Spell out the SHA-256-truncation algorithm previously described in
prose:
ack_crc = SHA-256(plaintext_prefix || sender_pub_key)[0..3]
where plaintext_prefix = timestamp(4) || txt_type_attempt(1) || text
(excluding any trailing NUL terminator). Add the SIGNED_PLAIN variant
(includes 4-byte signature prefix, hashes against receiver's pub_key).
Reference the upstream functions that implement each side.
Resolves the ACK CRC vagueness called out in the 2026-04-01 audit.
Paired with the new corpus/crypto/sha256/ack-crc.json vectors.
Validated against upstream meshcore-dev/MeshCore @ 1a7b361
(BaseChatMesh.cpp:222, 249, 407; Utils::sha256 two-fragment form).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
swaitsandClaude Opus 4.7
277d4463c1