# 6.2.8 re DefcampCTF2015 entry_language - [题目解析](#题目解析) - [参考资料](#参考资料) [下载文件](../src/writeup/6.2.8_re_defcampctf2015_entry_language) ## 题目解析 这是一题标准的密码验证题，输入一个字符串，程序验证对误。 ```text $ file entry_language defcamp_r100: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.24, BuildID[sha1]=0f464824cc8ee321ef9a80a799c70b1b6aec8168, stripped ``` ```text $ ./entry_language Enter the password: ABCD Incorrect password! ``` 为了与 angr 的自动化做对比，我们先使用传统的方法，逆向算法求解，`main` 函数和验证函数 `fcn.004006fd` 如下： ```text [0x00400610]> pdf @ main / (fcn) main 153 | main (); | ; var int local_110h @ rbp-0x110 | ; var int local_8h @ rbp-0x8 | ; DATA XREF from 0x0040062d (entry0) | 0x004007e8 55 push rbp | 0x004007e9 4889e5 mov rbp, rsp | 0x004007ec 4881ec100100. sub rsp, 0x110 | 0x004007f3 64488b042528. mov rax, qword fs:[0x28] ; [0x28:8]=-1 ; '(' ; 40 | 0x004007fc 488945f8 mov qword [local_8h], rax | 0x00400800 31c0 xor eax, eax | 0x00400802 bf37094000 mov edi, str.Enter_the_password: ; 0x400937 ; "Enter the password: " | 0x00400807 b800000000 mov eax, 0 | 0x0040080c e8affdffff call sym.imp.printf ; int printf(const char *format) | 0x00400811 488b15500820. mov rdx, qword [obj.stdin] ; [0x601068:8]=0 | 0x00400818 488d85f0feff. lea rax, [local_110h] | 0x0040081f beff000000 mov esi, 0xff ; 255 | 0x00400824 4889c7 mov rdi, rax | 0x00400827 e8b4fdffff call sym.imp.fgets ; char *fgets(char *s, int size, FILE *stream) | 0x0040082c 4885c0 test rax, rax | ,=< 0x0040082f 7435 je 0x400866 | | 0x00400831 488d85f0feff. lea rax, [local_110h] | | 0x00400838 4889c7 mov rdi, rax | | 0x0040083b e8bdfeffff call fcn.004006fd ; 调用验证函数 | | 0x00400840 85c0 test eax, eax | ,==< 0x00400842 7511 jne 0x400855 | || 0x00400844 bf4c094000 mov edi, str.Nice_ ; 0x40094c ; "Nice!" | || 0x00400849 e852fdffff call sym.imp.puts ; int puts(const char *s) | || 0x0040084e b800000000 mov eax, 0 | ,===< 0x00400853 eb16 jmp 0x40086b | ||| ; JMP XREF from 0x00400842 (main) | |`--> 0x00400855 bf52094000 mov edi, str.Incorrect_password_ ; 0x400952 ; "Incorrect password!" | | | 0x0040085a e841fdffff call sym.imp.puts ; int puts(const char *s) | | | 0x0040085f b801000000 mov eax, 1 | |,==< 0x00400864 eb05 jmp 0x40086b | ||| ; JMP XREF from 0x0040082f (main) | ||`-> 0x00400866 b800000000 mov eax, 0 | || ; JMP XREF from 0x00400864 (main) | || ; JMP XREF from 0x00400853 (main) | ``--> 0x0040086b 488b4df8 mov rcx, qword [local_8h] | 0x0040086f 6448330c2528. xor rcx, qword fs:[0x28] | ,=< 0x00400878 7405 je 0x40087f | | 0x0040087a e831fdffff call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void) | | ; JMP XREF from 0x00400878 (main) | `-> 0x0040087f c9 leave \ 0x00400880 c3 ret [0x00400610]> pdf @ fcn.004006fd / (fcn) fcn.004006fd 171 | fcn.004006fd (int arg_bh); | ; var int local_38h @ rbp-0x38 | ; var int local_24h @ rbp-0x24 | ; var int local_20h @ rbp-0x20 | ; var int local_18h @ rbp-0x18 | ; var int local_10h @ rbp-0x10 | ; arg int arg_bh @ rbp+0xb | ; CALL XREF from 0x0040083b (main) | 0x004006fd 55 push rbp | 0x004006fe 4889e5 mov rbp, rsp | 0x00400701 48897dc8 mov qword [local_38h], rdi | 0x00400705 c745dc000000. mov dword [local_24h], 0 | 0x0040070c 48c745e01409. mov qword [local_20h], str.Dufhbmf ; 0x400914 ; "Dufhbmf" | 0x00400714 48c745e81c09. mov qword [local_18h], str.pG_imos ; 0x40091c ; "pG`imos" | 0x0040071c 48c745f02409. mov qword [local_10h], str.ewUglpt ; 0x400924 ; "ewUglpt" | 0x00400724 c745dc000000. mov dword [local_24h], 0 | ,=< 0x0040072b eb6e jmp 0x40079b | | ; JMP XREF from 0x0040079f (fcn.004006fd) | .--> 0x0040072d 8b4ddc mov ecx, dword [local_24h] | :| 0x00400730 ba56555555 mov edx, 0x55555556 | :| 0x00400735 89c8 mov eax, ecx | :| 0x00400737 f7ea imul edx | :| 0x00400739 89c8 mov eax, ecx | :| 0x0040073b c1f81f sar eax, 0x1f | :| 0x0040073e 29c2 sub edx, eax | :| 0x00400740 89d0 mov eax, edx | :| 0x00400742 01c0 add eax, eax | :| 0x00400744 01d0 add eax, edx | :| 0x00400746 29c1 sub ecx, eax | :| 0x00400748 89ca mov edx, ecx | :| 0x0040074a 4863c2 movsxd rax, edx | :| 0x0040074d 488b74c5e0 mov rsi, qword [rbp + rax*8 - 0x20] | :| 0x00400752 8b4ddc mov ecx, dword [local_24h] | :| 0x00400755 ba56555555 mov edx, 0x55555556 | :| 0x0040075a 89c8 mov eax, ecx | :| 0x0040075c f7ea imul edx | :| 0x0040075e 89c8 mov eax, ecx | :| 0x00400760 c1f81f sar eax, 0x1f | :| 0x00400763 29c2 sub edx, eax | :| 0x00400765 89d0 mov eax, edx | :| 0x00400767 01c0 add eax, eax | :| 0x00400769 4898 cdqe | :| 0x0040076b 4801f0 add rax, rsi ; '+' | :| 0x0040076e 0fb600 movzx eax, byte [rax] | :| 0x00400771 0fbed0 movsx edx, al | :| 0x00400774 8b45dc mov eax, dword [local_24h] | :| 0x00400777 4863c8 movsxd rcx, eax | :| 0x0040077a 488b45c8 mov rax, qword [local_38h] | :| 0x0040077e 4801c8 add rax, rcx ; '&' | :| 0x00400781 0fb600 movzx eax, byte [rax] | :| 0x00400784 0fbec0 movsx eax, al | :| 0x00400787 29c2 sub edx, eax | :| 0x00400789 89d0 mov eax, edx | :| 0x0040078b 83f801 cmp eax, 1 ; 1 | ,===< 0x0040078e 7407 je 0x400797 ; = 1 时跳转，验证成功 | |:| 0x00400790 b801000000 mov eax, 1 ; 返回 1，验证失败 | ,====< 0x00400795 eb0f jmp 0x4007a6 | ||:| ; JMP XREF from 0x0040078e (fcn.004006fd) | |`---> 0x00400797 8345dc01 add dword [local_24h], 1 ; i = i + 1 | | :| ; JMP XREF from 0x0040072b (fcn.004006fd) | | :`-> 0x0040079b 837ddc0b cmp dword [local_24h], 0xb ; [0xb:4]=-1 ; 11 | | `==< 0x0040079f 7e8c jle 0x40072d ; i <= 11 时跳转 | | 0x004007a1 b800000000 mov eax, 0 ; 返回 0 | | ; JMP XREF from 0x00400795 (fcn.004006fd) | `----> 0x004007a6 5d pop rbp \ 0x004007a7 c3 ret ``` 整理后可以得到下面的伪代码： ```C int fcn_004006fd(int *passwd) { char *str_1 = "Dufhbmf"; char *str_2 = "pG`imos"; char *str_3 = "ewUglpt"; for (int i = 0; i <= 11; i++) { if((&str_3)[i % 3][2 * (1 / 3)] - *(i + passwd) != 1) { return 1; } } return 0; } ``` 然后写出逆向脚本： ```python str_list = ["Dufhbmf", "pG`imos", "ewUglpt"] passwd = [] for i in range(12): passwd.append(chr(ord(str_list[i % 3][2 * (i / 3)]) - 1)) print ''.join(passwd) ``` 逆向算法似乎也很简单，但如果连算法都不用逆的话，下面就是见证 angr 魔力的时刻，我们只需要指定让程序运行到 `0x400844`，即验证通过时的位置，而不用管验证的逻辑是怎么样的。 完整的脚本如下： ```python import angr project = angr.Project("entry_language", auto_load_libs=False) @project.hook(0x400844) def print_flag(state): print "FLAG SHOULD BE:", state.posix.dump_fd(0) project.terminate_execution() project.execute() ``` Bingo!!! ```text $ python2 solve_angr.py FLAG SHOULD BE: Code_Talkers $ ./entry_language Enter the password: Code_Talkers Nice! ``` ## 参考资料 -