buuctf 刮开有奖wp

buuctf 刮开有奖

1. 正常EXE文件，无壳，直接打开。
   

2. 毫无反应，ENTER退出，拖入IDA，F5。
   

3. 找到主要部分，开始分析程序。

BOOL __stdcall DialogFunc(HWND hDlg, UINT a2, WPARAM a3, LPARAM a4)
{
  const char *v4; // esi
  const char *v5; // edi
  int v7; // [esp+8h] [ebp-20030h]
  int v8; // [esp+Ch] [ebp-2002Ch]
  int v9; // [esp+10h] [ebp-20028h]
  int v10; // [esp+14h] [ebp-20024h]
  int v11; // [esp+18h] [ebp-20020h]
  int v12; // [esp+1Ch] [ebp-2001Ch]
  int v13; // [esp+20h] [ebp-20018h]
  int v14; // [esp+24h] [ebp-20014h]
  int v15; // [esp+28h] [ebp-20010h]
  int v16; // [esp+2Ch] [ebp-2000Ch]
  int v17; // [esp+30h] [ebp-20008h]
  CHAR String; // [esp+34h] [ebp-20004h]
  char v19; // [esp+35h] [ebp-20003h]
  char v20; // [esp+36h] [ebp-20002h]
  char v21; // [esp+37h] [ebp-20001h]
  char v22; // [esp+38h] [ebp-20000h]
  char v23; // [esp+39h] [ebp-1FFFFh]
  char v24; // [esp+3Ah] [ebp-1FFFEh]
  char v25; // [esp+3Bh] [ebp-1FFFDh]
  char v26; // [esp+10034h] [ebp-10004h]
  char v27; // [esp+10035h] [ebp-10003h]
  char v28; // [esp+10036h] [ebp-10002h]

  if ( a2 == 272 )
    return 1;
  if ( a2 != 273 )
    return 0;
  if ( (_WORD)a3 == 1001 )
  {
    memset(&String, 0, 0xFFFFu);
    GetDlgItemTextA(hDlg, 1000, &String, 0xFFFF);
    if ( strlen(&String) == 8 )
    {
      v7 = 90;
      v8 = 74;
      v9 = 83;
      v10 = 69;
      v11 = 67;
      v12 = 97;
      v13 = 78;
      v14 = 72;
      v15 = 51;
      v16 = 110;
      v17 = 103;
      sub_4010F0((int)&v7, 0, 10);
      memset(&v26, 0, 0xFFFFu);
      v26 = v23;
      v28 = v25;
      v27 = v24;
      v4 = (const char *)sub_401000(&v26, strlen(&v26));
      memset(&v26, 0, 0xFFFFu);
      v27 = v21;
      v26 = v20;
      v28 = v22;
      v5 = (const char *)sub_401000(&v26, strlen(&v26));
      if ( String == v7 + 34
        && v19 == v11
        && 4 * v20 - 141 == 3 * v9
        && v21 / 4 == 2 * (v14 / 9)
        && !strcmp(v4, "ak1w")
        && !strcmp(v5, "V1Ax") )
      {
        MessageBoxA(hDlg, "U g3t 1T!", "@_@", 0);
      }
    }
    return 0;
  }
  if ( (_WORD)a3 != 1 && (_WORD)a3 != 2 )
    return 0;
  EndDialog(hDlg, (unsigned __int16)a3);
  return 1;
}

4. sub_4010F0进行了一个数据的变换，直接尝试翻译成C语言执行。

   int __cdecl sub_4010F0(int a1, int a2, int a3)
   {
     int result; // eax
     int i; // esi
     int v5; // ecx
     int v6; // edx
   
     result = a3;
     for ( i = a2; i <= a3; a2 = i )
     {
       v5 = 4 * i;
       v6 = *(_DWORD *)(4 * i + a1);
       if ( a2 < result && i < result )
       {
         do
         {
           if ( v6 > *(_DWORD *)(a1 + 4 * result) )
           {
             if ( i >= result )
               break;
             ++i;
             *(_DWORD *)(v5 + a1) = *(_DWORD *)(a1 + 4 * result);
             if ( i >= result )
               break;
             while ( *(_DWORD *)(a1 + 4 * i) <= v6 )
             {
               if ( ++i >= result )
                 goto LABEL_13;
             }
             if ( i >= result )
               break;
             v5 = 4 * i;
             *(_DWORD *)(a1 + 4 * result) = *(_DWORD *)(4 * i + a1);
           }
           --result;
         }
         while ( i < result );
       }
   LABEL_13:
       *(_DWORD *)(a1 + 4 * result) = v6;
       sub_4010F0(a1, a2, i - 1);
       result = a3;
       ++i;
     }
     return result;
   }
   

   翻译的C语言程序

    #include
    int sub_40(int a[10],int a2,int a3)
    {
   int result; // eax
    int i; // esi
    int v5; // ecx
    int v6; // edx
   result = a3;
    for ( i = a2; i <= a3; a2 = i )
    {
      v5 = i;
      v6 = a[i];
      if ( a2 < result && i < result )
      {
        do
        {
          if ( v6 > a[result] )
          {
            if ( i >= result )
              break;
            ++i;
            a[v5] = a[result];
            if ( i >= result )
              break;
            while ( a[i] <= v6 )
            {
              if ( ++i >= result )
                goto LABEL_13;
            }
            if ( i >= result )
              break;
            v5 = i;
           a[result] = a[i];
          }
          --result;
        }
        while ( i < result );
      }
     LABEL_13:
     a[result] = v6;
      sub_40(a, a2, i - 1);
      result = a3;
      ++i;
     	}
    	return result;
    }
    void main()
    {
    	int i;
    	int a[20]={90,74,83,69,67,97,78,72,51,110,103};
    	sub_40(a,0,10);
    	for (i=0;i<=10;i++)
    		printf("%4c",a[i]);
    	printf("\n");
    	for (i=0;i<=10;i++)
    		printf("%4d",a[i]);
    	printf("\n");
    }
   

   运行结果
   

5. 继续分析，sub_401000，猜测是一个base64加密过程。
   
   

6. 通过base64解密和上面转换后的数值，得到最后flag中的内容为"UJWP1jMp"

2020年8月8日23点04分