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def main():
    key = [1, 2, 3, 4, 5]        # 准备一些变量
    key_len = len(key)
    plain = "i_am_plain_text"
    plain_len = len(plain)
    cipher = [0] * plain_len

    s = [i for i in range(256)]    # 初始化s盒
    rc4_init(s, key, key_len)      # 使用key打乱s盒
    key_stream = rc4_generate_keystream(s[:], plain_len) # 生成密钥流

    for i in range(plain_len):     # 逐字节异或加密
        cipher[i] = ord(plain[i]) ^ key_stream[i]

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def rc4_init(s, key, key_len):
    j = 0
    for i in range(256):
        j = (j + s[i] + key[i%key_len])%256
        tmp = s[i]
        s[i] = s[j]
        s[j] = tmp

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def rc4_generate_keystream(s, length):
    i = 0
    j = 0
    key_stream = []
    while length:
        i = (i + 1) % 256    # 可以保证每256次循环后s盒中的每个元素至少被交换一次
        j = (j + s[i]) % 256
        tmp = s[i]
        s[i] = s[j]
        s[j] = tmp
        key_stream.append(s[(s[i] + s[j]) % 256])
        length -= 1
    return key_stream