Introduce secp256k1_sha256_initialize_midstate() in the hash layer and use it at all tagged-hash midstate call sites across schnorrsig, musig, and ellswift.
Document the byte-counter contract at the declaration site in hash.h and add run_sha256_initialize_midstate_tests() to directly verify helper behavior against initialize_tagged.
Also switch the helper to take const uint32_t state[8] to reduce argument-order risk at call sites.
This gives the caller more control about whether the state should
be cleaned (= should be considered secret). Moreover, it gives the
caller the possibility to clean a hash struct without finalizing it.
Recent compilers compile the two new functions to very efficient code
on various platforms. In particular, already GCC >= 5 and clang >= 5
understand do this for the read function, which is the one critical
for performance (called 16 times per SHA256 transform).
Fixes#1080.
This avoids that the SHA256 implementation would produce wrong paddings
and thus wrong digests for messages of length >= 2^32 bytes on 32-bit
platforms.
This is not exploitable in any way since the SHA256 API is an internal
API and we never call it with that long messages.
Identifiers starting with an underscore and followed immediately by a capital letter are reserved by the C++ standard.
The only header guards not fixed are those in the headers auto-generated from java.
