[GXYCTF2019]luck_guy

unsigned __int64 get_flag()
{
  unsigned int v0; // eax
  int i; // [rsp+4h] [rbp-3Ch]
  int j; // [rsp+8h] [rbp-38h]
  __int64 s; // [rsp+10h] [rbp-30h] BYREF
  char v5; // [rsp+18h] [rbp-28h]
  unsigned __int64 v6; // [rsp+38h] [rbp-8h]

  v6 = __readfsqword(0x28u);
  v0 = time(0LL);  //得到时间
  srand(v0);   //使用时间作为种子生成随机数字
  for ( i = 0; i <= 4; ++i )
  {
    switch ( rand() % 200 ) //产生1-199随机数
    {
      case 1:
        puts("OK, it's flag:");
        memset(&s, 0, 0x28uLL);
        strcat((char *)&s, f1); // f1 = GXY{do_not_
        strcat((char *)&s, &f2); //初始化f2
        printf("%s", (const char *)&s);
        break;
      case 2:
        printf("Solar not like you");
        break;
      case 3:
        printf("Solar want a girlfriend");
        break;
      case 4:
        s = 9180147350284624745LL;
        v5 = 0;
        strcat(&f2, (const char *)&s);  //f2和s进行拼接
        break;
      case 5:  //对拼接后的f2进行处理
        for ( j = 0; j <= 7; ++j )
        {
          if ( j % 2 == 1 )
            *(&f2 + j) -= 2;
          else
            --*(&f2 + j);
        }
        break;
      default:
        puts("emmm,you can't find flag 23333");
        break;
    }
  }
  return __readfsqword(0x28u) ^ v6;
}

0x12 | 0x34 | 0x56 | 0x78

0x78 | 0x56 | 0x34 | 0x12
可见，大端模式和字符串的存储模式类似。

flag = 'GXY{do_not_'
f2 = [0x7F,0x66,0x6F,0x60,0x67,0x75,0x63,0x69][::-1]
for i in range(len(f2)):
    if(i%2==1):
        s = chr(f2[i] - 2)
    else:
        s = chr(f2[i] -1)
    flag = flag + s
print(flag)