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HMAC was there first (the RFC 2104 is from 1997, while CMAC is from 2006), which is reason enough to explain its primacy. If you use HMAC, you will more easily find test vectors and implementations against which to test, and with which to interoperate, which again explains continued primacy. Being the de facto standard is a very strong position. On many ...


Memory of the first round can't help the attacker win the second round. If it could: the attacker could simulate the first round on his own (picking his own key, pretending to play the game against himself), and then play the second round against the challenger. So, adversaries like this are not stronger -- not even if they have memory.


According to HMAC definition only one Key is used. The only thing is, the key is outer padded and inner padded HMAC(K,M) = H( (K⊕opad)| H( (K⊕ipad)| M)) H is a cryptographic hash function; K is a secret key padded to the right with extra zeros to the input block size of the hash function, or the hash of the original key if it's longer than that block ...

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