simple otp module

otp.go

package otp

import (
	"bytes"
	"crypto/hmac"
	"crypto/sha1"
	"encoding/binary"
	"strconv"
	"strings"
	"time"
)

func getHOTPToken(key []byte, interval int64) string {
	bs := make([]byte, 8)
	binary.BigEndian.PutUint64(bs, uint64(interval))
	hash := hmac.New(sha1.New, key)
	hash.Write(bs)
	h := hash.Sum(nil)
	o := (h[19] & 15)

	var header uint32
	r := bytes.NewReader(h[o : o+4])
	binary.Read(r, binary.BigEndian, &header)
	h12 := (int(header) & 0x7fffffff) % 1000000
	otp := strconv.Itoa(int(h12))

	return strings.Repeat("0", 6 - len(otp)) + otp
}

func getTOTPToken(key []byte, sep, delta int64) string {
	interval := time.Now().Unix()/sep + delta
	return getHOTPToken(key, interval)
}