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/*
Package dgoogauth implements the one-time password algorithms supported by Google Authenticator
This package supports the HMAC-Based One-time Password (HOTP) algorithm
specified in RFC 4226 and the Time-based One-time Password (TOTP) algorithm
specified in RFC 6238.
*/
package dgoogauth
import (
"crypto/hmac"
"crypto/sha1"
"encoding/base32"
"encoding/binary"
"errors"
"net/url"
"sort"
"strconv"
"time"
)
// Much of this code assumes int == int64, which probably is not the case.
// ComputeCode computes the response code for a 64-bit challenge 'value' using the secret 'secret'.
// To avoid breaking compatibility with the previous API, it returns an invalid code (-1) when an error occurs,
// but does not silently ignore them (it forces a mismatch so the code will be rejected).
func ComputeCode(secret string, value int64) int {
key, err := base32.StdEncoding.DecodeString(secret)
if err != nil {
return -1
}
hash := hmac.New(sha1.New, key)
err = binary.Write(hash, binary.BigEndian, value)
if err != nil {
return -1
}
h := hash.Sum(nil)
offset := h[19] & 0x0f
truncated := binary.BigEndian.Uint32(h[offset : offset+4])
truncated &= 0x7fffffff
code := truncated % 1000000
return int(code)
}
// ErrInvalidCode indicate the supplied one-time code was not valid
var ErrInvalidCode = errors.New("invalid code")
// OTPConfig is a one-time-password configuration. This object will be modified by calls to
// Authenticate and should be saved to ensure the codes are in fact only used
// once.
type OTPConfig struct {
Secret string // 80-bit base32 encoded string of the user's secret
WindowSize int // valid range: technically 0..100 or so, but beyond 3-5 is probably bad security
HotpCounter int // the current otp counter. 0 if the user uses time-based codes instead.
DisallowReuse []int // timestamps in the current window unavailable for re-use
ScratchCodes []int // an array of 8-digit numeric codes that can be used to log in
UTC bool // use UTC for the timestamp instead of local time
}
func (c *OTPConfig) checkScratchCodes(code int) bool {
for i, v := range c.ScratchCodes {
if code == v {
// remove this code from the list of valid ones
l := len(c.ScratchCodes) - 1
c.ScratchCodes[i] = c.ScratchCodes[l] // copy last element over this element
c.ScratchCodes = c.ScratchCodes[0:l] // and trim the list length by 1
return true
}
}
return false
}
func (c *OTPConfig) checkHotpCode(code int) bool {
for i := 0; i < c.WindowSize; i++ {
if ComputeCode(c.Secret, int64(c.HotpCounter+i)) == code {
c.HotpCounter += i + 1
// We don't check for overflow here, which means you can only authenticate 2^63 times
// After that, the counter is negative and the above 'if' test will fail.
// This matches the behaviour of the PAM module.
return true
}
}
// we must always advance the counter if we tried to authenticate with it
c.HotpCounter++
return false
}
func (c *OTPConfig) checkTotpCode(t0, code int) bool {
minT := t0 - (c.WindowSize / 2)
maxT := t0 + (c.WindowSize / 2)
for t := minT; t <= maxT; t++ {
if ComputeCode(c.Secret, int64(t)) == code {
if c.DisallowReuse != nil {
for _, timeCode := range c.DisallowReuse {
if timeCode == t {
return false
}
}
// code hasn't been used before
c.DisallowReuse = append(c.DisallowReuse, t)
// remove all time codes outside of the valid window
sort.Ints(c.DisallowReuse)
min := 0
for c.DisallowReuse[min] < minT {
min++
}
// FIXME: check we don't have an off-by-one error here
c.DisallowReuse = c.DisallowReuse[min:]
}
return true
}
}
return false
}
// Authenticate a one-time-password against the given OTPConfig
// Returns true/false if the authentication was successful.
// Returns error if the password is incorrectly formatted (not a zero-padded 6 or non-zero-padded 8 digit number).
func (c *OTPConfig) Authenticate(password string) (bool, error) {
var scratch bool
switch {
case len(password) == 6 && password[0] >= '0' && password[0] <= '9':
break
case len(password) == 8 && password[0] >= '1' && password[0] <= '9':
scratch = true
break
default:
return false, ErrInvalidCode
}
code, err := strconv.Atoi(password)
if err != nil {
return false, ErrInvalidCode
}
if scratch {
return c.checkScratchCodes(code), nil
}
// we have a counter value we can use
if c.HotpCounter > 0 {
return c.checkHotpCode(code), nil
}
var t0 int
// assume we're on Time-based OTP
if c.UTC {
t0 = int(time.Now().UTC().Unix() / 30)
} else {
t0 = int(time.Now().Unix() / 30)
}
return c.checkTotpCode(t0, code), nil
}
// ProvisionURI generates a URI that can be turned into a QR code to configure
// a Google Authenticator mobile app.
func (c *OTPConfig) ProvisionURI(user string) string {
return c.ProvisionURIWithIssuer(user, "")
}
// ProvisionURIWithIssuer generates a URI that can be turned into a QR code
// to configure a Google Authenticator mobile app. It respects the recommendations
// on how to avoid conflicting accounts.
//
// See https://code.google.com/p/google-authenticator/wiki/ConflictingAccounts
func (c *OTPConfig) ProvisionURIWithIssuer(user string, issuer string) string {
auth := "totp/"
q := make(url.Values)
if c.HotpCounter > 0 {
auth = "hotp/"
q.Add("counter", strconv.Itoa(c.HotpCounter))
}
q.Add("secret", c.Secret)
if issuer != "" {
q.Add("issuer", issuer)
auth += issuer + ":"
}
return "otpauth://" + auth + user + "?" + q.Encode()
}
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