[GXYCTF2019]luck_guy 

链接：https://pan.baidu.com/s/1hoU9 uQsSeGr-6p7RBSm56w
提取码：ed2z

 

0x01 查看有无加壳

 

没有加壳，64位文件

 

0x02 打开IDA64，查看main函数，F5反编译

 

 

0x03 进入patch_me函数，接着进入get_flag函数界面

 

 

 

0x04 进行代码分析 😞

 1 __int64 get_flag()
 2 {
 3   unsigned int v0; // eax@1
 4   signed int i; // [sp+4h] [bp-3Ch]@1
 5   signed int j; // [sp+8h] [bp-38h]@7
 6   __int64 s; // [sp+10h] [bp-30h]@3
 7   char v5; // [sp+18h] [bp-28h]@6
 8   __int64 v6; // [sp+38h] [bp-8h]@1
 9 
10   v6 = *MK_FP(__FS__, 40LL);
11   v0 = time(0LL);               //得到时间
12   srand(v0);                    //使用时间作为种子生成随机数字
13   for ( i = 0; i <= 4; ++i )
14   {
15     switch ( rand() % 200 )      // 产生1-199之间的随机数
16     {
17       case 1:
18         puts("OK, it's flag:");      
19         memset(&s, 0, 0x28uLL); 
20         strcat((char *)&s, f1);   // f1='GXY{do_not_'
21         strcat((char *)&s, &f2);  //f2初始为空
22         printf("%s", &s);
23         break;
24       case 2:
25         printf("Solar not like you");
26         break;
27       case 3:
28         printf("Solar want a girlfriend");
29         break;
30       case 4:
31         v5 = 0;
32         s = 9180147350284624745LL;  //在IDA里面选中9180147350284624745LL按H键转换16进制为0x69,0x63,0x75,0x67,0x60,0x6f,0x66,0x7f    
33         strcat(&f2, (const char *)&s);  //f2和s拼接
34         break;
35       case 5:
36         for ( j = 0; j <= 7; ++j )
37         {
38           if ( ((((unsigned int)((unsigned __int64)j >> 32) >> 31) + (_BYTE)j) & 1)
39              - ((unsigned int)((unsigned __int64)j >> 32) >> 31) == 1 )
40             *(&f2 + j) -= 2;
41           else
42             --*(&f2 + j);
43         }
44         break;
45       default:
46         puts("emmm,you can't find flag 23333");
47         break;
48     }
49   }
50   return *MK_FP(__FS__, 40LL) ^ v6;
51 }

 

 

flag就是由f1和f2组成，f1已经告诉，现在只需要求f2就行。

  • case4给f2赋值

  • case5，对f2进行处理

所以顺序是: case5>case4>case1

 

0x05 脚本构建 🍔

由于IDA是反编译C语言，s=0x69,0x63,0x75,0x67,0x60,0x6f,0x66,0x7f应该逆序成s = 0x7F666F6067756369LL作为小端储存，关于大小端推荐师傅的**文章**

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

 

 

GXY{do_not_hate_me}

 

posted @ 2021-04-08 21:56  原来是甘文川同学  阅读(1208)  评论(0)    收藏  举报