getHOTPToken.go

func getHOTPToken(secret string, interval int64) string {

	//Converts secret to base32 Encoding. Base32 encoding desires a 32-character
	//subset of the twenty-six letters A–Z and ten digits 0–9
	key, err := base32.StdEncoding.DecodeString(strings.ToUpper(secret))
	check(err)
	bs := make([]byte, 8)
	binary.BigEndian.PutUint64(bs, uint64(interval))

	//Signing the value using HMAC-SHA1 Algorithm
	hash := hmac.New(sha1.New, key)
	hash.Write(bs)
	h := hash.Sum(nil)

	// We're going to use a subset of the generated hash.
	// Using the last nibble (half-byte) to choose the index to start from.
	// This number is always appropriate as it's maximum decimal 15, the hash will
	// have the maximum index 19 (20 bytes of SHA1) and we need 4 bytes.
	o := (h[19] & 15)

	var header uint32
	//Get 32 bit chunk from hash starting at the o
	r := bytes.NewReader(h[o : o+4])
	err = binary.Read(r, binary.BigEndian, &header)

	check(err)
	//Ignore most significant bits as per RFC 4226.
	//Takes division from one million to generate a remainder less than < 7 digits
	h12 := (int(header) & 0x7fffffff) % 1000000

	//Converts number as a string
	otp := strconv.Itoa(int(h12))

	return prefix0(otp)
}