protocol_crypto.go
func (sender *encryptedTCPSender) Send(msg []byte) error {
	sender.Lock()
	defer sender.Unlock()
	encodedMsg := secretbox.Seal(nil, msg, &sender.state.nonce, sender.state.sessionKey)
	sender.state.advance()
	return sender.sender.Send(encodedMsg)
}
func (sender *encryptedTCPSender) Send(msg []byte) error {
	sender.Lock()
	defer sender.Unlock()
	encodedMsg := secretbox.Seal(nil, msg, &sender.state.nonce, sender.state.sessionKey)
	sender.state.advance()
	return sender.sender.Send(encodedMsg)
}
// Send implements TCPSender by sealing and sending the msg as-is.