凯撒加密解密

凯撒加密解密，偏移量为13

mode = input("输入 encode(解密) 或 decode(加密) ：")
if ('decode' or 'encode') in mode:
    tran = ''
    key = 13  #偏移量 13
    LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    mess = input('输入编码：')
    mess = mess.upper()  
    print("开始"+mode+':')
    for symbol in mess:
        if symbol in LETTERS:
            num = LETTERS.find(symbol)
            if mode == 'encode':
                num = num + key
            elif mode == 'decode':
                num = num - key
            if num >= len(LETTERS):
                num = num -len(LETTERS)
            elif num <  0:
                num = num + len(LETTERS)
            tran = tran + LETTERS[num]
        else:
            tran = tran + symbol
    print(tran.lower())
else:
    print('输入错误......')