/
yagotp.go
61 lines (50 loc) · 1.27 KB
/
yagotp.go
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package core
import (
"crypto/hmac"
"crypto/sha256"
"math"
"time"
)
const timePeriodSec = 30
const otpLength = 8
func toBigEndianArray(value int64) []byte {
var res [8]byte
for i := 7; i >= 0; i-- {
res[i] = byte(value & 0xFF)
value >>= 8
}
return res[:]
}
func fromBigEndianArray(startIndex int, array []byte) int64 {
var value int64 = 0
for i := startIndex; i <= startIndex+7; i++ {
value = value<<8 | int64(array[i])
}
return value
}
func toBase26(value int64, length int) string {
var chars = make([]rune, length, length)
var index = length - 1
for index >= 0 {
chars[index] = 'a' + rune(value%26)
value /= 26
index--
}
return string(chars)
}
func ComputeOtp(pin Pin, secret Secret) string {
keyHashTmp := sha256.Sum256(append(pin.data, secret.data...))
var keyHash []byte
if keyHashTmp[0] == 0 {
keyHash = keyHashTmp[1:]
} else {
keyHash = keyHashTmp[:]
}
var curPeriod = time.Now().UTC().Unix() / timePeriodSec
hash := hmac.New(sha256.New, keyHash[:])
hash.Write(toBigEndianArray(curPeriod))
periodHash := hash.Sum(nil)
periodHashSlice := fromBigEndianArray(int(periodHash[len(periodHash)-1]&15), periodHash) & int64(^uint64(0)>>1)
limitValue := int64(math.Pow(26, otpLength))
return toBase26(periodHashSlice%limitValue, otpLength)
}