OpenSSL::X509::Certificate#sign

static VALUE
ossl_x509_sign(VALUE self, VALUE key, VALUE digest)
{
    X509 *x509;
    EVP_PKEY *pkey;
    const EVP_MD *md;

    pkey = GetPrivPKeyPtr(key); /* NO NEED TO DUP */
    if (NIL_P(digest)) {
        md = NULL; /* needed for some key types, e.g. Ed25519 */
    } else {
        md = ossl_evp_get_digestbyname(digest);
    }
    GetX509(self, x509);
    if (!X509_sign(x509, pkey, md)) {
        ossl_raise(eX509CertError, NULL);
    }

    return self;
}