Question

Assignment 7:

Caesar Cipher Assignment 7 You will create a Caesar cipher which allows the user to specify the key and the text to be encrypted.

A Caesar cipher is a simple substitution cipher wherein each letter in the message is shifted a certain number of spaces down the alphabet -- this number is called the key.

a b c d e f g h i j k l m n o p q r s t u v w x y z

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

For example, let’s say we are encrypting the text ‘hello world’.

If the key is 5, the encrypted text would be ‘mjqqt btwqi’.

This is calculated letter by letter, ie ‘h’ corresponds to element 7, and 7+5=12, and 12 corresponds to ‘m’, and so on for each letter. Note that values loop around the alphabet, ie ‘w’ corresponds to 22, and 22+5=27, which will then correspond to 1 or ‘b’.

This can be accomplished computationally with a modulus ( %26 ).

Your program should:

1. Ask the user for the Caesar cipher key -- give clear instructions; a. If the key is equal to 100, then randomly pick the key (between 0 and 25); i. Print out the randomly decided value; b. Check that the user input a number between 0 and 25, or 100; i. If they did not, request a new key.

2. Ask the user for the text to encrypt -- again give clear instructions; a. Take the lowercase of the text and check that it only contains letters and spaces; i. If the check fails, request a new text from the user; b. Check that no more than 250 characters were input by the user; i. If the check fails, request a new text from the user.

3. Calculate the encrypted text; a. Remember to leave spaces untouched; b. Print the encrypted text to the screen.

4. Ask if the user wants to perform another Caesar cipher; a. If Yes, begin again at (1). Submit your program script called cipher.py. Use functions as appropriate for optimal abstraction and encapsulation. You may find nested while or for loops useful, as well as the string method find().

Answer 1

Screenshot of the code used:

The output obtained:

The code used:

import random   # this is used for randomly generating a number.


def cipher(text, key):  # this function will convert our text to encrypted text.
    # the dictionary below store the integers and the appropriate character
    dictionary = {0: 'a', 1: 'b', 2: 'c', 3: 'd', 4: 'e', 5: 'f', 6: 'g', 7: 'h',
                  8: 'i', 9: 'j', 10: 'k', 11: 'l', 12: 'm', 13: 'n', 14: 'o', 15: 'p', 16: 'q',
                  17: 'r', 18: 's', 19: 't', 20: 'u', 21: 'v', 22: 'w', 23: 'x', 24: 'y', 25: 'z'}

    encryptedText = ""
    for char in text:
        if char != " ":
            dictKey = ord(char) - 97
            encryptedText += dictionary[(dictKey + key) % 26]
        else:
            encryptedText += char

    return encryptedText


def validateText(text):  # this will check if the text has only alphabets and spaces.
    for char in text:
        if (not char.isalpha()) and (not char.isspace()):
            return False
    if len(text) > 250:
        return False
    return True


def validateKey(key):  # this will check if the key is between 0 to 25 and 100.
    if (key < 0 or key > 25) and key != 100:
        return False
    return True


def main():  # this is the driver part of the code, we will keep asking for input from the user util he/she want to quit
    Quit = False

    while not Quit:
        while True:
            key = int(input("Enter Caesar cipher key: "))
            if validateKey(key):
                break
            else:
                print("Invalid, Key!!")

        while True:
            text = input("Enter the text to be encrypted: ").lower()
            if validateText(text):
                break
            else:
                print("Invalid, Text!!")

        if key == 100:  # generating a random key.
            key = random.randint(0, 26)
            print(key, " is randomly selected as the key.")

        encryptedText = cipher(text, key)
        print("The encrypted text:", encryptedText)

        choice = input("Enter yes if you want to continue with another text or Enter no to quit(yes/no): ")
        if choice == "no":
            Quit = True


if __name__ == '__main__':
    main()

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