测试文件：https://adworld.xctf.org.cn/media/task/attachments/5d4117b968684b9483d0d4464e0a6fea
这道题要使用到gdb文件调试，gdb调试相关知识：https://www.cnblogs.com/Mayfly-nymph/p/11403150.html
1.准备
获得信息
- 32位文件
2.IDA打开
获得main函数的C语言代码
int __cdecl main(int argc, const char **argv, const char **envp) { setlocale(6, &locale); banner(); prompt_authentication(); authenticate(); return 0; }
2.1 代码分析
通过分析，可知authenticate();函数，储存着flag，进入函数
1 void authenticate() 2 { 3 int ws[8192]; // [esp+1Ch] [ebp-800Ch] 4 wchar_t *s2; // [esp+801Ch] [ebp-Ch] 5 6 s2 = decrypt(&s, &dword_8048A90); 7 if ( fgetws(ws, 0x2000, stdin) ) 8 { 9 ws[wcslen(ws) - 1] = 0; 10 if ( !wcscmp(ws, s2) ) 11 wprintf(&unk_8048B44); 12 else 13 wprintf(&unk_8048BA4); 14 } 15 free(s2); 16 }
通过第10~13行代码，我们可以知道s2就是我们需要flag(ws为输入值)
unk_8048B44内容
.rodata:08048B44 unk_8048B44 db 53h ; S ; DATA XREF: authenticate+78↑o .rodata:08048B45 db 0 .rodata:08048B46 db 0 .rodata:08048B47 db 0 .rodata:08048B48 db 75h ; u .rodata:08048B49 db 0 .rodata:08048B4A db 0 .rodata:08048B4B db 0 .rodata:08048B4C db 63h ; c .rodata:08048B4D db 0 .rodata:08048B4E db 0 .rodata:08048B4F db 0 .rodata:08048B50 db 63h ; c .rodata:08048B51 db 0 .rodata:08048B52 db 0 .rodata:08048B53 db 0 .rodata:08048B54 db 65h ; e .rodata:08048B55 db 0 .rodata:08048B56 db 0 .rodata:08048B57 db 0 .rodata:08048B58 db 73h ; s .rodata:08048B59 db 0 .rodata:08048B5A db 0 .rodata:08048B5B db 0 .rodata:08048B5C db 73h ; s .rodata:08048B5D db 0 .rodata:08048B5E db 0 .rodata:08048B5F db 0 .rodata:08048B60 db 21h ; ! .rodata:08048B61 db 0 .rodata:08048B62 db 0 .rodata:08048B63 db 0 .rodata:08048B64 db 20h .rodata:08048B65 db 0 .rodata:08048B66 db 0 .rodata:08048B67 db 0 .rodata:08048B68 db 57h ; W .rodata:08048B69 db 0 .rodata:08048B6A db 0 .rodata:08048B6B db 0 .rodata:08048B6C db 65h ; e .rodata:08048B6D db 0 .rodata:08048B6E db 0 .rodata:08048B6F db 0 .rodata:08048B70 db 6Ch ; l .rodata:08048B71 db 0 .rodata:08048B72 db 0 .rodata:08048B73 db 0 .rodata:08048B74 db 63h ; c .rodata:08048B75 db 0 .rodata:08048B76 db 0 .rodata:08048B77 db 0 .rodata:08048B78 db 6Fh ; o .rodata:08048B79 db 0 .rodata:08048B7A db 0 .rodata:08048B7B db 0 .rodata:08048B7C db 6Dh ; m .rodata:08048B7D db 0 .rodata:08048B7E db 0 .rodata:08048B7F db 0 .rodata:08048B80 db 65h ; e .rodata:08048B81 db 0 .rodata:08048B82 db 0 .rodata:08048B83 db 0 .rodata:08048B84 db 20h .rodata:08048B85 db 0 .rodata:08048B86 db 0 .rodata:08048B87 db 0 .rodata:08048B88 db 62h ; b .rodata:08048B89 db 0 .rodata:08048B8A db 0 .rodata:08048B8B db 0 .rodata:08048B8C db 61h ; a .rodata:08048B8D db 0 .rodata:08048B8E db 0 .rodata:08048B8F db 0 .rodata:08048B90 db 63h ; c .rodata:08048B91 db 0 .rodata:08048B92 db 0 .rodata:08048B93 db 0 .rodata:08048B94 db 6Bh ; k .rodata:08048B95 db 0 .rodata:08048B96 db 0 .rodata:08048B97 db 0 .rodata:08048B98 db 21h ; ! .rodata:08048B99 db 0 .rodata:08048B9A db 0 .rodata:08048B9B db 0 .rodata:08048B9C db 0Ah .rodata:08048B9D db 0 .rodata:08048B9E db 0 .rodata:08048B9F db 0 .rodata:08048BA0 db 0 .rodata:08048BA1 db 0 .rodata:08048BA2 db 0 .rodata:08048BA3 db 0
unk_8048BA4内容
.rodata:08048BA4 unk_8048BA4 db 41h ; A ; DATA XREF: authenticate:loc_804878F↑o .rodata:08048BA5 db 0 .rodata:08048BA6 db 0 .rodata:08048BA7 db 0 .rodata:08048BA8 db 63h ; c .rodata:08048BA9 db 0 .rodata:08048BAA db 0 .rodata:08048BAB db 0 .rodata:08048BAC db 63h ; c .rodata:08048BAD db 0 .rodata:08048BAE db 0 .rodata:08048BAF db 0 .rodata:08048BB0 db 65h ; e .rodata:08048BB1 db 0 .rodata:08048BB2 db 0 .rodata:08048BB3 db 0 .rodata:08048BB4 db 73h ; s .rodata:08048BB5 db 0 .rodata:08048BB6 db 0 .rodata:08048BB7 db 0 .rodata:08048BB8 db 73h ; s .rodata:08048BB9 db 0 .rodata:08048BBA db 0 .rodata:08048BBB db 0 .rodata:08048BBC db 20h .rodata:08048BBD db 0 .rodata:08048BBE db 0 .rodata:08048BBF db 0 .rodata:08048BC0 db 64h ; d .rodata:08048BC1 db 0 .rodata:08048BC2 db 0 .rodata:08048BC3 db 0 .rodata:08048BC4 db 65h ; e .rodata:08048BC5 db 0 .rodata:08048BC6 db 0 .rodata:08048BC7 db 0 .rodata:08048BC8 db 6Eh ; n .rodata:08048BC9 db 0 .rodata:08048BCA db 0 .rodata:08048BCB db 0 .rodata:08048BCC db 69h ; i .rodata:08048BCD db 0 .rodata:08048BCE db 0 .rodata:08048BCF db 0 .rodata:08048BD0 db 65h ; e .rodata:08048BD1 db 0 .rodata:08048BD2 db 0 .rodata:08048BD3 db 0 .rodata:08048BD4 db 64h ; d .rodata:08048BD5 db 0 .rodata:08048BD6 db 0 .rodata:08048BD7 db 0 .rodata:08048BD8 db 21h ; ! .rodata:08048BD9 db 0 .rodata:08048BDA db 0 .rodata:08048BDB db 0 .rodata:08048BDC db 0Ah .rodata:08048BDD db 0 .rodata:08048BDE db 0 .rodata:08048BDF db 0 .rodata:08048BE0 db 0 .rodata:08048BE1 db 0 .rodata:08048BE2 db 0 .rodata:08048BE3 db 0
2.2 authenticate()函数
回到authenticate()函数的汇编代码
1 .text:08048708 push ebp 2 .text:08048709 mov ebp, esp 3 .text:0804870B sub esp, 8028h 4 .text:08048711 mov dword ptr [esp+4], offset dword_8048A90 ; wchar_t * 5 .text:08048719 mov dword ptr [esp], offset s ; s 6 .text:08048720 call decrypt 7 .text:08048725 mov [ebp+s2], eax 8 .text:08048728 mov eax, ds:stdin@@GLIBC_2_0 9 .text:0804872D mov [esp+8], eax ; stream 10 .text:08048731 mov dword ptr [esp+4], 2000h ; n 11 .text:08048739 lea eax, [ebp+ws] 12 .text:0804873F mov [esp], eax ; ws 13 .text:08048742 call _fgetws 14 .text:08048747 test eax, eax 15 .text:08048749 jz short loc_804879C 16 .text:0804874B lea eax, [ebp+ws] 17 .text:08048751 mov [esp], eax ; s 18 .text:08048754 call _wcslen 19 .text:08048759 sub eax, 1 20 .text:0804875C mov [ebp+eax*4+ws], 0 21 .text:08048767 mov eax, [ebp+s2] 22 .text:0804876A mov [esp+4], eax ; s2 23 .text:0804876E lea eax, [ebp+ws] 24 .text:08048774 mov [esp], eax ; s1 25 .text:08048777 call _wcscmp 26 .text:0804877C test eax, eax 27 .text:0804877E jnz short loc_804878F 28 .text:08048780 mov eax, offset unk_8048B44 29 .text:08048785 mov [esp], eax 30 .text:08048788 call _wprintf 31 .text:0804878D jmp short loc_804879C
通过第6~7行代码，我们可以知道eax储存着decryp函数返回的flag值，再保存到s2
decrypt函数
wchar_t *__cdecl decrypt(wchar_t *s, wchar_t *a2) { size_t v2; // eax signed int v4; // [esp+1Ch] [ebp-1Ch] signed int i; // [esp+20h] [ebp-18h] signed int v6; // [esp+24h] [ebp-14h] signed int v7; // [esp+28h] [ebp-10h] wchar_t *dest; // [esp+2Ch] [ebp-Ch] v6 = wcslen(s); v7 = wcslen(a2); v2 = wcslen(s); dest = (wchar_t *)malloc(v2 + 1); wcscpy(dest, s); while ( v4 < v6 ) { for ( i = 0; i < v7 && v4 < v6; ++i ) dest[v4++] -= a2[i]; } return dest; }
2.3 gdb调试准备
综上所述，我们需要的flag保存在eax中，因此我们可以将断点设置在decrypt函数处，单步执行后，eax保存着我们需要的值，再读取eax值即可。
3.gdb调试
3.1 调试文件
gdb pro -q
设置断点
b decrypt
执行到断点
r
单步执行decrypt
n
显示寄存器
i r
eax 0x804e800 134539264 ecx 0x1480 5248 edx 0x7d 125 ebx 0x0 0 esp 0xffff5300 0xffff5300 ebp 0xffffd328 0xffffd328 esi 0xf7fac000 -134561792 edi 0xf7fac000 -134561792 eip 0x8048725 0x8048725 <authenticate+29> eflags 0x282 [ SF IF ] cs 0x23 35 ss 0x2b 43 ds 0x2b 43 es 0x2b 43 fs 0x0 0 gs 0x63 99
查看eax的值
x/6sw $eax
6：显示6行数据
s：字符串形式
w：word（4字节）形式
0x804e800: U"9447{you_are_an_international_mystery}" 0x804e89c: U"W 01xf7ade1e8xf7ade1eaxf7ade1ecxf7ade1eexf7ade1f0xf7ade1f2xf7ade1f4xf7ade1f6xf7ade1f8xf7ade1fa 01xf7ade200xf7ade204xf7ade208xf7ade20cxf7ade210xf7ade214xf7ade218xf7ade21cxf7ade220xf7ade224xf7ade228xf7ade22axf7ade22cxf7ade22exf7ade230xf7ade232xf7ade234xf7ade236xf7ade238xf7ade23a 60 61 62 63 64 65 66 67 70 71x175axf7ade268xf7ae3fd0xf7aefaa0xf7af5808 01xf7b07f4c" 0x804e968: U"xf7b07f54" 0x804e970: U"" 0x804e974: U"xf7b07f7cxf7b088b8xf7b09234xf7b0aa74xf7b0acecxf7b0af64xf7b0b29cxf7b0d194xf7b0f08cxf7b0f3c4xf7b0f67cxf7b10f74xf7b127b4xf7b138e4xf7b14854xf7b14b6cxf7b1a988xf7b1fba4 21x435f687ax54552e4ex382d46 21x435f687ax54552e4ex382d46!x804ea00 01x804ea80 01" 0x804e9f4: U""
4.get flag！
9447{you_are_an_international_mystery}