CTFshow-萌新赛逆向_签退

Posted 2021-03-06 anweilx

查看题目信息

 

 下载re3.pyc文件

使用uncompyle把re3.pyc反编译为re3.py

uncompyle6 re3.pyc > re3.py

 

 查看re3.py文件

# uncompyle6 version 3.6.4
# Python bytecode 2.7 (62211)
# Decompiled from: Python 2.7.15 (v2.7.15:ca079a3ea3, Apr 30 2018, 16:30:26) [MSC v.1500 64 bit (AMD64)]
# Embedded file name: re3.py
# Compiled at: 2020-03-06 17:43:28
import string
c_charset = string.ascii_uppercase + string.ascii_lowercase + string.digits + ‘()‘
flag = ‘BozjB3vlZ3ThBn9bZ2jhOH93ZaH9‘　　　　#这个是最后输出的密文

def encode(origin_bytes):　　　　#这个函数是base64加密
    c_bytes = [ (‘{:0>8}‘).format(str(bin(b)).replace(‘0b‘, ‘‘)) for b in origin_bytes ]
    resp = ‘‘
    nums = len(c_bytes) // 3
    remain = len(c_bytes) % 3
    integral_part = c_bytes[0:3 * nums]
    while integral_part:
        tmp_unit = (‘‘).join(integral_part[0:3])
        tmp_unit = [ int(tmp_unit[x:x + 6], 2) for x in [0, 6, 12, 18] ]
        resp += (‘‘).join([ c_charset[i] for i in tmp_unit ])
        integral_part = integral_part[3:]

    if remain:
        remain_part = (‘‘).join(c_bytes[3 * nums:]) + (3 - remain) * ‘0‘ * 8
        tmp_unit = [ int(remain_part[x:x + 6], 2) for x in [0, 6, 12, 18] ][:remain + 1]
        resp += (‘‘).join([ c_charset[i] for i in tmp_unit ]) + (3 - remain) * ‘.‘
    return rend(resp)　　　　#这个代码说明先进行base64加密，然后进行凯撒加密


def rend(s):

    def encodeCh(ch):　　　　#这个函数是凯撒加密
        f = lambda x: chr((ord(ch) - x + 2) % 26 + x)
        if ch.islower():
            return f(97)
        if ch.isupper():
            return f(65)
        return ch

    return (‘‘).join(encodeCh(c) for c in s)
# okay decompiling re3.pyc

文件分析在如上

首先使用凯撒密码进行解密

 

 把解密后的密文写入base.txt文件中

使用脚本进行解密

成功拿到flag

 

 

 

附上解密脚本

#! /usr/bin/env python
# _*_  coding:utf-8 _*_
import base64


filename = "base64.txt"
f = open(filename,‘r‘)
while True:
    text_base =f.readline()
    if not text_base:
        break
    else:
        text_str = str(base64.b64decode(text_base))
        if "flag" in text_str:
            text_str =text_str.replace("‘","").replace("b","")
            print(text_str)