crypto/mem_dbg.c: make it indent-friendly.

[openssl.git] / crypto / ec / ec2_mult.c

diff --git a/crypto/ec/ec2_mult.c b/crypto/ec/ec2_mult.c
index e12b9b284a00298cec51623ec81193c2e86499b9..9955948dfd1e6e282630b78d4b3925f73ac3e3e1 100644 (file)
--- a/crypto/ec/ec2_mult.c
+++ b/crypto/ec/ec2_mult.c
@@ -71,8 +71,11 @@
 
 #include "ec_lcl.h"
 
+#ifndef OPENSSL_NO_EC2M
 
-/* Compute the x-coordinate x/z for the point 2*(x/z) in Montgomery projective 
+
+/*-
+ * Compute the x-coordinate x/z for the point 2*(x/z) in Montgomery projective 
  * coordinates.
  * Uses algorithm Mdouble in appendix of 
  *     Lopez, J. and Dahab, R.  "Fast multiplication on elliptic curves over 
@@ -104,7 +107,8 @@ static int gf2m_Mdouble(const EC_GROUP *group, BIGNUM *x, BIGNUM *z, BN_CTX *ctx
        return ret;
        }
 
-/* Compute the x-coordinate x1/z1 for the point (x1/z1)+(x2/x2) in Montgomery 
+/*-
+ * Compute the x-coordinate x1/z1 for the point (x1/z1)+(x2/x2) in Montgomery 
  * projective coordinates.
  * Uses algorithm Madd in appendix of 
  *     Lopez, J. and Dahab, R.  "Fast multiplication on elliptic curves over 
@@ -138,7 +142,8 @@ static int gf2m_Madd(const EC_GROUP *group, const BIGNUM *x, BIGNUM *x1, BIGNUM
        return ret;
        }
 
-/* Compute the x, y affine coordinates from the point (x1, z1) (x2, z2) 
+/*-
+ * Compute the x, y affine coordinates from the point (x1, z1) (x2, z2) 
  * using Montgomery point multiplication algorithm Mxy() in appendix of 
  *     Lopez, J. and Dahab, R.  "Fast multiplication on elliptic curves over 
  *     GF(2^m) without precomputation" (CHES '99, LNCS 1717).
@@ -206,11 +211,16 @@ 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.
+
+/*-
+ * 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)
@@ -244,6 +254,11 @@ 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 */
@@ -268,16 +283,12 @@ 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;
@@ -308,7 +319,8 @@ static int ec_GF2m_montgomery_point_multiply(const EC_GROUP *group, EC_POINT *r,
        }
 
 
-/* Computes the sum
+/*-
+ * Computes the sum
  *     scalar*group->generator + scalars[0]*points[0] + ... + scalars[num-1]*points[num-1]
  * gracefully ignoring NULL scalar values.
  */
@@ -384,3 +396,5 @@ int ec_GF2m_have_precompute_mult(const EC_GROUP *group)
        {
        return ec_wNAF_have_precompute_mult(group);
        }
+
+#endif