Fix OPENSSL_EC_* constants and add EC_GROUP_get_asn1_flag

I got the values flipped around. Also cryptography.io wants
EC_GROUP_get_asn1_flag to check a curve's encoding. We (mostly) only
support named curves, so just return OPENSSL_EC_NAMED_CURVE.

Change-Id: I544e76b7380ecd8dceb1df3db4dd4cf5cb322352
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/46024
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/fipsmodule/ec/ec.c

@@ -1232,6 +1232,10 @@ void ec_set_to_safe_point(const EC_GROUP *group, EC_RAW_POINT *out) {
 
 void EC_GROUP_set_asn1_flag(EC_GROUP *group, int flag) {}
 
+int EC_GROUP_get_asn1_flag(const EC_GROUP *group) {
+  return OPENSSL_EC_NAMED_CURVE;
+}
+
 const EC_METHOD *EC_GROUP_method_of(const EC_GROUP *group) {
   // This function exists purely to give callers a way to call
   // |EC_METHOD_get_field_type|. cryptography.io crashes if |EC_GROUP_method_of|

include/openssl/ec.h

@@ -343,11 +343,14 @@ OPENSSL_EXPORT int EC_GROUP_set_generator(EC_GROUP *group,
 OPENSSL_EXPORT int EC_GROUP_get_order(const EC_GROUP *group, BIGNUM *order,
                                       BN_CTX *ctx);
 
+#define OPENSSL_EC_EXPLICIT_CURVE 0
+#define OPENSSL_EC_NAMED_CURVE 1
+
 // EC_GROUP_set_asn1_flag does nothing.
 OPENSSL_EXPORT void EC_GROUP_set_asn1_flag(EC_GROUP *group, int flag);
 
-#define OPENSSL_EC_NAMED_CURVE 0
-#define OPENSSL_EC_EXPLICIT_CURVE 1
+// EC_GROUP_get_asn1_flag returns |OPENSSL_EC_NAMED_CURVE|.
+OPENSSL_EXPORT int EC_GROUP_get_asn1_flag(const EC_GROUP *group);
 
 typedef struct ec_method_st EC_METHOD;