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/* $OpenBSD: bn_mod_words.c,v 1.1 2025/05/25 04:58:32 jsing Exp $ */
/*
* Copyright (c) 2024 Joel Sing <jsing@openbsd.org>
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
#include "bn_local.h"
#include "bn_internal.h"
/*
* bn_mod_add_words() computes r[] = (a[] + b[]) mod m[], where a, b, r and
* m are arrays of words with length n (r may be the same as a or b).
*/
#ifndef HAVE_BN_MOD_ADD_WORDS
void
bn_mod_add_words(BN_ULONG *r, const BN_ULONG *a, const BN_ULONG *b,
const BN_ULONG *m, size_t n)
{
BN_ULONG carry, mask;
/*
* Compute a + b, then compute r - m to determine if r >= m, considering
* any carry that resulted from the addition. Finally complete a
* conditional subtraction of r - m.
*/
/* XXX - change bn_add_words to use size_t. */
carry = bn_add_words(r, a, b, n);
mask = ~(carry - bn_sub_words_borrow(r, m, n));
bn_sub_words_masked(r, r, m, mask, n);
}
#endif
/*
* bn_mod_sub_words() computes r[] = (a[] - b[]) mod m[], where a, b, r and
* m are arrays of words with length n (r may be the same as a or b).
*/
#ifndef HAVE_BN_MOD_SUB_WORDS
void
bn_mod_sub_words(BN_ULONG *r, const BN_ULONG *a, const BN_ULONG *b,
const BN_ULONG *m, size_t n)
{
BN_ULONG borrow, mask;
/*
* Compute a - b, then complete a conditional addition of r + m
* based on the resulting borrow.
*/
/* XXX - change bn_sub_words to use size_t. */
borrow = bn_sub_words(r, a, b, n);
mask = (0 - borrow);
bn_add_words_masked(r, r, m, mask, n);
}
#endif
/*
* bn_mod_mul_words() computes r[] = (a[] * b[]) mod m[], where a, b, r and
* m are arrays of words with length n (r may be the same as a or b) in the
* Montgomery domain. The result remains in the Montgomery domain.
*/
#ifndef HAVE_BN_MOD_MUL_WORDS
void
bn_mod_mul_words(BN_ULONG *r, const BN_ULONG *a, const BN_ULONG *b,
const BN_ULONG *m, BN_ULONG *t, BN_ULONG m0, size_t n)
{
bn_montgomery_multiply_words(r, a, b, m, t, m0, n);
}
#endif
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