BUUCTF SimpleRev

Posted 2021-03-26 dirwang

SimpleRev

程序流程很清晰

 1 int __cdecl __noreturn main(int argc, const char **argv, const char **envp)
 2 {
 3   int v3; // eax
 4   char v4; // [rsp+Fh] [rbp-1h]
 5 
 6   while ( 1 )
 7   {
 8     while ( 1 )
 9     {
10       printf("Welcome to CTF game!
Please input d/D to start or input q/Q to quit this program: ", argv, envp);
11       v4 = getchar();
12       if ( v4 != ‘d‘ && v4 != ‘D‘ )
13         break;
14       Decry();
15     }
16     if ( v4 == ‘q‘ || v4 == ‘Q‘ )
17       Exit();
18     puts("Input fault format!");
19     v3 = getchar();
20     putchar(v3);
21   }
22 }

关键处理在Decry()中

 1 unsigned __int64 Decry()
 2 {
 3   char c; // [rsp+Fh] [rbp-51h]
 4   int j; // [rsp+10h] [rbp-50h]
 5   int index; // [rsp+14h] [rbp-4Ch]
 6   int i; // [rsp+18h] [rbp-48h]
 7   int v5; // [rsp+1Ch] [rbp-44h]
 8   char src[8]; // [rsp+20h] [rbp-40h]
 9   __int64 v7; // [rsp+28h] [rbp-38h]
10   int v8; // [rsp+30h] [rbp-30h]
11   __int64 v9; // [rsp+40h] [rbp-20h]
12   __int64 v10; // [rsp+48h] [rbp-18h]
13   int v11; // [rsp+50h] [rbp-10h]
14   unsigned __int64 v12; // [rsp+58h] [rbp-8h]
15 
16   v12 = __readfsqword(0x28u);
17   *(_QWORD *)src = ‘SLCDN‘;                     // NDCLS
18   v7 = 0LL;
19   v8 = 0;
20   v9 = ‘wodah‘;                                 // hadow
21   v10 = 0LL;
22   v11 = 0;
23   text = (char *)join(key3, &v9);               // text=killshadow
24   strcpy(key, key1);                            // ADSFK
25   strcat(key, src);                             // key=ADSFKNDCLS
26   j = 0;
27   index = 0;
28   getchar();
29   v5 = strlen(key);
30   for ( i = 0; i < v5; ++i )
31   {
32     if ( key[index % v5] > ‘@‘ && key[index % v5] <= ‘Z‘ )
33       key[i] = key[index % v5] + 32;            // key大写转小写
34     ++index;
35   }
36   printf("Please input your flag:", src);
37   while ( 1 )
38   {
39     c = getchar();
40     if ( c == ‘
‘ )
41       break;
42     if ( c == ‘ ‘ )
43     {
44       ++j;
45     }
46     else
47     {
48       if ( c <= 96 || c > ‘z‘ )
49       {
50         if ( c > 64 && c <= ‘Z‘ )               // 大写字母
51           str2[j] = (c - 39 - key[index++ % v5] + 97) % 26 + 97;// key=adsfkndcls
52       }
53       else                                      // 小写字母
54       {
55         str2[j] = (c - 39 - key[index++ % v5] + 97) % 26 + 97;
56       }
57       if ( !(index % v5) )
58         putchar(‘ ‘);
59       ++j;
60     }
61   }
62   if ( !strcmp(text, str2) )                    // killshadow
63     puts("Congratulation!
");
64   else
65     puts("Try again!
");
66   return __readfsqword(0x28u) ^ v12;
67 }

wp:

 1 lt=‘ABCDEFGHIJKLMNOPQRSTUVWXYZ‘
 2 key=list(‘ADSFKNDCLS‘.lower())
 3 klens=len(key)
 4 
 5 text=‘killshadow‘
 6 flag=‘‘
 7 for i in range(len(text)):
 8     str2=text[i]
 9     for c in lt:
10         if str2== chr((ord(c) - 39 - ord(key[i  % klens]) + 97) % 26 + 97):
11             flag+=c
12 print(‘flag{‘+flag+‘}‘)

flag{KLDQCUDFZO}

 

输入的大小写字母操作是一样的，按我理解这题多解，爆破时输入是大小写都可以，但这里只考虑了大写。本想linux下运行看看，结果提示 ：段错误（核心已转储）。0.0

 

 1 popk@popk-virtual-machine:~$ ulimit -c unlimited
 2 popk@popk-virtual-machine:~$ gdb ./SimpleRev core 
 3 GNU gdb (Ubuntu 8.1-0ubuntu3) 8.1.0.20180409-git
 4 Copyright (C) 2018 Free Software Foundation, Inc.
 5 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
 6 This is free software: you are free to change and redistribute it.
 7 There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
 8 and "show warranty" for details.
 9 This GDB was configured as "x86_64-linux-gnu".
10 Type "show configuration" for configuration details.
11 For bug reporting instructions, please see:
12 <http://www.gnu.org/software/gdb/bugs/>.
13 Find the GDB manual and other documentation resources online at:
14 <http://www.gnu.org/software/gdb/documentation/>.
15 For help, type "help".
16 Type "apropos word" to search for commands related to "word"...
17 Reading symbols from ./SimpleRev...(no debugging symbols found)...done.
18 [New LWP 2376]
19 Core was generated by `./SimpleRev‘.
20 Program terminated with signal SIGSEGV, Segmentation fault.
21 #0  0x0000000000000000 in ?? ()
22 (gdb) 

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