Add blinding to an ECDSA signature

Keegan Ryan (NCC Group) has demonstrated a side channel attack on an
ECDSA signature operation. During signing the signer calculates:

s:= k^-1 * (m + r * priv_key) mod order

The addition operation above provides a sufficient signal for a
flush+reload attack to derive the private key given sufficient signature
operations.

As a mitigation (based on a suggestion from Keegan) we add blinding to
the operation so that:

s := k^-1 * blind^-1 (blind * m + blind * r * priv_key) mod order

Since this attack is a localhost side channel only no CVE is assigned.

Reviewed-by: Rich Salz <rsalz@openssl.org>

2 files changed, 75 insertions, 16 deletions

diff --git a/crypto/ecdsa/ecdsatest.c b/crypto/ecdsa/ecdsatest.c
index 0f301f86d9..a130fc9117 100644
--- a/crypto/ecdsa/ecdsatest.c
+++ b/crypto/ecdsa/ecdsatest.c
@@ -137,7 +137,7 @@ int restore_rand(void)
         return 1;
 }
 
-static int fbytes_counter = 0;
+static int fbytes_counter = 0, use_fake = 0;
 static const char *numbers[8] = {
     "651056770906015076056810763456358567190100156695615665659",
     "6140507067065001063065065565667405560006161556565665656654",
@@ -158,6 +158,11 @@ int fbytes(unsigned char *buf, int num)
     int ret;
     BIGNUM *tmp = NULL;
 
+    if (use_fake == 0)
+        return old_rand->bytes(buf, num);
+
+    use_fake = 0;
+
     if (fbytes_counter >= 8)
         return 0;
     tmp = BN_new();
@@ -199,11 +204,13 @@ int x9_62_test_internal(BIO *out, int nid, const char *r_in, const char *s_in)
     /* create the key */
     if ((key = EC_KEY_new_by_curve_name(nid)) == NULL)
         goto x962_int_err;
+    use_fake = 1;
     if (!EC_KEY_generate_key(key))
         goto x962_int_err;
     BIO_printf(out, ".");
     (void)BIO_flush(out);
     /* create the signature */
+    use_fake = 1;
     signature = ECDSA_do_sign(digest, 20, key);
     if (signature == NULL)
         goto x962_int_err;
diff --git a/crypto/ecdsa/ecs_ossl.c b/crypto/ecdsa/ecs_ossl.c
index 16d4f59b9b..1d37551803 100644
--- a/crypto/ecdsa/ecs_ossl.c
+++ b/crypto/ecdsa/ecs_ossl.c
@@ -252,6 +252,7 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len,
 {
     int ok = 0, i;
     BIGNUM *kinv = NULL, *s, *m = NULL, *tmp = NULL, *order = NULL;
+    BIGNUM *blind = NULL, *blindm = NULL;
     const BIGNUM *ckinv;
     BN_CTX *ctx = NULL;
     const EC_GROUP *group;
@@ -269,14 +270,25 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len,
     }
 
     ret = ECDSA_SIG_new();
-    if (!ret) {
+    if (ret == NULL) {
         ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_MALLOC_FAILURE);
         return NULL;
     }
     s = ret->s;
 
-    if ((ctx = BN_CTX_new()) == NULL || (order = BN_new()) == NULL ||
-        (tmp = BN_new()) == NULL || (m = BN_new()) == NULL) {
+    ctx = BN_CTX_new();
+    if (ctx == NULL) {
+        ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_MALLOC_FAILURE);
+        goto err;
+    }
+
+    BN_CTX_start(ctx);
+    order = BN_CTX_get(ctx);
+    tmp = BN_CTX_get(ctx);
+    m = BN_CTX_get(ctx);
+    blind = BN_CTX_get(ctx);
+    blindm = BN_CTX_get(ctx);
+    if (blindm == NULL) {
         ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_MALLOC_FAILURE);
         goto err;
     }
@@ -315,26 +327,70 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len,
             }
         }
 
-        if (!BN_mod_mul(tmp, priv_key, ret->r, order, ctx)) {
+        /*
+         * The normal signature calculation is:
+         *
+         *   s := k^-1 * (m + r * priv_key) mod order
+         *
+         * We will blind this to protect against side channel attacks
+         *
+         *   s := k^-1 * blind^-1 * (blind * m + blind * r * priv_key) mod order
+         */
+
+        /* Generate a blinding value */
+        do {
+            if (!BN_rand(blind, BN_num_bits(order) - 1, -1, 0))
+                goto err;
+        } while (BN_is_zero(blind));
+        BN_set_flags(blind, BN_FLG_CONSTTIME);
+        BN_set_flags(blindm, BN_FLG_CONSTTIME);
+        BN_set_flags(tmp, BN_FLG_CONSTTIME);
+
+        /* tmp := blind * priv_key * r mod order */
+        if (!BN_mod_mul(tmp, blind, priv_key, order, ctx)) {
+            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
+            goto err;
+        }
+        if (!BN_mod_mul(tmp, tmp, ret->r, order, ctx)) {
+            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
+            goto err;
+        }
+
+        /* blindm := blind * m mod order */
+        if (!BN_mod_mul(blindm, blind, m, order, ctx)) {
+            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
+            goto err;
+        }
+
+        /* s : = (blind * priv_key * r) + (blind * m) mod order */
+        if (!BN_mod_add_quick(s, tmp, blindm, order)) {
+            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
+            goto err;
+        }
+
+        /* s:= s * blind^-1 mod order */
+        if (BN_mod_inverse(blind, blind, order, ctx) == NULL) {
             ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
             goto err;
         }
-        if (!BN_mod_add_quick(s, tmp, m, order)) {
+        if (!BN_mod_mul(s, s, blind, order, ctx)) {
             ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
             goto err;
         }
+
+        /* s := s * k^-1 mod order */
         if (!BN_mod_mul(s, s, ckinv, order, ctx)) {
             ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
             goto err;
         }
+
         if (BN_is_zero(s)) {
             /*
              * if kinv and r have been supplied by the caller don't to
              * generate new kinv and r values
              */
             if (in_kinv != NULL && in_r != NULL) {
-                ECDSAerr(ECDSA_F_ECDSA_DO_SIGN,
-                         ECDSA_R_NEED_NEW_SETUP_VALUES);
+                ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ECDSA_R_NEED_NEW_SETUP_VALUES);
                 goto err;
             }
         } else
@@ -349,15 +405,11 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len,
         ECDSA_SIG_free(ret);
         ret = NULL;
     }
-    if (ctx)
+    if (ctx != NULL) {
+        BN_CTX_end(ctx);
         BN_CTX_free(ctx);
-    if (m)
-        BN_clear_free(m);
-    if (tmp)
-        BN_clear_free(tmp);
-    if (order)
-        BN_free(order);
-    if (kinv)
+    }
+    if (kinv != NULL)
         BN_clear_free(kinv);
     return ret;
 }