Parse the Facebook "signed_request" param

signed_request.rb

# Found here: http://stackoverflow.com/questions/4987772/decoding-facebooks-signed-request-in-ruby-sinatra

require 'base64'
require 'openssl'
require 'crack/json'

def base64_url_decode str
  encoded_str = str.gsub('-','+').gsub('_','/')
  encoded_str += '=' while !(encoded_str.size % 4).zero?
  Base64.decode64(encoded_str)
end

def decode_data(signed_request)
  encoded_sig, payload = signed_request.split('.')
  data = base64_url_decode(payload)
end

json_request = decode_data(params[:signed_request])
signed_request = Crack::JSON.parse(json_request)

# I needed the profile_id
fb_id = signed_request["profile_id"]