RC4算法：

RC4算法：

RC4（来自Rivest Cipher 4的缩写）是一种流加密算法，密钥长度可变。它加解密使用相同的密钥

基本原理：

生成一个密钥流，然后与密文进行异或。

大致过程：

生成密钥流：

先利用Key生成S盒——The key-scheduling algorithm (KSA)

再利用S盒生成密钥流——The pseudo-random generation algorithm(PRGA)

加密：

将密文与密钥流异或。

解密：

将明文与密钥流异或。

代码实现：

KSA(生成S盒)：

#生成一个长度为256的S盒。注！：这是标准的，可以为别的，例如128，但安全会有问题
def KSA(key)
key =list(key)
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]
    return s_box

PRGA（生成密钥流）:

def PRGA(s_box)
res=[]
i=0
j=0
for a in plain:
    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
    yield s_box[t]

加密同解密：

def rc4_encrypt (date,key):
    s = KSA(key)
    keystream =PRGA(s)
    encrypted = bytes([byte^next(keystream) for b in date])
    return encrypted

完整代码：

def KSA(key):
    key =list(key)
    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]
        return s_box
    
def PRGA(s_box)
    res=[]
    i=0
    j=0
    for a in plain:
        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
    yield s_box[t]
def rc4_encrypt (date,key):
    s = KSA(key)
    keystream =PRGA(s)
    encrypted = bytes([byte^next(keystream) for b in date])
    return encrypted
#密文密钥
key =b"key"
plaintext+b"Hello,world"
#加密
ciphertext = rc4_encrypt(plaintext,key)
#解密
decrypted = rc4_encrypt(ciphertext,key)

例题：
[SWPUCTF 2021 新生赛]简简单单的解密：

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

WP：

import base64,urllib.parse
a ="%C2%A6n%C2%87Y%1Ag%3F%C2%A01.%C2%9C%C3%B7%C3%8A%02%C3%80%C2%92W%C3%8C%C3%BA"
a = urllib.parse.unquote(a)
key = "HereIsFlagggg"
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 a:
    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)
print(cipher)

NSSCTF{REAL_EZ_RC4}