I'm looking into refactoring hmac, mainly to trim context size https://git.lysator.liu.se/nettle/nettle/-/issues/2.
Two questions somewhat related to this:
1. The hmac spec allows arbitrarily large keys; if key size exceeds underlying block size, the key is hashed and the digest is used as the hmac key. Effectively the same as if caller would hash the key, and pass in the digest. Is this a feature that anyone is using, or in other words, what would break if nettle's hmac implementation were restricted to key size <= block size, and leave the obscure(?) prehashing needed for support of arbitrary large keys to the application?
2. That kind of prehashing is also a usecase for all-in-one hashing. Would it be useful to add convenience functions for all-in-one-hashing, e.g.,
void sha256_sum (uint8_t *digest, size_t length, const uint8_t *data);
to hash a contiguous string without having to bother with init/update/digest? Implementation would typically need to allocate a context struct on the stack.
Regards, /Niels