothy Meade for commenting and pushing me in the right direction.

Node’s Buffer type generates standard Base64 with +, / and =

There is a URL safe base64 encoding as mentioned here: http://en.wikipedia.org/wiki/Base64

It replaces + with -, / with _ and = is optional. The token that is passed on the QueryString (d’uh) is a URL safe version. Hence the difference.

Code was fixed by a simple:

out = out.replace('+','-').replace('/','_').replace('=','');