-@node Transport protocol
+@node Transport
+@cindex Transport
+@cindex Transport protocol
+@cindex Salsa20
+@cindex PRP
+@cindex Nonce
+@cindex Poly1305
+@cindex XTEA
+@cindex Serial
@section Transport protocol
@verbatim
-ENCn(SERIAL) + ENC(KEY, ENCn(SERIAL), DATA_SIZE+DATA+NOISE) +
- AUTH(ENCn(SERIAL) + ENC(KEY, ENCn(SERIAL), DATA_SIZE+DATA+NOISE))
+TAG || ENCRYPTED || NONCE --> PACKET
+ ^ ^ ^
+ | | |
+ | | +-------------+
+ | | |
+ | +-------------+ |
+ | | |
+ +--< AUTH(AUTH_KEY, ENCRYPTED || NONCE)
+ ^ ^
+ | |
++------------------------+ |
+| |
+| +---------------+
+| |
++--< ENCRYPT(KEY, NONCE, PAYLOAD)
+ ^ ^
+ | |
+ | +--< DATA || PAD [|| ZEROS]
+ |
+ +--< PRP(PRP_KEY, SERIAL)
@end verbatim
-All transport and handshake messages are indistinguishable from
-pseudo random noise.
-
@code{SERIAL} is message's serial number. Odds are reserved for
-client(→server) messages, evens for server(→client) messages.
+client (to server) messages, evens for server (to client) messages.
-@code{ENCn} is XTEA block cipher algorithm used here as PRP (pseudo
-random permutation) to randomize, obfuscate @code{SERIAL}. Plaintext
+@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 is compact and fast enough. Salsa20 is PRF
-function and requires much more code to create PRP from it. 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).
+before transmission.
-Encrypted @code{SERIAL} is used as a nonce for @code{DATA} encryption:
-encryption key is different during each handshake, so (key, nonce) pair
-is always used only once. @code{ENC} is Salsa20 cipher, with established
-session @code{KEY} and encrypted @code{SERIAL} used as a nonce.
-@code{DATA_SIZE} is @emph{uint16} storing length of the @code{DATA}.
+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).
-@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.
+@verbatim
+PRP_KEY = 128bit(ENCRYPT(KEY, 0))
+@end verbatim
-@code{AUTH} is Poly1305 authentication function. First 256 bits of
-Salsa20 output are used as a one-time key for @code{AUTH}. Next 256 bits
-of Salsa20 are ignored. All remaining output is XORed with the data,
+@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
-if meet one, then drop it. Basically we could just store latest number
-and check if received one is greater, but because of UDP packets
-reordering this can lead to valid packets dropping and overall
-performance degradation. We store up to 256 seen nonces in hash
-structure, in two swapping buckets.
+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.