[alex]

What

vg_aes_gcm_stream_encrypt_to, which vg_aes_gcm_seal_gather (AesGcm::encrypt) calls once per slice on its streaming path, encrypted a slice straight from src to dst only when the text before it ended a block. A slice after one that ends mid-block was copied to dst byte by byte and encrypted in place by vg_aes_gcm_stream_encrypt, slow path and all. For example, encrypt(&[&p[..1], &p[1..]]) copied almost the whole message one byte at a time.

It now starts with a head step. If the text so far ends inside a block, the (-text_len) mod 16 bytes that finish it (or all of len, if fewer) are copied and encrypted by vg_aes_gcm_stream_encrypt. Then the whole blocks after them go to vg_aes_gcm_encrypt_blocks_to straight from src, and at most 15 bytes are left for the copy. The head is skipped when the text so far plus the head would pass 2⁶⁴ bytes, so the text length stays exact. blocks now works past the bytes done instead of from offset 0. Every instance of stream_encrypt_to gets this, for every variant: it is generic code.

This is part of #1228. The workload is the one from the rustls analysis in #991 (encrypt with a few slices, reused key).

Measurements

AesGcm::encrypt timed with a reused key, 13-byte AAD and 12-byte nonce, on an AVX-512 Xeon (VAES, VPCLMULQDQ). Each cell is the best of 9 runs in ns, base → this PR. Texts under 768 bytes still take the copy-and-seal path, so this change doesn't affect them.

bytes	[n]	[1, n-1]	[n-1, 1]	[5, n-5]	[100, n-100]	three thirds
768	165 → 166	504 → 236	171 → 173	509 → 236	516 → 275	191 → 193
1024	182 → 182	615 → 250	187 → 188	618 → 247	619 → 290	603 → 359
1360	224 → 226	755 → 269	185 → 187	759 → 272	762 → 317	748 → 407
2048	259 → 258	1047 → 323	263 → 263	1051 → 323	1054 → 356	958 → 445
4096	376 → 375	1925 → 456	403 → 403	1937 → 461	1923 → 500	1547 → 563
16384	1201 → 1203	7176 → 1193	1211 → 1206	7170 → 1236	7079 → 1268	5257 → 1346

With a slice after one that ends mid-block, encrypt is now 2.1–6.0× faster: within 1.0–1.7× of a single slice instead of 2.5–6×. Inputs where every slice boundary is block-aligned ([n], [n-1, 1]) are unchanged.

Proofs

Assembly comes only from the Lean code. Spec, TCB and contracts are unchanged; the proofs are against the existing streamEncryptTo contract.

• Steps.lean: per-block lemmas for the new code (headLen, headOver, headPtrs, headArgs, headDone, blocksLoad, blocksCount). blocksArgs now works from any offset.
• BlkCall.lean, EncCall.lean: the calls of encrypt_blocks_to and stream_encrypt now take an offset and a count, not just "from 0" or "the rest".
• Blocks.lean: call_mid and blocks_ok now start from Mid s o instead of Mid s 0.
• Head.lean (new): head_ok, split into per-piece lemmas that the constant-time proof reuses. The head's byte count stays in the free slot work + 72 across its call.
• CT.lean: head_rel, plus blocks_rel from any offset. Every branch in the head depends only on text_len and len, which are public.

Checks

• lake build (all 13,546 jobs), then the emitter. Only src/asm/x86_64/gcm.rs changes.
• ci/check_lean_speed.py, check_lean_imports.py, check_vectors.py, check_arch_gates.py, check_variants.py, check_mcdt.py, cpu_tests.py lint, algorithms_table.py.
• cargo fmt --check, cargo clippy --all-targets -- -D warnings.
• cargo test with Wycheproof, with all CPU features, and the AES-GCM tests again with VG_CPU_FEATURES=none.
• backends_agree now also covers slices after one that ends mid-block: a slice shorter than the bytes that end the open block, one exactly that long, and one longer.

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https://claude.ai/code/session_01FoYWDDEDFq1DFev5X7n2aq

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