加密解密

choices =input('你要加密(a)或解密(b)？')
alphabet = 'abcdefghijklmnopqrstuvwxyz'

if choices == 'a':
  key = input(' 你要什么密钥？')
  newMessage = ' '

  message = input('请输入你的讯息:')

  for character in message:
    if character in alphabet:
      position = alphabet.find(character)
      newPosition = (position + int(key)) % 26
      newCharacter = alphabet[newPosition]
      newMessage += newCharacter
    else:
        newMessage += character

  print '你的加密讯息是:',newMessage

else:
  key = input('你的密钥是？')
  newMessage = ' '

  message = input('请输入你的加密讯息:')

  for character in message:
    if character in alphabet:
      position = alphabet.find(character)
      newPosition = (position - int(key)) % 26
      newCharacter = alphabet[newPosition]
      newMessage += newCharacter
    else:
        newMessage += character

  print '你的讯息是:',newMessage