Get signed_request in Node.js (Express) Facebook canvas app

I did this a little while ago, and ended up writing a small library to do it. The original CoffeeScript can be found at https://gist.github.com/fbef51815ab6f062b51a#file_signed_request.coffee, here is a JavaScript translation:

var crypto = require('crypto');SignedRequest = (function() {  function SignedRequest(secret, request) {    this.secret = secret;    this.request = request;    this.verify = this.verify.bind(this);    var parts = this.request.split('.');    this.encodedSignature = parts[0];    this.encoded = parts[1];    this.signature = this.base64decode(this.encodedSignature);    this.decoded = this.base64decode(this.encoded);    this.data = JSON.parse(this.decoded);  }  SignedRequest.prototype.verify = function() {    if (this.data.algorithm !== 'HMAC-SHA256') {      return false;    }    var hmac = crypto.createHmac('SHA256', this.secret);    hmac.update(this.encoded);    var result = hmac.digest('base64').replace(/\//g, '_').replace(/\+/g, '-').replace(/\=/g, '');    return result === this.encodedSignature;  };  SignedRequest.prototype.base64encode = function(data) {    return new Buffer(data, 'utf8').toString('base64').replace(/\//g, '_').replace(/\+/g, '-').replace(/\=/g, '');  };  SignedRequest.prototype.base64decode = function(data) {    while (data.length % 4 !== 0) {      data += '=';    }    data = data.replace(/-/g, '+').replace(/_/g, '/');    return new Buffer(data, 'base64').toString('utf-8');  };  return SignedRequest;})();module.exports = SignedRequest;

Which you can use like this:

var verifier = new SignedRequest(clientSecret, signedRequest);verifier.verify() // whether or not the signed request verifiesverifier.data // the data from the signed request