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).

It's not used outside xmss_commons

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.

As a result, performs various refactors that also impact the verification
function, since cleaner signing functions exposed more overlap.

Using global defines for parameters (as seems to be typical in
academic crypto code) does not play nice with multithreading at all.

A large number of functions was repeated in xmss_fast; these are now
shared between the two implementations via the xmss_commons file.
Notably, we ensure compatability by sharing the verification functions.

Versions are incompatible due to different address formats and differing message compression!