[noise] expected length of messages during handshake

[David Wong]

> It is assumed you have a transport layer underneath that ReadMessage
> talks to.

Yes, but this transport layer doesn't tell you how long a Noise
ciphertext is. See my example.


> You might not be using tcp. TCP usually needs a length, yes, although
> you can also pad all the messages to fixed lengths instead. Other transports
> already may include framing information.

UDP doesn't fix the problem, see my example again where two
concatenated Noise messages can fit in one UDP message.