def decode_ack_plain(ciphertext)
      message = begin
                  @ecc.ecies_decrypt ciphertext[0,210]
                rescue
                  raise AuthenticationError, $!
                end
      raise RLPxSessionError, 'invalid message length' unless message.size == 64+32+1

      ephemeral_pubkey = message[0,64]
      nonce = message[64,32]
      known = message[-1].ord
      raise RLPxSessionError, 'invalid known byte' unless known == 0

      [210, ephemeral_pubkey, nonce, 4]
    end
def decode_ack_plain(ciphertext)
      message = begin
                  @ecc.ecies_decrypt ciphertext[0,210]
                rescue
                  raise AuthenticationError, $!
                end
      raise RLPxSessionError, 'invalid message length' unless message.size == 64+32+1

      ephemeral_pubkey = message[0,64]
      nonce = message[64,32]
      known = message[-1].ord
      raise RLPxSessionError, 'invalid known byte' unless known == 0

      [210, ephemeral_pubkey, nonce, 4]
    end
decode legacy pre-EIP-8 ack message format