Caesar cipher with Python

*This post comes from my blog

I created Caesar cipher with Python.

import sys

def caesar(plaintext, key):
    """
    plaintext: original data
    key: number of character shift
    """
    ciphertext = ""

    for ch in plaintext:
        asc = ord(ch)   # ASCIIに変換
        if 65 <= asc <= 90:
            num = asc - 65  # A=0, B=1, ..., Z=25
            shift_asc = (num + key) % 26
            cipherchar = chr(shift_asc + 65)
            ciphertext += cipherchar
        elif 97 <= asc <= 122:
            num = asc - 97  # a=0, b=1, ..., z=25
            shift_asc = (num + key) % 26
            cipherchar = chr(shift_asc + 97)
            ciphertext += cipherchar
        else:
            sys.stderr("Error: alphabet only")

    return ciphertext

First of all, it processes a given string( plaintext ) character by character.

Convert to ASCII after that substitute A=0, B=1, ..., Z=25 for easy to use numbers, and then slide by the given number( key ).

Run the code:

t = caeser("flower", 3)     # 第一引数: 暗号化したい文字列、第二引数: 3文字ずらす
print(t)

It generates the cipher text as below:

iorzhu

Caesar cipher is very simple algorithm that shift by the specified number.

Because of its simplicity, it is useless in modern times but often appears in the history of cipher because it played a role in the development of cryptography.