instance method decrypt

# File activesupport/lib/active_support/message_encryptor.rb, line 295
      def decrypt(encrypted_message)
        cipher = new_cipher
        encrypted_data, iv, auth_tag = extract_parts(encrypted_message)

        # Currently the OpenSSL bindings do not raise an error if auth_tag is
        # truncated, which would allow an attacker to easily forge it. See
        # https://github.com/ruby/openssl/issues/63
        if aead_mode? && auth_tag.bytesize != AUTH_TAG_LENGTH
          throw :invalid_message_format, "truncated auth_tag"
        end

        cipher.decrypt
        cipher.key = @secret
        cipher.iv  = iv
        if aead_mode?
          cipher.auth_tag = auth_tag
          cipher.auth_data = ""
        end

        decrypted_data = cipher.update(encrypted_data)
        decrypted_data << cipher.final
      rescue OpenSSLCipherError => error
        throw :invalid_message_format, error
      end