[C++知识库]2022DASCTF X SU 三月春季挑战赛-reverse-easyre（复现）

先用PE查壳，一开始没看清楚，以为没壳，动调了半天，后来才看到是个ASP壳

用工具脱壳后分析main函数

int __cdecl main(int argc, const char **argv, const char **envp)
{
  _BYTE v4[50]; // [esp+1Ch] [ebp-74h] BYREF
  _BYTE v5[50]; // [esp+4Eh] [ebp-42h] BYREF
  _DWORD v6[3]; // [esp+80h] [ebp-10h] BYREF
  int v7; // [esp+8Ch] [ebp-4h]

  sub_40DCF0();
  sub_41A070((int)v6);
  v7 = sub_41A078((int)v6);
  sub_47C7D0(&dword_487F00, v5);                // 输入函数，输入到v5
  if ( sub_41A038((int)v5) != 42 )              // 长度为42
  {
    sub_47BAB0(&dword_488140, (int)aWrong);
    sub_41A060(0);
  }
  if ( v5[0] != 'D' || v5[1] != 'A' || v5[2] != 'S' || v5[3] != 'C' || v5[4] != 'T' || v5[5] != 'F' )
  {
    sub_47BAB0(&dword_488140, (int)aWrong);
    sub_41A060(0);
  }
  sub_41A080((int)v4, (int)v5);                 // 拷贝
  v6[2] = *(_DWORD *)(v7 + 20) + 1900;
  v6[1] = 0;
  sub_4019BE((int)v5);                          // 
  sub_401771((int)v4);                          // 加密函数
  sub_41A088(aPause);
  return 0;
}

加密函数

_DWORD *__cdecl sub_401771(int a1)
{
  int v2[50]; // [esp+1Ch] [ebp-DCh] BYREF
  int v3; // [esp+E4h] [ebp-14h]
  int j; // [esp+E8h] [ebp-10h]
  int i; // [esp+ECh] [ebp-Ch]

  v3 = sub_41A038(a1);                          // 长度
  sub_401500();                                 // dword_492040[i] = i,最大为255
  sub_40152B();                                 // byte_492440[i] = 1~6,最大为255
  sub_401593();
  sub_401619(a1, v3);
  for ( i = 0; i < v3; ++i )
    byte_492A60[i] = (LOBYTE(dword_492940[i]) ^ *(_BYTE *)(i + a1)) + 71;
  memset(v2, 0, sizeof(v2));
  v2[0] = -61;
  v2[1] = -128;
  v2[2] = -43;
  v2[3] = -14;
  v2[4] = -101;
  v2[5] = 48;
  v2[6] = 11;
  v2[7] = -76;
  v2[8] = 85;
  v2[9] = -34;
  v2[10] = 34;
  v2[11] = -125;
  v2[12] = 47;
  v2[13] = -105;
  v2[14] = -72;
  v2[15] = 32;
  v2[16] = 29;
  v2[17] = 116;
  v2[18] = -47;
  v2[19] = 1;
  v2[20] = 115;
  v2[21] = 26;
  v2[22] = -78;
  v2[23] = -56;
  v2[24] = -59;
  v2[25] = 116;
  v2[26] = -64;
  v2[27] = 91;
  v2[28] = -9;
  v2[29] = 15;
  v2[30] = -45;
  v2[31] = 1;
  v2[32] = 85;
  v2[33] = -78;
  v2[34] = -92;
  v2[35] = -82;
  v2[36] = 123;
  v2[37] = -84;
  v2[38] = 92;
  v2[39] = 86;
  v2[40] = -68;
  v2[41] = 35;
  for ( j = 0; j <= 41; ++j )
  {
    if ( v2[j] != byte_492A60[j] )
      sub_41A060(0);
  }
  return sub_47BAB0(&dword_488140, (int)aRight);
}

把里面这些复制出来运行就能看到flag，但是我比赛的时候得到的只有大部分的flag，有些字符不正确，然后我就一直在那调代码，结果还是如下图

后面调着调着感觉问题越来越大，然后卡那了

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int v2[50] = {0};   //v2是加密后的密文
    int i,j,k;
  v2[0] = -61;
  v2[1] = -128;
  v2[2] = -43;
  v2[3] = -14;
  v2[4] = -101;
  v2[5] = 48;
  v2[6] = 11;
  v2[7] = -76;
  v2[8] = 85;
  v2[9] = -34;
  v2[10] = 34;
  v2[11] = -125;
  v2[12] = 47;
  v2[13] = -105;
  v2[14] = -72;
  v2[15] = 32;
  v2[16] = 29;
  v2[17] = 116;
  v2[18] = -47;
  v2[19] = 1;
  v2[20] = 115;
  v2[21] = 26;
  v2[22] = -78;
  v2[23] = -56;
  v2[24] = -59;
  v2[25] = 116;
  v2[26] = -64;
  v2[27] = 91;
  v2[28] = -9;
  v2[29] = 15;
  v2[30] = -45;
  v2[31] = 1;
  v2[32] = 85;
  v2[33] = -78;
  v2[34] = -92;
  v2[35] = -82;
  v2[36] = 123;
  v2[37] = -84;
  v2[38] = 92;
  v2[39] = 86;
  v2[40] = -68;
  v2[41] = 35;
   int dword_492040[257];
   char byte_492440[257];
   char  v1[8] = "123456";
   for ( i = 0; i <= 255; ++i )
   {
     dword_492040[i] = i;
     byte_492440[i] = v1[i % 6];
   }
   int  v3 = 0;
   int v8 = 0;
   for ( i = 0; i <= 255; ++i )
  {
    v3 = ((char)byte_492440[i] + v3 + dword_492040[i]) % 256;
    v8 = dword_492040[i];                       // 交换顺序
    dword_492040[i] = dword_492040[v3];
    dword_492040[v3] = v8;

  }
  int  v5 = 0;
  int v6 = 0;
  int v4 = 0;
  int a2 = 42;
  int dword_492940[73];
  for ( i = 0; a2--; dword_492940[v5++] = dword_492040[(dword_492040[v6] + dword_492040[i]) % 256] )
  {

    i = (i + 1) % 256;
    v6 = (v6 + dword_492040[i] ) % 256;
    v4 = dword_492040[i] + 66;

    dword_492040[i] = dword_492040[v6] - 33;
    dword_492040[i] ^= 2;
    dword_492040[v6] = 5 * v4;
    dword_492040[v6] = dword_492040[i] - 10;
    dword_492040[v6] += dword_492040[i];
    dword_492040[i] -= 18;
  }
   unsigned char flag[43] = {0};
   for ( j = 0; j <= 41; ++j )
  {
    flag[j] = ((v2[j] - 71) ^ dword_492940[j]);
   //printf("%d %c %d\n",flag[j],flag[j],j);
   printf("%c",flag[j]);
  }



  return 0;

}

最后运行出来得到

DASCbF{Welc0me-t0?j01n-SU l0ve-suyuQ1Sg1e}

参考大佬博客1
参考大佬博客2

我看了下别人的wp，他们的脚本运行出来都不能完整的得到flag，总会有几个字符对不上，就很奇怪，他们的flag都是最后一个个试的么。

最终flag是：DASCTF{Welc0me-t0-j01n-SU-l0ve-suyug1eg1e}