Claude
3bd81097e1
The header scan (Captain Bob, 2026-11-10): 788 source files -- 351 with the
SL-1.0 header, 12 with the old name (docs/working/experiments, kept by
ruling), 31 under someone else's licence (tools/kconfig, Linux's, GPL-2.0:
kept), and 394 with none. Ruled: the SPDX form; capsules (.4th, .f18) and
proofs (.thy) left for a ruling of their own (mkcapsule's 64x16 block format,
content-addressed capsules).
253 files now begin
SPDX-License-Identifier: LicenseRef-Starship-1.0
Copyright (c) 2023-2026 Robert A. James
as a /* */ block (C, headers, assembly, linker scripts) or # lines (scripts,
Makefiles, Kconfig; below a shebang or a coding line); tools/mkcapsule.c,
which had the copyright line, gets the SPDX line only.
Not touched: docs/working/ (archive and old campaign material, as the
ruling kept the experiments); v4/system/post_cases.c (generated by
v4/tools/mkpost.py, "do not edit" -- the generator has the header).
Comments only: the kernel builds on amd64, aarch64, riscv64; make -C v4
test-32: 39 programs, 0 failures; every changed script parses.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GE3SDvQn5EoxdGufk84zz2
64 lines
3.0 KiB
C
64 lines
3.0 KiB
C
/* SPDX-License-Identifier: LicenseRef-Starship-1.0
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* Copyright (c) 2023-2026 Robert A. James
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*/
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/* fe25519.h -- arithmetic mod p = 2^255-19, for Ed25519.
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*
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* Five-limb representation, uniform radix 2^51 (value =
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* sum(limb[i] * 2^(51*i)), limb i in roughly [0, 2^51)) -- the standard
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* Ed25519 reference layout (matches the widely-reviewed "amd64-51"-style
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* implementations), not something invented for this codebase. 51*5=255
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* exactly, so unlike a mismatched limb-count/width choice, the reduction
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* constant is the clean 2^255 mod p = 19 with no extra scaling.
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*
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* Uses __int128 for multiply-accumulate (product of two ~51-bit limbs is
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* up to ~102 bits, summed across up to 5 terms per bucket -- needs a
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* wide type). Confirmed safe in this kernel's freestanding -nostdlib
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* build by direct toolchain testing (gcc/aarch64-linux-gnu-gcc/
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* riscv64-linux-gnu-gcc, matching Makefile.starkernel's exact flags):
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* __int128 multiply, add, and shift-by-constant all compile with zero
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* undefined symbols on all three target architectures. This is DIFFERENT
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* from __int128 DIVISION, which src/starkernel/arch/amd64/timer.c
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* documents as broken (needs libgcc's __udivti3, undefined in this
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* -nostdlib build) -- this file never divides __int128 values, so that
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* restriction doesn't apply here. An earlier draft of this file avoided
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* __int128 entirely (10 limbs, radix 2^26, int64_t only) out of
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* over-caution before this was checked directly; abandoned after running
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* into real bugs from that scheme's own complexity, not from __int128
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* unavailability -- __int128 was never actually the constraint once
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* verified.
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*/
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#ifndef FE25519_H
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#define FE25519_H
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#include <stdint.h>
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/* int64_t, not uint64_t: fe25519_sub produces negative intermediate
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* limbs (a[i] - b[i] can be < 0 for a specific limb even when the total
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* value a-b, mod p, is what's wanted), and fe25519_carry() relies on
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* arithmetic right shift to propagate negative "borrows" the same way
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* it propagates positive carries -- proven correct by the property-based
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* host test, not just assumed. */
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typedef struct { int64_t v[5]; } fe25519;
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void fe25519_0(fe25519 *r);
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void fe25519_1(fe25519 *r);
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void fe25519_copy(fe25519 *r, const fe25519 *a);
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void fe25519_add(fe25519 *r, const fe25519 *a, const fe25519 *b);
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void fe25519_sub(fe25519 *r, const fe25519 *a, const fe25519 *b);
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void fe25519_neg(fe25519 *r, const fe25519 *a);
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void fe25519_mul(fe25519 *r, const fe25519 *a, const fe25519 *b);
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void fe25519_sq(fe25519 *r, const fe25519 *a);
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void fe25519_invert(fe25519 *r, const fe25519 *a);
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void fe25519_mul_small(fe25519 *r, const fe25519 *a, uint32_t c);
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/* Pack to 32 little-endian bytes (fully reduced mod p) / unpack from same. */
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void fe25519_pack(uint8_t out[32], const fe25519 *a);
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void fe25519_unpack(fe25519 *r, const uint8_t in[32]);
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/* 1 if a == b (as field elements, after full reduction), else 0. */
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int fe25519_eq(const fe25519 *a, const fe25519 *b);
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/* Parity of the fully-reduced value's low bit (used for point decompression's sign bit). */
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int fe25519_parity(const fe25519 *a);
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#endif /* FE25519_H */
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