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Use Fermat's Little Theorem when converting points to affine.

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Fermat's Little Theorem is already used for the custom curve implementations.
Use it, for the same reasons, for the ec_montgomery-based implementations.

I tested the performance (only) on x86-64 Windows.

Change-Id: Ibf770fd3f2d3e2cfe69f06bc12c81171624ff557
Reviewed-on: https://boringssl-review.googlesource.com/8924
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
This commit is contained in:
Brian Smith 2016-07-25 18:24:21 -10:00 committed by CQ bot account: commit-bot@chromium.org
parent 286fbf2ce0
commit 92d60c2059
1 changed files with 13 additions and 3 deletions
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16
crypto/ec/ec_montgomery.c
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@ -230,9 +230,11 @@ static int ec_GFp_mont_point_get_affine_coordinates(const EC_GROUP *group,
BIGNUM *Z_1 = BN_CTX_get(ctx);
BIGNUM *Z_2 = BN_CTX_get(ctx);
BIGNUM *Z_3 = BN_CTX_get(ctx);
BIGNUM *field_minus_2 = BN_CTX_get(ctx);
if (Z_1 == NULL ||
Z_2 == NULL ||
Z_3 == NULL) {
Z_3 == NULL ||
field_minus_2 == NULL) {
goto err;
}
@ -243,10 +245,18 @@ static int ec_GFp_mont_point_get_affine_coordinates(const EC_GROUP *group,
*
* This is equivalent, but more efficient, because |BN_from_montgomery|
* is more efficient (at least in theory) than |BN_to_montgomery|, since it
* doesn't have to do the multiplication before the reduction. */
* doesn't have to do the multiplication before the reduction.
*
* Use Fermat's Little Theorem with |BN_mod_exp_mont_consttime| instead of
* |BN_mod_inverse| since this inversion may be done as the final step of
* private key operations. Unfortunately, this is suboptimal for ECDSA
* verification. */
if (!BN_from_montgomery(Z_1, &point->Z, group->mont, ctx) ||
!BN_from_montgomery(Z_1, Z_1, group->mont, ctx) ||
!BN_mod_inverse(Z_1, Z_1, &group->field, ctx)) {
!BN_copy(field_minus_2, &group->field) ||
!BN_sub_word(field_minus_2, 2) ||
!BN_mod_exp_mont_consttime(Z_1, Z_1, field_minus_2, &group->field,
ctx, group->mont)) {
goto err;
}