攻防世界-Reverse新手区WP--no-strings-attached

int __cdecl main(int argc, const char **argv, const char **envp)
{
  setlocale(6, &locale);   //locale = 0
  banner();
  prompt_authentication();
  authenticate();
  return 0;
}

int banner()
{
  unsigned int v0; // eax


  v0 = time(0);
  srand(v0);
  wprintf(&unk_80488B0);   //Welcome to cyber malware control software.
  rand();
  return wprintf(&unk_8048960);    //Currently tracking %d bots worldwide
/*-----------------------------------------------------*/
int prompt_authentication()
{
  return wprintf(&unk_80489F8);     //Please enter authentication details:
}
/*------------------------------------------------------*/
void authenticate()
{
  wchar_t ws[8192];// [esp+1Ch] [ebp-800Ch]
  wchar_t *s2;// [esp+801Ch] [ebp-Ch]


  s2 = (wchar_t *)decrypt(&s, &dword_8048A90);       //s2由decrypt函数加密而来
  if ( fgetws(ws, 0x2000, stdin) )        //输入字符串ws
  {
    ws[wcslen(ws) - 1] = 0;
    if ( !wcscmp(ws, s2) )        //字符串ws和s2比较 s2为flag
      wprintf(&unk_8048B44);//Sucess! Welcome back!
    else
      wprintf(&unk_8048BA4);//Access dinied!
  }
  free(s2);
}
/*--------------------------------------------------*/
wchar_t *__cdecl decrypt(wchar_t *s, wchar_t *a2)
{
  size_t v2; // eax
  signed int v4; // [esp+1Ch] [ebp-1Ch]
  signed int i; // [esp+20h] [ebp-18h]
  signed int v6; // [esp+24h] [ebp-14h]
  signed int v7; // [esp+28h] [ebp-10h]
  wchar_t *dest; // [esp+2Ch] [ebp-Ch]


  v6 = wcslen(s);
  v7 = wcslen(a2);
  v2 = wcslen(s);
  dest = (wchar_t *)malloc(v2 + 1);
  wcscpy(dest, s);
  while ( v4 < v6 )
  {
    for ( i = 0; i < v7 && v4 < v6; ++i )
      dest[v4++] -= a2[i];
  }
  return dest;  //这里的返回值dest就是s2的值，即flag
}

text:08048658 ; int __cdecl decrypt(wchar_t *s, wchar_t *)
.text:08048658                 public decrypt
.text:08048658 decrypt         proc near               ; CODE XREF: authenticate+18↓p
.text:08048658
.text:08048658 var_1C          = dword ptr -1Ch
.text:08048658 var_18          = dword ptr -18h
.text:08048658 var_14          = dword ptr -14h
.text:08048658 var_10          = dword ptr -10h
.text:08048658 dest            = dword ptr -0Ch
.text:08048658 s               = dword ptr  8
.text:08048658 arg_4           = dword ptr  0Ch
.text:08048658
.text:08048658 ; __unwind {
.text:08048658                 push    ebp
.text:08048659                 mov     ebp, esp
.text:0804865B                 push    ebx
.text:0804865C                 sub     esp, 34h
.text:0804865F                 mov     eax, [ebp+s]
.text:08048662                 mov     [esp], eax      ; s
.text:08048665                 call    _wcslen
.text:0804866A                 mov     [ebp+var_14], eax
.text:0804866D                 mov     eax, [ebp+arg_4]
.text:08048670                 mov     [esp], eax      ; s
.text:08048673                 call    _wcslen
.text:08048678                 mov     [ebp+var_10], eax
.text:0804867B                 mov     ebx, [ebp+s]
.text:0804867E                 mov     eax, [ebp+s]
.text:08048681                 mov     [esp], eax      ; s
.text:08048684                 call    _wcslen
.text:08048689                 add     eax, 1
.text:0804868C                 mov     [esp], eax      ; size
.text:0804868F                 call    _malloc
.text:08048694                 mov     [ebp+dest], eax
.text:08048697                 mov     [esp+4], ebx    ; src
.text:0804869B                 mov     eax, [ebp+dest]        #这里可以看到，dest值是传进了寄存器eax
.text:0804869E                 mov     [esp], eax      ; dest
.text:080486A1                 call    _wcscpy
.text:080486A6                 mov     [ebp+var_18], 0
.text:080486AD                 jmp     short loc_80486F7

(gdb) file a          # file命令 读取文件（文件名为我随便重命名的a）
Reading symbols from a...(no debugging symbols found)...done.
(gdb) b decrypt          # b 在decrypt处设置断点
Breakpoint 1 at 0x804865c
(gdb) r                # r 运行程序(run) 显示在断点处停止
Starting program: /home/terra/11 
Welcome to cyber malware control software.
Currently tracking 1092326991 bots worldwide


Breakpoint 1, 0x0804865c in decrypt ()
(gdb) n                # n 单步步入
Single stepping until exit from function decrypt,
which has no line number information.
0x08048725 in authenticate ()
(gdb) x/5sw $eax  # x就是查看内存内容 5表示查看的单元个数 s是以字符串形式 w是按字单元(word) $eax表示查看eax寄存器内容 
0x804cfd0:  U"9447{you_are_an_international_mystery}"
0x804d06c:  U""
0x804d070:  U""
0x804d074:  U""
0x804d078:  U""