# File lib/openid/consumer/associationmanager.rb, line 272
      def self.extract_expires_in(message)
        # expires_in should be a base-10 string.
        expires_in_str = message.get_arg(OPENID_NS, 'expires_in', NO_DEFAULT)
        if !(/\A\d+\Z/ =~ expires_in_str)
          raise ProtocolError, "Invalid expires_in field: #{expires_in_str}"
        end
        expires_in_str.to_i
      end