X52219/X448 export public key coordinates

[ORNEST Matej - Contractor via openssl-users]

Hi all,

I need to implement support for X52219/X448 for DH key exchange (and 
Ed52219/Ed448 for DSA) elliptic curves in our project. I need to export public 
key for DH exchange in form of DER encoded chunk in form 
tag+X-coordinate+Y-coordinate. Thus I need to get EC_POINT from EVP_PKEY and 
encode it as needed. I understand that those key types differs from EC types in 
way that I need just X coordinate and a flag bit to reconstruct the key, but 
still, how do I get the X coordinate?
My solution works for all other EC types such as SecpX and Brainpool families, 
but not for X52219/X448 keys and I do not completely understand why. 
Specifically when I decode public key previously encoded with i2d_PUBKEY() to 
EVP_PEKY and try to get EC_KEY by calling EVP_PKEY_get0_EC_KEY(), it returns 
NULL and issues an error that it’s not an EC key…

I’m using following code:


EVP_PKEY *key = … // Decode from DER encoded public key



    if(key != nil) {



        EC_KEY *ecKey = EVP_PKEY_get0_EC_KEY(key);

         /// When X52219 or X448 key is passed, ecKey is NULL

        if(ecKey != NULL) {

            const EC_POINT *point = EC_KEY_get0_public_key(ecKey);

            const EC_GROUP *group = EC_KEY_get0_group(ecKey);



            if(point != NULL && group != NULL) {

                BIGNUM *bnX = BN_new();

                BIGNUM *bnY = BN_new();



                if(EC_POINT_get_affine_coordinates(group, point, bnX, bnY, 
NULL)) {

                    char *hexX = BN_bn2hex(bnX);

                    char *hexY = BN_bn2hex(bnY);



                    // Convert to custom data structures

                      …

                }



                BN_free(bnX);

                BN_free(bnY);

            }

        }

    }


Is there any way how to export those key types in desired format?  I’m using 
OpenSSL version 1.1.1q.

Thank you very much for any hint
Matt