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/* $OpenBSD: ec_field.c,v 1.1 2025/05/25 05:12:05 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 <string.h>
#include <openssl/ec.h>
#include "bn_local.h"
#include "bn_internal.h"
#include "ec_local.h"
#include "ec_internal.h"
int
ec_field_modulus_from_bn(EC_FIELD_MODULUS *fm, const BIGNUM *bn, BN_CTX *ctx)
{
BN_MONT_CTX *mctx = NULL;
size_t i;
int ret = 0;
if (BN_is_negative(bn))
goto err;
if (BN_num_bits(bn) > EC_FIELD_ELEMENT_MAX_BITS)
goto err;
memset(fm, 0, sizeof(*fm));
fm->n = (BN_num_bits(bn) + BN_BITS2 - 1) / BN_BITS2;
for (i = 0; i < bn->top; i++)
fm->m.w[i] = bn->d[i];
/* XXX - implement this without BN_MONT_CTX. */
if ((mctx = BN_MONT_CTX_new()) == NULL)
goto err;
if (!BN_MONT_CTX_set(mctx, bn, ctx))
goto err;
for (i = 0; i < mctx->RR.top; i++)
fm->rr.w[i] = mctx->RR.d[i];
fm->minv0 = mctx->n0[0];
ret = 1;
err:
BN_MONT_CTX_free(mctx);
return ret;
}
int
ec_field_element_from_bn(const EC_FIELD_MODULUS *fm, const EC_GROUP *group,
EC_FIELD_ELEMENT *fe, const BIGNUM *bn, BN_CTX *ctx)
{
BN_ULONG t[EC_FIELD_ELEMENT_MAX_WORDS * 2 + 2];
BIGNUM *tmp;
size_t i;
int ret = 0;
BN_CTX_start(ctx);
if ((tmp = BN_CTX_get(ctx)) == NULL)
goto err;
/* XXX - enforce 0 <= n < p. */
if (BN_num_bits(bn) > EC_FIELD_ELEMENT_MAX_BITS)
goto err;
/* XXX - do this without BN. */
if (!BN_nnmod(tmp, bn, group->p, ctx))
goto err;
if (BN_num_bits(tmp) > EC_FIELD_ELEMENT_MAX_BITS)
abort();
memset(fe->w, 0, sizeof(fe->w));
for (i = 0; i < tmp->top; i++)
fe->w[i] = tmp->d[i];
bn_mod_mul_words(fe->w, fe->w, fm->rr.w, fm->m.w, t, fm->minv0, fm->n);
ret = 1;
err:
BN_CTX_end(ctx);
return ret;
}
int
ec_field_element_to_bn(const EC_FIELD_MODULUS *fm, const EC_FIELD_ELEMENT *fe,
BIGNUM *bn, BN_CTX *ctx)
{
BN_ULONG t[EC_FIELD_ELEMENT_MAX_WORDS * 2 + 2];
size_t i;
if (!bn_wexpand(bn, fm->n))
return 0;
memset(t, 0, sizeof(t));
for (i = 0; i < fm->n; i++)
t[i] = fe->w[i];
bn_montgomery_reduce_words(bn->d, t, fm->m.w, fm->minv0, fm->n);
bn->top = fm->n;
bn_correct_top(bn);
return 1;
}
void
ec_field_element_copy(EC_FIELD_ELEMENT *dst, const EC_FIELD_ELEMENT *src)
{
memcpy(dst, src, sizeof(EC_FIELD_ELEMENT));
}
int
ec_field_element_equal(const EC_FIELD_MODULUS *fm, const EC_FIELD_ELEMENT *a,
const EC_FIELD_ELEMENT *b)
{
BN_ULONG v = 0;
int i;
for (i = 0; i < fm->n; i++)
v |= a->w[i] ^ b->w[i];
return bn_ct_eq_zero(v);
}
int
ec_field_element_is_zero(const EC_FIELD_MODULUS *fm, const EC_FIELD_ELEMENT *fe)
{
BN_ULONG v = 0;
int i;
for (i = 0; i < fm->n; i++)
v |= fe->w[i];
return bn_ct_eq_zero(v);
}
void
ec_field_element_add(const EC_FIELD_MODULUS *m, EC_FIELD_ELEMENT *r,
const EC_FIELD_ELEMENT *a, const EC_FIELD_ELEMENT *b)
{
bn_mod_add_words(r->w, a->w, b->w, m->m.w, m->n);
}
void
ec_field_element_sub(const EC_FIELD_MODULUS *m, EC_FIELD_ELEMENT *r,
const EC_FIELD_ELEMENT *a, const EC_FIELD_ELEMENT *b)
{
bn_mod_sub_words(r->w, a->w, b->w, m->m.w, m->n);
}
void
ec_field_element_mul(const EC_FIELD_MODULUS *m, EC_FIELD_ELEMENT *r,
const EC_FIELD_ELEMENT *a, const EC_FIELD_ELEMENT *b)
{
BN_ULONG t[EC_FIELD_ELEMENT_MAX_WORDS * 2 + 2];
bn_mod_mul_words(r->w, a->w, b->w, m->m.w, t, m->minv0, m->n);
}
void
ec_field_element_sqr(const EC_FIELD_MODULUS *m, EC_FIELD_ELEMENT *r,
const EC_FIELD_ELEMENT *a)
{
BN_ULONG t[EC_FIELD_ELEMENT_MAX_WORDS * 2 + 2];
bn_mod_mul_words(r->w, a->w, a->w, m->m.w, t, m->minv0, m->n);
}
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