BUUCTF [GXYCTF2019]luck_guy RE

无壳64位

用 IDA64 打开，查看主函数

int __cdecl main(int argc, const char **argv, const char **envp)
{
  int v4; // [rsp+14h] [rbp-Ch] BYREF
  unsigned __int64 v5; // [rsp+18h] [rbp-8h]

  v5 = __readfsqword(0x28u);
  welcome(argc, argv, envp);
  puts("_________________");
  puts("try to patch me and find flag");
  v4 = 0;
  puts("please input a lucky number");
  __isoc99_scanf("%d", &v4);
  patch_me(v4);
  puts("OK,see you again");
  return 0;
}

一眼找到patch_me()函数

int __fastcall patch_me(int a1)
{
  int result; // eax

  if ( a1 % 2 == 1 )
    result = puts("just finished");
  else
    result = get_flag();
  return result;
}

如果输入的是偶数就进入get_flag()函数，继续跟踪

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);
  srand(v0);
  for ( i = 0; i <= 4; ++i )
  {
    switch ( rand() % 200 )
    {
      case 1:
        puts("OK, it's flag:");
        memset(&s, 0, 0x28uLL);
        strcat((char *)&s, f1);
        strcat((char *)&s, &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 = 0x7F666F6067756369LL;
        v5 = 0;
        strcat(&f2, (const char *)&s);
        break;
      case 5:
        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;
}

随机生成五个数然后执行switch语句
case2、case3、default不重要
case1将f1和f2连接作为flag
其中f1已知为GXY{do_not_

f2为空

case4将s = 0x7F666F6067756369LL复制给f2
由于小端序所以f2=[0x69,0x63,0x75,0x67,0x60,0x6F,0x66,0x7F]
case5对f2进行调整
奇数位-2，偶数位-1
猜测执行顺序应该是case4，case5，case1
脚本如下

f2=[0x69,0x63,0x75,0x67,0x60,0x6F,0x66,0x7F]
flag="GXY{do_not_"
for i in range(8):
    if(i%2): flag+=chr(f2[i]-2)
    else: flag+=chr(f2[i]-1)
print(flag)

flagGXY{do_not_hate_me}
提交上去是错的，改成flag{do_not_hate_me}就对了