From a2c9ac6897e8b64e05821689710e57203db774ed Mon Sep 17 00:00:00 2001
From: CDFN <codefun@protonmail.com>
Date: Thu, 22 Oct 2020 12:08:19 +0200
Subject: [PATCH] add topt token generation

---
 totp/totp.go | 85 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)
 create mode 100644 totp/totp.go

diff --git a/totp/totp.go b/totp/totp.go
new file mode 100644
index 00000000..97a7b1d2
--- /dev/null
+++ b/totp/totp.go
@@ -0,0 +1,85 @@
+package totp
+
+import (
+	"crypto/hmac"
+	"crypto/sha1"
+	"encoding/base64"
+	"encoding/binary"
+	"errors"
+	"time"
+)
+
+// ErrInvalidSharedSecret is returned when shared secret isn't in base64 form
+var ErrInvalidSharedSecret error = errors.New("invalid base64 shared secret")
+
+const (
+	// Range of possible chars for auth code.
+	chars    string = "23456789BCDFGHJKMNPQRTVWXY"
+	charsLen int    = len(chars)
+)
+
+// Totp is structure holding data needed for generating TOTP token.
+type Totp struct {
+	sharedSecret string
+	Time         time.Time
+}
+
+// NewTotp creates new Totp structure with current time.
+func NewTotp(sharedSecret string) *Totp {
+	return &Totp{sharedSecret, time.Now()}
+}
+
+// NewTimedTotp creates new Totp structure with custom time.
+func NewTimedTotp(sharedSecret string, time time.Time) *Totp {
+	return &Totp{sharedSecret, time}
+}
+
+// GenerateCode generates Steam TOTP code which is always 5 symbols.
+func (totp *Totp) GenerateCode() (string, error) {
+	return GenerateTotpCode(totp.sharedSecret, totp.Time)
+}
+
+// SharedSecret returns shared secret of Totp structure.
+func (totp *Totp) SharedSecret() string {
+	return totp.sharedSecret
+}
+
+// Function below is originally made by https://github.com/fortis/ and used in https://github.com/fortis/go-steam-totp/.
+
+// GenerateTotpCode generates steam TOTP code which is always 5 symbols.
+func GenerateTotpCode(sharedSecret string, time time.Time) (string, error) {
+	key, err := base64.StdEncoding.DecodeString(sharedSecret)
+	if err != nil {
+		return "", ErrInvalidSharedSecret
+	}
+
+	// Converting time for any reason
+	// 00 00 00 00 00 00 00 00
+	// 00 00 00 00 xx xx xx xx
+	ut := uint64(time.Unix()) / 30
+	tb := make([]byte, 8)
+	binary.BigEndian.PutUint64(tb, ut)
+
+	// Evaluate hash code for `tb` by key
+	mac := hmac.New(sha1.New, key)
+	mac.Write(tb)
+	hashcode := mac.Sum(nil)
+
+	// Last 4 bits provide initial position
+	// len(hashcode) = 20 bytes
+	start := hashcode[19] & 0xf
+
+	// Extract 4 bytes at `start` and drop first bit
+	fc32 := binary.BigEndian.Uint32(hashcode[start : start+4])
+	fc32 &= 1<<31 - 1
+	fullcode := int(fc32)
+
+	// Generate auth code
+	code := make([]byte, 5)
+	for i := range code {
+		code[i] = chars[fullcode%charsLen]
+		fullcode /= charsLen
+	}
+
+	return string(code[:]), nil
+}