EC2M Lopez-Dahab ladder: use it also for ECDSA verify

By default `ec_scalar_mul_ladder` (which uses the Lopez-Dahab ladder
implementation) is used only for (k * Generator) or (k * VariablePoint).
ECDSA verification uses (a * Generator + b * VariablePoint): this commit
forces the use of `ec_scalar_mul_ladder` also for the ECDSA verification
path, while using the default wNAF implementation for any other case.

With this commit `ec_scalar_mul_ladder` loses the static attribute, and
is added to ec_lcl.h so EC_METHODs can directly use it.

While working on a new custom EC_POINTs_mul implementation, I realized
that many checks (e.g. all the points being compatible with the given
EC_GROUP, creating a temporary BN_CTX if `ctx == NULL`, check for the
corner case `scalar == NULL && num == 0`) were duplicated again and
again in every single implementation (and actually some
implementations lacked some of the tests).
I thought that it makes way more sense for those checks that are
independent from the actual implementation and should always be done, to
be moved in the EC_POINTs_mul wrapper: so this commit also includes
these changes.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6690)

1 files changed, 30 insertions, 3 deletions

diff --git a/crypto/ec/ec_lib.c b/crypto/ec/ec_lib.c
index 6a2d1b5800..457cd3593c 100644
--- a/crypto/ec/ec_lib.c
+++ b/crypto/ec/ec_lib.c
@@ -919,11 +919,38 @@ int EC_POINTs_mul(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
                   size_t num, const EC_POINT *points[],
                   const BIGNUM *scalars[], BN_CTX *ctx)
 {
-    if (group->meth->mul == 0)
+    int ret = 0;
+    size_t i = 0;
+    BN_CTX *new_ctx = NULL;
+
+    if ((scalar == NULL) && (num == 0)) {
+        return EC_POINT_set_to_infinity(group, r);
+    }
+
+    if (!ec_point_is_compat(r, group)) {
+        ECerr(EC_F_EC_POINTS_MUL, EC_R_INCOMPATIBLE_OBJECTS);
+        return 0;
+    }
+    for (i = 0; i < num; i++) {
+        if (!ec_point_is_compat(points[i], group)) {
+            ECerr(EC_F_EC_POINTS_MUL, EC_R_INCOMPATIBLE_OBJECTS);
+            return 0;
+        }
+    }
+
+    if (ctx == NULL && (ctx = new_ctx = BN_CTX_secure_new()) == NULL) {
+        ECerr(EC_F_EC_POINTS_MUL, ERR_R_INTERNAL_ERROR);
+        return 0;
+    }
+
+    if (group->meth->mul != NULL)
+        ret = group->meth->mul(group, r, scalar, num, points, scalars, ctx);
+    else
         /* use default */
-        return ec_wNAF_mul(group, r, scalar, num, points, scalars, ctx);
+        ret = ec_wNAF_mul(group, r, scalar, num, points, scalars, ctx);
 
-    return group->meth->mul(group, r, scalar, num, points, scalars, ctx);
+    BN_CTX_free(new_ctx);
+    return ret;
 }
 
 int EC_POINT_mul(const EC_GROUP *group, EC_POINT *r, const BIGNUM *g_scalar,