1// Copyright 2012 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// Package xsrftoken provides methods for generating and validating secure XSRF tokens.
6package xsrftoken // import "golang.org/x/net/xsrftoken"
7
8import (
9	"crypto/hmac"
10	"crypto/sha1"
11	"crypto/subtle"
12	"encoding/base64"
13	"fmt"
14	"strconv"
15	"strings"
16	"time"
17)
18
19// Timeout is the duration for which XSRF tokens are valid.
20// It is exported so clients may set cookie timeouts that match generated tokens.
21const Timeout = 24 * time.Hour
22
23// clean sanitizes a string for inclusion in a token by replacing all ":" with "::".
24func clean(s string) string {
25	return strings.Replace(s, `:`, `::`, -1)
26}
27
28// Generate returns a URL-safe secure XSRF token that expires in 24 hours.
29//
30// key is a secret key for your application; it must be non-empty.
31// userID is an optional unique identifier for the user.
32// actionID is an optional action the user is taking (e.g. POSTing to a particular path).
33func Generate(key, userID, actionID string) string {
34	return generateTokenAtTime(key, userID, actionID, time.Now())
35}
36
37// generateTokenAtTime is like Generate, but returns a token that expires 24 hours from now.
38func generateTokenAtTime(key, userID, actionID string, now time.Time) string {
39	if len(key) == 0 {
40		panic("zero length xsrf secret key")
41	}
42	// Round time up and convert to milliseconds.
43	milliTime := (now.UnixNano() + 1e6 - 1) / 1e6
44
45	h := hmac.New(sha1.New, []byte(key))
46	fmt.Fprintf(h, "%s:%s:%d", clean(userID), clean(actionID), milliTime)
47
48	// Get the padded base64 string then removing the padding.
49	tok := string(h.Sum(nil))
50	tok = base64.URLEncoding.EncodeToString([]byte(tok))
51	tok = strings.TrimRight(tok, "=")
52
53	return fmt.Sprintf("%s:%d", tok, milliTime)
54}
55
56// Valid reports whether a token is a valid, unexpired token returned by Generate.
57func Valid(token, key, userID, actionID string) bool {
58	return validTokenAtTime(token, key, userID, actionID, time.Now())
59}
60
61// validTokenAtTime reports whether a token is valid at the given time.
62func validTokenAtTime(token, key, userID, actionID string, now time.Time) bool {
63	if len(key) == 0 {
64		panic("zero length xsrf secret key")
65	}
66	// Extract the issue time of the token.
67	sep := strings.LastIndex(token, ":")
68	if sep < 0 {
69		return false
70	}
71	millis, err := strconv.ParseInt(token[sep+1:], 10, 64)
72	if err != nil {
73		return false
74	}
75	issueTime := time.Unix(0, millis*1e6)
76
77	// Check that the token is not expired.
78	if now.Sub(issueTime) >= Timeout {
79		return false
80	}
81
82	// Check that the token is not from the future.
83	// Allow 1 minute grace period in case the token is being verified on a
84	// machine whose clock is behind the machine that issued the token.
85	if issueTime.After(now.Add(1 * time.Minute)) {
86		return false
87	}
88
89	expected := generateTokenAtTime(key, userID, actionID, issueTime)
90
91	// Check that the token matches the expected value.
92	// Use constant time comparison to avoid timing attacks.
93	return subtle.ConstantTimeCompare([]byte(token), []byte(expected)) == 1
94}
95