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, 13 insertions, 28 deletions

diff --git a/crypto/ec/ec_mult.c b/crypto/ec/ec_mult.c
index 55cbfa105d..7e1b3650e7 100644
--- a/crypto/ec/ec_mult.c
+++ b/crypto/ec/ec_mult.c
@@ -121,6 +121,9 @@ void EC_ec_pre_comp_free(EC_PRE_COMP *pre)
  * `scalar` cannot be NULL and should be in the range [0,n) otherwise all
  * constant time bets are off (where n is the cardinality of the EC group).
  *
+ * This function expects `group->order` and `group->cardinality` to be well
+ * defined and non-zero: it fails with an error code otherwise.
+ *
  * NB: This says nothing about the constant-timeness of the ladder step
  * implementation (i.e., the default implementation is based on EC_POINT_add and
  * EC_POINT_dbl, which of course are not constant time themselves) or the
@@ -128,9 +131,11 @@ void EC_ec_pre_comp_free(EC_PRE_COMP *pre)
  *
  * The product is stored in `r`.
  *
+ * This is an internal function: callers are in charge of ensuring that the
+ * input parameters `group`, `r`, `scalar` and `ctx` are not NULL.
+ *
  * Returns 1 on success, 0 otherwise.
  */
-static
 int ec_scalar_mul_ladder(const EC_GROUP *group, EC_POINT *r,
                          const BIGNUM *scalar, const EC_POINT *point,
                          BN_CTX *ctx)
@@ -141,15 +146,20 @@ int ec_scalar_mul_ladder(const EC_GROUP *group, EC_POINT *r,
     BIGNUM *k = NULL;
     BIGNUM *lambda = NULL;
     BIGNUM *cardinality = NULL;
-    BN_CTX *new_ctx = NULL;
     int ret = 0;
 
     /* early exit if the input point is the point at infinity */
     if (point != NULL && EC_POINT_is_at_infinity(group, point))
         return EC_POINT_set_to_infinity(group, r);
 
-    if (ctx == NULL && (ctx = new_ctx = BN_CTX_secure_new()) == NULL)
+    if (BN_is_zero(group->order)) {
+        ECerr(EC_F_EC_SCALAR_MUL_LADDER, EC_R_UNKNOWN_ORDER);
         return 0;
+    }
+    if (BN_is_zero(group->cofactor)) {
+        ECerr(EC_F_EC_SCALAR_MUL_LADDER, EC_R_UNKNOWN_COFACTOR);
+        return 0;
+    }
 
     BN_CTX_start(ctx);
 
@@ -370,7 +380,6 @@ int ec_scalar_mul_ladder(const EC_GROUP *group, EC_POINT *r,
     EC_POINT_free(p);
     EC_POINT_free(s);
     BN_CTX_end(ctx);
-    BN_CTX_free(new_ctx);
 
     return ret;
 }
@@ -402,7 +411,6 @@ int ec_wNAF_mul(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
                 size_t num, const EC_POINT *points[], const BIGNUM *scalars[],
                 BN_CTX *ctx)
 {
-    BN_CTX *new_ctx = NULL;
     const EC_POINT *generator = NULL;
     EC_POINT *tmp = NULL;
     size_t totalnum;
@@ -427,15 +435,6 @@ int ec_wNAF_mul(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
                                  * precomputation is not available */
     int ret = 0;
 
-    if (!ec_point_is_compat(r, group)) {
-        ECerr(EC_F_EC_WNAF_MUL, EC_R_INCOMPATIBLE_OBJECTS);
-        return 0;
-    }
-
-    if ((scalar == NULL) && (num == 0)) {
-        return EC_POINT_set_to_infinity(group, r);
-    }
-
     if (!BN_is_zero(group->order) && !BN_is_zero(group->cofactor)) {
         /*-
          * Handle the common cases where the scalar is secret, enforcing a
@@ -465,19 +464,6 @@ int ec_wNAF_mul(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
         }
     }
 
-    for (i = 0; i < num; i++) {
-        if (!ec_point_is_compat(points[i], group)) {
-            ECerr(EC_F_EC_WNAF_MUL, EC_R_INCOMPATIBLE_OBJECTS);
-            return 0;
-        }
-    }
-
-    if (ctx == NULL) {
-        ctx = new_ctx = BN_CTX_new();
-        if (ctx == NULL)
-            goto err;
-    }
-
     if (scalar != NULL) {
         generator = EC_GROUP_get0_generator(group);
         if (generator == NULL) {
@@ -784,7 +770,6 @@ int ec_wNAF_mul(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
     ret = 1;
 
  err:
-    BN_CTX_free(new_ctx);
     EC_POINT_free(tmp);
     OPENSSL_free(wsize);
     OPENSSL_free(wNAF_len);