instance method decrypt

# File activesupport/lib/active_support/message_encryptor.rb, line 36
    def decrypt(encrypted_message)
      cipher = new_cipher
      encrypted_data, iv = encrypted_message.split("--").map {|v| ActiveSupport::Base64.decode64(v)}

      cipher.decrypt
      cipher.key = @secret
      cipher.iv  = iv

      decrypted_data = cipher.update(encrypted_data)
      decrypted_data << cipher.final

      Marshal.load(decrypted_data)
    rescue OpenSSLCipherError, TypeError
      raise InvalidMessage
    end