validate facebook signed_request in ruby

validate_facebook_signature.rb

require 'base64'

def base64_url_decode(str)
  str += '=' * (4 - str.length.modulo(4))
  Base64.decode64(str.gsub("-", "+").gsub("_", "/"))
end # source: https://github.com/ptarjan/base64url/blob/master/ruby.rb


require 'hmac'
require 'hmac-sha2' ## used to decode facebook return values


# used to validate signed requests from facebook http://developers.facebook.com/docs/authentication/canvas
# 
# replace FACEBOOK[:secret] with your facebook secret
def valid_facebook_signature?(signed_request)
  signature, encoded_data = signed_request.split(".")
  expected_signature = base64_url_decode(signature) 
  computed_signature = HMAC::SHA256.digest(FACEBOOK[:secret], encoded_data) 
  return expected_signature == computed_signature
end

Where can I get the FACEBOOK[:secret] ?