【攻防世界】Reverse——BABYRE writeup

下面的代码可知：flag的长度是14，通过judge来判断是否成功。但jugde函数是加密过，也就是代码是被混淆过的。所以不可以直接查看。

int __cdecl main(int argc, const char **argv, const char **envp)
{
  char s[24]; // [rsp+0h] [rbp-20h] BYREF
  int v5; // [rsp+18h] [rbp-8h]
  int i; // [rsp+1Ch] [rbp-4h]

  for ( i = 0; i <= 181; ++i )
    *((_BYTE *)judge + i) ^= 0xCu;
  printf("Please input flag:");
  __isoc99_scanf("%20s", s);
  v5 = strlen(s);
  if ( v5 == 14 && (unsigned int)judge((__int64)s) )
    puts("Right!");
  else
    puts("Wrong!");
  return 0;
}

main函数中有下面这段代码，它是用来还原或解密代码的。

  for ( i = 0; i <= 181; ++i )
    *((_BYTE *)judge + i) ^= 0xCu;

解决方法有两个：

1. 静态方法

使用脚本还原代码。在File-》script command中输入脚本：

在“run”后，可看到judge的数值已经发生改变。

选择judge，按“c” 然后“p”，得到下面的代码：

.data:0000000000600B00                 public judge
.data:0000000000600B00 judge           proc near               ; CODE XREF: main+80↑p
.data:0000000000600B00                                         ; DATA XREF: main+16↑r ...
.data:0000000000600B00
.data:0000000000600B00 var_28          = qword ptr -28h
.data:0000000000600B00 var_20          = byte ptr -20h
.data:0000000000600B00 var_1F          = byte ptr -1Fh
.data:0000000000600B00 var_1E          = byte ptr -1Eh
.data:0000000000600B00 var_1D          = byte ptr -1Dh
.data:0000000000600B00 var_1C          = byte ptr -1Ch
.data:0000000000600B00 var_1B          = byte ptr -1Bh
.data:0000000000600B00 var_1A          = byte ptr -1Ah
.data:0000000000600B00 var_19          = byte ptr -19h
.data:0000000000600B00 var_18          = byte ptr -18h
.data:0000000000600B00 var_17          = byte ptr -17h
.data:0000000000600B00 var_16          = byte ptr -16h
.data:0000000000600B00 var_15          = byte ptr -15h
.data:0000000000600B00 var_14          = byte ptr -14h
.data:0000000000600B00 var_13          = byte ptr -13h
.data:0000000000600B00 var_4           = dword ptr -4
.data:0000000000600B00
.data:0000000000600B00                 push    rbp
.data:0000000000600B01                 mov     rbp, rsp
.data:0000000000600B04                 mov     [rbp+var_28], rdi
.data:0000000000600B08                 mov     [rbp+var_20], 66h ; 'f'
.data:0000000000600B0C                 mov     [rbp+var_1F], 6Dh ; 'm'
.data:0000000000600B10                 mov     [rbp+var_1E], 63h ; 'c'
.data:0000000000600B14                 mov     [rbp+var_1D], 64h ; 'd'
.data:0000000000600B18                 mov     [rbp+var_1C], 7Fh
.data:0000000000600B1C                 mov     [rbp+var_1B], 6Bh ; 'k'
.data:0000000000600B20                 mov     [rbp+var_1A], 37h ; '7'
.data:0000000000600B24                 mov     [rbp+var_19], 64h ; 'd'
.data:0000000000600B28                 mov     [rbp+var_18], 3Bh ; ';'
.data:0000000000600B2C                 mov     [rbp+var_17], 56h ; 'V'
.data:0000000000600B30                 mov     [rbp+var_16], 60h ; '`'
.data:0000000000600B34                 mov     [rbp+var_15], 3Bh ; ';'
.data:0000000000600B38                 mov     [rbp+var_14], 6Eh ; 'n'
.data:0000000000600B3C                 mov     [rbp+var_13], 70h ; 'p'
.data:0000000000600B40                 mov     [rbp+var_4], 0
.data:0000000000600B47                 jmp     short loc_600B71
.data:0000000000600B49 ; ---------------------------------------------------------------------------
.data:0000000000600B49
.data:0000000000600B49 loc_600B49:                             ; CODE XREF: judge+75↓j
.data:0000000000600B49                 mov     eax, [rbp+var_4]
.data:0000000000600B4C                 movsxd  rdx, eax
.data:0000000000600B4F                 mov     rax, [rbp+var_28]
.data:0000000000600B53                 add     rax, rdx
.data:0000000000600B56                 mov     edx, [rbp+var_4]
.data:0000000000600B59                 movsxd  rcx, edx
.data:0000000000600B5C                 mov     rdx, [rbp+var_28]
.data:0000000000600B60                 add     rdx, rcx
.data:0000000000600B63                 movzx   edx, byte ptr [rdx]
.data:0000000000600B66                 mov     ecx, [rbp+var_4]
.data:0000000000600B69                 xor     edx, ecx
.data:0000000000600B6B                 mov     [rax], dl
.data:0000000000600B6D                 add     [rbp+var_4], 1
.data:0000000000600B71
.data:0000000000600B71 loc_600B71:                             ; CODE XREF: judge+47↑j
.data:0000000000600B71                 cmp     [rbp+var_4], 0Dh
.data:0000000000600B75                 jle     short loc_600B49
.data:0000000000600B77                 mov     [rbp+var_4], 0
.data:0000000000600B7E                 jmp     short loc_600BA9
.data:0000000000600B80 ; ---------------------------------------------------------------------------
.data:0000000000600B80
.data:0000000000600B80 loc_600B80:                             ; CODE XREF: judge+AD↓j
.data:0000000000600B80                 mov     eax, [rbp+var_4]
.data:0000000000600B83                 movsxd  rdx, eax
.data:0000000000600B86                 mov     rax, [rbp+var_28]
.data:0000000000600B8A                 add     rax, rdx
.data:0000000000600B8D                 movzx   edx, byte ptr [rax]
.data:0000000000600B90                 mov     eax, [rbp+var_4]
.data:0000000000600B93                 cdqe
.data:0000000000600B95                 movzx   eax, [rbp+rax+var_20]
.data:0000000000600B9A                 cmp     dl, al
.data:0000000000600B9C                 jz      short loc_600BA5
.data:0000000000600B9E                 mov     eax, 0
.data:0000000000600BA3                 jmp     short loc_600BB4
.data:0000000000600BA5 ; ---------------------------------------------------------------------------
.data:0000000000600BA5
.data:0000000000600BA5 loc_600BA5:                             ; CODE XREF: judge+9C↑j
.data:0000000000600BA5                 add     [rbp+var_4], 1
.data:0000000000600BA9
.data:0000000000600BA9 loc_600BA9:                             ; CODE XREF: judge+7E↑j
.data:0000000000600BA9                 cmp     [rbp+var_4], 0Dh
.data:0000000000600BAD                 jle     short loc_600B80
.data:0000000000600BAF                 mov     eax, 1
.data:0000000000600BB4
.data:0000000000600BB4 loc_600BB4:                             ; CODE XREF: judge+A3↑j
.data:0000000000600BB4                 pop     rbp
.data:0000000000600BB5                 retn
.data:0000000000600BB5 judge           endp

反编译后可得：

__int64 __fastcall judge(__int64 a1)
{
  char v2[5]; // [rsp+8h] [rbp-20h] BYREF
  char v3[9]; // [rsp+Dh] [rbp-1Bh] BYREF
  int i; // [rsp+24h] [rbp-4h]

  qmemcpy(v2, "fmcd", 4);
  v2[4] = 127;
  qmemcpy(v3, "k7d;V`;np", sizeof(v3));
  for ( i = 0; i <= 13; ++i )
    *(_BYTE *)(i + a1) ^= i;
  for ( i = 0; i <= 13; ++i )
  {
    if ( *(_BYTE *)(i + a1) != v2[i] )
      return 0LL;
  }
  return 1LL;
}

输出flag：

v2='fmcd'+chr(127)+'k7d;V`;np'

flag = ''
for i in range(14):
   flag+= chr(ord(v2[i])^i)
   
print(flag)

方法2：动态方法

在?if ( v5 == 14 && (unsigned int)judge((__int64)s) ) 这行下断点，这时解密方法已经执行，也就是可以跳转到judge函数查看解密后的代码。