Fix for CVE-2014-0076 backported to 0.9.8 branch

Fix for the attack described in the paper "Recovering OpenSSL
ECDSA Nonces Using the FLUSH+RELOAD Cache Side-channel Attack"
by Yuval Yarom and Naomi Benger. Details can be obtained from:
http://eprint.iacr.org/2014/140

Thanks to Yuval Yarom and Naomi Benger for discovering this
flaw and to Yuval Yarom for supplying a fix.

Thanks for mancha for backporting the fix to OpenSSL 0.9.8 branch.

diff --git a/CHANGES b/CHANGES
index 6a305e14ef3ad6a07032cff5124f3d7ba5b6dfa9..27c65984c18b0a35bdbdea1b9d2877c44ca281e1 100644 (file)
--- a/CHANGES
+++ b/CHANGES
@@ -4,6 +4,17 @@
 
  Changes between 0.9.8y and 0.9.8za [xx XXX xxxx]
 
+  *) Fix for the attack described in the paper "Recovering OpenSSL
+     ECDSA Nonces Using the FLUSH+RELOAD Cache Side-channel Attack"
+     by Yuval Yarom and Naomi Benger. Details can be obtained from:
+     http://eprint.iacr.org/2014/140
+
+     Thanks to Yuval Yarom and Naomi Benger for discovering this
+     flaw and to Yuval Yarom for supplying a fix (CVE-2014-0076)
+     [Yuval Yarom and Naomi Benger]
+
+     Thanks to mancha for backporting the fix to the 0.9.8 branch.
+
   *) Fix handling of warning-level alerts in SSL23 client mode so they
      don't cause client-side termination (eg. on SNI unrecognized_name
      warnings). Add client and server support for six additional alerts

diff --git a/crypto/bn/bn.h b/crypto/bn/bn.h
index f1719a5877f754d13682878c859ece1a72e70a6c..688a4e7e86fb37268a2ff84e9d974fa508564c23 100644 (file)
--- a/crypto/bn/bn.h
+++ b/crypto/bn/bn.h
@@ -511,6 +511,8 @@ BIGNUM *BN_mod_inverse(BIGNUM *ret,
 BIGNUM *BN_mod_sqrt(BIGNUM *ret,
        const BIGNUM *a, const BIGNUM *n,BN_CTX *ctx);
 
+void   BN_consttime_swap(BN_ULONG swap, BIGNUM *a, BIGNUM *b, int nwords);
+
 /* Deprecated versions */
 #ifndef OPENSSL_NO_DEPRECATED
 BIGNUM *BN_generate_prime(BIGNUM *ret,int bits,int safe,
@@ -740,11 +742,20 @@ int RAND_pseudo_bytes(unsigned char *buf,int num);
 
 #define bn_fix_top(a)          bn_check_top(a)
 
+#define bn_check_size(bn, bits) bn_wcheck_size(bn, ((bits+BN_BITS2-1))/BN_BITS2)
+#define bn_wcheck_size(bn, words) \
+       do { \
+               const BIGNUM *_bnum2 = (bn); \
+               assert(words <= (_bnum2)->dmax && words >= (_bnum2)->top); \
+       } while(0)
+
 #else /* !BN_DEBUG */
 
 #define bn_pollute(a)
 #define bn_check_top(a)
 #define bn_fix_top(a)          bn_correct_top(a)
+#define bn_check_size(bn, bits)
+#define bn_wcheck_size(bn, words)
 
 #endif
 

diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c
index 32a8fbaf51ee29e1e93003497da1e08bad720180..b66f50752b6c5bd5e45d6ca2d435dfe46434d816 100644 (file)
--- a/crypto/bn/bn_lib.c
+++ b/crypto/bn/bn_lib.c
@@ -824,3 +824,55 @@ int bn_cmp_part_words(const BN_ULONG *a, const BN_ULONG *b,
                }
        return bn_cmp_words(a,b,cl);
        }
+
+/* 
+ * Constant-time conditional swap of a and b.  
+ * a and b are swapped if condition is not 0.  The code assumes that at most one bit of condition is set.
+ * nwords is the number of words to swap.  The code assumes that at least nwords are allocated in both a and b,
+ * and that no more than nwords are used by either a or b.
+ * a and b cannot be the same number
+ */
+void BN_consttime_swap(BN_ULONG condition, BIGNUM *a, BIGNUM *b, int nwords)
+       {
+       BN_ULONG t;
+       int i;
+
+       bn_wcheck_size(a, nwords);
+       bn_wcheck_size(b, nwords);
+
+       assert(a != b);
+       assert((condition & (condition - 1)) == 0);
+       assert(sizeof(BN_ULONG) >= sizeof(int));
+
+       condition = ((condition - 1) >> (BN_BITS2 - 1)) - 1;
+
+       t = (a->top^b->top) & condition;
+       a->top ^= t;
+       b->top ^= t;
+
+#define BN_CONSTTIME_SWAP(ind) \
+       do { \
+               t = (a->d[ind] ^ b->d[ind]) & condition; \
+               a->d[ind] ^= t; \
+               b->d[ind] ^= t; \
+       } while (0)
+
+
+       switch (nwords) {
+       default:
+               for (i = 10; i < nwords; i++) 
+                       BN_CONSTTIME_SWAP(i);
+               /* Fallthrough */
+       case 10: BN_CONSTTIME_SWAP(9); /* Fallthrough */
+       case 9: BN_CONSTTIME_SWAP(8); /* Fallthrough */
+       case 8: BN_CONSTTIME_SWAP(7); /* Fallthrough */
+       case 7: BN_CONSTTIME_SWAP(6); /* Fallthrough */
+       case 6: BN_CONSTTIME_SWAP(5); /* Fallthrough */
+       case 5: BN_CONSTTIME_SWAP(4); /* Fallthrough */
+       case 4: BN_CONSTTIME_SWAP(3); /* Fallthrough */
+       case 3: BN_CONSTTIME_SWAP(2); /* Fallthrough */
+       case 2: BN_CONSTTIME_SWAP(1); /* Fallthrough */
+       case 1: BN_CONSTTIME_SWAP(0);
+       }
+#undef BN_CONSTTIME_SWAP
+}

diff --git a/crypto/ec/ec2_mult.c b/crypto/ec/ec2_mult.c
index 7dca5e4bcd4364e43b6a3ec6af199f83e3e538b3..6b570a3f91733facb0ac0e115aa6df8160c064ff 100644 (file)
--- a/crypto/ec/ec2_mult.c
+++ b/crypto/ec/ec2_mult.c
@@ -208,9 +208,12 @@ static int gf2m_Mxy(const EC_GROUP *group, const BIGNUM *x, const BIGNUM *y, BIG
 
 /* Computes scalar*point and stores the result in r.
  * point can not equal r.
- * Uses algorithm 2P of
+ * Uses a modified algorithm 2P of
  *     Lopex, J. and Dahab, R.  "Fast multiplication on elliptic curves over 
  *     GF(2^m) without precomputation".
+ *
+ * To protect against side-channel attack the function uses constant time
+ * swap avoiding conditional branches.
  */
 static int ec_GF2m_montgomery_point_multiply(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
        const EC_POINT *point, BN_CTX *ctx)
@@ -244,6 +247,11 @@ static int ec_GF2m_montgomery_point_multiply(const EC_GROUP *group, EC_POINT *r,
        x2 = &r->X;
        z2 = &r->Y;
 
+       bn_wexpand(x1, group->field.top);
+       bn_wexpand(z1, group->field.top);
+       bn_wexpand(x2, group->field.top);
+       bn_wexpand(z2, group->field.top);
+
        if (!BN_GF2m_mod_arr(x1, &point->X, group->poly)) goto err; /* x1 = x */
        if (!BN_one(z1)) goto err; /* z1 = 1 */
        if (!group->meth->field_sqr(group, z2, x1, ctx)) goto err; /* z2 = x1^2 = x^2 */
@@ -266,16 +274,12 @@ static int ec_GF2m_montgomery_point_multiply(const EC_GROUP *group, EC_POINT *r,
                {
                for (; j >= 0; j--)
                        {
-                       if (scalar->d[i] & mask)
-                               {
-                               if (!gf2m_Madd(group, &point->X, x1, z1, x2, z2, ctx)) goto err;
-                               if (!gf2m_Mdouble(group, x2, z2, ctx)) goto err;
-                               }
-                       else
-                               {
-                               if (!gf2m_Madd(group, &point->X, x2, z2, x1, z1, ctx)) goto err;
-                               if (!gf2m_Mdouble(group, x1, z1, ctx)) goto err;
-                               }
+                       BN_consttime_swap(scalar->d[i] & mask, x1, x2, group->field.top);
+                       BN_consttime_swap(scalar->d[i] & mask, z1, z2, group->field.top);
+                       if (!gf2m_Madd(group, &point->X, x2, z2, x1, z1, ctx)) goto err;
+                       if (!gf2m_Mdouble(group, x1, z1, ctx)) goto err;
+                       BN_consttime_swap(scalar->d[i] & mask, x1, x2, group->field.top);
+                       BN_consttime_swap(scalar->d[i] & mask, z1, z2, group->field.top);
                        mask >>= 1;
                        }
                j = BN_BITS2 - 1;