[ctf.show.reverse] re3

逻辑很简单，但用python写的话由于python没有长度限制，直接用会有些问题需要解决。所以用gdb跟一下。

  v7 = 0x50;
  v8 = 0xFAE3;
  v9 = 0xD7D3F7B;
  v10 = 0xA43499F6;
  v11 = 5;
  v12 = 0x10;
  v13 = 0xEF9;
  v5 = 0;
  puts("plz input the key:");
  __isoc99_scanf("%s", s);
  v3 = strlen(s);
  strncpy(dest, v19, v3 - 6);                   // 去掉flag{
  dest[strlen(s) - 6] = 0;
  __isoc99_sscanf(dest, "%x", &v5);
  v17[0] = v7;
  v17[1] = v8;
  v17[2] = v9;
  v17[3] = v10;
  v17[4] = (v11 << 12) + v12;
  v17[5] = v13;
  v17[6] = v5;
  v16 = 0LL;
  for ( i = 0; i <= 6; ++i )
  {
    for ( v16 += (unsigned int)v17[i]; v16 > 0xFFFF; v16 = v15 + (unsigned int)(unsigned __int16)v16 )
    {
      v14 = (unsigned __int16)v16;
      v15 = v16 >> 16;
    }
  }
  if ( v16 == 0xFFFF )
    puts("OK");
  else
    puts("Error");   //断点下在这

在puts("Error")下断点，输入flag{0000}然后看v16的值，由于输入的是0，退出里得到v16的值与0xffff的差就是flag的值。

shi@ubuntu:~/xctf$ gdb ./r3
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
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Reading symbols from ./r3...
(No debugging symbols found in ./r3)
gdb-peda$ r
Starting program: /home/shi/xctf/r3 
plz input the key:
^C
Program received signal SIGINT, Interrupt.
......
gdb-peda$ vmmap
Start              End                Perm	Name
0x0000555555554000 0x0000555555555000 r-xp	/home/shi/xctf/r3
0x0000555555754000 0x0000555555755000 r--p	/home/shi/xctf/r3
0x0000555555755000 0x0000555555756000 rw-p	/home/shi/xctf/r3
......
gdb-peda$ b *0x00005555555549fc
Breakpoint 1 at 0x5555555549fc
gdb-peda$ r
Starting program: /home/shi/xctf/r3 
plz input the key:
flag{0000}
......
gdb-peda$ x/wx $rsp+0x38
0x7fffffffdee8:	0x0000e560
gdb-peda$ p 0xffff-0x0000e560
$1 = 0x1a9f
gdb-peda$ 

#flag{1a9f}