totp-code

  private static long getCode(byte[] secret, long timeIndex) 
	          throws NoSuchAlgorithmException, InvalidKeyException {
	    SecretKeySpec signKey = new SecretKeySpec(secret, "HmacSHA1");
	    ByteBuffer buffer = ByteBuffer.allocate(8);
	    buffer.putLong(timeIndex);
	    byte[] timeBytes = buffer.array();
	    Mac mac = Mac.getInstance("HmacSHA1");
	    mac.init(signKey);
	    byte[] hash = mac.doFinal(timeBytes);
	    int offset = hash[19] & 0xf;
	    long truncatedHash = hash[offset] & 0x7f;
	    for (int i = 1; i < 4; i++) {
	        truncatedHash <<= 8;
	        truncatedHash |= hash[offset + i] & 0xff;
	    }
	    return (truncatedHash %= 1000000);
	}