Ceaser Cipher Program

import string

# Create the alphabet list to be used for encryption
alphabet = list((string.ascii_lowercase + string.ascii_uppercase)*2)

# Create the main function
def caesar_cipher():

    # Get the user's intention to encrypt or decrypt
    code_route = input('Enter "encrypt" to encrypt, or "decrypt" to decrypt:\n').lower()

    # Create a list of expected keywords from users
    code_route_response = ['decrypt', 'encrypt']

    # Ensure that the user only enters the required information
    while code_route not in code_route_response:
        code_route = input('Please enter the correct keyword❗, "encrypt" to encrypt, or "decrypt" to decrypt:\n')

    # Get the key to be used for encryption or decryption and catch invalid user input
    while True:
        try:
            shift_num = int(input('Please enter the key for encryption/decryption, it should be composed of only numbers:\n'))
        except:
            print('Please enter only an integer for encryption/decryption:\n')
            continue
        else:
            print('You entered a great integer\n')
            break

    # Mod the user's preferred shift number by half of the total number of characters in the alphabet in case a high shift number is used to avoid exceeding array limits
    shift_num %= 52
    shift_num += 3

    # If the user selects the decryption option, convert the shift number to a negative shift number
    if code_route == 'decrypt':
        shift_num = shift_num *-1

    # Get the text to be encrypted/decrypted from the user
    user_text = input("Please enter the text you want to encrypt/decrypt: ")

    # Create an empty string to add the encrypted/decrypted characters
    end_result = ''

    # Perform the encryption/decryption process
    for char in user_text:
        if char in alphabet:
            position = alphabet.index(char)
            end_position = position + shift_num
            end_result += alphabet[end_position]
        else:
            end_result += char

    # Print the encrypted/decrypted message
    print(f'\nYour {code_route.capitalize()}ed message is as follows:\n{end_result}')

# Run the function
caesar_cipher()

# Ask the user if they want to run the program again or exit
rerun_response = ['yes', 'no']

# Automate the program to continue or end
rerun = True
while rerun:
    cipher_rerun = input('\nHello! Do you want to use this program again? Enter "yes" to continue or "no" to exit\n').lower()

    while cipher_rerun not in rerun_response:
        cipher_rerun = input('\nPlease enter the correct keyword❗, Enter "yes" to continue or "no" to exit\n').lower()
    if cipher_rerun == 'yes':
        caesar_cipher()
    else:
        print('Thank you for using the Caesar Cipher Program developed by Geoff N. Have a nice day!\n')
        rerun = False

This is a Python program that implements the Ceaser cipher encryption and decryption algorithm.

Introduction

The Ceaser cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is shifted a certain number of places down the alphabet.

Implementation

The program first creates a list of letters from the alphabet. It then prompts the user to input whether they want to encrypt or decrypt a message. The user is also asked to input a key which is an integer between 0 and 9. The program then applies the key to the input text and outputs the resulting encrypted or decrypted message.

Usage

To use the program, follow the steps below:

1. Run the program in a Python environment.
2. Choose whether you want to encrypt or decrypt a message by typing “encrypt” or “decrypt” when prompted.
3. Enter the key, which is an integer between 0 and 9.
4. Enter the text you want to encrypt or decrypt.
5. The program will output the resulting message.

Example

Suppose you want to encrypt the message “Hello, World!” with a key of 3.

1. Run the program and type “encrypt” when prompted.
2. Enter the key “3”.
3. Enter the text “Hello, World!”.
4. The program will output the encrypted message “Khoor, Zruog!”.

Conclusion

This Ceaser cipher program provides a simple and effective way to encrypt and decrypt messages. It can be used for basic encryption needs, but it is not recommended for highly sensitive information.