[jasonsopko]

What

datum_siphash and datum_siphash_mod8 read the key and the message words with upk_u64le instead of through a uint64_t cast.

Why

The short txn list reply (0x90) hashes hash_bin, which sits at offset 178 of T_DATUM_TEMPLATE_TXN, so every word load there is misaligned. x86 tolerates it, but it is undefined behavior, and every Linux and FreeBSD CI build has -fsanitize=undefined -fno-sanitize-recover=all. No test sends that request today, so CI is green; the test I suggested on #46 does, and fails under UBSan without this.

How I tested

• The new test checks the published SipHash-2-4 vectors (lengths 0, 8 and 15) and a 32-byte case with the key and the message misaligned. On master it passes in a plain build and fails under UBSan at datum_utils.c:694, the key load. With this change it passes in the gcc, clang, Umbrel, no-API and ASan+UBSan configs with -Wall -Werror.
• 2,000 random keys and messages against an independent SipHash-2-4 in Python: master matches at aligned offsets, this change at every offset from 0 to 7, for datum_siphash and, on the 1,116 messages whose length is a multiple of 8, datum_siphash_mod8.
• Release build, 16,383 32-byte hashes at the hash_bin offset, best of 200, three runs each: 340.0 to 340.7 us with this change and 339.9 to 341.9 us on master, with the same checksum.

Risk and rollback

Output on little-endian hosts is unchanged, and every CI target is little-endian. A big-endian host now reads the words in SipHash's byte order rather than its own. One commit to revert.

Scripts and logs: https://gist.github.com/jasonsopko/7df5dc75053fc4abb65cb7b927c816a8

[Copilot]

🟢 Approval recommended

The focused decoding changes preserve SipHash semantics, and regression tests cover misaligned inputs with no unresolved findings.

No open issues 🎉

What changed in this PR

Makes both SipHash implementations safe for unaligned inputs while preserving little-endian results.

Changes:

• Replaces cast-based word loads with upk_u64le.
• Assembles trailing bytes explicitly in little-endian order.
• Adds reference-vector tests with misaligned keys and messages.

File	Description
src/​datum_utils.c	Uses alignment-safe, little-endian decoding in both SipHash implementations.
src/​datum_utils_tests.c	Adds and registers misaligned-input regression tests.

_{Review effort: Balanced}