HackingReversing

Christmas CTF - Simple_Bit

크리스마스 CTF

역연산 해주면 되는 문제였다.

먼저 ida hexray를 이용해서 함수들을 살펴보자

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__int64 __fastcall main(__int64 a1, char **a2, char **a3)
{
  __int64 result; // rax@4
  __int64 v4; // rsi@4
  char buf[264]; // [sp+10h] [bp-110h]@1
  __int64 canary; // [sp+118h] [bp-8h]@1

  canary = *MK_FP(__FS__, 40LL);
  memset(buf, 0, 0x100uLL);
  printf("Input : ", a2, buf);
  fflush(stdout);
  buf[(signed int)((unsigned __int64)read(0, buf, 0x100uLL) - 1)] = 0;
  if ( (unsigned int)check(buf) )
    puts("Correct");
  else
    puts("Wrong");
  result = 0LL;
  v4 = *MK_FP(__FS__, 40LL) ^ canary;
  return result;
}

signed __int64 __fastcall check(const char *input)
{
  signed __int64 result; // rax@17
  __int64 v2; // rsi@21
  int v3; // [sp+14h] [bp-402Ch]@8
  int i; // [sp+18h] [bp-4028h]@1
  int j; // [sp+1Ch] [bp-4024h]@2
  signed int k; // [sp+20h] [bp-4020h]@7
  int l; // [sp+24h] [bp-401Ch]@8
  signed int m; // [sp+28h] [bp-4018h]@15
  int length; // [sp+2Ch] [bp-4014h]@1
  int arr[2048]; // [sp+30h] [bp-4010h]@1
  int goal[288]; // [sp+2030h] [bp-2010h]@1
  __int64 canary; // [sp+4038h] [bp-8h]@1

  canary = *MK_FP(__FS__, 40LL);
  memset(arr, 0, sizeof(arr));
  memset(goal, 0, 0x2000uLL);
  goal[2] = 1;
  goal[3] = 1;
  goal[4] = 1;
  goal[8] = 1;
  goal[9] = 1;
  goal[11] = 1;
  goal[13] = 1;
  goal[16] = 1;
  goal[20] = 1;
  goal[21] = 1;
  goal[22] = 1;
  goal[28] = 1;
  goal[30] = 1;
  goal[33] = 1;
  goal[35] = 1;
  goal[36] = 1;
  goal[37] = 1;
  goal[40] = 1;
  goal[41] = 1;
  goal[48] = 1;
  goal[49] = 1;
  goal[51] = 1;
  goal[53] = 1;
  goal[54] = 1;
  goal[60] = 1;
  goal[61] = 1;
  goal[63] = 1;
  goal[64] = 1;
  goal[65] = 1;
  goal[67] = 1;
  goal[68] = 1;
  goal[73] = 1;
  goal[76] = 1;
  goal[78] = 1;
  goal[80] = 1;
  goal[85] = 1;
  goal[86] = 1;
  goal[87] = 1;
  goal[89] = 1;
  goal[90] = 1;
  goal[91] = 1;
  goal[95] = 1;
  goal[96] = 1;
  goal[99] = 1;
  goal[100] = 1;
  goal[101] = 1;
  goal[102] = 1;
  goal[104] = 1;
  goal[110] = 1;
  goal[111] = 1;
  goal[116] = 1;
  goal[117] = 1;
  goal[120] = 1;
  goal[121] = 1;
  goal[122] = 1;
  goal[125] = 1;
  goal[126] = 1;
  goal[132] = 1;
  goal[135] = 1;
  goal[140] = 1;
  goal[142] = 1;
  goal[144] = 1;
  goal[145] = 1;
  goal[146] = 1;
  goal[147] = 1;
  goal[149] = 1;
  goal[150] = 1;
  goal[151] = 1;
  goal[154] = 1;
  goal[157] = 1;
  goal[162] = 1;
  goal[167] = 1;
  goal[171] = 1;
  goal[173] = 1;
  goal[174] = 1;
  goal[175] = 1;
  goal[176] = 1;
  goal[179] = 1;
  goal[180] = 1;
  goal[182] = 1;
  goal[184] = 1;
  goal[185] = 1;
  goal[186] = 1;
  goal[191] = 1;
  goal[193] = 1;
  goal[196] = 1;
  goal[198] = 1;
  goal[201] = 1;
  goal[203] = 1;
  goal[204] = 1;
  goal[205] = 1;
  goal[206] = 1;
  goal[208] = 1;
  goal[209] = 1;
  goal[210] = 1;
  goal[216] = 1;
  goal[218] = 1;
  goal[221] = 1;
  goal[222] = 1;
  goal[226] = 1;
  goal[227] = 1;
  goal[228] = 1;
  goal[229] = 1;
  goal[230] = 1;
  goal[231] = 1;
  goal[234] = 1;
  goal[237] = 1;
  goal[239] = 1;
  goal[240] = 1;
  goal[242] = 1;
  goal[245] = 1;
  goal[247] = 1;
  goal[251] = 1;
  goal[254] = 1;
  goal[259] = 1;
  goal[263] = 1;
  goal[264] = 1;
  goal[268] = 1;
  goal[269] = 1;
  goal[271] = 1;
  goal[274] = 1;
  goal[276] = 1;
  goal[277] = 1;
  goal[279] = 1;
  goal[280] = 1;
  goal[282] = 1;
  goal[283] = 1;
  goal[284] = 1;
  goal[287] = 1;
  length = strlen(input);
  for ( i = 0; i < length; ++i )
  {
    for ( j = 8 * i; j < 8 * (i + 1); ++j )
    {
      arr[j] = input[i] & 1;
      input[i] >>= 1;
    }
  }
  for ( k = 0; k <= 15; ++k )
  {
    v3 = 1;
    for ( l = 0; l < 8 * length; ++l )
    {
      if ( arr[l] )
        v3 ^= 1u;
      arr[l] = v3;
    }
    shuffle(arr, 8 * length);
  }
  for ( m = 0; m <= 0x7FF; ++m )
  {
    if ( arr[m] != goal[m] )
    {
      result = 0LL;
      goto LABEL_21;
    }
  }
  result = 1LL;
LABEL_21:
  v2 = *MK_FP(__FS__, 40LL) ^ canary;
  return result;
}

_DWORD *__fastcall shuffle(_DWORD *arr, signed int length)
{
  _DWORD *result; // rax@1
  int i; // [sp+18h] [bp-8h]@1
  int r; // [sp+1Ch] [bp-4h]@2

  result = (_DWORD *)(unsigned int)(length - 1);
  for ( i = length - 1; i >= 0; --i )
  {
    r = rand() % length;
    result = (_DWORD *)(unsigned int)i;
    if ( i != r )
    {
      arr[i] ^= arr[r];                         // swap(arr[i], arr[r])
      arr[r] ^= arr[i];
      result = &arr[i];
      *result ^= arr[r];
    }
  }
  return result;
}

루틴을 정리해보면 유저에게 입력을 받고 그 글자를 2진수로 바꾼다음 뒤집어서 넣는다.
그리고 어떠한 연산을 한 뒤에 배열을 섞어준다. 이걸 16번 반복하고 goal배열과 결과를 비교해본다.

해당 프로그램에서 shuffle에 rand()를 사용하고 있는 것을 볼 수 있지만 srand를 사용하지 않았기에 rand는 항상 동일한 값이 나오며 이것을 이용해서 우리는 shuffle을 역연산 하는 행위도 할 수 있다.

solve.py

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from ctypes import *
import sys

def shuffle(arr, rand):
    rand = rand[::-1]
    for i in range(len(rand)):
        arr[i], arr[rand[i]] = arr[rand[i]], arr[i]

length = 36

libc = CDLL('libc.so.6')

rands = []
for i in range(16):
    rand = []
    for j in range(length * 8):
        rand.append(libc.rand() % (length * 8))
    rands.append(rand)

arr = [0, 0, 1, 1, 1, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 1, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 1, 0, 0, 0, 0, 0, 1, 1, 0, 1, 1, 1, 0, 1, 1, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 0, 1, 1, 1, 0, 1, 1, 1, 0, 0, 0, 1, 1, 0, 0, 1, 1, 1, 1, 0, 1, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 0, 1, 0, 1, 1, 1, 1, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0, 0, 1, 0, 1, 1, 1, 1, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 1, 0, 0, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 0, 1]
arr += [0] * (0x800 - len(arr))

for i in range(15, -1, -1):
    shuffle(arr, rands[i])
    for j in range(8 * length - 1, -1, -1):
        if j == 0:
            arr[j] = arr[j] ^ 1
        else:
            arr[j] = arr[j] ^ arr[j-1]

bins = ''.join(map(str, arr))
output = ''
for i in range(0, len(bins), 8):
    output += chr(int(bins[i:i+8][::-1], 2))

print output # XMAS{S1mp1e_diff3r3nt1cal_c0d1ng_:p}
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