2020 | 强网杯 | 强网先锋

bank

解题思路¶

nc连过去，要求输入三个字符，其与剩下的随机字符串进行 SHA256 加密的结果等于等号后的随机字符串

$ nc 39.101.134.52 8005
sha256(XXX+f2DXa00fbrrtXrZAV) == fad0b6f4dc03f907b999f15db8b467b17139189d2cc61f9fc37e213d91d0a2aa
Give me XXX:

使用 Python 破解

from hashlib import sha256
import itertools

table = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'

for ch in itertools.permutations(table, 3):
    m = ''.join(ch) + 'f2DXa00fbrrtXrZAV'
    h = sha256(m.encode()).hexdigest()
    if (h == 'fad0b6f4dc03f907b999f15db8b467b17139189d2cc61f9fc37e213d91d0a2aa'):
        print(m[0:3])

进入系统后，要求输入名字，随便输一个就可以

Welcome to the challenge!


give me your name:yanhui

your cash:10
you can choose: transact, view records, provide a record, get flag, hint

首先选择get flag查看，提示现金不足

> get flag
you need pay 1000 for the flag!
don't have enough money!

查看hint，给出了transact的加密方式

def transact_ecb(key, sender, receiver, amount):
    aes = AES.new(key, AES.MODE_ECB)
    ct = b""
    ct += aes.encrypt(sender)
    ct += aes.encrypt(receiver)
    ct += aes.encrypt(amount)
    return ct

view records给出的是经过加密的交易记录，provide a record则给出My system is secure if you can give me other records, the receiver can also get the money.

首要目标是要获得足够的现金，看看交易

> transact

please give me the trader and the amount(for example:Alice 1)
> Alice 1
16afee8cc0a88bf0478fefe229e68418a9a0627e2da31f1d055e01d7000a3b3a1794bbe06d2ed80b02b1c90ba2c4606f

your cash:9
you can choose: transact, view records, provide a record, get flag, hint
> transact

please give me the trader and the amount(for example:Alice 1)
> Alice 0
16afee8cc0a88bf0478fefe229e68418a9a0627e2da31f1d055e01d7000a3b3ab55eefd708d39ed2b4ce4d561c34e5ec

your cash:9

输入金额为 \(0\) 的话，交易也可以成功进行！尝试一下负数，看能不能获得现金

> transact

please give me the trader and the amount(for example:Alice 1)
> Alice -1
dd664379452c6af73b382cc5cd076754c70214708e2c0071ed74906852ecc510f2f0bd5754e02a2b03c635178e0200c5

your cash:11

发现可以成功增加持有现金，并没有被过滤之类的，获得足够现金之后就可以『购买』Flag 了！【之前还以为要考虑 ECB 加密……】

最后更新: 2020年10月26日 10:36:26

Contributors: YanhuiJessica

bank

解题思路¶

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