protected function createClientCredentials(array $clientCredentials)
    {
        $keys = array('identifier', 'secret');

        foreach ($keys as $key) {
            if (!isset($clientCredentials[$key])) {
                throw new \InvalidArgumentException("Missing client credentials key [$key] from options.");
            }
        }

        if (isset($clientCredentials['rsa_private_key']) && isset($clientCredentials['rsa_public_key'])) {
            $_clientCredentials = new RsaClientCredentials();
            $_clientCredentials->setRsaPrivateKey($clientCredentials['rsa_private_key']);
            $_clientCredentials->setRsaPublicKey($clientCredentials['rsa_public_key']);
        } else {
            $_clientCredentials = new ClientCredentials();
        }

        $_clientCredentials->setIdentifier($clientCredentials['identifier']);
        $_clientCredentials->setSecret($clientCredentials['secret']);

        if (isset($clientCredentials['callback_uri'])) {
            $_clientCredentials->setCallbackUri($clientCredentials['callback_uri']);
        }

        return $_clientCredentials;
    }