Validate and decode the user context served to an app using the Optimizely Canvas.
testing_tools/common/utils.py
@param {String} clientSecret the OAuth client secret provided by Optimizely to the app developer. The secret is
specific to the app.
@param  {String} signedRequest the value of the signed request provided by Optimizely to the app (that is, the
value of the "signed_request" parameter provided by Optimizely).
@return {Object} containing the user context provided by Optimizely, including user email, current project, etc.
@throws {Error} if the provided signedRequest is malformed or does not appear to be authentic since it was not
correctly signed with the provided client secret.
function(clientSecret, signedRequest) {
    var signedRequestParts = signedRequest.split('.');

    var hashedContext = signedRequestParts[0],
        unhashedContext = signedRequestParts[1];

    var rehashedContext =
        new Buffer(crypto.createHmac(CANVAS_HMAC_ALGORITHM, clientSecret).update(unhashedContext).digest('hex'))
        .toString('base64');

    if (rehashedContext !== hashedContext) {
      throw Error('Request not properly signed.')
    }

    return JSON.parse(new Buffer(unhashedContext, 'base64'));
  }