-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).
-
-@code{ENC} is Salsa20 stream cipher, with established session @code{KEY}
-and obfuscated @code{SERIAL} used as a nonce. First 256 bits of
-Salsa20's output is used as Poly1305 authentication key, next 256 bits
-are ignored. All remaining output is XORed with the data, encrypting it.