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package main
import (
"crypto/hmac"
"crypto/sha1"
"encoding/base32"
"fmt"
"os"
"strings"
"time"
)
func toBytes(value int64) []byte {
var result []byte
mask := int64(0xFF)
shifts := [8]uint16{56, 48, 40, 32, 24, 16, 8, 0}
for _, shift := range shifts {
result = append(result, byte((value>>shift)&mask))
}
return result
}
func toUint32(bytes []byte) uint32 {
return (uint32(bytes[0]) << 24) + (uint32(bytes[1]) << 16) +
(uint32(bytes[2]) << 8) + uint32(bytes[3])
}
func oneTimePassword(key []byte, value []byte) uint32 {
// sign the value using HMAC-SHA1
hmacSha1 := hmac.New(sha1.New, key)
hmacSha1.Write(value)
hash := hmacSha1.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.
offset := hash[len(hash)-1] & 0x0F
// get a 32-bit (4-byte) chunk from the hash starting at offset
hashParts := hash[offset : offset+4]
// ignore the most significant bit as per RFC 4226
hashParts[0] = hashParts[0] & 0x7F
number := toUint32(hashParts)
// size to 6 digits
// one million is the first number with 7 digits so the remainder
// of the division will always return < 7 digits
pwd := number % 1000000
return pwd
}
// all []byte in this program are treated as Big Endian
func main() {
if len(os.Args) < 2 {
fmt.Fprintln(os.Stderr, "must specify key to use")
os.Exit(1)
}
input := os.Args[1]
// decode the key from the first argument
inputNoSpaces := strings.Replace(input, " ", "", -1)
inputNoSpacesUpper := strings.ToUpper(inputNoSpaces)
key, err := base32.StdEncoding.DecodeString(inputNoSpacesUpper)
if err != nil {
fmt.Fprintln(os.Stderr, err.Error())
os.Exit(1)
}
// generate a one-time password using the time at 30-second intervals
epochSeconds := time.Now().Unix()
pwd := oneTimePassword(key, toBytes(epochSeconds/30))
secondsRemaining := 30 - (epochSeconds % 30)
fmt.Printf("%06d (%d second(s) remaining)\n", pwd, secondsRemaining)
}
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