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rust-secp256k1-unsafe-fast/secp256k1-sys/depend/secp256k1/src/ecmult_const_impl.h

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/***********************************************************************
* Copyright (c) 2015 Pieter Wuille, Andrew Poelstra *
* Distributed under the MIT software license, see the accompanying *
* file COPYING or https://www.opensource.org/licenses/mit-license.php.*
***********************************************************************/
#ifndef SECP256K1_ECMULT_CONST_IMPL_H
#define SECP256K1_ECMULT_CONST_IMPL_H
#include "scalar.h"
#include "group.h"
#include "ecmult_const.h"
#include "ecmult_impl.h"
/* This is like `ECMULT_TABLE_GET_GE` but is constant time */
#define ECMULT_CONST_TABLE_GET_GE(r,pre,n,w) do { \
int m = 0; \
/* Extract the sign-bit for a constant time absolute-value. */ \
int mask = (n) >> (sizeof(n) * CHAR_BIT - 1); \
int abs_n = ((n) + mask) ^ mask; \
int idx_n = abs_n >> 1; \
rustsecp256k1_v0_4_1_fe neg_y; \
VERIFY_CHECK(((n) & 1) == 1); \
VERIFY_CHECK((n) >= -((1 << ((w)-1)) - 1)); \
VERIFY_CHECK((n) <= ((1 << ((w)-1)) - 1)); \
VERIFY_SETUP(rustsecp256k1_v0_4_1_fe_clear(&(r)->x)); \
VERIFY_SETUP(rustsecp256k1_v0_4_1_fe_clear(&(r)->y)); \
/* Unconditionally set r->x = (pre)[m].x. r->y = (pre)[m].y. because it's either the correct one \
* or will get replaced in the later iterations, this is needed to make sure `r` is initialized. */ \
(r)->x = (pre)[m].x; \
(r)->y = (pre)[m].y; \
for (m = 1; m < ECMULT_TABLE_SIZE(w); m++) { \
/* This loop is used to avoid secret data in array indices. See
* the comment in ecmult_gen_impl.h for rationale. */ \
rustsecp256k1_v0_4_1_fe_cmov(&(r)->x, &(pre)[m].x, m == idx_n); \
rustsecp256k1_v0_4_1_fe_cmov(&(r)->y, &(pre)[m].y, m == idx_n); \
} \
(r)->infinity = 0; \
rustsecp256k1_v0_4_1_fe_negate(&neg_y, &(r)->y, 1); \
rustsecp256k1_v0_4_1_fe_cmov(&(r)->y, &neg_y, (n) != abs_n); \
} while(0)
/** Convert a number to WNAF notation.
* The number becomes represented by sum(2^{wi} * wnaf[i], i=0..WNAF_SIZE(w)+1) - return_val.
* It has the following guarantees:
* - each wnaf[i] an odd integer between -(1 << w) and (1 << w)
* - each wnaf[i] is nonzero
* - the number of words set is always WNAF_SIZE(w) + 1
*
* Adapted from `The Width-w NAF Method Provides Small Memory and Fast Elliptic Scalar
* Multiplications Secure against Side Channel Attacks`, Okeya and Tagaki. M. Joye (Ed.)
* CT-RSA 2003, LNCS 2612, pp. 328-443, 2003. Springer-Verlag Berlin Heidelberg 2003
*
* Numbers reference steps of `Algorithm SPA-resistant Width-w NAF with Odd Scalar` on pp. 335
*/
static int rustsecp256k1_v0_4_1_wnaf_const(int *wnaf, const rustsecp256k1_v0_4_1_scalar *scalar, int w, int size) {
int global_sign;
int skew = 0;
int word = 0;
/* 1 2 3 */
int u_last;
int u;
int flip;
int bit;
rustsecp256k1_v0_4_1_scalar s;
int not_neg_one;
VERIFY_CHECK(w > 0);
VERIFY_CHECK(size > 0);
/* Note that we cannot handle even numbers by negating them to be odd, as is
* done in other implementations, since if our scalars were specified to have
* width < 256 for performance reasons, their negations would have width 256
* and we'd lose any performance benefit. Instead, we use a technique from
* Section 4.2 of the Okeya/Tagaki paper, which is to add either 1 (for even)
* or 2 (for odd) to the number we are encoding, returning a skew value indicating
* this, and having the caller compensate after doing the multiplication.
*
* In fact, we _do_ want to negate numbers to minimize their bit-lengths (and in
* particular, to ensure that the outputs from the endomorphism-split fit into
* 128 bits). If we negate, the parity of our number flips, inverting which of
* {1, 2} we want to add to the scalar when ensuring that it's odd. Further
* complicating things, -1 interacts badly with `rustsecp256k1_v0_4_1_scalar_cadd_bit` and
* we need to special-case it in this logic. */
flip = rustsecp256k1_v0_4_1_scalar_is_high(scalar);
/* We add 1 to even numbers, 2 to odd ones, noting that negation flips parity */
bit = flip ^ !rustsecp256k1_v0_4_1_scalar_is_even(scalar);
/* We check for negative one, since adding 2 to it will cause an overflow */
rustsecp256k1_v0_4_1_scalar_negate(&s, scalar);
not_neg_one = !rustsecp256k1_v0_4_1_scalar_is_one(&s);
s = *scalar;
rustsecp256k1_v0_4_1_scalar_cadd_bit(&s, bit, not_neg_one);
/* If we had negative one, flip == 1, s.d[0] == 0, bit == 1, so caller expects
* that we added two to it and flipped it. In fact for -1 these operations are
* identical. We only flipped, but since skewing is required (in the sense that
* the skew must be 1 or 2, never zero) and flipping is not, we need to change
* our flags to claim that we only skewed. */
global_sign = rustsecp256k1_v0_4_1_scalar_cond_negate(&s, flip);
global_sign *= not_neg_one * 2 - 1;
skew = 1 << bit;
/* 4 */
u_last = rustsecp256k1_v0_4_1_scalar_shr_int(&s, w);
do {
int even;
/* 4.1 4.4 */
u = rustsecp256k1_v0_4_1_scalar_shr_int(&s, w);
/* 4.2 */
even = ((u & 1) == 0);
/* In contrast to the original algorithm, u_last is always > 0 and
* therefore we do not need to check its sign. In particular, it's easy
* to see that u_last is never < 0 because u is never < 0. Moreover,
* u_last is never = 0 because u is never even after a loop
* iteration. The same holds analogously for the initial value of
* u_last (in the first loop iteration). */
VERIFY_CHECK(u_last > 0);
VERIFY_CHECK((u_last & 1) == 1);
u += even;
u_last -= even * (1 << w);
/* 4.3, adapted for global sign change */
wnaf[word++] = u_last * global_sign;
u_last = u;
} while (word * w < size);
wnaf[word] = u * global_sign;
VERIFY_CHECK(rustsecp256k1_v0_4_1_scalar_is_zero(&s));
VERIFY_CHECK(word == WNAF_SIZE_BITS(size, w));
return skew;
}
static void rustsecp256k1_v0_4_1_ecmult_const(rustsecp256k1_v0_4_1_gej *r, const rustsecp256k1_v0_4_1_ge *a, const rustsecp256k1_v0_4_1_scalar *scalar, int size) {
rustsecp256k1_v0_4_1_ge pre_a[ECMULT_TABLE_SIZE(WINDOW_A)];
rustsecp256k1_v0_4_1_ge tmpa;
rustsecp256k1_v0_4_1_fe Z;
int skew_1;
rustsecp256k1_v0_4_1_ge pre_a_lam[ECMULT_TABLE_SIZE(WINDOW_A)];
int wnaf_lam[1 + WNAF_SIZE(WINDOW_A - 1)];
int skew_lam;
rustsecp256k1_v0_4_1_scalar q_1, q_lam;
int wnaf_1[1 + WNAF_SIZE(WINDOW_A - 1)];
int i;
/* build wnaf representation for q. */
int rsize = size;
if (size > 128) {
rsize = 128;
/* split q into q_1 and q_lam (where q = q_1 + q_lam*lambda, and q_1 and q_lam are ~128 bit) */
rustsecp256k1_v0_4_1_scalar_split_lambda(&q_1, &q_lam, scalar);
skew_1 = rustsecp256k1_v0_4_1_wnaf_const(wnaf_1, &q_1, WINDOW_A - 1, 128);
skew_lam = rustsecp256k1_v0_4_1_wnaf_const(wnaf_lam, &q_lam, WINDOW_A - 1, 128);
} else
{
skew_1 = rustsecp256k1_v0_4_1_wnaf_const(wnaf_1, scalar, WINDOW_A - 1, size);
skew_lam = 0;
}
/* Calculate odd multiples of a.
* All multiples are brought to the same Z 'denominator', which is stored
* in Z. Due to secp256k1' isomorphism we can do all operations pretending
* that the Z coordinate was 1, use affine addition formulae, and correct
* the Z coordinate of the result once at the end.
*/
rustsecp256k1_v0_4_1_gej_set_ge(r, a);
rustsecp256k1_v0_4_1_ecmult_odd_multiples_table_globalz_windowa(pre_a, &Z, r);
for (i = 0; i < ECMULT_TABLE_SIZE(WINDOW_A); i++) {
rustsecp256k1_v0_4_1_fe_normalize_weak(&pre_a[i].y);
}
if (size > 128) {
for (i = 0; i < ECMULT_TABLE_SIZE(WINDOW_A); i++) {
rustsecp256k1_v0_4_1_ge_mul_lambda(&pre_a_lam[i], &pre_a[i]);
}
}
/* first loop iteration (separated out so we can directly set r, rather
* than having it start at infinity, get doubled several times, then have
* its new value added to it) */
i = wnaf_1[WNAF_SIZE_BITS(rsize, WINDOW_A - 1)];
VERIFY_CHECK(i != 0);
ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, i, WINDOW_A);
rustsecp256k1_v0_4_1_gej_set_ge(r, &tmpa);
if (size > 128) {
i = wnaf_lam[WNAF_SIZE_BITS(rsize, WINDOW_A - 1)];
VERIFY_CHECK(i != 0);
ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a_lam, i, WINDOW_A);
rustsecp256k1_v0_4_1_gej_add_ge(r, r, &tmpa);
}
/* remaining loop iterations */
for (i = WNAF_SIZE_BITS(rsize, WINDOW_A - 1) - 1; i >= 0; i--) {
int n;
int j;
for (j = 0; j < WINDOW_A - 1; ++j) {
rustsecp256k1_v0_4_1_gej_double(r, r);
}
n = wnaf_1[i];
ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, n, WINDOW_A);
VERIFY_CHECK(n != 0);
rustsecp256k1_v0_4_1_gej_add_ge(r, r, &tmpa);
if (size > 128) {
n = wnaf_lam[i];
ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a_lam, n, WINDOW_A);
VERIFY_CHECK(n != 0);
rustsecp256k1_v0_4_1_gej_add_ge(r, r, &tmpa);
}
}
rustsecp256k1_v0_4_1_fe_mul(&r->z, &r->z, &Z);
{
/* Correct for wNAF skew */
rustsecp256k1_v0_4_1_ge correction = *a;
rustsecp256k1_v0_4_1_ge_storage correction_1_stor;
rustsecp256k1_v0_4_1_ge_storage correction_lam_stor;
rustsecp256k1_v0_4_1_ge_storage a2_stor;
rustsecp256k1_v0_4_1_gej tmpj;
rustsecp256k1_v0_4_1_gej_set_ge(&tmpj, &correction);
rustsecp256k1_v0_4_1_gej_double_var(&tmpj, &tmpj, NULL);
rustsecp256k1_v0_4_1_ge_set_gej(&correction, &tmpj);
rustsecp256k1_v0_4_1_ge_to_storage(&correction_1_stor, a);
if (size > 128) {
rustsecp256k1_v0_4_1_ge_to_storage(&correction_lam_stor, a);
}
rustsecp256k1_v0_4_1_ge_to_storage(&a2_stor, &correction);
/* For odd numbers this is 2a (so replace it), for even ones a (so no-op) */
rustsecp256k1_v0_4_1_ge_storage_cmov(&correction_1_stor, &a2_stor, skew_1 == 2);
if (size > 128) {
rustsecp256k1_v0_4_1_ge_storage_cmov(&correction_lam_stor, &a2_stor, skew_lam == 2);
}
/* Apply the correction */
rustsecp256k1_v0_4_1_ge_from_storage(&correction, &correction_1_stor);
rustsecp256k1_v0_4_1_ge_neg(&correction, &correction);
rustsecp256k1_v0_4_1_gej_add_ge(r, r, &correction);
if (size > 128) {
rustsecp256k1_v0_4_1_ge_from_storage(&correction, &correction_lam_stor);
rustsecp256k1_v0_4_1_ge_neg(&correction, &correction);
rustsecp256k1_v0_4_1_ge_mul_lambda(&correction, &correction);
rustsecp256k1_v0_4_1_gej_add_ge(r, r, &correction);
}
}
}
#endif /* SECP256K1_ECMULT_CONST_IMPL_H */
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