1 files changed, 16 insertions, 11 deletions

diff --git a/src/lib/libcrypto/ec/ec2_mult.c b/src/lib/libcrypto/ec/ec2_mult.c
index 26f4a783fc..1c575dc47a 100644
--- a/src/lib/libcrypto/ec/ec2_mult.c
+++ b/src/lib/libcrypto/ec/ec2_mult.c
@@ -208,11 +208,15 @@ static int gf2m_Mxy(const EC_GROUP *group, const BIGNUM *x, const BIGNUM *y, BIG
         return ret;
         }
 
+
 /* Computes scalar*point and stores the result in r.
  * point can not equal r.
- * Uses algorithm 2P of
+ * Uses a modified algorithm 2P of
  *     Lopez, J. and Dahab, R.  "Fast multiplication on elliptic curves over 
  *     GF(2^m) without precomputation" (CHES '99, LNCS 1717).
+ *
+ * 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)
@@ -246,6 +250,11 @@ 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 */
@@ -270,16 +279,12 @@ static int ec_GF2m_montgomery_point_multiply(const EC_GROUP *group, EC_POINT *r,
                 word = scalar->d[i];
                 while (mask)
                         {
-                        if (word & 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(word & mask, x1, x2, group->field.top);
+                        BN_consttime_swap(word & 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(word & mask, x1, x2, group->field.top);
+                        BN_consttime_swap(word & mask, z1, z2, group->field.top);
                         mask >>= 1;
                         }
                 mask = BN_TBIT;