凯撒密码，不知道密钥

Question

嘿伙计，如果你看下面我的代码，你将能够看到我能够创建一个程序，可以打开文件解码文件的内容并将其保存到另一个文件，但我需要输入密钥每一次，但如果我不知道他们关键如何使用频率分析来计算密钥。

import sys

def decrypt(cipher, key):

plain = ""
for index in range(len(cipher)):
    if cipher[index].isalpha():

        if cipher[index].isupper():
            plain = plain + chr((ord(cipher[index]) - 64 - key) % 26 + 64)


        elif cipher[index].islower():
            plain = plain + chr((ord(cipher[index]) - 96 - key) % 26 + 96)

    else:
            plain = plain + cipher[index]

return plain

in_filename = sys.argv[1]
key = int(sys.argv[2])
out_filename = sys.argv[3] 

with open(in_filename, "r") as f:
    encrypted = f.read()

decrypted = decrypt(encrypted, key)

with open(out_filename, "w+") as f:
    f.write(decrypted)