SCTF2020 signin writeup

1.丢进ida，查看字符串。发现带有python相关字眼，且是pyinstaller打包的。

2.用pyinstxtractor将exe编译回pyc

3.使用工具uncompyle6反编译pyc。由于是python3.8写的，所以存在几个情况

• 1.在python3.8环境下编译，可以直接打开。
• 2.缺乏python3.8环境，打不开。winhex添加pyc头部magic number，换成3.8的。经师傅提醒，可以查看同文件夹下的pyc文件，参考并修改。（可留多个版本的pyc文件，作为参考）
• 3.python版本不同，pyc头部magic number不同。

4.打开uncompyle6反编译成功后的main.py

# uncompyle6 version 3.7.2
# Python bytecode 3.8 (3413)
# Decompiled from: Python 3.7.1 (v3.7.1:260ec2c36a, Oct 20 2018, 14:57:15) [MSC v.1915 64 bit (AMD64)]
# Embedded file name: main.py
# Compiled at: 1995-09-28 00:18:56
# Size of source mod 2**32: 184549648 bytes
import sys
from PyQt5.QtCore import *
from PyQt5.QtWidgets import *
from signin import *
from mydata import strBase64
from ctypes import *
import _ctypes
from base64 import b64decode
import os

class AccountChecker:
# 可知： 1.算法在tmp.dll的enc里  2.用户名为SCTFer 
#        密码最终结果为  b'PLHCu+fujfZmMOMLGHCyWWOq5H5HDN2R5nHnlV30Q0EA'

    def __init__(self):
        self.dllname = './tmp.dll'
        self.dll = self._AccountChecker__release_dll()
        self.enc = self.dll.enc
        self.enc.argtypes = (c_char_p, c_char_p, c_char_p, c_int)
        self.enc.restype = c_int
        self.accounts = {b'SCTFer': b64decode(b'PLHCu+fujfZmMOMLGHCyWWOq5H5HDN2R5nHnlV30Q0EA')}

        self.try_times = 0

    def __release_dll(self):
        with open(self.dllname, 'wb') as (f):
            f.write(b64decode(strBase64.encode('ascii')))
        return WinDLL(self.dllname)

    def clean(self):
        _ctypes.FreeLibrary(self.dll._handle)
        if os.path.exists(self.dllname):
            os.remove(self.dllname)

    def _error(self, error_code):
        errormsg = {0:'Unknown Error', 
         1:'Memory Error'}
        QMessageBox.information(None, 'Error', errormsg[error_code], QMessageBox.Abort, QMessageBox.Abort)
        sys.exit(1)

    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


class SignInWnd(QMainWindow, Ui_QWidget):

    def __init__(self, checker, parent=None):
        super().__init__(parent)
        self.checker = checker
        self.setupUi(self)
        self.PB_signin.clicked.connect(self.on_confirm_button_clicked)

    @pyqtSlot()
    def on_confirm_button_clicked(self):
        username = bytes((self.LE_usrname.text()), encoding='ascii')
        password = bytes((self.LE_pwd.text()), encoding='ascii')
        if username == b'' or password == b'':
            self.check_input_msgbox()
        else:
            self.msgbox(self.checker.check(username, password))

    def check_input_msgbox(self):
        QMessageBox.information(None, 'Error', 'Check Your Input!', QMessageBox.Ok, QMessageBox.Ok)

    def msgbox(self, status):
        msg_ex = {0:'', 
         1:'', 
         2:"It's no big deal, try again!", 
         3:'Useful information is in the binary, guess what?'}
        msg = 'Succeeded! Flag is your password' if status else 'Failed to sign in\n' + msg_ex[(self.checker.try_times % 4)]
        QMessageBox.information(None, 'SCTF2020', msg, QMessageBox.Ok, QMessageBox.Ok)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    checker = AccountChecker()
    sign_in_wnd = SignInWnd(checker)
    sign_in_wnd.show()
    app.exec()
    checker.clean()
    sys.exit()
# okay decompiling C:\Users\admin\Desktop\main.pyc

运行时将动态链接tmp.dll复制出来，丢入ida分析。如下

值得注意的是：如果按v6>=0去编写代码，结果会是错的。需要是==1.如下

贴上官方代码，本人代码太丑。

from base64 import *
import struct


def u_qword(a):
    return struct.unpack('>= 1
            qword |= 0x8000000000000000
            continue
        else:
            qword >>= 1
        qwords[i] = qword
print(qwords)   # [13105084052108931695, 2298581059073635890, 16408946688824694790, 148766366331253429]
pwd = []
for i in range(4):
    pwd.append(p_qword(qwords[i]).decode())

flag = ''.join(pwd)
print(flag)
#print(pwd)

• python
• CRC32基本算法