public static function _validateNonce($secret, $nonce)
    {
        if (is_string($nonce) == false) {
            return false;
        }

        $a = explode(':', $nonce);
        if (count($a) != 3) {
            return false;
        }

        $salt     = $a[0];
        $lifetime = intval($a[1]);
        $hash     = $a[2];
        $back     = hash_hmac( 'sha1', $salt.$lifetime,  $secret );

        if ($back != $hash) {
            return false;
        }

        if (time() > $lifetime) {
            return false;
        }

        return true;
    }