public function getKey()
    {
        $key = '';

        if ($this->clientCredentials instanceof ClientCredentials) {
            $key .= rawurlencode($this->clientCredentials->getSecret());
        }

        // Keep the ampersand even if both keys are empty.
        $key .= '&';

        if ($this->serverIssuedCredentials instanceof ServerIssuedCredentials) {
            $key .= rawurlencode($this->serverIssuedCredentials->getSecret());
        }

        return $key;
    }