OpenSSL::X509::Certificate#sign

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

    pkey = GetPrivPKeyPtr(key); /* NO NEED TO DUP */
    /* NULL needed for some key types, e.g. Ed25519 */
    md = NIL_P(digest) ? NULL : ossl_evp_md_fetch(digest, &md_holder);
    GetX509(self, x509);
    if (!X509_sign(x509, pkey, md))
        ossl_raise(eX509CertError, "X509_sign");

    return self;
}