bn: avoid ossl_bn_new(NULL)

Currently, calling ossl_bn_new() with a NULL argument allocates a new
OpenSSL::BN instance representing 0. This behavior is confusing. Raise
an exception if this is attempted, instead.

Author: rhenium

ext/openssl/ossl_bn.c

@@ -61,10 +61,9 @@ ossl_bn_new(const BIGNUM *bn)
     VALUE obj;
 
     obj = NewBN(cBN);
-    newbn = bn ? BN_dup(bn) : BN_new();
-    if (!newbn) {
-	ossl_raise(eBNError, NULL);
-    }
+    newbn = BN_dup(bn);
+    if (!newbn)
+        ossl_raise(eBNError, "BN_dup");
     SetBN(obj, newbn);
 
     return obj;

ext/openssl/ossl_pkey_ec.c

@@ -805,11 +805,10 @@ static VALUE ossl_ec_group_get_order(VALUE self)
 {
     VALUE bn_obj;
     BIGNUM *bn;
-    EC_GROUP *group = NULL;
+    EC_GROUP *group;
 
     GetECGroup(self, group);
-
-    bn_obj = ossl_bn_new(NULL);
+    bn_obj = ossl_bn_new(BN_value_one());
     bn = GetBNPtr(bn_obj);
 
     if (EC_GROUP_get_order(group, bn, ossl_bn_ctx) != 1)
@@ -830,11 +829,10 @@ static VALUE ossl_ec_group_get_cofactor(VALUE self)
 {
     VALUE bn_obj;
     BIGNUM *bn;
-    EC_GROUP *group = NULL;
+    EC_GROUP *group;
 
     GetECGroup(self, group);
-
-    bn_obj = ossl_bn_new(NULL);
+    bn_obj = ossl_bn_new(BN_value_one());
     bn = GetBNPtr(bn_obj);
 
     if (EC_GROUP_get_cofactor(group, bn, ossl_bn_ctx) != 1)