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-rw-r--r--crypto/ecdsa/ecs_ossl.c24
1 files changed, 19 insertions, 5 deletions
diff --git a/crypto/ecdsa/ecs_ossl.c b/crypto/ecdsa/ecs_ossl.c
index dd769609be4c..16d4f59b9ba9 100644
--- a/crypto/ecdsa/ecs_ossl.c
+++ b/crypto/ecdsa/ecs_ossl.c
@@ -95,6 +95,7 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp,
EC_POINT *tmp_point = NULL;
const EC_GROUP *group;
int ret = 0;
+ int order_bits;
if (eckey == NULL || (group = EC_KEY_get0_group(eckey)) == NULL) {
ECDSAerr(ECDSA_F_ECDSA_SIGN_SETUP, ERR_R_PASSED_NULL_PARAMETER);
@@ -126,6 +127,13 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp,
goto err;
}
+ /* Preallocate space */
+ order_bits = BN_num_bits(order);
+ if (!BN_set_bit(k, order_bits)
+ || !BN_set_bit(r, order_bits)
+ || !BN_set_bit(X, order_bits))
+ goto err;
+
do {
/* get random k */
do
@@ -139,13 +147,19 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp,
/*
* We do not want timing information to leak the length of k, so we
* compute G*k using an equivalent scalar of fixed bit-length.
+ *
+ * We unconditionally perform both of these additions to prevent a
+ * small timing information leakage. We then choose the sum that is
+ * one bit longer than the order. This guarantees the code
+ * path used in the constant time implementations elsewhere.
+ *
+ * TODO: revisit the BN_copy aiming for a memory access agnostic
+ * conditional copy.
*/
-
- if (!BN_add(k, k, order))
+ if (!BN_add(r, k, order)
+ || !BN_add(X, r, order)
+ || !BN_copy(k, BN_num_bits(r) > order_bits ? r : X))
goto err;
- if (BN_num_bits(k) <= BN_num_bits(order))
- if (!BN_add(k, k, order))
- goto err;
/* compute r the x-coordinate of generator * k */
if (!EC_POINT_mul(group, tmp_point, k, NULL, NULL, ctx)) {