Add blinding to a DSA signature

This extends the recently added ECDSA signature blinding to blind DSA too.

This is based on side channel attacks demonstrated by Keegan Ryan (NCC
Group) for ECDSA which are likely to be able to be applied to DSA.

Normally, as in ECDSA, during signing the signer calculates:

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

In ECDSA, 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.

This commit also tweaks the previous ECDSA blinding so that blinding is
only removed at the last possible step.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6524)

diff --git a/CHANGES b/CHANGES
index a3861abac14c66f6e67e8d72ed746cd3d238c1b7..f9562dd712c53ae75b83124e1b1fbb7a18275acb 100644 (file)
--- a/CHANGES
+++ b/CHANGES
@@ -9,8 +9,8 @@
 
  Changes between 1.0.2o and 1.0.2p [xx XXX xxxx]
 
-  *) Add blinding to an ECDSA signature to protect against side channel attacks
-     discovered by Keegan Ryan (NCC Group).
+  *) Add blinding to ECDSA and DSA signatures to protect against side channel
+     attacks discovered by Keegan Ryan (NCC Group).
      [Matt Caswell]
 
   *) When unlocking a pass phrase protected PEM file or PKCS#8 container, we

diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c
index aa10dd12f6f8c5400964686fdf0ab510fa9eae3e..2dcfedeeee7e5c9bf7a0ce63965f03edb6cdf1dd 100644 (file)
--- a/crypto/dsa/dsa_ossl.c
+++ b/crypto/dsa/dsa_ossl.c
@@ -133,17 +133,13 @@ const DSA_METHOD *DSA_OpenSSL(void)
 static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
 {
     BIGNUM *kinv = NULL, *r = NULL, *s = NULL;
-    BIGNUM m;
-    BIGNUM xr;
+    BIGNUM *m, *blind, *blindm, *tmp;
     BN_CTX *ctx = NULL;
     int reason = ERR_R_BN_LIB;
     DSA_SIG *ret = NULL;
     int noredo = 0;
 
-    BN_init(&m);
-    BN_init(&xr);
-
-    if (!dsa->p || !dsa->q || !dsa->g) {
+    if (dsa->p == NULL || dsa->q == NULL || dsa->g == NULL) {
         reason = DSA_R_MISSING_PARAMETERS;
         goto err;
     }
@@ -154,6 +150,13 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
     ctx = BN_CTX_new();
     if (ctx == NULL)
         goto err;
+    m = BN_CTX_get(ctx);
+    blind = BN_CTX_get(ctx);
+    blindm = BN_CTX_get(ctx);
+    tmp = BN_CTX_get(ctx);
+    if (tmp == NULL)
+        goto err;
+
  redo:
     if ((dsa->kinv == NULL) || (dsa->r == NULL)) {
         if (!DSA_sign_setup(dsa, ctx, &kinv, &r))
@@ -173,20 +176,52 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
          * 4.2
          */
         dlen = BN_num_bytes(dsa->q);
-    if (BN_bin2bn(dgst, dlen, &m) == NULL)
+    if (BN_bin2bn(dgst, dlen, m) == NULL)
         goto err;
 
-    /* Compute  s = inv(k) (m + xr) mod q */
-    if (!BN_mod_mul(&xr, dsa->priv_key, r, dsa->q, ctx))
-        goto err;               /* s = xr */
-    if (!BN_add(s, &xr, &m))
-        goto err;               /* s = m + xr */
-    if (BN_cmp(s, dsa->q) > 0)
-        if (!BN_sub(s, s, dsa->q))
+    /*
+     * The normal signature calculation is:
+     *
+     *   s := k^-1 * (m + r * priv_key) mod q
+     *
+     * We will blind this to protect against side channel attacks
+     *
+     *   s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod q
+     */
+
+    /* Generate a blinding value */
+    do {
+        if (!BN_rand(blind, BN_num_bits(dsa->q) - 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 q */
+    if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->q, ctx))
+        goto err;
+    if (!BN_mod_mul(tmp, tmp, r, dsa->q, ctx))
+        goto err;
+
+    /* blindm := blind * m mod q */
+    if (!BN_mod_mul(blindm, blind, m, dsa->q, ctx))
+        goto err;
+
+    /* s : = (blind * priv_key * r) + (blind * m) mod q */
+    if (!BN_mod_add_quick(s, tmp, blindm, dsa->q))
+        goto err;
+
+    /* s := s * k^-1 mod q */
     if (!BN_mod_mul(s, s, kinv, dsa->q, ctx))
         goto err;
 
+    /* s:= s * blind^-1 mod q */
+    if (BN_mod_inverse(blind, blind, dsa->q, ctx) == NULL)
+        goto err;
+    if (!BN_mod_mul(s, s, blind, dsa->q, ctx))
+        goto err;
+
     /*
      * Redo if r or s is zero as required by FIPS 186-3: this is very
      * unlikely.
@@ -210,13 +245,9 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
         BN_free(r);
         BN_free(s);
     }
-    if (ctx != NULL)
-        BN_CTX_free(ctx);
-    BN_clear_free(&m);
-    BN_clear_free(&xr);
-    if (kinv != NULL)           /* dsa->kinv is NULL now if we used it */
-        BN_clear_free(kinv);
-    return (ret);
+    BN_CTX_free(ctx);
+    BN_clear_free(kinv);
+    return ret;
 }
 
 static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,

diff --git a/crypto/ecdsa/ecs_ossl.c b/crypto/ecdsa/ecs_ossl.c
index 1d3755180335a0fd8619e7c10bb317411fc07a7c..6115df74079493ce0bfa507da0b3220641ac6e31 100644 (file)
--- a/crypto/ecdsa/ecs_ossl.c
+++ b/crypto/ecdsa/ecs_ossl.c
@@ -334,7 +334,7 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len,
          *
          * We will blind this to protect against side channel attacks
          *
-         *   s := k^-1 * blind^-1 * (blind * m + blind * r * priv_key) mod order
+         *   s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod order
          */
 
         /* Generate a blinding value */
@@ -368,18 +368,18 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len,
             goto err;
         }
 
-        /* s:= s * blind^-1 mod order */
-        if (BN_mod_inverse(blind, blind, order, ctx) == NULL) {
+        /* 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_mod_mul(s, s, blind, order, ctx)) {
+
+        /* 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;
         }
-
-        /* s := s * k^-1 mod order */
-        if (!BN_mod_mul(s, s, ckinv, order, ctx)) {
+        if (!BN_mod_mul(s, s, blind, order, ctx)) {
             ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);
             goto err;
         }