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Description: https://en.wikipedia.org/wiki/Caesar_cipher

#!/usr/bin/python
# -*- coding: UTF-8 -*-

# 恺撒挪移式密码法
## 密码字母移动三位
def caesar_crypt(plain):
    a, z = ord('a'), ord('z')
    cipher = []
    for char in plain:
        c = char
        if c != ' ':
            asc = ord(c) - 3
            if asc < a:
                asc = z - (a - asc) + 1
            c = chr(asc)
        cipher.append(c)
    return "".join(cipher)

def caesar_decrypt(cipher):
    a, z = ord('a'), ord('z')
    plain = []
    for char in cipher:
        c = char
        if c != ' ':
            asc = ord(c) + 3
            if asc > z:
                asc = a + (asc - z) - 1
            c = chr(asc)
        plain.append(c)
    return "".join(plain)


def main():
    plain = "the quick brown fox jumps over the lazy dog"
    print(plain)
    cipher = caesar_crypt(plain)
    print(cipher)
    plain = caesar_decrypt(cipher)
    print(plain)


if __name__ == '__main__':
    main()