python反编译全流程

[NISACTF 2022]ezpython

1、将exe文件转换为pyc文件格式

此题附件下载下来后为exe文件格式，我们需要用到pyinstxtractor.py这个工具来将exe文件转成pyc格式

在pyinstxtractor.py的文件夹中cmd，输入

python pyinstxtractor.py 文件名

2、修改magic number

经过以上操作后会生成一个文件夹，打开文件夹后找到struct和src文件，添加pyc后缀，再用winhex把他俩打开

在struct.pyc中把E3前的magic number复制到src.pyc开头,注意！是复制不是修改！

3、pyc反编译成py文件

推荐python反编译在线工具

反编译后得到以下代码

# Visit https://www.lddgo.net/string/pyc-compile-decompile for more information
# Version : Python 3.4

import rsa
import base64
key1 = rsa.PrivateKey.load_pkcs1(base64.b64decode('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'))
key2 = rsa.PublicKey.load_pkcs1(base64.b64decode('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'))

def encrypt1(message):
    crypto_text = rsa.encrypt(message.encode(), key2)
    return crypto_text


def decrypt1(message):
    message_str = rsa.decrypt(message, key1).decode()
    return message_str


def encrypt2(tips, key):
    ltips = len(tips)
    lkey = len(key)
    secret = []
    num = 0
    for each in tips:
        if num >= lkey:
            num = num % lkey
        secret.append(chr(ord(each) ^ ord(key[num])))
        num += 1
    
    return base64.b64encode(''.join(secret).encode()).decode()


def decrypt2(secret, key):
    tips = base64.b64decode(secret.encode()).decode()
    ltips = len(tips)
    lkey = len(key)
    secret = []
    num = 0
    for each in tips:
        if num >= lkey:
            num = num % lkey
        secret.append(chr(ord(each) ^ ord(key[num])))
        num += 1
    
    return ''.join(secret)

flag = 'IAMrG1EOPkM5NRI1cChQDxEcGDZMURptPzgHJHUiN0ASDgUYUB4LGQMUGAtLCQcJJywcFmddNno/PBtQbiMWNxsGLiFuLwpiFlkyP084Ng0lKj8GUBMXcwEXPTJrRDMdNwMiHVkCBFklHgIAWQwgCz8YQhp6E1xUHgUELxMtSh0xXzxBEisbUyYGOx1DBBZWPg1CXFkvJEcxO0ADeBwzChIOQkdwXQRpQCJHCQsaFE4CIjMDcwswTBw4BS9mLVMLLDs8HVgeQkscGBEBFSpQFQQgPTVRAUpvHyAiV1oPE0kyADpDbF8AbyErBjNkPh9PHiY7O1ZaGBADMB0PEVwdCxI+MCcXARZiPhwfH1IfKitGOF42FV8FTxwqPzBPAVUUOAEKAHEEP2QZGjQVV1oIS0QBJgBDLx1jEAsWKGk5Nw03MVgmWSE4Qy5LEghoHDY+OQ9dXE44Th0='
key = 'this is key'

try:
    result = input('please input key: ')
    if result == decrypt2('AAAAAAAAAAAfFwwRSAIWWQ==', key):
        print(decrypt1(base64.b64decode(decrypt2(flag, result))))
    elif result == key:
        print('flag{0e26d898-b454-43de-9c87-eb3d122186bc}')
    else:
        print('key is error.')
except Exception:
    e = None
    
    try:
        pass
    finally:
        e = None
        del e

4、开始操作

分析到这段代码

其中elif是假的flag，而真的flag就藏在第一个if语句中，我们先要获得他的key，然后就可以得到flag了

在所有代码的左后一行加上

print(decrypt2('AAAAAAAAAAAfFwwRSAIWWQ==',key))

就可以获得真正的key‘this is true key’

然后再将真正的key输入进去

总结

首先利用pyinstxtractor.py将exe文件换成pyc文件，再将src的magic number补充完整，接着将补充好的src.pyc拖入在线反编译工具中，得到py文件，最后分析源码得到key.