[SWPUCTF 2021 新生赛]简简单单的解密

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)

运行得到flag

NSSCTF{REAL_EZ_RC4}