Question 1

Amina is writing a Python program to encrypt messages using a Caesar cipher.

The Caesar cipher works by shifting each letter of a message forward in the alphabet by a set number of places (the key).

For example, with a key of +3, the word code becomes frgh:

Plaintext	c	o	d	e
Ciphertext	f	r	g	h

Amina has already written the helper function encrypt(text, key) which performs the character shifting.

She needs you to complete the rest of the program to meet these requirements:

Input a plaintext message from the user.
Input an integer key from the user.
Validate the key: it must be between 1 and 15 inclusive. The program must repeatedly prompt the user until a valid key is entered.
Call the encrypt function with the plaintext and valid key to generate the ciphertext.
Write the ciphertext to a text file named secret.txt.
Display the ciphertext on the screen.

Complete the Python 3 program code below:

def encrypt(text, key):
    ciphertext = ""
    for char in text:
        if char.isalpha():
            start = ord('a') if char.islower() else ord('A')
            ciphertext += chr((ord(char) - start + key) % 26 + start)
        else:
            ciphertext += char
    return ciphertext

# --- COMPLETE THE PROGRAM BELOW THIS LINE ---