public function withIssuerCertificate(Certificate $cert): self
    {
        $obj = clone $this;
        // set issuer DN from cert's subject
        $obj->_issuer = $cert->tbsCertificate()->subject();
        // add authority key identifier extension
        $key_id = $cert->tbsCertificate()
            ->subjectPublicKeyInfo()
            ->keyIdentifier();
        $obj->_extensions = $obj->_extensions->withExtensions(
            new AuthorityKeyIdentifierExtension(false, $key_id));
        return $obj;
    }