In the public comments to draft version of NIST Special Publication 800-208, ETSI TC CYBER WG QSC identified a multi-target attack against the method of pseudorandom key generation used in this referrence implementation. ETSI TC CYBER WG QSC suggested using the pseudorandom key generation method from SPHINCS+, however, there is still a multi-user attack against that key generation method.
This commit revises the pseudorandom key generation method by using the method from SPINCS+, but adding SEED as an input in order to protect against multi-user attacks. Since prf() only accepts 32-byte inputs, the new key generation method uses a new PRF. The resulting key generation method is sk[i] = prf_keygen(sk_seed, pub_seed || adrs).
The reference implemention of XMSS currently assumes that n bytes of padding is used for the prefix in the functions prf, hash_message, thash_h, and thash_f. While this is the case for all of the parameter sets in RFC 8391, the draft version of NIST Special Publication 800-208 specifies paramter sets in which the amount of padding is different than n.
This commit allows for the padding length for a parameter set to be specified separately from n.
Previous code allocated an array on the stack of mlen bytes, but
it should be possible to also sign heap-space messages. By relying
on the fact that sm and m fit the message + signature, we move
the message so that 4*n bytes of prefix can be added.
