re | [NPUCTF2020]BasicASM

这个题很神奇，给了一个.s文件和一个txt文件。

打开一看，txt文件是输出，.s文件是汇编代码，x86-64的汇编，完全不难，大概看看就知道干了些啥了，这里贴出分析：

  1 00007FF7A8AC5A50  push        rbp  
  2 00007FF7A8AC5A52  push        rdi  
  3 00007FF7A8AC5A53  sub         rsp,238h  
  4 00007FF7A8AC5A5A  lea         rbp,[rsp+20h]  
  5 00007FF7A8AC5A5F  mov         rdi,rsp  
  6 00007FF7A8AC5A62  mov         ecx,8Eh  
  7 00007FF7A8AC5A67  mov         eax,0CCCCCCCCh  
  8 00007FF7A8AC5A6C  rep stos    dword ptr [rdi]  
  9 00007FF7A8AC5A6E  mov         rax,qword ptr [__security_cookie (07FF7A8AD3018h)]  
 10 00007FF7A8AC5A75  xor         rax,rbp  
 11 00007FF7A8AC5A78  mov         qword ptr [rbp+208h],rax  
 12 00007FF7A8AC5A7F  lea         rcx,[__06A15900_ConsoleApplication@cpp (07FF7A8AD902Ah)]  
 13 00007FF7A8AC5A86  call        __CheckForDebuggerJustMyCode (07FF7A8AC1122h)                                ;反调试
 14 00007FF7A8AC5A8B  lea         rdx,[string "flag{this_is_a_fake_flag}" (07FF7A8ACF450h)]                    ;取假flag地址
 15 00007FF7A8AC5A92  lea         rcx,[flag]                                                                  ; flag地址
 16 00007FF7A8AC5A96  call        std::basic_string<char,std::char_traits<char>,std::allocator<char> >::basic_string<char,std::char_traits<char>,std::allocator<char> > (07FF7A8AC15E1h) 
 17                                                                                                         ;创建字符串？ 
 18 00007FF7A8AC5A9B  nop  
 19 00007FF7A8AC5A9C  mov         dword ptr [p],0  
 20 00007FF7A8AC5AA3  mov         dword ptr [rbp+64h],0  
 21 00007FF7A8AC5AAA  jmp         main+64h (07FF7A8AC5AB4h)                    ;跳到四行下
 22 00007FF7A8AC5AAC  mov         eax,dword ptr [rbp+64h]  
 23 00007FF7A8AC5AAF  inc         eax  
 24 00007FF7A8AC5AB1  mov         dword ptr [rbp+64h],eax  
 25 00007FF7A8AC5AB4  movsxd      rax,dword ptr [rbp+64h]                       ;跳转到这里 取DWORD 
 26 00007FF7A8AC5AB8  mov         qword ptr [rbp+1F8h],rax                      ; rbp+64h的DWORD --> rbp+1F8h的QWORD
 27 00007FF7A8AC5ABF  lea         rcx,[flag]                                    ;取flag地址
 28 00007FF7A8AC5AC3  call        std::basic_string<char,std::char_traits<char>,std::allocator<char> >::length (07FF7A8AC122Bh)  
 29                                                                                             ;获取字符串长度
 30 00007FF7A8AC5AC8  mov         rcx,qword ptr [rbp+1F8h]                     ;rcx = QWORD[rbp+1F8h]
 31 00007FF7A8AC5ACF  cmp         rcx,rax                                      ; 比较长度
 32 00007FF7A8AC5AD2  jae         main+1B2h (07FF7A8AC5C02h)                   ;若大于等于 -> 跳转到倒数几行
 33 00007FF7A8AC5AD8  mov         eax,dword ptr [rbp+64h]  
 34 00007FF7A8AC5ADB  and         eax,1                                       ; 取奇数位
 35 00007FF7A8AC5ADE  cmp         eax,1  
 36 00007FF7A8AC5AE1  jne         main+126h (07FF7A8AC5B76h)                   ;不是奇数位就跳转
 37 00007FF7A8AC5AE7  movsxd      rax,dword ptr [rbp+64h]                     ;DWORD [rbp+64h] --> i --> flag[i]
 38 00007FF7A8AC5AEB  mov         rdx,rax                                      ;第二个参数
 39 00007FF7A8AC5AEE  lea         rcx,[flag]  
 40 00007FF7A8AC5AF2  call        std::basic_string<char,std::char_traits<char>,std::allocator<char> >::operator[] (07FF7A8AC1442h) 
 41                                                                             ;依次取flag 
 42 00007FF7A8AC5AF7  movsx       eax,byte ptr [rax]                            ;将flag[i] 放进eax
 43 00007FF7A8AC5AFA  xor         eax,42h                                      ; flag[i] ^ 0x42
 44 00007FF7A8AC5AFD  mov         dword ptr [p],eax                             ; 异或后的字符放进p
 45 00007FF7A8AC5B00  mov         dl,30h  
 46 00007FF7A8AC5B02  lea         rcx,[rbp+144h]  
 47 00007FF7A8AC5B09  call        std::setfill<char> (07FF7A8AC1046h)         ;设置输出 
 48 00007FF7A8AC5B0E  mov         qword ptr [rbp+1F8h],rax  
 49 00007FF7A8AC5B15  mov         edx,2  
 50 00007FF7A8AC5B1A  lea         rcx,[rbp+168h]  
 51 00007FF7A8AC5B21  call        std::setw (07FF7A8AC10D2h)             ;设置输出宽度
 52 00007FF7A8AC5B26  mov         qword ptr [rbp+200h],rax  
 53 00007FF7A8AC5B2D  lea         rdx,[std::hex (07FF7A8AC1488h)]               ;输出16进制
 54 00007FF7A8AC5B34  mov         rcx,qword ptr [__imp_std::cout (07FF7A8AD71C0h)]  
 55 00007FF7A8AC5B3B  call        qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF7A8AD7160h)]  
 56                                                                                     ;输出
 57 00007FF7A8AC5B41  mov         rcx,qword ptr [rbp+200h]  
 58 00007FF7A8AC5B48  mov         rdx,rcx  
 59 00007FF7A8AC5B4B  mov         rcx,rax  
 60 00007FF7A8AC5B4E  call        std::operator<<<char,std::char_traits<char>,__int64> (07FF7A8AC12F8h)  
 61 00007FF7A8AC5B53  mov         rcx,qword ptr [rbp+1F8h]  
 62 00007FF7A8AC5B5A  mov         rdx,rcx  
 63 00007FF7A8AC5B5D  mov         rcx,rax  
 64 00007FF7A8AC5B60  call        std::operator<<<char,std::char_traits<char>,char> (07FF7A8AC11A4h)  
 65 00007FF7A8AC5B65  mov         edx,dword ptr [p]  
 66 00007FF7A8AC5B68  mov         rcx,rax  
 67 00007FF7A8AC5B6B  call        qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF7A8AD7158h)]  
 68 00007FF7A8AC5B71  jmp         main+1ADh (07FF7A8AC5BFDh)                      ;跳回循环开头
 69 00007FF7A8AC5B76  movsxd      rax,dword ptr [rbp+64h]                      ;这里应该是偶数位
 70 00007FF7A8AC5B7A  mov         rdx,rax  
 71 00007FF7A8AC5B7D  lea         rcx,[flag]  
 72 00007FF7A8AC5B81  call        std::basic_string<char,std::char_traits<char>,std::allocator<char> >::operator[] (07FF7A8AC1442h)  
 73 00007FF7A8AC5B86  movsx       eax,byte ptr [rax]  
 74 00007FF7A8AC5B89  mov         dword ptr [p],eax  
 75 00007FF7A8AC5B8C  mov         dl,30h  
 76 00007FF7A8AC5B8E  lea         rcx,[rbp+194h]  
 77 00007FF7A8AC5B95  call        std::setfill<char> (07FF7A8AC1046h)  
 78 00007FF7A8AC5B9A  mov         qword ptr [rbp+1F8h],rax  
 79 00007FF7A8AC5BA1  mov         edx,2  
 80 00007FF7A8AC5BA6  lea         rcx,[rbp+1B8h]  
 81 00007FF7A8AC5BAD  call        std::setw (07FF7A8AC10D2h)  
 82 00007FF7A8AC5BB2  mov         qword ptr [rbp+200h],rax  
 83 00007FF7A8AC5BB9  lea         rdx,[std::hex (07FF7A8AC1488h)]  
 84 00007FF7A8AC5BC0  mov         rcx,qword ptr [__imp_std::cout (07FF7A8AD71C0h)]  
 85 00007FF7A8AC5BC7  call        qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF7A8AD7160h)] 
 86                                                                             ;输出 
 87 00007FF7A8AC5BCD  mov         rcx,qword ptr [rbp+200h]  
 88 00007FF7A8AC5BD4  mov         rdx,rcx  
 89 00007FF7A8AC5BD7  mov         rcx,rax  
 90 00007FF7A8AC5BDA  call        std::operator<<<char,std::char_traits<char>,__int64> (07FF7A8AC12F8h)  
 91 00007FF7A8AC5BDF  mov         rcx,qword ptr [rbp+1F8h]  
 92 00007FF7A8AC5BE6  mov         rdx,rcx  
 93 00007FF7A8AC5BE9  mov         rcx,rax  
 94 00007FF7A8AC5BEC  call        std::operator<<<char,std::char_traits<char>,char> (07FF7A8AC11A4h)  
 95 00007FF7A8AC5BF1  mov         edx,dword ptr [p]  
 96 00007FF7A8AC5BF4  mov         rcx,rax  
 97 00007FF7A8AC5BF7  call        qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF7A8AD7158h)]  
 98 00007FF7A8AC5BFD  jmp         main+5Ch (07FF7A8AC5AACh)  
 99 00007FF7A8AC5C02  mov         dword ptr [rbp+1E4h],0                                                ;跳转到这里
100 00007FF7A8AC5C0C  lea         rcx,[flag]  
101 00007FF7A8AC5C10  call        std::basic_string<char,std::char_traits<char>,std::allocator<char> >::~basic_string<char,std::char_traits<char>,std::allocator<char> > (07FF7A8AC1302h)  
102 00007FF7A8AC5C15  mov         eax,dword ptr [rbp+1E4h]  
103 00007FF7A8AC5C1B  mov         edi,eax  
104 00007FF7A8AC5C1D  lea         rcx,[rbp-20h]  
105 00007FF7A8AC5C21  lea         rdx,[__xt_z+540h (07FF7A8ACEFE0h)]  
106 00007FF7A8AC5C28  call        _RTC_CheckStackVars (07FF7A8AC1596h)  
107 00007FF7A8AC5C2D  mov         eax,edi  
108 00007FF7A8AC5C2F  mov         rcx,qword ptr [rbp+208h]  
109 00007FF7A8AC5C36  xor         rcx,rbp  
110 00007FF7A8AC5C39  call        __security_check_cookie (07FF7A8AC1190h)  
111 00007FF7A8AC5C3E  lea         rsp,[rbp+218h]  
112 00007FF7A8AC5C45  pop         rdi  
113 00007FF7A8AC5C46  pop         rbp  
114 00007FF7A8AC5C47  ret

简单的说就是下标奇数做异或嘛，随便写个脚本好啦：

 1 Python 3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 21:26:53) [MSC v.1916 32 bit (Intel)] on win32
 2 Type "help", "copyright", "credits" or "license" for more information.
 3 >>>
 4 >>> a = "662e61257b26301d7972751d6b2c6f355f3a38742d74341d61776d7d7d"
 5 >>> b = bytes.fromhex(a)
 6 >>> b
 7 b'f.a%{&0\x1dyru\x1dk,o5_:8t-t4\x1dawm}}'
 8 >>> flag = []
 9 >>>
10 >>> for i in range(len(b)):
11 ...     if ((i & 1)-1) != 0:
12 ...             flag.append(chr(b[i]))
13 ...     else:
14 ...             for j in range(33,127):
15 ...                     if j^0x42 == b[i]:
16 ...                             flag.append(chr(j))
17 ...                             break
18 ...
19 >>> flag
20 ['f', 'l', 'a', 'g', '{', 'd', '0', '_', 'y', '0', 'u', '_', 'k', 'n', 'o', 'w', '_', 'x', '8', '6', '-', '6', '4', '_', 'a', '5', 'm', '?', '}']
21 >>> "".join(flag)
22 'flag{d0_y0u_know_x86-64_a5m?}'

这种给汇编的题第一次见，很有意思，还好我不是只会f5，233333333333.

over.

posted @ 2020-12-04 22:00  Mz1  阅读(396)  评论(0)  编辑  收藏  举报