Fix timing side-channel in ECDSA signature computation

There is a timing signal of around 300 nanoseconds when the top word of
the inverted ECDSA nonce value is zero. This can happen with significant
probability only for some of the supported elliptic curves. In particular
the NIST P-521 curve is affected. To be able to measure this leak, the
attacker process must either be located in the same physical computer or
must have a very fast network connection with low latency.

Attacks on ECDSA nonce are also known as Minerva attack.

Fixes CVE-2024-13176

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/26429)

(cherry picked from commit 63c40a66c5dc287485705d06122d3a6e74a6a203)

diff --git a/crypto/bn/bn_exp.c b/crypto/bn/bn_exp.c
index 598a592ca13972b6415f2dc2855dba1592d118e3..d84c7de18a6b608acc5a69127d7a133ad4e00a91 100644 (file)
--- a/crypto/bn/bn_exp.c
+++ b/crypto/bn/bn_exp.c
@@ -606,7 +606,7 @@ static int MOD_EXP_CTIME_COPY_FROM_PREBUF(BIGNUM *b, int top,
  * out by Colin Percival,
  * http://www.daemonology.net/hyperthreading-considered-harmful/)
  */
-int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
+int bn_mod_exp_mont_fixed_top(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
                               const BIGNUM *m, BN_CTX *ctx,
                               BN_MONT_CTX *in_mont)
 {
@@ -623,10 +623,6 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
     unsigned int t4 = 0;
 #endif
 
-    bn_check_top(a);
-    bn_check_top(p);
-    bn_check_top(m);
-
     if (!BN_is_odd(m)) {
         ERR_raise(ERR_LIB_BN, BN_R_CALLED_WITH_EVEN_MODULUS);
         return 0;
@@ -1146,7 +1142,7 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
             goto err;
     } else
 #endif
-    if (!BN_from_montgomery(rr, &tmp, mont, ctx))
+    if (!bn_from_mont_fixed_top(rr, &tmp, mont, ctx))
         goto err;
     ret = 1;
  err:
@@ -1160,6 +1156,19 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
     return ret;
 }
 
+int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
+                              const BIGNUM *m, BN_CTX *ctx,
+                              BN_MONT_CTX *in_mont)
+{
+    bn_check_top(a);
+    bn_check_top(p);
+    bn_check_top(m);
+    if (!bn_mod_exp_mont_fixed_top(rr, a, p, m, ctx, in_mont))
+        return 0;
+    bn_correct_top(rr);
+    return 1;
+}
+
 int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
                          const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *in_mont)
 {

diff --git a/crypto/ec/ec_lib.c b/crypto/ec/ec_lib.c
index b1696d93bd6dd0dab0d613e41fe5e1c78fbbd665..1f0bf1ec795fa08582e08f068a7595c4be4be5c8 100644 (file)
--- a/crypto/ec/ec_lib.c
+++ b/crypto/ec/ec_lib.c
@@ -20,6 +20,7 @@
 #include <openssl/err.h>
 #include <openssl/opensslv.h>
 #include "crypto/ec.h"
+#include "crypto/bn.h"
 #include "internal/nelem.h"
 #include "ec_local.h"
 
@@ -1262,10 +1263,10 @@ static int ec_field_inverse_mod_ord(const EC_GROUP *group, BIGNUM *r,
     if (!BN_sub(e, group->order, e))
         goto err;
     /*-
-     * Exponent e is public.
-     * No need for scatter-gather or BN_FLG_CONSTTIME.
+     * Although the exponent is public we want the result to be
+     * fixed top.
      */
-    if (!BN_mod_exp_mont(r, x, e, group->order, ctx, group->mont_data))
+    if (!bn_mod_exp_mont_fixed_top(r, x, e, group->order, ctx, group->mont_data))
         goto err;
 
     ret = 1;

diff --git a/include/crypto/bn.h b/include/crypto/bn.h
index c5f328156d3a9c06dcfd423e3a5ec3ea738d2d7b..59a629b9f62882a9429ccdeef31455cc881491d8 100644 (file)
--- a/include/crypto/bn.h
+++ b/include/crypto/bn.h
@@ -73,6 +73,9 @@ int bn_set_words(BIGNUM *a, const BN_ULONG *words, int num_words);
  */
 int bn_mul_mont_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
                           BN_MONT_CTX *mont, BN_CTX *ctx);
+int bn_mod_exp_mont_fixed_top(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
+                              const BIGNUM *m, BN_CTX *ctx,
+                              BN_MONT_CTX *in_mont);
 int bn_to_mont_fixed_top(BIGNUM *r, const BIGNUM *a, BN_MONT_CTX *mont,
                          BN_CTX *ctx);
 int bn_from_mont_fixed_top(BIGNUM *r, const BIGNUM *a, BN_MONT_CTX *mont,