@node Transport
@section Transport protocol

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

@code{SERIAL} is message's serial number. Odds are reserved for
client (to server) messages, evens for server (to 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 @code{PRP_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 = 128bit(ENCRYPT(KEY, 0))
@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{DATA} is padded with @code{PAD} (0x80 byte). Optional @code{ZEROS}
may follow, to fill up packet to conceal payload packet 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 = 256bit(ENCRYPT(KEY, NONCE))
@end verbatim

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

In @ref{Encless, encryptionless mode} this scheme is slightly different:

@verbatim
 PACKET = ENCODED || NONCE
ENCODED = ENCLESS(DATA || PAD || ZEROS)
  NONCE = PRP(PRP_KEY, SERIAL)
@end verbatim

@code{ENCLESS} is AONT and chaffing function. There is no need in
explicit separate authentication.