Robert Allan JamesandClaude Sonnet 5
53e6c5709f
Extends the previously verify-only ed25519.c with ed25519_keygen() and ed25519_sign() per RFC 8032 5.1.5/5.1.6, reusing every point-arithmetic primitive verify already had -- only seed expansion/clamping and per-message nonce derivation are new. Signing is deterministic; only keygen ever touches entropy, via a caller-supplied seed (virtio_rng, Phase A) -- keygen still generates nothing itself. New scalar_muladd() (scalar25519.c) for signing's S = (k*a + r) mod L, the one scalar op verify never needed. Schoolbook multiply into a u128 wide accumulator with one final carry pass -- deliberately the same shape as fe25519.c's existing multiply, which has a documented history of a real bug from carrying mid-accumulation instead of in one pass. Verified against an independent implementation, not self-consistency: a throwaway host harness against Python's cryptography library (OpenSSL- backed) across 6 trials (5 random seed/message pairs + the empty-message case) produced byte-for-byte identical pubkeys and signatures every time. Clean compile on all three architectures and a full 3-arch QEMU acceptance boot, conservation intact, no panics or guest errors. Nothing calls the new functions from the live kernel path yet -- that's Phase C (the MINT word itself), still open, documented in FABRIC-3.md. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01U14ET9CWAtbQMbYqomKgXd
38 lines
1.7 KiB
C
38 lines
1.7 KiB
C
/* scalar25519.h -- arithmetic mod L (the Ed25519 base point's order),
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* for reducing SHA-512 output to a valid scalar and checking a
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* signature's S component for the RFC 8032 malleability requirement
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* (S < L, not just S < 2^256).
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*
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* Deliberately NOT the intricate hand-tuned "sc_reduce" reduction most
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* reference implementations use (a bespoke Barrett-style reduction with
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* constants specific to L, notoriously easy to transcribe wrong) --
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* this is a plain binary long-division reduction, one bit at a time.
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* O(512) steps per reduction; this is a verify-only, non-hot-path
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* library (one reduction per signature check), so the simpler,
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* more obviously-correct approach is the right tradeoff here.
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*/
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#ifndef SCALAR25519_H
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#define SCALAR25519_H
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#include <stdint.h>
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/* 32-byte little-endian scalars, reduced mod L where noted. */
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/* Reduce a 64-byte little-endian value (e.g. raw SHA-512 output) mod L,
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* producing a 32-byte little-endian result < L. */
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void scalar_reduce512(uint8_t out[32], const uint8_t in[64]);
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/* 1 if the 32-byte little-endian scalar is < L (a well-formed,
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* non-malleable signature component per RFC 8032), else 0. */
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int scalar_lt_L(const uint8_t s[32]);
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/* out = (a*b + c) mod L, all 32-byte little-endian scalars (a, b, c need
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* not already be reduced mod L, though every caller in this codebase
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* passes already-reduced inputs). Needed for EdDSA signing's
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* S = (k*a + r) mod L step -- verify never needed scalar multiplication,
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* only reduction, so this didn't exist until signing did. */
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void scalar_muladd(uint8_t out[32], const uint8_t a[32], const uint8_t b[32],
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const uint8_t c[32]);
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#endif /* SCALAR25519_H */
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