Always use Fermat's Little Theorem in ecdsa_sign_setup.
The case where ec_group_get_mont_data is NULL is only for arbitrary groups
which we now require to be prime order. BN_mod_exp_mont is fine with a NULL
BN_MONT_CTX. It will just compute it. Saves a bit of special-casing.
Also don't mark p-2 as BN_FLG_CONSTTIME as the exponent is public anyway.
(cherry picked from commit 8cf79af7d1497c07bd684764b96c9659e7b32ae1)
Bug: 33752052
Change-Id: Iedaf2f40028ef703078262ae5e971cc715d49866
(cherry picked from commit 5e7ef724aead3c33184f34ac7c684e9c2a859b87)
diff --git a/src/crypto/ec/ec.c b/src/crypto/ec/ec.c
index 827cc57..a7097f2 100644
--- a/src/crypto/ec/ec.c
+++ b/src/crypto/ec/ec.c
@@ -379,6 +379,12 @@
return 0;
}
+ /* Require a cofactor of one for custom curves, which implies prime order. */
+ if (!BN_is_one(cofactor)) {
+ OPENSSL_PUT_ERROR(EC, EC_R_WRONG_CURVE_PARAMETERS);
+ return 0;
+ }
+
if (group->generator == NULL) {
group->generator = EC_POINT_new(group);
if (group->generator == NULL) {
diff --git a/src/crypto/ecdsa/ecdsa.c b/src/crypto/ecdsa/ecdsa.c
index 16760ed..2a655b6 100644
--- a/src/crypto/ecdsa/ecdsa.c
+++ b/src/crypto/ecdsa/ecdsa.c
@@ -234,7 +234,7 @@
BIGNUM **rp, const uint8_t *digest,
size_t digest_len) {
BN_CTX *ctx = NULL;
- BIGNUM *k = NULL, *r = NULL, *X = NULL;
+ BIGNUM *k = NULL, *r = NULL, *tmp = NULL;
EC_POINT *tmp_point = NULL;
const EC_GROUP *group;
int ret = 0;
@@ -255,8 +255,8 @@
k = BN_new(); /* this value is later returned in *kinvp */
r = BN_new(); /* this value is later returned in *rp */
- X = BN_new();
- if (k == NULL || r == NULL || X == NULL) {
+ tmp = BN_new();
+ if (k == NULL || r == NULL || tmp == NULL) {
OPENSSL_PUT_ERROR(ECDSA, ERR_R_MALLOC_FAILURE);
goto err;
}
@@ -305,33 +305,25 @@
OPENSSL_PUT_ERROR(ECDSA, ERR_R_EC_LIB);
goto err;
}
- if (!EC_POINT_get_affine_coordinates_GFp(group, tmp_point, X, NULL, ctx)) {
+ if (!EC_POINT_get_affine_coordinates_GFp(group, tmp_point, tmp, NULL,
+ ctx)) {
OPENSSL_PUT_ERROR(ECDSA, ERR_R_EC_LIB);
goto err;
}
- if (!BN_nnmod(r, X, order, ctx)) {
+ if (!BN_nnmod(r, tmp, order, ctx)) {
OPENSSL_PUT_ERROR(ECDSA, ERR_R_BN_LIB);
goto err;
}
} while (BN_is_zero(r));
- /* compute the inverse of k */
- if (ec_group_get_mont_data(group) != NULL) {
- /* We want inverse in constant time, therefore we use that the order must
- * be prime and thus we can use Fermat's Little Theorem. */
- if (!BN_set_word(X, 2) ||
- !BN_sub(X, order, X)) {
- OPENSSL_PUT_ERROR(ECDSA, ERR_R_BN_LIB);
- goto err;
- }
- BN_set_flags(X, BN_FLG_CONSTTIME);
- if (!BN_mod_exp_mont_consttime(k, k, X, order, ctx,
- ec_group_get_mont_data(group))) {
- OPENSSL_PUT_ERROR(ECDSA, ERR_R_BN_LIB);
- goto err;
- }
- } else if (!BN_mod_inverse(k, k, order, ctx)) {
+ /* Compute the inverse of k. The order is a prime, so use Fermat's Little
+ * Theorem. */
+ if (!BN_set_word(tmp, 2) ||
+ !BN_sub(tmp, order, tmp) ||
+ /* Note |ec_group_get_mont_data| may return NULL but |BN_mod_exp_mont|
+ * allows it to be. */
+ !BN_mod_exp_mont(k, k, tmp, order, ctx, ec_group_get_mont_data(group))) {
OPENSSL_PUT_ERROR(ECDSA, ERR_R_BN_LIB);
goto err;
}
@@ -353,7 +345,7 @@
BN_CTX_free(ctx);
}
EC_POINT_free(tmp_point);
- BN_clear_free(X);
+ BN_clear_free(tmp);
return ret;
}