Lesson 8.4 | Griffin CS

Introduction

Caesar Cipher Problem

Write a function called cipher that uses the Caesar cipher to encrypt a message. The Caesar cipher works like a substitution cipher but each character is replaced by the character 13 characters to “the right” in the alphabet. So for example the letter a becomes the letter n. If a letter is past the middle of the alphabet then the counting wraps around to the letter a again, so n becomes a, o becomes b and so on. Hint: Whenever you talk about things wrapping around its a good idea to think of modulo division.

# caesar_cipher.py

import string

def encrypt(message):
    # set encryption key
    alphabet = string.ascii_lowercase           # all lowercase letters
    punctuation = string.punctuation + " "  # all punctuation (need to add a space)

    # encrypt message
    encryption = ""
    for character in message.lower():
        if character in punctuation:                        # keep punctuation the same
            encryption = encryption + character
        else:
            index = alphabet.find(character)         # find the letter's index in alphabet
            code_index = (index + 13) % 26           # shift the index 13 spaces
            encryption = encryption + alphabet[code_index] # replace with the new letter

    return encryption

def decrypt(message):
    # set decryption key
    alphabet = string.ascii_lowercase
    key = "qwertyuiopasdfghjklzxcvbnm"
    punctuation = string.punctuation + " "

    # decrypt message
    decryption = ""
    for character in message.lower():
        if character in punctuation:
            decryption = decryption + character
        else:
            code_index = alphabet.find(character)
            index = code_index - 13                   # negative indexes aren't a problem
            decryption = decryption + alphabet[index] # replace with same index in alphabet

    return decryption

message = "Storm the walls at dawn!"

# encrypt message and return to user
encrypted = (encrypt(message))
print(encrypted)

# decrypt message and return to user
decrypted = decrypt(encrypted)
print(decrypted)

HW: begin working on Project 7