# -*- coding:utf-8 -*-
import os
#==================================================================#
#     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 = raw_input("请输入明文：")
  k = int(raw_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 = raw_input("请输入密文：")
  k = int(raw_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 = raw_input("请选择：")
  if choice == "1":
    encryption()
  elif choice == "2":
    decryption()
  else:
    print (u"您的输入有误！")