package main

import (
    "crypto/rand"
    "crypto/hmac"
    "crypto/sha1"
    "encoding/base32"
    "time"
    "fmt"
)

func encodeHMACSHA1(data []byte, secret []byte) []byte {
    h := hmac.New(sha1.New, []byte(secret))
    h.Write(data)
    sha := h.Sum(nil)
    return sha
}

func secret() string {
    secretKey := make([]byte, 5)
    rand.Read(secretKey)

    return base32.StdEncoding.EncodeToString(secretKey)
}

func makeTimestamp() int64 {
    return time.Now().UnixNano() / int64(time.Millisecond)
}

func generate(secretKey string) (string, error) {
    key, err := base32.StdEncoding.DecodeString(secretKey)
    if err != nil {
        return "", err
    }

    l := makeTimestamp() / (1000 * 30)
    return getHash(key, l), nil
}

func getHash(secretKey []byte, l int64) (string) {
    data := make([]byte, 8)
    for i := 7; i >= 0; i-- {
        data[i] = byte(l)
        l = l >> 8
    }

    hash := encodeHMACSHA1(data, secretKey)
    offset := int(hash[20 - 1] & 0xF)

    truncatedHash := 0
    for i := 0; i < 4; i++ {
        truncatedHash <<= 8
        truncatedHash |= int(hash[offset + i] & 0xFF)
    }

    truncatedHash &= 0x7FFFFFFF
    truncatedHash %= 1000000

    return fmt.Sprintf("%06d", truncatedHash)
}

func main() {
    secretKey := secret()
    println(secretKey)

    otp, _ := generate(secretKey)
    println(otp)
}