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[SWPUCTF 2021 新生赛]简简单单的解密

news 来源：原创 2025/5/24 12:30:38

附件下载完是python代码

看到256想到rc4

关于rc4的逆向技巧

首先根据原理我们可以看到会初始化一个256字节的数组

其次会将一个key也填充到数组中

函数的话大概率都是两个参数,一个是key 一个是keylen

import base64,urllib.parse
key = "HereIsFlagggg"
flag = "xxxxxxxxxxxxxxxxxxx"

s_box = list(range(256))
j = 0
for i in range(256):
    j = (j + s_box[i] + ord(key[i % len(key)])) % 256
    s_box[i], s_box[j] = s_box[j], s_box[i]
res = []
i = j = 0
for s in flag:
    i = (i + 1) % 256
    j = (j + s_box[i]) % 256
    s_box[i], s_box[j] = s_box[j], s_box[i]
    t = (s_box[i] + s_box[j]) % 256
    k = s_box[t]
    res.append(chr(ord(s) ^ k))
cipher = "".join(res)
crypt = (str(base64.b64encode(cipher.encode('utf-8')), 'utf-8'))
enc = str(base64.b64decode(crypt),'utf-8')
enc = urllib.parse.quote(enc)
print(enc)
# enc = %C2%A6n%C2%87Y%1Ag%3F%C2%A01.%C2%9C%C3%B7%C3%8A%02%C3%80%C2%92W%C3%8C%C3%BA

Exp

import base64,urllib.parse
enc = "%C2%A6n%C2%87Y%1Ag%3F%C2%A01.%C2%9C%C3%B7%C3%8A%02%C3%80%C2%92W%C3%8C%C3%BA"
key = "HereIsFlagggg"
flag = ""

enc = urllib.parse.unquote(enc)
s_box = list(range(256))
j = 0
for i in range(256):
    j = (j + s_box[i] + ord(key[i % len(key)])) % 256
    s_box[i], s_box[j] = s_box[j], s_box[i]
i = j = 0
for s in enc:
    i = (i + 1) % 256
    j = (j + s_box[i]) % 256
    s_box[i], s_box[j] = s_box[j], s_box[i]
    t = (s_box[i] + s_box[j]) % 256
    k = s_box[t]
    flag += (chr(ord(s) ^ k))
print(flag)