Robert Allan JamesandClaude Sonnet 5
2e7e957680
New freestanding, verify-only Ed25519 (RFC 8032) implementation:
include/starkernel/{sha512,fe25519,scalar25519,ed25519}.h +
src/starkernel/crypto/{sha512,fe25519,scalar25519,ed25519}.c, wired into
Makefile.starkernel. Kernel never signs or generates keys -- only
ed25519_verify() is needed; signing happens in the host-side mkcapsule
build tool via libsodium/OpenSSL.
Confirmed __int128 multiply/add/shift-by-constant compile with zero
undefined symbols on all three target toolchains (only division needs
libgcc's __udivti3, per timer.c's existing documented finding -- that
file's comment updated to narrow the claim, since it had been read as
"avoid __int128 entirely"). This enabled the standard 5-limb radix-2^51
field arithmetic representation.
An abandoned first attempt (10-limb radix-2^26, avoiding __int128 out of
premature caution) hit two real bugs, both invisible on inspection and
found only by property-based testing against Python's own bignum
arithmetic: a non-uniform-radix limb misalignment in multiplication, and
a double-counted carry. Verification chain: SHA-512 against known +
boundary vectors (7/7); field arithmetic property-tested 25,045 cases;
scalar-mod-L arithmetic 300 cases (L confirmed prime via Miller-Rabin
first); full verify() end-to-end against 110 real signatures from
Python's cryptography library, including tampered inputs and the RFC
8032 S>=L malleability attack -- all correctly accepted/rejected.
Compiles clean (zero warnings) and links on all three architectures,
confirmed via the mandatory three-arch QEMU boot. The code is linked but
not yet called from anywhere -- wiring into capsule_birth.c needs a
from-scratch X.509/DER parser first (Captain Bob chose real X.509 over a
raw-blob cert format this session), which is the next open item.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HZ8kNoTuP63pbQtro4qvrm
24 lines
975 B
C
24 lines
975 B
C
/* ed25519.h -- EdDSA signature VERIFICATION only (RFC 8032), freestanding
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* C99. No signing, no key generation, no RNG -- this kernel never signs;
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* signing happens in the host-side build tool, which can link
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* libsodium/OpenSSL because it's a normal Linux binary. That halves the
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* implementation surface here: no scalar clamping, no key derivation, no
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* constant-time discipline (verify operates only on public data --
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* public key, message, signature -- there's no secret-dependent branch
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* to leak).
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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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#endif /* ED25519_H */
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