Gacha

Gacha:Web:90pts

ガチャを作ったらしいけど, そのseedの使い方間違ってない？
http://34.82.49.144:3334/
main.go

Solution

URLにアクセスするとガチャサイト？のようだ。
複数回wgetしてみる。

$ wget -q -O - http://34.82.49.144:3334/
{"flag":"You might not have a luck...","sum":"11843"}
$ wget -q -O - http://34.82.49.144:3334/
{"flag":"You might not have a luck...","sum":"11845"}
$ wget -q -O - http://34.82.49.144:3334/
{"flag":"You might not have a luck...","sum":"11846"}
$ wget -q -O - http://34.82.49.144:3334/
{"flag":"You might not have a luck...","sum":"11848"}

秒数が経過するごとにsumが増加しているようだ。
ソースの以下に注目する。

~~~
	seed := r.FormValue("seed")
	if len(seed) == 0 {
		seed = "1"
	}
	seedInt, err := strconv.Atoi(seed)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	// get current time(HHmmss)
	jst := time.FixedZone("Asia/Tokyo", 9*60*60)
	nowStr := time.Now().In(jst).Format("150405")
	log.Println(nowStr)
	nowInt, err := strconv.Atoi(nowStr)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	sm := (seedInt + nowInt) % 100000
	log.Println(sm)
	var flag map[string]string

	if sm == 1337 {
		flag = map[string]string{
			"flag": "taskctf{this_is_dummy_flag}",
		}
	} else {
		flag = map[string]string{
			"flag": "You might not have a luck...",
			"sum":  strconv.Itoa(sm),
		}
	}
~~~

seedが負の数の場合の対応がされていない。

$ wget -q -O - http://34.82.49.144:3334/
{"flag":"You might not have a luck...","sum":"12627"}
$ wget -q -O - http://34.82.49.144:3334/?seed=-10000
{"flag":"You might not have a luck...","sum":"2631"}

負の数で調整する。

$ wget -q -O - http://34.82.49.144:3334/?seed=-10000
{"flag":"You might not have a luck...","sum":"4846"}
$ wget -q -O - http://34.82.49.144:3334/?seed=-13000
{"flag":"You might not have a luck...","sum":"1853"}
$ wget -q -O - http://34.82.49.144:3334/?seed=-13500
{"flag":"You might not have a luck...","sum":"1358"}
$ wget -q -O - http://34.82.49.144:3334/?seed=-13570
{"flag":"You might not have a luck...","sum":"1334"}
$ wget -q -O - http://34.82.49.144:3334/?seed=-13570
{"flag":"taskctf{Y0u_h4ve_4_gre4t_luck}"}
$ wget -q -O - http://34.82.49.144:3334/?seed=-13570
{"flag":"You might not have a luck...","sum":"1339"}

flagが表示された。

taskctf{Y0u_h4ve_4_gre4t_luck}