X52219/X448 export public key coordinates

[Kyle Hamilton]

X25519?

On Mon, Nov 14, 2022, 05:23 ORNEST Matej - Contractor via openssl-users <
openssl-users at openssl.org> wrote:

> 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
>
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