How to create a secp256k1 key pair with OpenSSL? - c++

Question

Since the EC_KEY functions are marked deprecated in OpenSSL 3 I tried to do it with EVP_PKEY functions.
Here is my OpenSSL 1.1 code:
void MainWindowOpenSsl::createKeyPair()
{
EC_KEY *key_pair_obj = nullptr;;
BIGNUM *priv_key;
EC_POINT *pub_key;
EC_GROUP *secp256k1_group;
char *priv_key_char;
char *pub_key_char;
char *pub_key_char_comp;
int ret_error;
// Generate secp256k1 key pair
key_pair_obj = EC_KEY_new_by_curve_name(NID_secp256k1);
ret_error = EC_KEY_generate_key(key_pair_obj);
// Get private key
priv_key = (BIGNUM *)EC_KEY_get0_private_key(key_pair_obj);
priv_key_char = BN_bn2hex(priv_key);
// Get public key
pub_key = (EC_POINT *)EC_KEY_get0_public_key(key_pair_obj);
secp256k1_group = EC_GROUP_new_by_curve_name(NID_secp256k1);
// Uncompressed
pub_key_char = EC_POINT_point2hex(secp256k1_group, pub_key, POINT_CONVERSION_UNCOMPRESSED, nullptr);
// Compressed
pub_key_char_comp = EC_POINT_point2hex(secp256k1_group, pub_key, POINT_CONVERSION_COMPRESSED, nullptr);
EC_GROUP_free(secp256k1_group);
qDebug().noquote() << "\n";
qDebug().noquote() << Q_FUNC_INFO;
printf("Private key : %s\n\n", priv_key_char);
printf("Public key ompressed : %s\n\n", pub_key_char);
printf("Public key uncompressed: %s\n", pub_key_char_comp);
printf("Error code : %d\n", ret_error);
}
With the EVP_PKEY functions I managed to get a private key, but I couldn't figure out how to extract the public key from it. That is how I started:
void MainWindowOpenSsl::createKeyPairNew()
{
char *priv_key_char;
// Private Key
EVP_PKEY *evp_pkey = EVP_EC_gen("secp256k1");
BIGNUM *bignum_priv_key = nullptr;
EVP_PKEY_get_bn_param(evp_pkey, OSSL_PKEY_PARAM_PRIV_KEY, &bignum_priv_key);
priv_key_char = BN_bn2hex(bignum_priv_key);
printf("Private key : %s\n\n", priv_key_char);
// Extract the compressed and uncompressed public key from the private key
}
but then I got stuck.
Thank you very much for your help.