I understand cryptonight uses keccak and aes. I don't understand what the tail-hash means though.
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You must have read what I said on IRC - there's a randomly chosen hash function used at the end to process the output, one of four. – Wolf0 Dec 18 '16 at 17:41
There is a diagram on page 6 and 7 here which labels the last (tail) hash as "Chosen hash".
And described:
Then, the 2 low-order bits of the first byte of the state are used to
select a hash function: 0=BLAKE-256 [BLAKE], 1=Groestl-256 [GROESTL],
2=JH-256 [JH], and 3=Skein-256 [SKEIN]. The chosen hash function is
then applied to the Keccak state, and the resulting hash is the
output of CryptoNight.
