Christmas CTF 2016 with noname cat

README.md

It's RPG game on server. They gave the server's binary on me. The pseudo code is..

main {
  butler: HP 1000 ATK 74 LUK 5 DEF 50
  Well512 random with timestamp seed
  A = 0
  B = 0
  if input is n {
    A++
  }
  if input is m {
    prints timestamp and B++
  }
  if input is y {
    I = input between 0 ~ 22. if input not on the range, exit.
    cat: HP timestamp%300 ATK Well512(timestamp%1000,A)%60 LUK Well512(timestamp%1000,A)%10 DEFWell512(timestamp%1000,B)%76
    Fight
    if cat wins {
      gives I-th letter of flag.
    }
  }
}
Well512(seed, round) {
  returns different random value if seed or round is different.
}
Fight {
  Attack cat player
  Attack player cat
}

Attack(A, B) {
  if A.ATK > B.DEF and Well512(new timestamp, 1) % 100 > LUK {
    B.HP -= A.ATK - B.DEF
  }
  if B.HP < 0 {
    A win
  }
}

There are Well512 PRNG which determines the cat's PRNG. I've just ported it to python and tested the solution locally, and it worked.

But on server, it didn't work. So I just brute forced every letter of flag.

python withnonamecat.py | tee|grep hereisflag &
# executed about 8 times

withnonamecat.c

#include <stdint.h>

unsigned int well512(int ts, unsigned int a2)
{
  int v2; // ST1C_4@5
  int v3; // ST20_4@5
  unsigned int v4; // ST1C_4@5
  int v5; // ST1C_4@5
  unsigned int v6; // ST24_4@5
  int v8; // [sp+10h] [bp-58h]@1
  unsigned int i; // [sp+14h] [bp-54h]@1
  unsigned int j; // [sp+14h] [bp-54h]@4
  int state[16]; // [sp+28h] [bp-40h]@2

  v8 = 0;
  for ( i = 0; i <= 0xF; ++i )
    state[(uint64_t)i] = i + ts;
  for ( j = 0; j < a2 % 0x2710; ++j )
  {
    v2 = state[(uint64_t)(((uint8_t)v8 + 13) & 0xF)];
    v3 = (state[(uint64_t)(unsigned int)v8] << 16) ^ v2 ^ state[(uint64_t)(unsigned int)v8] ^ (v2 << 15);
    v4 = state[(uint64_t)(((uint8_t)v8 + 9) & 0xF)];
    v5 = (v4 >> 11) ^ v4;
    state[(uint64_t)(unsigned int)v8] = v5 ^ v3;
    v6 = state[(uint64_t)(unsigned int)v8] ^ 32 * state[(uint64_t)(unsigned int)v8] & 0xDA442D20;
    v8 = ((uint8_t)v8 + 15) & 0xF;
    state[(uint64_t)(unsigned int)v8] ^= (v5 << 28) ^ (v3 << 18) ^ v6 ^ v3 ^ 4
                                                                                   * state[(uint64_t)(unsigned int)v8];
  }
  return (unsigned int)state[(uint64_t)(unsigned int)v8];
}

withnonamecat.py

#coding: utf8
from pwn import *
import datetime
import time
import ctypes
import random

lib = ctypes.CDLL('./withnonamecat.so')
FLAG_LEN = 22
HP, LUK, ATK, DEF = 1000, 5, 74, 50
flag = ''
HOST, PORT = '0.0.0.0', 9000
HOST, PORT = 'devslave.com', 9001
u = lambda x: x & 0xffffffff
timer = None
while timer is None or timer in [0, 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 15, 16, 17, 18, 19, 20, 21]:
	timer = random.randint(0, FLAG_LEN)
while timer < FLAG_LEN:
	r = remote(HOST, PORT)
	r.recvuntil('체크해본다.')
	r.sendline('m')
	years = int(r.recvuntil('년', drop=True))
	months = int(r.recvuntil('월', drop=True))
	days = int(r.recvuntil('일', drop=True))
	hours = int(r.recvuntil('시', drop=True))
	minutes = int(r.recvuntil('분', drop=True))
	seconds = int(r.recvuntil('초', drop=True))
	d = datetime.datetime(years, months, days, hours, minutes, seconds)
	ts = int(time.mktime(d.timetuple()))
	# print ts, time.time()
	forward = 1
	checktime = 2
	catHP = ts % 0x12C
	found = False
	while True:
		catLUK = u(lib.well512(ts % 0x3e8, forward)) % 0xA
		catATK = u(lib.well512(ts % 0x3e8, forward)) % 0x3C
		catDEF = u(lib.well512(ts % 0x3e8, checktime)) % 0x4C
		if (catATK > DEF and catDEF > ATK) and (catATK - DEF) * 180 > 1000 and catLUK > 5:
			found = True
			break
		if forward == 200:
			forward = 0
			checktime += 1
		if checktime > 200:
			found = False
			break
		forward += 1
	if found == False and 0:
		time.sleep(1)
		r.close()
		continue
	forward = random.randint(0, 30)
	checktime = random.randint(1, 30)
	for i in xrange(checktime - 1):
		r.recvuntil('체크해본다.')
		r.sendline('m')
	for i in xrange(forward):
		r.recvuntil('체크해본다.')
		r.sendline('n')
	r.recvuntil('체크해본다.')
	r.sendline('y')
	r.recvuntil('...')
	r.sendline(str(timer + 1))
	#print r.recvuntil('==', drop=True)
	#r.interactive()
	#print 1337, hex(u(lib.well512(ts % 0x3e8, forward)))
	#print 1337 ,hex(u(lib.well512(ts % 0x3e8, checktime)))
	#print 1337, catHP, catATK, catLUK, catDEF
	#print 1337, forward, checktime
	data = r.recvall()
	if '{' not in data:
		r.close()
		continue
	# print `data`
	data = data.split('\xed\x94\x8c\xeb\x9e\x98\xea\xb7\xb8 {')[1]
	c = data[0]
	flag += c
	print 'hereisflag', `flag`, timer
	timer += 1

I think that replacing this code with line 22 in withnonamecat.py might be a better way to improve readability.
int(time.mktime(time.strptime(P, '\n%Y년 %m월 %d일 %H시 %M분 %S초'))) - time.timezone