pkey/ec: fix OpenSSL::PKey::EC::Group#curve_name for unknown curves

EC_GROUP_get_curve_name() returns NID_undef when OpenSSL does not
recognize the curve and there is no associated OID.

Handle this case explicitly and return nil instead of the string
"UNDEF", which should not be exposed outside the extension.

Author: rhenium

ext/openssl/ossl_pkey_ec.c

@@ -849,25 +849,23 @@ static VALUE ossl_ec_group_get_cofactor(VALUE self)
 
 /*
  * call-seq:
- *   group.curve_name  => String
+ *    group.curve_name -> string or nil
  *
- * Returns the curve name (sn).
+ * Returns the curve name (short name) corresponding to this group, or +nil+
+ * if \OpenSSL does not have an OID associated with the group.
  *
  * See the OpenSSL documentation for EC_GROUP_get_curve_name()
  */
 static VALUE ossl_ec_group_get_curve_name(VALUE self)
 {
-    EC_GROUP *group = NULL;
+    EC_GROUP *group;
     int nid;
 
     GetECGroup(self, group);
-    if (group == NULL)
-        return Qnil;
-
     nid = EC_GROUP_get_curve_name(group);
-
-/* BUG: an nid or asn1 object should be returned, maybe. */
-    return rb_str_new2(OBJ_nid2sn(nid));
+    if (nid == NID_undef)
+        return Qnil;
+    return rb_str_new_cstr(OBJ_nid2sn(nid));
 }
 
 /*

test/openssl/test_pkey_ec.rb

@@ -369,18 +369,26 @@ def test_ec_point
       point2.to_octet_string(:uncompressed)
     assert_equal point2.to_octet_string(:uncompressed),
       point3.to_octet_string(:uncompressed)
+  end
 
+  def test_small_curve
     begin
       group = OpenSSL::PKey::EC::Group.new(:GFp, 17, 2, 2)
       group.point_conversion_form = :uncompressed
       generator = OpenSSL::PKey::EC::Point.new(group, B(%w{ 04 05 01 }))
       group.set_generator(generator, 19, 1)
-      point = OpenSSL::PKey::EC::Point.new(group, B(%w{ 04 06 03 }))
     rescue OpenSSL::PKey::EC::Group::Error
       pend "Patched OpenSSL rejected curve" if /unsupported field/ =~ $!.message
       raise
     end
-
+    assert_equal 17.to_bn.num_bits, group.degree
+    assert_equal B(%w{ 04 05 01 }),
+      group.generator.to_octet_string(:uncompressed)
+    assert_equal 19.to_bn, group.order
+    assert_equal 1.to_bn, group.cofactor
+    assert_nil group.curve_name
+
+    point = OpenSSL::PKey::EC::Point.new(group, B(%w{ 04 06 03 }))
     assert_equal 0x040603.to_bn, point.to_bn
     assert_equal 0x040603.to_bn, point.to_bn(:uncompressed)
     assert_equal 0x0306.to_bn, point.to_bn(:compressed)