加密原理 :
明文、密鑰：由字母組成的字串
把 A 到 Z 編號成 0 到 25
明文中第一個字母的位移量就是密鑰第一個字母的編號、
第二個字母的位移量就是密鑰第二個字母的編號、……
如果密鑰長度比明文字母數短
就再從第一個字母開始用

明文：HSNU CRC
密鑰：CRSC

明文 H S N U C R C
密鑰 C R S C C R S
密文 J J F W E I U

因為每一個字母的位移量都不盡相同
所以不知道密鑰的情況下
要還原出明文是很困難的事
幾乎不可能

維吉尼亞表格

要怎麼把字母位移?

我們利用ASCII(美國資訊交換標準代碼)

找到字元 c 對應的數字：ord（c）
找到數字 i 對應的字元：chr（i）

純字母版

def encryption(key, password):
    cipher = ""
    now = 0
    for char in password:
        t = ord(char) - ord('A')
        k = ord( key[now] ) - ord('A')
        print(t+k)
        cipher += chr( ord('A') + (t+k)%26 )
        now = (now + 1) % len(key)
    
    return cipher
 
 
def decryption(key, password):
    cipher = ""
    now = 0
    for char in password:
        t = ord(char) - ord('A')
        k = ord( key[now] ) - ord('A')
        print(t-k)
        cipher += chr( ord('A') + (t-k)%26)
        now = (now + 1) % len(key)
 
    return cipher 

廣泛版(不局限於字母)

def encryption(key, password):
    cipher = ""
    now = 0
    for char in password:
        k = ord( key[now] ) - ord('A')
        cipher += chr( (ord(char) + k)%126 )
        now = (now + 1) % len(key)
    
    return cipher
 
 
def decryption(key, password):
    cipher = ""
    now = 0
    for char in password:
        k = ord( key[now] ) - ord('A')
        cipher += chr( (ord(char) - k)%126 )
        now = (now + 1) % len(key)
    
    return cipher

我們將用它來加密/解密我們的密碼

有時候在很多網站創建很多帳號
其中每種帳號的密碼限制都不一樣
時間久了很容易忘記

這種時候就很需要Password Manager的幫忙

Process:

1. 輸入Master password (用戶名，也是到時候加解密的key)
2. 選擇瀏覽密碼還是新增密碼
3. 如果新增密碼，輸入帳號名稱跟密碼，把密碼依照維吉尼亞密碼規定加密後連同帳號名稱存入記事本
4. 如果瀏覽密碼，把目前所有的密碼解密之後連同帳號名稱秀出來
5. 都沒事了就按q退出

如果master password輸入錯誤，就會無法看到既有的密碼

1. 輸入master password

key = input("What is the master password?")

2. 選擇瀏覽密碼還是加新密碼

while True:
        mode= input("Would you like to add a new password or view existing ones (view, add), press q to quit? ").lower()
        
        if mode == 'q':
            break
 
        if mode == "view":
            view(key)
        elif mode == "add":
            add(key)
        else:
            print("Valid mode.")
            continue

加解密函式

def encryption(key, password):
    cipher = ""
    now = 0
    for char in password:
        k = ord( key[now] ) - ord('A')
        cipher += chr( (ord(char) + k)%126 )
        now = (now + 1) % len(key)
    
    return cipher
 
 
def decryption(key, password):
    cipher = ""
    now = 0
    for char in password:
        k = ord( key[now] ) - ord('A')
        cipher += chr( (ord(char) - k)%126 )
        now = (now + 1) % len(key)
    
    return cipher

add()函式

在password.txt 中新加一條密碼，包含用戶名稱跟密碼

• with open("password.txt", 'a') as file:
  打開一個文件檔並做事情，把他想成一個if
  執行結束之後password.txt就會自動被關掉，很方便

輸入完會請使用者再確認一次

def add(key):
    print('')
    Done = False
 
    while not Done:
        name = input("Account name : ")
        password = input("Password : ")
 
        print('')
        print("Please double check the account name and password you\'ve typed in :")
        print("Account name :", name)
        print("Password :", password)
        correct = input("Type \"yes\" if they're correct, \"no\" if they're not : ").lower()
        print('')
        
        if correct == "yes":
            encrypted_password = encryption(key, password)
            # like a function. Once it's done, the file will be closed automatically.
            # a stands for "pen". Add sth to an existing file or create a new file if the file doesn't exist.
            # w stands for "write". Create a file or overwrite the file
            # r stands for "read". Just read.
            with open("password.txt", 'a') as file:
                file.write(name + '|' + encrypted_password + '\n') # 寫入密碼
            Done = True
            
        elif correct == "no":
            print("Alright. Please type again.")
            continue
        else:
            print("Valid reaction.")
            continue

view()函式

把目前所有密碼都依照輸入的key解密後秀出來

• file.readlines(len) : 取得文字檔file的所有文字，len默認是-1，代表所有行數
• line.split('|') : 把line依照'|'切割成n個不同字元，大多數密碼不會包含'|'，所以假設只利用原先用來分割的'|'切割出兩個字串
• line.rstrip() : 去除line最後面的指定部分，默認是空白、換行等等

def view(key):
    print('')
    with open("password.txt", 'r') as file:
        for line in file.readlines():
            data = line.rstrip() # delete the '\n' behind ecery line
            user, password = data.split('|') # "FB|123456789" -> ["FB", "123456789"]
            password = decryption(key, password)
            print("Account name :", user, "| Password :", password)
 
    print('')

恭喜各位完成Password Manager，
可以自己多加一些有趣的功能