攻防世界--re2-cpp-is-awesome

测试文件：https://adworld.xctf.org.cn/media/task/attachments/c5802869b8a24033b4a80783a67c858b

 

1.准备

获取信息

1. 64位文件

 

2.IDA打开

 1 __int64 __fastcall main(int a1, char **a2, char **a3)
 2 {
 3   char *v3; // rbx
 4   __int64 v4; // rax
 5   __int64 v5; // rdx
 6   __int64 v6; // rax
 7   __int64 v7; // rdx
 8   _BYTE *v8; // rax
 9   __int64 i; // [rsp+10h] [rbp-60h]
10   char v11; // [rsp+20h] [rbp-50h]
11   char v12; // [rsp+4Fh] [rbp-21h]
12   __int64 v13; // [rsp+50h] [rbp-20h]
13   int v14; // [rsp+5Ch] [rbp-14h]
14 
15   if ( a1 != 2 )
16   {
17     v3 = *a2;
18     v4 = std::operator<<char>>(&std::cout, "Usage: ", a3);
19     v6 = std::operator<<char>>(v4, v3, v5);
20     std::operator<<char>>(v6, " flag\n", v7);
21     exit(0);
22   }
23   std::allocator<char>::allocator(&v12, a2, a3);
24   std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string(&v11, a2[1], &v12);
25   std::allocator<char>::~allocator(&v12);
26   v14 = 0;
27   for ( i = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::begin(&v11); ; sub_400D7A(&i) )
28   {
29     v13 = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::end(&v11);
30     if ( !(unsigned __int8)sub_400D3D(&i, &v13) )
31       break;
32     v8 = (_BYTE *)sub_400D9A(&i);
33     if ( *v8 != off_6020A0[dword_6020C0[v14]] )
34       sub_400B56();
35     ++v14;
36   }
37   sub_400B73();
38   std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::~basic_string(&v11);
39   return 0LL;
40 }

 

3.代码分析

实际上这堆代码，很大部分“垃圾代码”--不需要我们关注的

我们只需要关注26行代码以下部分即可

第27行代码，是一个for循环，没有结束条件，每次增加sub_400D7A(&i)，即1字节

_QWORD *__fastcall sub_400D7A(_QWORD *a1)
{
  ++*a1;
  return a1;
}

 

  for ( i = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::begin(&v11); ; sub_400D7A(&i) )
  {
    v13 = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::end(&v11);
    if ( !sub_400D3D((__int64)&i, (__int64)&v13) )// 循环结束条件
      break;
    v8 = (_BYTE *)sub_400D9A((__int64)&i);      // 进行某种赋值
    if ( *v8 != off_6020A0[dword_6020C0[v14]] ) // 重点！这里实际上就是一个数组套着数组
      sub_400B56(&i, &v13);
    ++v14;                                      // 数组下标
  }

 

3.1条件中找flag

通过对比sub_400B56(&i, &v13);和 sub_400B73(&i, &v13);函数，我们能够得到flag实际藏在if判断条件中

void __fastcall __noreturn sub_400B56(__int64 a1, __int64 a2, __int64 a3)
{
  std::operator<<char>>(&std::cout, "Better luck next time\n", a3);
  exit(0);
}

 

__int64 __fastcall sub_400B73(__int64 a1, __int64 a2, __int64 a3)
{
  return std::operator<<char>>(&std::cout, "You should have the flag by now\n", a3);
}

 

进入off_6020A0和dword_6020C0我们可以看到

.data:00000000006020A0 off_6020A0      dq offset aL3tMeT3llY0uS0
.data:00000000006020A0                                         ; DATA XREF: main+D1↑r
.data:00000000006020A0                                         ; "L3t_ME_T3ll_Y0u_S0m3th1ng_1mp0rtant_A_{"...
.data:00000000006020A8                 align 20h

.rodata:0000000000400E58 aL3tMeT3llY0uS0 db 'L3t_ME_T3ll_Y0u_S0m3th1ng_1mp0rtant_A_{FL4G}_W0nt_b3_3X4ctly_th4t'
.rodata:0000000000400E58                                         ; DATA XREF: .data:off_6020A0↓o

这实际上就是一段字符串

 

.data:00000000006020C0 dword_6020C0    dd 24h                  ; DATA XREF: main+DD↑r
.data:00000000006020C4                 align 8
.data:00000000006020C8                 db    5
.data:00000000006020C9                 db    0
.data:00000000006020CA                 db    0
.data:00000000006020CB                 db    0
.data:00000000006020CC                 db  36h ; 6
.data:00000000006020CD                 db    0
.data:00000000006020CE                 db    0
.data:00000000006020CF                 db    0
.data:00000000006020D0                 db  65h ; e
.data:00000000006020D1                 db    0
.data:00000000006020D2                 db    0
.data:00000000006020D3                 db    0
.data:00000000006020D4                 db    7
.data:00000000006020D5                 db    0
.data:00000000006020D6                 db    0
.data:00000000006020D7                 db    0
.data:00000000006020D8                 db  27h ; '
.data:00000000006020D9                 db    0
.data:00000000006020DA                 db    0
.data:00000000006020DB                 db    0
.data:00000000006020DC                 db  26h ; &
.data:00000000006020DD                 db    0
.data:00000000006020DE                 db    0
.data:00000000006020DF                 db    0
.data:00000000006020E0                 db  2Dh ; -
.data:00000000006020E1                 db    0
.data:00000000006020E2                 db    0
.data:00000000006020E3                 db    0
.data:00000000006020E4                 db    1
.data:00000000006020E5                 db    0
.data:00000000006020E6                 db    0
.data:00000000006020E7                 db    0
.data:00000000006020E8                 db    3
.data:00000000006020E9                 db    0
.data:00000000006020EA                 db    0
.data:00000000006020EB                 db    0
.data:00000000006020EC                 db    0
.data:00000000006020ED                 db    0
.data:00000000006020EE                 db    0
.data:00000000006020EF                 db    0
.data:00000000006020F0                 db  0Dh
.data:00000000006020F1                 db    0
.data:00000000006020F2                 db    0
.data:00000000006020F3                 db    0
.data:00000000006020F4                 db  56h ; V
.data:00000000006020F5                 db    0
.data:00000000006020F6                 db    0
.data:00000000006020F7                 db    0
.data:00000000006020F8                 db    1
.data:00000000006020F9                 db    0
.data:00000000006020FA                 db    0
.data:00000000006020FB                 db    0
.data:00000000006020FC                 db    3
.data:00000000006020FD                 db    0
.data:00000000006020FE                 db    0
.data:00000000006020FF                 db    0
.data:0000000000602100                 db  65h ; e
.data:0000000000602101                 db    0
.data:0000000000602102                 db    0
.data:0000000000602103                 db    0
.data:0000000000602104                 db    3
.data:0000000000602105                 db    0
.data:0000000000602106                 db    0
.data:0000000000602107                 db    0
.data:0000000000602108                 db  2Dh ; -
.data:0000000000602109                 db    0
.data:000000000060210A                 db    0
.data:000000000060210B                 db    0
.data:000000000060210C                 db  16h
.data:000000000060210D                 db    0
.data:000000000060210E                 db    0
.data:000000000060210F                 db    0
.data:0000000000602110                 db    2
.data:0000000000602111                 db    0
.data:0000000000602112                 db    0
.data:0000000000602113                 db    0
.data:0000000000602114                 db  15h
.data:0000000000602115                 db    0
.data:0000000000602116                 db    0
.data:0000000000602117                 db    0
.data:0000000000602118                 db    3
.data:0000000000602119                 db    0
.data:000000000060211A                 db    0
.data:000000000060211B                 db    0
.data:000000000060211C                 db  65h ; e
.data:000000000060211D                 db    0
.data:000000000060211E                 db    0
.data:000000000060211F                 db    0
.data:0000000000602120                 db    0
.data:0000000000602121                 db    0
.data:0000000000602122                 db    0
.data:0000000000602123                 db    0
.data:0000000000602124                 db  29h ; )
.data:0000000000602125                 db    0
.data:0000000000602126                 db    0
.data:0000000000602127                 db    0
.data:0000000000602128                 db  44h ; D
.data:0000000000602129                 db    0
.data:000000000060212A                 db    0
.data:000000000060212B                 db    0
.data:000000000060212C                 db  44h ; D
.data:000000000060212D                 db    0
.data:000000000060212E                 db    0
.data:000000000060212F                 db    0
.data:0000000000602130                 db    1
.data:0000000000602131                 db    0
.data:0000000000602132                 db    0
.data:0000000000602133                 db    0
.data:0000000000602134                 db  44h ; D
.data:0000000000602135                 db    0
.data:0000000000602136                 db    0
.data:0000000000602137                 db    0
.data:0000000000602138                 db  2Bh ; +
.data:0000000000602139                 db    0
.data:000000000060213A                 db    0
.data:000000000060213B                 db    0
.data:000000000060213B _data           ends

这里就相当于是一连串的整数

 

因此我们可以分析得出，通过v15/v14作为内部数组下标，循环获到的整数再作为外部数组的下标，获取到需要的字符串。

这里值得注意的一点是，algn 8表示两个数之间间隔8位，相当于在两个数之间插了7个0，也就相当于在头两个数之间还有一个'0'

 

4.脚本获取flag

S = 'L3t_ME_T3ll_Y0u_S0m3th1ng_1mp0rtant_A_{FL4G}_W0nt_b3_3X4ctly_th4t_345y_t0_c4ptur3_H0wev3r_1T_w1ll_b3_C00l_'

N = [0x24,0x0,0x5,0x36,0x65,0x7,0x27,0x26,0x2d,0x1,0x3,0x0,0x0d,0x56,0x1,0x3,0x65,0x3,0x2d,0x16,0x2,0x15,0x3,0x65,0x0,0x29,0x44,0x44,0x1,0x44,0x2b]

x = ''

for i in N:
    x += S[i]

print(x)

 

5.get flag！

ALEXCTF{W3_L0v3_C_W1th_CL45535}