public function verifyClientNonce($cnonce)
    {
        $cnonce_cache_key = 'cnonce__'.$cnonce;

        if (empty($this->cache)) {
            return;
        }

        $cnonce_data = $this->cache->get($cnonce_cache_key);
        if ($cnonce_data == 'USED') {
            throw new NonceException('That nonce has already been used');
        }

        $this->cache->add($cnonce_cache_key, 'USED');
    }