1 files changed, 25 insertions, 3 deletions

diff --git a/drivers/builtin_openssl2/crypto/ecdsa/ecs_ossl.c b/drivers/builtin_openssl2/crypto/ecdsa/ecs_ossl.c
index 4c5fa6b92..dd769609b 100644
--- a/drivers/builtin_openssl2/crypto/ecdsa/ecs_ossl.c
+++ b/drivers/builtin_openssl2/crypto/ecdsa/ecs_ossl.c
@@ -179,10 +179,32 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp,
     while (BN_is_zero(r));
 
     /* compute the inverse of k */
-    if (!BN_mod_inverse(k, k, order, ctx)) {
-        ECDSAerr(ECDSA_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
-        goto err;
+    if (EC_GROUP_get_mont_data(group) != NULL) {
+        /*
+         * We want inverse in constant time, therefore we utilize the fact
+         * order must be prime and use Fermats Little Theorem instead.
+         */
+        if (!BN_set_word(X, 2)) {
+            ECDSAerr(ECDSA_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
+            goto err;
+        }
+        if (!BN_mod_sub(X, order, X, order, ctx)) {
+            ECDSAerr(ECDSA_F_ECDSA_SIGN_SETUP, 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))) {
+            ECDSAerr(ECDSA_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
+            goto err;
+        }
+    } else {
+        if (!BN_mod_inverse(k, k, order, ctx)) {
+            ECDSAerr(ECDSA_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
+            goto err;
+        }
     }
+
     /* clear old values if necessary */
     if (*rp != NULL)
         BN_clear_free(*rp);