EC_KEY_priv2buf
privlen = EC_KEY_priv2buf(x, &priv);
privlen = EC_KEY_priv2buf(a, &priv);
OSSL_DEPRECATEDIN_3_0 size_t EC_KEY_priv2buf(const EC_KEY *eckey,
priv_len = EC_KEY_priv2buf(ec, &priv);