Google Authenticator-TOTP.go

package main

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

func generate_topt(secret string, interval int64, digits int) (string, error) {
	key, err := base32.StdEncoding.DecodeString(secret)
	if err != nil {
		return "", fmt.Errorf("failed to decode secret: %w", err)
	}

	time_step := time.Now().Unix() / interval

	var counter_bytes [8]byte
	binary.BigEndian.PutUint64(counter_bytes[:], uint64(time_step))

	hmac_hash := hmac.New(sha1.New, key)
	if _, err := hmac_hash.Write(counter_bytes[:]); err != nil {
		return "", fmt.Errorf("failed to write HMAC: %w", err)
	}
	hash := hmac_hash.Sum(nil)

	offset := hash[len(hash)-1] & 0x0F
	code := (int(hash[offset])&0x7F)<<24 |
		(int(hash[offset+1])&0xFF)<<16 |
		(int(hash[offset+2])&0xFF)<<8 |
		(int(hash[offset+3]) & 0xFF)
	otp := code % int(math.Pow10(digits))
	return fmt.Sprintf("%0*d", digits, otp), nil
}

func main() {
	// secret key
	secret := "JBSWY3DPEHPK3PXP"
	// Generate a 6-digit TOTP with 30-second intervals
	totp, err := generate_topt(secret, 30, 6)
	if err != nil {
		fmt.Println("Error generating TOTP:", err)
		return
	}

	fmt.Println("Your TOTP code is:", totp)
}