public static class GoogleAuthenticator {
        public static String generateSecret() {
            SecureRandom random = new SecureRandom();
            byte bytes[] = new byte[32];
            random.nextBytes(bytes);

            Base32 codec = new Base32();
            byte[] secretKey = Arrays.copyOf(bytes, GoogleAuthentictorSecretSize);
            byte[] bEncodedKey = codec.encode(secretKey);
            return new String(bEncodedKey);
        }

        public static String getQRBarcodeURL(String user, String host, String secret) {
            return "otpauth://totp/"+user+"@"+host+"?secret="+secret;
        }

        private static int verify_code(byte[] key,long t) throws NoSuchAlgorithmException, InvalidKeyException {
            byte[] data = new byte[8];
            long value = t;
            for (int i = 8; i-- > 0; value >>>= 8) {
                data[i] = (byte) value;
            }

            SecretKeySpec signKey = new SecretKeySpec(key, "HmacSHA1");
            Mac mac = Mac.getInstance("HmacSHA1");
            mac.init(signKey);
            byte[] hash = mac.doFinal(data);


            int offset = hash[20 - 1] & 0xF;

            // We're using a long because Java hasn't got unsigned int.
            long truncatedHash = 0;
            for (int i = 0; i < 4; ++i) {
                truncatedHash <<= 8;
                // We are dealing with signed bytes:
                // we just keep the first byte.
                truncatedHash |= (hash[offset + i] & 0xFF);
            }

            truncatedHash &= 0x7FFFFFFF;
            truncatedHash %= 1000000;

            return (int) truncatedHash;
        }

        public static boolean check_code(String secret, long code, long t) throws NoSuchAlgorithmException, InvalidKeyException {
            Base32 codec = new Base32();
            byte[] decodedKey = codec.decode(secret);


            // Window is used to check codes generated in the near past.
            // You can use this value to tune how far you're willing to go.
            int window = 0;
            for (int i = -window; i <= window; ++i) {
                long hash = verify_code(decodedKey, t + i);
                if (hash == code) {
                    return true;
                }
            }
            // The validation code is invalid.
            return false;
        }
    }