問題文

てけいさんのプロのために作りました。
えくすとりーむなので300秒でタイムアウトします。
$ nc tekeisan-ekusutoriim.chall.beginners.seccon.jp 8690

writeup

まずは接続してみる。

root@kali:~# nc tekeisan-ekusutoriim.chall.beginners.seccon.jp 8690
Welcome to TEKEISAN for Beginners -extreme edition-
---------------------------------------------------------------
Please calculate. You need to answered 100 times.
e.g.
(Stage.1)
4 + 5 = 9
...
(Stage.99)
4 * 4 = 869
[!!] Wrong, see you.
---------------------------------------------------------------
(Stage.1)
840 + 779 = 1619
(Stage.2)
505 + 756 = 1111
[!!] Wrong, see you.

延々と計算問題をこなしていくようだ。
100回も手動でやってられないので、スクリプトを書く。
式部分を抽出してevalで計算して送信する。

import socket
import re

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('tekeisan-ekusutoriim.chall.beginners.seccon.jp', 8690))

while True:
    receivedata = s.recv(1024).decode('utf-8')
    print("[+]receivedata=",receivedata)

    splitreceivedata = receivedata.split("\n")
    formula = splitreceivedata[len(splitreceivedata)-1]

    pattern = '(.*) ='
    m = re.search(pattern, formula)
    if m:
        formula = m.group(1)
        print("[+]formula=",formula)

    senddata = str(eval(formula))
    print("[+]senddata=",senddata)
    s.sendall((senddata+"\n").encode("utf-8"))

実行結果。

[+]receivedata= Welcome to TEKEISAN for Beginners -extreme edition-
---------------------------------------------------------------
Please calculate. You need to answered 100 times.
e.g.
(Stage.1)
4 + 5 = 9
...
(Stage.99)
4 * 4 = 869
[!!] Wrong, see you.
---------------------------------------------------------------
(Stage.1)
565 + 997 =
[+]formula= 565 + 997
[+]senddata= 1562
[+]receivedata= (Stage.2)
559 - 527 =
[+]formula= 559 - 527
[+]senddata= 32
(snip)
[+]receivedata= (Stage.99)
624 * 588 =
[+]formula= 624 * 588
[+]senddata= 366912
[+]receivedata= (Stage.100)
729 - 810 =
[+]formula= 729 - 810
[+]senddata= -81
[+]receivedata= Congrats.
Flag is: "ctf4b{ekusutori-mu>tekeisann>bigina-zu>2018}"

フラグゲット。
ctf4b{ekusutori-mu>tekeisann>bigina-zu>2018}