module ActiveSupport # MessageVerifier makes it easy to generate and verify messages which are signed # to prevent tampering. # # This is useful for cases like remember-me tokens and auto-unsubscribe links where the # session store isn't suitable or available. # # Remember Me: # cookies[:remember_me] = @verifier.generate([@user.id, 2.weeks.from_now]) # # In the authentication filter: # # id, time = @verifier.verify(cookies[:remember_me]) # if time < Time.now # self.current_user = User.find(id) # end # class MessageVerifier class InvalidSignature < StandardError; end def initialize(secret, digest = 'SHA1') @secret = secret @digest = digest end def verify(signed_message) data, digest = signed_message.split("--") if secure_compare(digest, generate_digest(data)) Marshal.load(ActiveSupport::Base64.decode64(data)) else raise InvalidSignature end end def generate(value) data = ActiveSupport::Base64.encode64s(Marshal.dump(value)) "#{data}--#{generate_digest(data)}" end private # constant-time comparison algorithm to prevent timing attacks def secure_compare(a, b) return false unless a.bytesize == b.bytesize l = a.unpack "C#{a.bytesize}" res = 0 b.each_byte { |byte| res |= byte ^ l.shift } res == 0 end def generate_digest(data) require 'openssl' unless defined?(OpenSSL) OpenSSL::HMAC.hexdigest(OpenSSL::Digest::Digest.new(@digest), @secret, data) end end end