从循环更改功能？

Question

首次发布。 我有一个学校作业，试图从Ceasar的密码中解密密码。 我获得了可以使用的功能。我不知道如何获得相反的加密密钥值。

编辑，这是我的代码。我不知道您需要什么，所以这里大部分。

import csv
import sys

#The password list - We start with it populated for testing purposes
passwords = [["yahoo","XqffoZeo"],["google","CoIushujSetu"]]


#The password file name to store the passwords to
passwordFileName = "samplePasswordFile"

#The encryption key for the caesar cypher
encryptionKey=16

#Caesar Cypher Encryption
def passwordEncrypt (unencryptedMessage, key):

    #We will start with an empty string as our encryptedMessage
    encryptedMessage = ''

    #For each symbol in the unencryptedMessage we will add an encrypted 
    #symbol into the encryptedMessage
    for symbol in unencryptedMessage:
        if symbol.isalpha():
            num = ord(symbol)
            num += key

            if symbol.isupper():
                if num > ord('Z'):
                    num -= 26
                elif num < ord('A'):
                    num += 26
            elif symbol.islower():
                if num > ord('z'):
                    num -= 26
                elif num < ord('a'):
                    num += 26

            encryptedMessage += chr(num)
        else:
            encryptedMessage += symbol

    return encryptedMessage

def loadPasswordFile(fileName):

    with open(fileName, newline='') as csvfile:
        passwordreader = csv.reader(csvfile)
        passwordList = list(passwordreader)

    return passwordList

def savePasswordFile(passwordList, fileName):

    with open(fileName, 'w+', newline='') as csvfile:
        passwordwriter = csv.writer(csvfile)
        passwordwriter.writerows(passwordList)

if(choice == '2'): #Lookup at password
    print("Which website do you want to lookup the password for?")
    for keyvalue in passwords:
        print(keyvalue[0])
        passwordToLookup = input()
        for keyvalue in passwords:
            website = keyvalue[0]
            password = keyvalue[1]
            if website == passwordToLookup:
                encryptionKey = encryptionKey -16
                passwordEncrypt(password, encryptionKey)
                print(password)

我尝试了多种方法，但仍然无法得出结论。