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
48 lines
2.3 KiB
C
48 lines
2.3 KiB
C
/* ed25519.h -- EdDSA (RFC 8032), freestanding C99.
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*
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* Originally verify-only ("this kernel never signs; signing happens in
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* the host-side build tool") -- that was correct for capsule signing
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* (build-time, offline, a normal Linux binary can link libsodium/
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* OpenSSL) but conflicts with an on-device Zuse session minting new
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* user certs live at runtime, which requires the kernel itself to sign.
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* Decided (Phase 8, 2026-08-26): add real keygen/signing rather than
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* reshape that flow around verify-only. Entropy for keygen comes from
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* virtio_rng.h -- this header still has no RNG of its own, and takes a
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* caller-supplied seed rather than generating one, deliberately: keygen
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* has no business deciding how the seed's randomness quality is
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* guaranteed, that's the caller's job.
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*
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* Signing is NOT constant-time (same non-constant-time double-and-add
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* scalar_mult() verify already used) -- acceptable for this project's
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* actual threat model (an emulated/embedded kernel with no untrusted
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* co-tenant able to observe timing), not acceptable if this code is
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* ever reused somewhere with a real timing-attack surface.
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*/
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#ifndef ED25519_H
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#define ED25519_H
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#include <stdint.h>
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#include <stddef.h>
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/* Returns 1 if signature (64 bytes: R || S) is a valid Ed25519 signature
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* by pubkey (32 bytes, compressed point) over msg, else 0. Rejects
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* malformed inputs (S >= L, an undecodable point) as invalid rather than
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* faulting. */
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int ed25519_verify(const uint8_t pubkey[32], const uint8_t *msg, size_t msg_len,
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const uint8_t sig[64]);
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/* Derive the public key (compressed point A = [a]B) from a 32-byte
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* seed. seed must be real, uniformly random entropy -- see this file's
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* header comment; ed25519_keygen() does not check or generate it. */
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void ed25519_keygen(const uint8_t seed[32], uint8_t pubkey_out[32]);
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/* Sign msg with the keypair derived from seed (the same seed passed to
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* ed25519_keygen() to obtain the matching public key). Deterministic
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* per RFC 8032 (the nonce is derived from seed + message, not fresh
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* randomness at sign time) -- only keygen needs real entropy, signing
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* needs none. */
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void ed25519_sign(const uint8_t seed[32], const uint8_t *msg, size_t msg_len,
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uint8_t sig_out[64]);
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#endif /* ED25519_H */
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