public static byte[] md5digest(String key) {
        if (key == null) {
            return null;
        }
        MessageDigest md5;
        try {
            md5 = (MessageDigest) md5Digest.clone();
        } catch (CloneNotSupportedException e) {
            throw new RuntimeException("clone of MD5 not supported", e);
        }
        md5.update(key.getBytes());
        return md5.digest();
    }