凯撒密码
一种最简单且最广为人知的加密技术。它是一种替换加密的技术

"""Crypto: tool for encrypting and decrypting messages.

Exercises   加深练习

1. Review 'ord' and 'chr' functions and letter-to-number mapping.   转数字  和  数字转字符
2. Explain what happens if you use key 26.   与原码保持不变   移动了26位
3. Find a way to decode a message without a key.   其实就是暴力解码  1-25都移动次
4. Encrypt numbers.    加密数字   加一个key数
5. Make the encryption harder to decode.   使用更难的加密算法   怎么乱怎么来   要保持加密和解密算法一致

Adapted from code in https://inventwithpython.com/chapter14.html

"""


def encrypt(message, key):
    """Encrypt message with key."""
    result = ''

    # Iterate letters in message and encrypt each individually.

    for letter in message:
        if letter.isalpha():

            # Letters are numbered like so:
            # A, B, C - Z is 65, 66, 67 - 90
            # a, b, c - z is 97, 98, 99 - 122

            num = ord(letter)
            base = None
            if letter.isupper():
                base = ord('A')
            elif letter.islower():
                base = ord('a')

            # The encryption equation:
            # print(base)
            num = (num - base + key) % 26 + base

            result += chr(num)

        elif letter.isdigit():

            # TODO: Encrypt digits.
            result += letter

        else:
            result += letter

    return result


def decrypt(message, key):
    """Decrypt message with key."""
    return encrypt(message, -key)


def decode(message):
    """Decode message without key."""
    pass  # TODO


def get_key():
    """Get key from user."""
    try:
        text = input('Enter a key (1 - 25): ')
        key = int(text)
        return key
    except:
        print('Invalid key. Using key: 0.')
        return 0


# print(ord('a'))
# print(chr(122))
print('Do you wish to encrypt, decrypt, or decode a message?')
choice = input()


if choice == 'encrypt':
    phrase = input('Message: ')
    code = get_key()
    print('Encrypted message:', encrypt(phrase, code))
elif choice == 'decrypt':
    phrase = input('Message: ')
    code = get_key()
    print('Decrypted message:', decrypt(phrase, code))
elif choice == 'decode':
    phrase = input('Message: ')
    print('Decoding message:')
    decode(phrase)
else:
    print('Error: Unrecognized Command')