Webcounter with cookies

2017-07-28 20:47:26 +02:00 · 2017-07-28 20:47:26 +02:00 · 4dc2e681c4
commit 4dc2e681c4
parent b9344dcf82
3 changed files with 92 additions and 0 deletions
--- a/.gitignore
+++ b/.gitignore
@ -59,3 +59,4 @@ tpl-ex2/tpl-ex2
 tpl-ex3/tpl-ex3
 form-ex1/form-ex1
 form-ex2/form-ex2
+webcounter/webcounter
--- a/webcounter/main.go
+++ b/webcounter/main.go
@ -0,0 +1,83 @@
+package main
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"fmt"
+	"hash"
+	"html/template"
+	"log"
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+var TPL *template.Template
+
+const (
+	SECRET                = "QDFuO2FyRHhGKEg5QlI4XDMlM2sK"
+	COUNTER_COOKIE string = "counter"
+)
+
+func displayPage(res http.ResponseWriter, req *http.Request) {
+	count, cookie := updateCounter(req)
+	http.SetCookie(res, cookie)
+	TPL.Execute(res, count)
+}
+
+func updateCounter(req *http.Request) (uint64, *http.Cookie) {
+	count := fromCookie(req)
+	count++
+	val := toCookieVal(count)
+	cookie := &http.Cookie{
+		Name:  COUNTER_COOKIE,
+		Value: val,
+	}
+	return count, cookie
+}
+
+func fromCookie(req *http.Request) uint64 {
+	cookie, err := req.Cookie(COUNTER_COOKIE)
+	if err != nil {
+		return 0
+	}
+	sp := strings.Split(cookie.Value, "|")
+	if len(sp) != 2 {
+		return 0
+	}
+	c, err := strconv.Atoi(sp[0])
+	if err != nil {
+		return 0
+	}
+	if !checkMac(sp[0], sp[1]) {
+		return 0
+	}
+	return uint64(c)
+}
+
+func checkMac(val string, macStr string) bool {
+	var mac []byte
+	fmt.Sscanf(macStr, "%x", &mac)
+	return hmac.Equal(mac, cookieHash(val).Sum(nil))
+}
+
+func cookieHash(val string) hash.Hash {
+	mac := hmac.New(sha256.New, []byte(SECRET))
+	mac.Write([]byte(val))
+	return mac
+}
+
+func toCookieVal(count uint64) string {
+	val := strconv.FormatUint(count, 10)
+	return val + "|" + fmt.Sprintf("%x", cookieHash(val).Sum(nil))
+}
+
+func main() {
+	var err error
+	TPL, err = template.ParseFiles("tpl.html")
+	if err != nil {
+		log.Fatalln(err)
+	}
+	http.HandleFunc("/", displayPage)
+	http.ListenAndServe(":9000", nil)
+}
--- a/webcounter/tpl.html
+++ b/webcounter/tpl.html
@ -0,0 +1,8 @@
+<!DOCTYPE html>
+<html>
+  <head><title>Webcounter</title></head>
+  <body>
+    <p>Number of visits: {{ . }}</p>
+  </body>
+</html>
+