python恺撒密码求解

import os
#-*-coding:utf-8-*-
#==================================================================#
#         凯撒密码(caesar)是最早的代换密码,对称密码的一种                #
#   算法：将每个字母用字母表中它之后的第k个字母（称作位移值）替代            #
#==================================================================#
def encryption():
    str_raw = input("请输入明文：")
    k = int(input("请输入位移值："))
    str_change = str_raw.lower()
    str_list = list(str_change)
    str_list_encry = str_list
    i = 0
    while i < len(str_list):
        if ord(str_list[i]) < 123-k:
            str_list_encry[i] = chr(ord(str_list[i]) + k)
        else:
            str_list_encry[i] = chr(ord(str_list[i]) + k - 26)
        i = i+1
    print ("加密结果为："+"".join(str_list_encry))
def decryption():
    str_raw = input("请输入密文：")
    k = int(input("请输入位移值："))
    str_change = str_raw.lower()
    str_list = list(str_change)
    str_list_decry = str_list
    i = 0
    while i < len(str_list):
        if ord(str_list[i]) >= 97+k:
            str_list_decry[i] = chr(ord(str_list[i]) - k)
        else:
            str_list_decry[i] = chr(ord(str_list[i]) + 26 - k)
        i = i+1
    print ("解密结果为："+"".join(str_list_decry))
while True:
    print (u"1. 加密")
    print (u"2. 解密")
    choice = input("请选择：")
    if choice == "1":
        encryption()
    elif choice == "2":
        decryption()
    else:
        print (u"您的输入有误！")

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