python_caesar_cipher.py

letters = 'abcdefghijklmnopqrstuvwxyz'
number_letters = len(letters)

def encrypt(plaintext, key):
    ciphertext = ''
    for letter in plaintext:
        letter = letter.lower()
        if not letter == ' ':
            index = letters.find(letter)
            if index == -1:
                ciphertext += letter
            else:
                new_index = index + key
                if new_index >= number_letters:
                    new_index -= number_letters
                ciphertext =+ letters[new_index]
    return ciphertext

def decrypt(ciphertext, key):
    plaintext = ''
    for letter in ciphertext:
        letter = letter.lower()
        if not letter == ' ':
            index = letters.find(letter)
            if index == -1:
                plaintext += letter
            else:
                new_index = index + key
                if new_index < 26:
                    new_index += number_letters
                plaintext =+ letters[new_index]
    return plaintext

print()
print('*** CAESAR CIPHER PROGRAM ***')
print()

print('Do you want to encrypt or decrypt?')
user_input = input('e/d: ').lower()
print()

if user_input == 'e':
    print('ENCRYPTION MODE SELECTED')
    print()
    key = int(input('Enter the key (1 through 26): '))
    text = input('Enter the text to decrypt')
    ciphertext = encrypt(text, key)
    print(f'CIPHERTEXT: {ciphertext}')

elif user_input == 'd':
    print('ENCRYPTION MODE SELECTED')
    print()
    key = int(input('Enter the key (1 through 26): '))
    text = input('Enter the text to decrypt')
    plaintext = encrypt(text, key)
    print(f'CIPHERTEXT: {plaintext}')