// Package xsrftoken provides methods for generating and validating secure XSRF tokens.
package xsrftoken // import "golang.org/x/net/xsrftoken"

import (
    "crypto/hmac"
    "crypto/sha1"
    "crypto/subtle"
    "encoding/base64"
    "fmt"
    "strconv"
    "strings"
    "time"
)

// 设置xsrf存活周期
// 也许会设置和cookie生命周期一样
const Timeout = 24 * time.Hour

// 清理字符串中：替换为_ 。替换所有
func clean(s string) string {
    return strings.Replace(s, ":", "_", -1)
}

// Generate returns a URL-safe secure XSRF token that expires in 24 hours.
//返回一个安全且加密的url  默认存活周期为24小时
// key  是应用程序的密钥.
// userID  唯一标识符.
// actionID 用户实际的动作(例如 ：  访问的资源的地址).
func Generate(key, userID, actionID string) string {
    return generateTokenAtTime(key, userID, actionID, time.Now())
}

// generateTokenAtTime is like Generate, but returns a token that expires 24 hours from now.
func generateTokenAtTime(key, userID, actionID string, now time.Time) string {
    // now转化为毫秒
    milliTime := (now.UnixNano() + 1e6 - 1) / 1e6

    h := hmac.New(sha1.New, []byte(key))  //使用hmac进行加密
    fmt.Fprintf(h, "%s:%s:%d", clean(userID), clean(actionID), milliTime)

    // Get the padded base64 string then removing the padding.
    tok := string(h.Sum(nil))
    tok = base64.URLEncoding.EncodeToString([]byte(tok))
    tok = strings.TrimRight(tok, "=")

    return fmt.Sprintf("%s:%d", tok, milliTime)
}

// Valid reports whether a token is a valid, unexpired token returned by Generate.
//验证 token 对应的key  userid  actionID  是否正确  并且在存活周期中
func Valid(token, key, userID, actionID string) bool {
    return validTokenAtTime(token, key, userID, actionID, time.Now())
}

// validTokenAtTime reports whether a token is valid at the given time.
func validTokenAtTime(token, key, userID, actionID string, now time.Time) bool {
    // Extract the issue time of the token.
    sep := strings.LastIndex(token, ":")
    if sep < 0 {
        return false
    }
    millis, err := strconv.ParseInt(token[sep+1:], 10, 64)
    if err != nil {
        return false
    }
    issueTime := time.Unix(0, millis*1e6)

    // Check that the token is not expired.
    if now.Sub(issueTime) >= Timeout {
        return false
    }

    // Check that the token is not from the future.
    // Allow 1 minute grace period in case the token is being verified on a
    // machine whose clock is behind the machine that issued the token.
    if issueTime.After(now.Add(1 * time.Minute)) {
        return false
    }

    expected := generateTokenAtTime(key, userID, actionID, issueTime)

    // Check that the token matches the expected value.
    // Use constant time comparison to avoid timing attacks.
    return subtle.ConstantTimeCompare([]byte(token), []byte(expected)) == 1
}