Allow the previous OTP

1 files changed, 10 insertions, 5 deletions

diff --git a/roseh.moe.go b/roseh.moe.go
index 49f62d3..40bc076 100644
--- a/roseh.moe.go
+++ b/roseh.moe.go
@@ -522,10 +522,10 @@ func cookieAuth(w http.ResponseWriter, r *http.Request) bool {
 	return true
 }
 
-func totp(key []byte) []byte {
+func totp(t int64) []byte {
 	ts := make([]byte, 8)
-	binary.BigEndian.PutUint64(ts, uint64(time.Now().Unix()/30))
-	hash := hmac.New(sha1.New, key)
+	binary.BigEndian.PutUint64(ts, uint64(t))
+	hash := hmac.New(sha1.New, authenticatorKey)
 	hash.Write(ts)
 	mac := hash.Sum(nil)
 	offset := mac[len(mac)-1] & 0xf
@@ -536,13 +536,18 @@ func totp(key []byte) []byte {
 	return strconv.AppendInt(nil, int64(n%1_000_000), 10)
 }
 
+func checkOTP(auth string) int {
+	now := time.Now().Unix() / 30
+	return subtle.ConstantTimeCompare([]byte(auth), totp(now)) | subtle.ConstantTimeCompare([]byte(auth), totp(now-1))
+}
+
 func checkPassword(password, auth string) bool {
-	otp := totp(authenticatorKey)
+	otpMatches := checkOTP(auth)
 	hash, err := pwhash.Hash(password, notepadPassword[:pwhash.SaltSize])
 	if err != nil {
 		return false
 	}
-	return subtle.ConstantTimeCompare(hash, notepadPassword[pwhash.SaltSize:])&subtle.ConstantTimeCompare([]byte(auth), otp) != 0
+	return subtle.ConstantTimeCompare(hash, notepadPassword[pwhash.SaltSize:])&otpMatches != 0
 }
 
 var (