2020 SCTF 部分WriteUp
signin
准备
signin.exe：https://wwa.lanzous.com/inIQdec11zi
程序分析
可以判断出，这个程序实际上是由Python打包成的可执行文件，且在运行这个程序时，在同目录下产生了一个tmp.dll文件，猜测是程序调用某些函数的接口。
反编译
使用archive_viewer.py反编译为字节码文件
python archive_viewer.py signin.exe
修补文件
55 0D 0D 0A 00 00 00 00 70 79 69 30 10 01 00 00
程序是在Python3.8环境下打包，因此我们需要在Python3.8下使用uncompyle6
uncompyle6 main.pyc > main.py
得到py文件
1 # uncompyle6 version 3.7.2
2 # Python bytecode 3.8 (3413)
3 # Decompiled from: Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)]
4 # Embedded file name: main.py
5 # Compiled at: 1995-09-28 00:18:56
6 # Size of source mod 2**32: 272 bytes
7 import sys
8 from PyQt5.QtCore import *
9 from PyQt5.QtWidgets import *
10 from signin import *
11 from mydata import strBase64
12 from ctypes import *
13 import _ctypes
14 from base64 import b64decode
15 import os
16
17 class AccountChecker:
18
19 def __init__(self):
20 self.dllname = './tmp.dll'
21 self.dll = self._AccountChecker__release_dll()
22 self.enc = self.dll.enc
23 self.enc.argtypes = (c_char_p, c_char_p, c_char_p, c_int)
24 self.enc.restype = c_int
25 self.accounts = {b'SCTFer': b64decode(b'PLHCu+fujfZmMOMLGHCyWWOq5H5HDN2R5nHnlV30Q0EA')}
26 self.try_times = 0
27
28 def __release_dll(self):
29 with open(self.dllname, 'wb') as (f):
30 f.write(b64decode(strBase64.encode('ascii')))
31 return WinDLL(self.dllname)
32
33 def clean(self):
34 _ctypes.FreeLibrary(self.dll._handle)
35 if os.path.exists(self.dllname):
36 os.remove(self.dllname)
37
38 def _error(self, error_code):
39 errormsg = {0:'Unknown Error',
40 1:'Memory Error'}
41 QMessageBox.information(None, 'Error', errormsg[error_code], QMessageBox.Abort, QMessageBox.Abort)
42 sys.exit(1)
43
44 def __safe(self, username: bytes, password: bytes):
45 pwd_safe = b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
46 status = self.enc(username, password, pwd_safe, len(pwd_safe))
47 return (pwd_safe, status)
48
49 def check(self, username, password):
50 self.try_times += 1
51 if username not in self.accounts:
52 return False
53 encrypted_pwd, status = self._AccountChecker__safe(username, password)
54 if status == 1:
55 self._AccountChecker__error(1)
56 if encrypted_pwd != self.accounts[username]:
57 return False
58 self.try_times -= 1
59 return True
60
61
62 class SignInWnd(QMainWindow, Ui_QWidget):
63
64 def __init__(self, checker, parent=None):
65 super().__init__(parent)
66 self.checker = checker
67 self.setupUi(self)
68 self.PB_signin.clicked.connect(self.on_confirm_button_clicked)
69
70 @pyqtSlot()
71 def on_confirm_button_clicked(self):
72 username = bytes((self.LE_usrname.text()), encoding='ascii')
73 password = bytes((self.LE_pwd.text()), encoding='ascii')
74 if username == b'' or password == b'':
75 self.check_input_msgbox()
76 else:
77 self.msgbox(self.checker.check(username, password))
78
79 def check_input_msgbox(self):
80 QMessageBox.information(None, 'Error', 'Check Your Input!', QMessageBox.Ok, QMessageBox.Ok)
81
82 def msgbox(self, status):
83 msg_ex = {0:'',
84 1:'',
85 2:"It's no big deal, try again!",
86 3:'Useful information is in the binary, guess what?'}
87 msg = 'Succeeded! Flag is your password' if status else 'Failed to sign in\n' + msg_ex[(self.checker.try_times % 4)]
88 QMessageBox.information(None, 'SCTF2020', msg, QMessageBox.Ok, QMessageBox.Ok)
89
90
91 if __name__ == '__main__':
92 app = QApplication(sys.argv)
93 checker = AccountChecker()
94 sign_in_wnd = SignInWnd(checker)
95 sign_in_wnd.show()
96 app.exec()
97 checker.clean()
98 sys.exit()
99 # okay decompiling main.pyc
代码分析
通过代码我们能够了解到这些信息
1.
elf.dllname = './tmp.dll'
调用了tmp.dll文件作为接口。
2.
def __safe(self, username: bytes, password: bytes):
pwd_safe = b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
status = self.enc(username, password, pwd_safe, len(pwd_safe))
return (pwd_safe, status)
def check(self, username, password):
self.try_times += 1
if username not in self.accounts:
return False
encrypted_pwd, status = self._AccountChecker__safe(username, password)
if status == 1:
self._AccountChecker__error(1)
if encrypted_pwd != self.accounts[username]:
return False
self.try_times -= 1
return True
self.accounts = {b'SCTFer': b64decode(b'PLHCu+fujfZmMOMLGHCyWWOq5H5HDN2R5nHnlV30Q0EA')}
调用tmp.dll文件中的enc函数，传入username, password, pwd_safe, len(pwd_safe)，实际就是将password加密后存储到pwd_safe字节码中。最后用pwd_safe与b64decode(b'PLHCu+fujfZmMOMLGHCyWWOq5H5HDN2R5nHnlV30Q0EA')比较，且我们能够了解到用户名应该是SCTFer，且最后返回的status一个为非1。
打开tmp.dll文件，找到enc函数
观察代码，实际操作可以分为两部分，逆向分析
异或操作
第47~54行代码实际上就是将Dst与用户名循环异或，最后得到b64decode(b'PLHCu+fujfZmMOMLGHCyWWOq5H5HDN2R5nHnlV30Q0EA')，因此我们只需要逆向异或就能得到加密后的Dst
from base64 import *
username = "SCTFer"
pwd_safe = b64decode('PLHCu+fujfZmMOMLGHCyWWOq5H5HDN2R5nHnlV30Q0EA')
# print (len(pwd_safe))
num = ["%02x" % x for x in pwd_safe]
hex_num = [int(x, 16) for x in num]
print(num)
# print (len(num))
for i in range(32):
hex_num[i] ^= ord(username[i % len(username)])
# print (hex_num)
hex_nums = bytes.fromhex(''.join([hex(x)[2:].rjust(2, '0') for x in hex_num]))
print (hex_nums)
得到
b'o\xf2\x96\xfd\x82\x9c\xde\xb52v\x86yK3\xe6\x1f\x06\xd8\xb7=\x13J\xb8\xe3\xb52\xb3\xd38\x86\x10\x02\x00'
加密操作
每次传入了8字节数据进行加密（总共64字节），打开sub_180011311函数
仔细观察代码，实际上这部分代码是使用CRC32的查表法，对数据进行加密。
加密原理实际上就是CRC32算法---输入一组长度32的字符串，每8个字节分为1组，共4组。对每一组取首位，判断正负。正值，左移一位；负值，左移一位，再异或0xB0004B7679FA26B3。重复判断操作64次，得到查表法所用的表。
因此我们只需要将整个加密过程逆向操作得到查表法的表，再进行CRC32计算，就能得到输入。
secret = []
# for i in range(4):
# secret.append(int(hex_nums[i*8:(i + 1) * 8][::-1].hex(),16))
for i in range(4):
secret.append(int.from_bytes(hex_nums[i*8:(i + 1) * 8], byteorder="little"))
print (secret)
key = 0xB0004B7679FA26B3
flag = ""
for s in secret:
for i in range(64):
sign = s & 1
if sign == 1:
s ^= key
s //= 2
if sign == 1:
s |= 0x8000000000000000
print(hex(s))
j = 0
while j < 8:
flag += chr(s&0xFF)
s >>= 8
j += 1
print(flag)
因为计算机中采用小端排序，因此需要注意分组倒序。得到
脚本
from base64 import *
username = "SCTFer"
pwd_safe = b64decode('PLHCu+fujfZmMOMLGHCyWWOq5H5HDN2R5nHnlV30Q0EA')
# print (len(pwd_safe))
num = ["%02x" % x for x in pwd_safe]
hex_num = [int(x, 16) for x in num]
print(num)
# print (len(num))
for i in range(32):
hex_num[i] ^= ord(username[i % len(username)])
# print (hex_num)
hex_nums = bytes.fromhex(''.join([hex(x)[2:].rjust(2, '0') for x in hex_num]))
print (hex_nums)
secret = []
# for i in range(4):
# secret.append(int(hex_nums[i*8:(i + 1) * 8][::-1].hex(),16))
for i in range(4):
secret.append(int.from_bytes(hex_nums[i*8:(i + 1) * 8], byteorder="little"))
print (secret)
key = 0xB0004B7679FA26B3
flag = ""
for s in secret:
for i in range(64):
sign = s & 1
if sign == 1:
s ^= key
s //= 2
if sign == 1:
s |= 0x8000000000000000
print(hex(s))
j = 0
while j < 8:
flag += chr(s&0xFF)
s >>= 8
j += 1
print(flag)
get flag!
username:SCTFer
password:SCTF{We1c0m3_To_Sctf_2020_re_!!}
get_up
这是实际就是在说明输入字符串长度不大于6，且通过sub_401DF0返回1
返回1，实际就是满足加密后的Dst与32c1d123c193aecc4280a5d7925a2504相同，实际是MD5加密，得到输入为sycsyc
接下来，程序查找.reioc段，并将.reioc段数据与sycsyc循环异或，写出IDAPython脚本
beg_adr = 0x405000
dst = "sycsyc"
for i in range(0,0x200,16):
for j in range(16):
PatchByte(beg_adr+i+j,Byte(beg_adr+i+j)^ord(dst[j%6]))然后跟去混淆方法类似，先转换为Data，再强制分析数据，转换为函数。
打开sub_4027F0函数
这里实际和上面差不多，输入长度为30的flag，取前5个字符，与.ebata段的部分循环异或。（实际就是得到下面sub_404000函数的代码）。这里可以合理猜测flag的前五个字符为SCTF{,写出脚本
beg_adr = 0x404000
dst = "SCTF{"
for i in range(16,96):
PatchByte(beg_adr+i,Byte(beg_adr+i)^ord(dst[i%5]))
跟上面一样将.ebata段转换为data，再分析代码，转换为函数，开头如果提示栈不平衡，重新单独分析一下就行。
这里一个函数实际就是RC4生成S-BOX，另一个函数就是加密函数，key值为syclover
#include
#include
#include
typedef unsigned longULONG;
#pragma warning(disable:4996)
/*初始化函数*/
void rc4_init(unsigned char* s, unsigned char* key, unsigned long Len)
{
int i = 0, j = 0;
char k[256] = { 0 };
unsigned char tmp = 0;
for (i = 0; i < 256; i++)
{
s[i] = i;
k[i] = key[i % Len];
}
for (i = 0; i < 256; i++)
{
j = (j + s[i] + k[i]) % 256;
tmp = s[i];
s[i] = s[j]; // 交换s[i]和s[j]
s[j] = tmp;
}
}
/*加解密*/
void rc4_crypt(unsigned char* s, unsigned char* Data, unsigned long Len)
{
int i = 0, j = 0, t = 0;
unsigned long k = 0;
unsigned char tmp;
for (k = 0; k < Len; k++)
{
i = (i + 1) % 256;
j = (j + s[i]) % 256;
tmp = s[i];
s[i] = s[j]; // 交换s[x]和s[y]
s[j] = tmp;
t = (s[i] + s[j]) % 256;
Data[k] ^= s[t];
}
}
int main()
{
unsigned char s[256] = { 0 }, s2[256] = { 0 }; // S-box
char key[256] = { "syclover" };
char pData[512] = { 0 };
int v12[] = { 128,85,126,45,209,9,37,171,60,86,149,196,54,19,237,114,36,147,178,200,69,236,22,107,103,29,249,163,150,217 };
int i;
//char m[32] = { 0 };
for (int i = 0; i < 30; ++i) {
pData[i] = (char)v12[i];
}
unsigned long len = strlen(pData);
printf("pData=%s\n", pData);
printf("key=%s,length=%d\n\n", key, strlen(key));
rc4_init(s, (unsigned char*)key, strlen(key)); // 已经完成了初始化
printf("完成对S[i]的初始化，如下：\n\n");
for (i = 0; i < 256; i++)
{
printf("%02X", s[i]);
if (i && (i + 1) % 16 == 0)putchar('\n');
}
printf("\n\n");
for (i = 0; i < 256; i++) // 用s2[i]暂时保留经过初始化的s[i]，很重要的！！！
{
s2[i] = s[i];
}
printf("已经初始化，现在解密:\n\n");
printf("len = %d\n", len);
rc4_crypt(s, (unsigned char*)pData, len);
printf("pData=%s\n\n", pData);
system("PAUSE");
return 0;
}
SCTF{zzz~(|3[___]_rc4_5o_e4sy}
Can you hear
音频文件： https://wwa.lanzous.com/iMv4zecuw3g
音频是SSTV，用软件Robot36接受音频中的数据
doudizhu
只要将手里牌出完，就能获得flag
