main.py

letters = 'abcdefghijklmnopqrstuvwxyz'
num_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 >= num_letters:
                    new_index -= num_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 < 0:
                    new_index += num_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('ENCRYPION MODE SELECTED')
    print()
    key = int(input('enter your key(1 through 26): '))
    text = input('enter the text to encrypt: ')
    ciphertext = encrypt(text, key)
    print(f'ciphertext: {ciphertext}')
elif user_input == 'd':
    print('DECRYPION MODE SELECTED')
    print()
    key = int(input('enter your key(1 through 26): '))
    text = input('enter the text to decrypt: ')
    plaintext = decrypt(text, key)
    print(f'plaintext: {plaintext}')

else:
    print('invalid input')