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ecmult_gen: Make code consistent with comment 

This also fixes a typo in the comment.
This commit is contained in:
Tim Ruffing 2021-10-07 19:41:26 +02:00
parent 3b0c2185ea
commit 00d2fa116e
1 changed files with 7 additions and 8 deletions
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15
src/ecmult_gen_impl.h
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@ -40,22 +40,21 @@ static void secp256k1_ecmult_gen_context_clear(secp256k1_ecmult_gen_context *ctx
* precomputed (call it prec(i, n_i)). The formula now becomes sum(prec(i, n_i), i=0 ... PREC_N-1).
* None of the resulting prec group elements have a known scalar, and neither do any of
* the intermediate sums while computing a*G.
* The prec values are stored in secp256k1_ecmult_gen_prec_table[j][i] = (PREC_G)^j * i * G + U_i.
* The prec values are stored in secp256k1_ecmult_gen_prec_table[i][n_i] = n_i * (PREC_G)^i * G + U_i.
*/
static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp256k1_gej *r, const secp256k1_scalar *gn) {
secp256k1_ge add;
secp256k1_ge_storage adds;
secp256k1_scalar gnb;
int bits;
int i, j;
int i, j, n_i;
memset(&adds, 0, sizeof(adds));
*r = ctx->initial;
/* Blind scalar/point multiplication by computing (n-b)G + bG instead of nG. */
secp256k1_scalar_add(&gnb, gn, &ctx->blind);
add.infinity = 0;
for (j = 0; j < ECMULT_GEN_PREC_N; j++) {
bits = secp256k1_scalar_get_bits(&gnb, j * ECMULT_GEN_PREC_B, ECMULT_GEN_PREC_B);
for (i = 0; i < ECMULT_GEN_PREC_G; i++) {
for (i = 0; i < ECMULT_GEN_PREC_N; i++) {
n_i = secp256k1_scalar_get_bits(&gnb, i * ECMULT_GEN_PREC_B, ECMULT_GEN_PREC_B);
for (j = 0; j < ECMULT_GEN_PREC_G; j++) {
/** This uses a conditional move to avoid any secret data in array indexes.
* _Any_ use of secret indexes has been demonstrated to result in timing
* sidechannels, even when the cache-line access patterns are uniform.
@ -66,12 +65,12 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
* by Dag Arne Osvik, Adi Shamir, and Eran Tromer
* (https://www.tau.ac.il/~tromer/papers/cache.pdf)
*/
secp256k1_ge_storage_cmov(&adds, &secp256k1_ecmult_gen_prec_table[j][i], i == bits);
secp256k1_ge_storage_cmov(&adds, &secp256k1_ecmult_gen_prec_table[i][j], j == n_i);
}
secp256k1_ge_from_storage(&add, &adds);
secp256k1_gej_add_ge(r, r, &add);
}
bits = 0;
n_i = 0;
secp256k1_ge_clear(&add);
secp256k1_scalar_clear(&gnb);
}