python-trade

题目来源： NJUPT CTF 2017

题目描述：菜鸡和菜猫进行了一场Py交易

 

 

下载下来发现是pyc文件，也就是python的字节码文件

首先通过命令uncompyle6 python_trade.pyc > trade.py 将字节码反编译成源码

源码如下：

# uncompyle6 version 3.7.4
# Python bytecode 2.7 (62211)
# Decompiled from: Python 3.8.5 (tags/v3.8.5:580fbb0, Jul 20 2020, 15:43:08) [MSC v.1926 32 bit (Intel)]
# Embedded file name: 1.py
# Compiled at: 2017-06-03 10:20:43
import base64

def encode(message):
    s = ''
    for i in message:
        x = ord(i) ^ 32
        x = x + 16
        s += chr(x)

    return base64.b64encode(s)


correct = 'XlNkVmtUI1MgXWBZXCFeKY+AaXNt'
flag = ''
print 'Input flag:'
flag = raw_input()
if encode(flag) == correct:
    print 'correct'
else:
    print 'wrong'
# okay decompiling python-trade.pyc

 

可以看到correct是由正确的flag将每个字符先异或32再加上16组成的字符串base64编码得到的

因此反过来，只需要把它base64解码，然后每个字符先减16再异或32即可得到flag

nctf{d3c0mpil1n9_PyC}