@node Transport
@section Transport protocol

@verbatim
TAG || ENCRYPTED || NONCE <-- PACKET
 ^         ^          ^
 |         |          |
 |         |          +------------+
 |         |                       |
 |         +------------+          |
 |                      |          |
 +-->AUTH(AUTH_KEY, ENCRYPTED || NONCE)
                        ^          ^
                        |          |
+-----------------------+          |
|                                  |
|                   +--------------+
|                   |
+--> ENCRYPT(KEY, NONCE, PAYLOAD)
                    ^       ^
                    |       |
                    |       +--> SIZE || DATA [|| NOISE]
                    |
                    +--> PRP(PRP_KEY, SERIAL)
@end verbatim

@code{SERIAL} is message's serial number. Odds are reserved for
client(->server) messages, evens for server(->client) messages.

@code{PRP} is XTEA block cipher algorithm used here as PRP (pseudo
random permutation function) to obfuscate @code{SERIAL}. Plaintext
@code{SERIAL} state is kept in peers internal state, but encrypted
before transmission.

XTEA's encryption key is the first 128-bit of Salsa20's output with
established common key and zero nonce (message nonces start from 1).

@verbatim
PRP_KEY = ENCRYPT(KEY, 0, 128-bit)
@end verbatim

@code{ENCRYPT} is Salsa20 stream cipher, with established session
@code{KEY} and obfuscated @code{SERIAL} used as a nonce. 512 bit of
Salsa20's output is ignored and only remaining is XORed with ther data,
encrypting it.

@code{SIZE} is big-endian @emph{uint16} storing length of the
@code{DATA}.

@code{NOISE} is optional. It is just some junk data, intended to fill up
packet to MTU size. This is useful for concealing payload packets length.

@code{AUTH} is Poly1305 authentication function. First 256 bits of
Salsa20's output are used as a one-time key for @code{AUTH}.

@verbatim
AUTH_KEY = ENCRYPT(KEY, NONCE, 256 bit)
@end verbatim

To prevent replay attacks we must remember received @code{SERIAL}s and
drop when receiving duplicate ones.