【python密码学编程】6.凯撒加密法

# _*_ coding:utf-8 _*_
#Caeser Ciper
import pyperclip
messgae = 'this is my secret messgae.'
key = 13
mode = 'encrypt'    #模式是加密而非解密（decrypt）
LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'    #常量要大写
translated = ''
messgae = messgae.upper()
for symbol in messgae:
    if symbol in LETTERS:
        num = LETTERS.find(symbol)    #find()返回的是该字符在字符串中的位置

        if mode == 'encrypt':#判断当前是加密还是解密过程
            num = num+key
        elif mode == 'decrypt':
            num = num-key

        if num > len(LETTERS):#当处理后的数字脱离26个字母的数字范围后的处理
            num = num - len(LETTERS)
        elif num < 0:
            num = num + len(LETTERS)

        translated = translated + LETTERS[num]#空字符串一个个加上翻译后得到的字符

    else:
        #如果要翻译的字符不在26个大写字母的范围内
        translated =translated + symbol

print translated
pyperclip.copy(translated)

>>>
GUVF VF ZL FRPERG ZRFFTNR.
[Finished in 0.8s]