I'd forgotten that the SSH-2 BPP uses a defensive measure of
generating the MAC for successive prefixes of an incoming packet,
which means that ssh_mac_genresult needs to be nondestructive.
While I'm at it, I've also made all of hmac's hash objects exist all
the time - they're created up front, destroyed unconditionally on
free, and in between, whenever one is destroyed at all it's
immediately recreated. I think this simplifies things in general, and
in particular, creating at least one hash object immediately will come
in useful when I add selector vtables in a few commits' time.
Keeping that information alongside the hashes themselves seems more
sensible than having the HMAC code know that fact about everything it
can work with.
This replaces all the separate HMAC-implementing wrappers in the
various source files implementing the underlying hashes.
The new HMAC code also correctly handles the case of a key longer than
the underlying hash's block length, by replacing it with its own hash.
This means I can reinstate the test vectors in RFC 6234 which exercise
that case, which I didn't add to cryptsuite before because they'd have
failed.
It also allows me to remove the ad-hoc code at the call site in
cproxy.c which turns out to have been doing the same thing - I think
that must have been the only call site where the question came up
(since MAC keys invented by the main SSH-2 BPP are always shorter than
that).
