【攻防世界】Crypto系列之easychallenge

这道题的附件是一个pyc文件，pyc是python代码经过编译器python虚拟机编译后生成的二进制文件，所以需要对其进行反编译获取其源代码

这里使用第三方python反编译器uncompyle6来进行反编译，首先使用pip安装

pip install uncompyle6

安装完成后，对pyc文件进行反编译获取源代码

uncompyle6 -o easychallenge.py easychallenge.pyc

源代码如下

# uncompyle6 version 3.9.0
# Python bytecode version base 2.7 (62211)
# Decompiled from: Python 3.8.9 (tags/v3.8.9:a743f81, Apr  6 2021, 14:02:34) [MSC v.1928 64 bit (AMD64)]
# Embedded file name: ans.py
# Compiled at: 2018-08-09 11:29:44
import base64

def encode1(ans):
    s = ''
    for i in ans:
        x = ord(i) ^ 36
        x = x + 25
        s += chr(x)

    return s


def encode2(ans):
    s = ''
    for i in ans:
        x = ord(i) + 36
        x = x ^ 36
        s += chr(x)

    return s


def encode3(ans):
    return base64.b32encode(ans)


flag = ' '
print 'Please Input your flag:'
flag = raw_input()
final = 'UC7KOWVXWVNKNIC2XCXKHKK2W5NLBKNOUOSK3LNNVWW3E==='
if encode3(encode2(encode1(flag))) == final:
    print 'correct'
else:
    print 'wrong'

上述代码就是将flag经过encode1()、encode2()和encode3()这三个函数处理后得到一串base64编码，只要将其一步一步进行反推复原即可推出flag

反推步骤：

• 调用base64.b32decode()解base64编码
• 调用decode2()对字符串进行**(i ^ 36) - 36**计算操作
• 调用decode1()对字符串进行**(ord(i) -25)^36**计算操作

代码如下

import base64

final = 'UC7KOWVXWVNKNIC2XCXKHKK2W5NLBKNOUOSK3LNNVWW3E==='
def decode3(ans):
    return base64.b32decode(ans)

def decode2(ans):
    s = ''
    for i in ans:
        i = (i ^ 36) - 36
        s += chr(i)
    return s
def decode1(ans):
    s = ''
    for i in ans:
        i = (ord(i) -25)^36
        s += chr(i)
    return s


print(decode1(decode2(decode3(final))))

得到flag:

cyberpeace{interestinghhhhh}

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