From 663f5f6b7617919acf0662b622c8cf3f4c493951 Mon Sep 17 00:00:00 2001
From: Samuel Lorch <sam@lorch.net>
Date: Mon, 20 Sep 2021 10:24:52 +0200
Subject: [PATCH] add totp generator from:
 https://github.com/yitsushi/totp-cli/blob/master/security/otp.go

---
 helper/totp.go | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 helper/totp.go

diff --git a/helper/totp.go b/helper/totp.go
new file mode 100644
index 0000000..42cf7b9
--- /dev/null
+++ b/helper/totp.go
@@ -0,0 +1,60 @@
+package helper
+
+import (
+	"crypto/hmac"
+	"crypto/sha1"
+	"encoding/base32"
+	"encoding/binary"
+	"fmt"
+	"math"
+	"strings"
+	"time"
+)
+
+const (
+	mask1              = 0xf
+	mask2              = 0x7f
+	mask3              = 0xff
+	timeSplitInSeconds = 30
+	shift24            = 24
+	shift16            = 16
+	shift8             = 8
+	codeLength         = 6
+)
+
+// GenerateOTPCode generates a 6 digit TOTP from the secret Token.
+func GenerateOTPCode(token string, when time.Time) (string, error) {
+	timer := uint64(math.Floor(float64(when.Unix()) / float64(timeSplitInSeconds)))
+	// Remove spaces, some providers are giving us in a readable format
+	// so they add spaces in there. If it's not removed while pasting in,
+	// remove it now.
+	token = strings.Replace(token, " ", "", -1)
+
+	// It should be uppercase always
+	token = strings.ToUpper(token)
+
+	secretBytes, err := base32.StdEncoding.WithPadding(base32.NoPadding).DecodeString(token)
+	if err != nil {
+		return "", fmt.Errorf("Decoding token string: %w", err)
+	}
+
+	buf := make([]byte, 8)
+	mac := hmac.New(sha1.New, secretBytes)
+
+	binary.BigEndian.PutUint64(buf, timer)
+	_, _ = mac.Write(buf)
+	sum := mac.Sum(nil)
+
+	// http://tools.ietf.org/html/rfc4226#section-5.4
+	offset := sum[len(sum)-1] & mask1
+	value := int64(((int(sum[offset]) & mask2) << shift24) |
+		((int(sum[offset+1] & mask3)) << shift16) |
+		((int(sum[offset+2] & mask3)) << shift8) |
+		(int(sum[offset+3]) & mask3))
+
+	modulo := int32(value % int64(math.Pow10(codeLength)))
+
+	format := fmt.Sprintf("%%0%dd", codeLength)
+
+	return fmt.Sprintf(format, modulo), nil
+}