This is a helper class that will take a signed request from Facebook's registration system and parse into a json hash.

example_usage.rb

registration_request = FacebookRegistration::SignedRequest.new(params[:signed_request])
registration_params = registration_request.parse_request
pp registration_params

facebook_registration.rb

require 'rubygems'
require 'openssl'
require 'base64'

module FacebookRegistration
  class SignedRequest
    attr_accessor :signed_request

    def initialize(signed_request)
      @signed_request = signed_request
    end

    def parse_request
      signature, signed_params = @signed_request.split('.')
      signed_params = JSON(base64_url_decode(signed_params))

      signed_params
    end

    private

    def signed_request_is_valid?(secret, signature, params)
      signature = base64_url_decode(signature)
      expected_signature = OpenSSL::HMAC.digest('SHA256', secret, params.tr("-_", "+/"))
      signature == expected_signature
    end

    def base64_url_decode(str)
      str = str + "=" * (6 - str.size % 6) unless str.size % 6 == 0
      Base64.decode64(str.tr("-_", "+/"))
    end

  end
end