An encryption/decryption program that uses the Caesar and Affine encryption methods to encrypt a string of text.

Caesar-Affine Encryption and Decryption.py

###########################################################
#  Computer Project #3
#
#  Algorithm
#    Prompt to encrypt, decrypt, or quit
#    Initialize variables that need to be appended to or used in the program
#    Generate string of all lowercase alphabetic characters
#    Ask for keyword of only alphabetic charactes and less than 26 characters
#    Ask for a message to encrypt/decrypt
#    If Encrypt:
#       Create Caesar cipher:
#          Append each letter in the keyword (if not already there)
#          Append each letter of the alphabet (if not already there)
#       Create the Affine cipher:
#          Loop through the Caesar key
#          For each index perform the Affine operation ((5x + 8) % 26)
#          Append the character in the Caesar key index from Affine operation
#       Encrypt with Caesar cipher:
#          Just add all non-alphabetic characters
#          If it is a letter, find its index in the alphabet and replace it
#          with the letter in that index in the Caesar cipher key
#       Encrypt with Affine cipher:
#          Just add all non-alphabetic characters
#          If it is a letter, find its index in the Caesar key and replace it
#          with the letter in that index in the Affine cipher key
#       Display encrypted message
#    If Decrypt:
#       Create Caesar cipher:
#          Append each letter in the keyword (if not already there)
#          Append each letter of the alphabet (if not already there)
#       Create the Affine cipher:
#          Loop through the Caesar key
#          For each index perform the Affine operation ((5x + 8) % 26)
#          Append the character in the Caesar key index from Affine operation
#       Decrypt with Affine cipher:
#          Just add all non-alphabetic characters
#          If it is a letter, find its index in the Affine cipher key and 
#          replace it with the letter in that index in the Caesar cipher key
#       Decrypt with Caesar cipher:
#          Just add all non-alphabetic characters
#          If it is a letter, find its index in the Caesar key and replace it
#          with the letter in that index in the alphabet string
#       Display decrypted message
#    Display closing message
###########################################################

import string

# Ask if user wants to quit or encrypt/decrypt
answer = input("Would you like to (D)ecrypt, (E)ncrypt or (Q)uit? ").upper()
while answer != 'Q':
    
    # Initialize variables that need to be appended to or used in the program
    ALPHABET_STR = string.ascii_lowercase
    c_key = ""
    c_encrypted_text = ""
    a_key = ""
    a_encrypted_text = ""
    A = 5
    B = 8
    M = 26
    a_decrypted_text = ""
    c_decrypted_text = ""
    
    # Encrypt
    if answer == 'E':
        keyword = input("Please enter a keyword: ").lower()
        
        # Make sure the keyword is all letters and not longer than 26 
        # characters.
        while keyword.isalpha() == False or len(keyword) > 26:
            print("There is an error in the keyword. It must be all letters\
 and a maximum length of 26")
            keyword = input("Please enter a keyword: ").lower()
        
        message = input("Enter your message: ").lower()
    
        # Create the Ceasar cipher key. 
        # With the keyword first.
        for index, letter in enumerate(keyword):
            if letter not in c_key:
                c_key += letter
        # Then the rest of the letters of the alphabet.
        for index, letter in enumerate(ALPHABET_STR):
            if letter not in c_key:
                c_key += letter
        
        # Create the Affine cipher key. 
        # Loop through each letter in the Caesar key, performing the Affine 
        # encryption operation, and then append the letter value to the Affine 
        # key.
        for index, letter in enumerate(c_key):
            affine_index = (A * index + B) % M
            a_key += c_key[affine_index]

        # Encrypt using the Caesar cipher.
        for letter in message:
            # If the string in this index is not a letter, just add it to the
            # ecrypted message.
            if letter.isalpha() == False:
                c_encrypted_text += letter
            # If the string in this index is a letter, find the index where
            # that letter normally appears in the alphabet. Then, substitute
            # that letter for whatever letter is in that index in the Caeasr
            # cipher.
            else:
                index_in_alphabet = ALPHABET_STR.find(letter)
                c_encrypted_text += c_key[index_in_alphabet]
            
        # Encrypt using the Affine cipher.
        for letter in c_encrypted_text:
            # If the string in this index is not a letter, just add it to the
            # ecrypted message.
            if letter.isalpha() == False:
                a_encrypted_text += letter
            # If the string in this index is a letter, find the index where
            # that letter appears in the Caesar cipher. Then, substitute that
            # letter for whatever letter is in that index in the Affine cipher.
            else:
                index_in_c_key = c_key.find(letter)
                a_encrypted_text += a_key[index_in_c_key]
        
        print("your encoded message: ", a_encrypted_text)
        
    # Decrypt  
    if answer == 'D':
        keyword = input("Please enter a keyword: ").lower()
        
        # Make sure the keyword is all letters and not longer than 26 
        # characters.
        while keyword.isalpha() == False or len(keyword) > 26:
            print("There is an error in the keyword. It must be all letters\
 and a maximum length of 26")
            keyword = input("Please enter a keyword: ").lower()
        
        message = input("Enter your message: ").lower()
        
        # Create the Ceasar cipher key. 
        # With the keyword first.
        for index, letter in enumerate(keyword):
            if letter not in c_key:
                c_key += letter
        # Then the rest of the letters of the alphabet.
        for index, letter in enumerate(ALPHABET_STR):
            if letter not in c_key:
                c_key += letter
        
        # Create the Affine cipher key. 
        # Loop through each letter in the Caesar key, performing the Affine 
        # encryption operation, and then append the letter value to the Affine 
        # key.
        for index, letter in enumerate(c_key):
            affine_index = (A * index + B) % M
            a_key += c_key[affine_index]

        # Decrypt text using Affine cipher
        for index, letter in enumerate(message):
           # If the string in this index is not a letter, just add it to the
           # decrypted message.
           if letter.isalpha() == False:
               a_decrypted_text += letter
            # If the string in this index is a letter, find the index where
            # that letter appears in the Affine cipher key. Then, substitute
            # that letter for whatever letter in is that index in the Caeasr
            # cipher.
           else:
               affine_index = a_key.find(letter)
               a_decrypted_text += c_key[affine_index]
        
        # Decrypt using Caesar cipher
        for index, letter in enumerate(a_decrypted_text):
            # If the string in this index is not a letter, just add it to the
            # ecrypted message.
            if letter.isalpha() == False:
                c_decrypted_text += letter
            # If the string in this index is a letter, find the index where
            # that letter appears in the Caesar cipher key. Then, substitute
            # that letter for whatever letter in is that index in the alphabet
            # string.
            else:
                caesar_index = c_key.find(letter)
                c_decrypted_text += ALPHABET_STR[caesar_index]
    
        print("your decoded message: ", c_decrypted_text)
        
    # Ask if user wants to quit or encrypt/decrypt
    answer = input("Would you like to (D)ecrypt, (E)ncrypt or (Q)uit? ").upper()

print("See you again soon!")