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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#include "ecp.h"
#include "mpi.h"
#include "mplogic.h"
#include "mpi-priv.h"
#define ECP521_DIGITS ECL_CURVE_DIGITS(521)
/* Fast modular reduction for p521 = 2^521 - 1. a can be r. Uses
* algorithm 2.31 from Hankerson, Menezes, Vanstone. Guide to
* Elliptic Curve Cryptography. */
static mp_err
ec_GFp_nistp521_mod(const mp_int *a, mp_int *r, const GFMethod *meth)
{
mp_err res = MP_OKAY;
int a_bits = mpl_significant_bits(a);
unsigned int i;
/* m1, m2 are statically-allocated mp_int of exactly the size we need */
mp_int m1;
mp_digit s1[ECP521_DIGITS] = { 0 };
MP_SIGN(&m1) = MP_ZPOS;
MP_ALLOC(&m1) = ECP521_DIGITS;
MP_USED(&m1) = ECP521_DIGITS;
MP_DIGITS(&m1) = s1;
if (a_bits < 521) {
if (a == r)
return MP_OKAY;
return mp_copy(a, r);
}
/* for polynomials larger than twice the field size or polynomials
* not using all words, use regular reduction */
if (a_bits > (521 * 2)) {
MP_CHECKOK(mp_mod(a, &meth->irr, r));
} else {
#define FIRST_DIGIT (ECP521_DIGITS - 1)
for (i = FIRST_DIGIT; i < MP_USED(a) - 1; i++) {
s1[i - FIRST_DIGIT] = (MP_DIGIT(a, i) >> 9) | (MP_DIGIT(a, 1 + i) << (MP_DIGIT_BIT - 9));
}
s1[i - FIRST_DIGIT] = MP_DIGIT(a, i) >> 9;
if (a != r) {
MP_CHECKOK(s_mp_pad(r, ECP521_DIGITS));
for (i = 0; i < ECP521_DIGITS; i++) {
MP_DIGIT(r, i) = MP_DIGIT(a, i);
}
}
MP_USED(r) = ECP521_DIGITS;
MP_DIGIT(r, FIRST_DIGIT) &= 0x1FF;
MP_CHECKOK(s_mp_add(r, &m1));
if (MP_DIGIT(r, FIRST_DIGIT) & 0x200) {
MP_CHECKOK(s_mp_add_d(r, 1));
MP_DIGIT(r, FIRST_DIGIT) &= 0x1FF;
} else if (s_mp_cmp(r, &meth->irr) == 0) {
mp_zero(r);
}
s_mp_clamp(r);
}
CLEANUP:
return res;
}
/* Compute the square of polynomial a, reduce modulo p521. Store the
* result in r. r could be a. Uses optimized modular reduction for p521.
*/
static mp_err
ec_GFp_nistp521_sqr(const mp_int *a, mp_int *r, const GFMethod *meth)
{
mp_err res = MP_OKAY;
MP_CHECKOK(mp_sqr(a, r));
MP_CHECKOK(ec_GFp_nistp521_mod(r, r, meth));
CLEANUP:
return res;
}
/* Compute the product of two polynomials a and b, reduce modulo p521.
* Store the result in r. r could be a or b; a could be b. Uses
* optimized modular reduction for p521. */
static mp_err
ec_GFp_nistp521_mul(const mp_int *a, const mp_int *b, mp_int *r,
const GFMethod *meth)
{
mp_err res = MP_OKAY;
MP_CHECKOK(mp_mul(a, b, r));
MP_CHECKOK(ec_GFp_nistp521_mod(r, r, meth));
CLEANUP:
return res;
}
/* Divides two field elements. If a is NULL, then returns the inverse of
* b. */
static mp_err
ec_GFp_nistp521_div(const mp_int *a, const mp_int *b, mp_int *r,
const GFMethod *meth)
{
mp_err res = MP_OKAY;
mp_int t;
/* If a is NULL, then return the inverse of b, otherwise return a/b. */
if (a == NULL) {
return mp_invmod(b, &meth->irr, r);
} else {
/* MPI doesn't support divmod, so we implement it using invmod and
* mulmod. */
MP_CHECKOK(mp_init(&t));
MP_CHECKOK(mp_invmod(b, &meth->irr, &t));
MP_CHECKOK(mp_mul(a, &t, r));
MP_CHECKOK(ec_GFp_nistp521_mod(r, r, meth));
CLEANUP:
mp_clear(&t);
return res;
}
}
/* Wire in fast field arithmetic and precomputation of base point for
* named curves. */
mp_err
ec_group_set_gfp521(ECGroup *group, ECCurveName name)
{
if (name == ECCurve_NIST_P521) {
group->meth->field_mod = &ec_GFp_nistp521_mod;
group->meth->field_mul = &ec_GFp_nistp521_mul;
group->meth->field_sqr = &ec_GFp_nistp521_sqr;
group->meth->field_div = &ec_GFp_nistp521_div;
}
return MP_OKAY;
}
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