Vigenère

tl;dr

Implement a program that encrypts messages using Vigenère’s cipher, per the below.

$ python vigenere.py ABC
plaintext:  HELLO
ciphertext: HFNLP

Design and implement a program that encrypts messages using Vigenère’s cipher, exactly as you did in Problem Set 2, except that your program this time should be written (a) in Python and (b) in CS50 IDE.

Implement your program in a file called vigenere.py in your ~/workspace/pset6/vigenere directory (if it doesn’t already exist, create it now!).
Your program must accept a single command-line argument: a keyword, k, composed entirely of alphabetical characters.
If your program is executed without any command-line arguments, with more than one command-line argument, or with one command-line argument that contains any non-alphabetical character, your program should print an error (of your choice) and exit immediately with a status code of 1.
Otherwise, your program must proceed to prompt the user for a string of plaintext, p, (as by a prompt for plaintext:) which it must then encrypt according to Vigenère’s cipher with k, ultimately printing the result (prepended with ciphertext: and ending with a newline) and exiting.
With respect to the characters in k, you must treat A and a as 0, B and b as 1, … , and Z and z as 25.
Your program must only apply Vigenère’s cipher to a character in p if that character is a letter. All other characters (numbers, symbols, spaces, punctuation marks, etc.) must be outputted unchanged. Moreover, if your code is about to apply the j^th character of k to the i^th character of p, but the latter proves to be a non-alphabetical character, you must wait to apply that j^th character of k to the next alphabetical character in p; you must not yet advance to the next character in k.
Your program must preserve the case of each letter in p.

Walkthrough

Usage

Your program should behave per the examples below. Assume that the underlined text is what some user has typed.

$ python vigenere.py 13
Usage: python vigenere.py k

$ python vigenere.py
Usage: python vigenere.py k

$ python vigenere.py bacon and eggs
Usage: python vigenere.py k

$ python vigenere.py bacon
plaintext: Meet me at the park at eleven am
ciphertext: Negh zf av huf pcfx bt gzrwep oz

Testing

To help you test vigenere, we’ve written a program called devigenere for you that also takes one and only one command-line argument (a keyword) but whose job is to take ciphertext as input and produce plaintext as output. To use our program, execute

~cs50/pset2/devigenere k

at your prompt, where k is some keyword. Presumably you’ll want to paste your program’s output as input to our program; be sure, of course, to use the same key. Note that you do not need to implement devigenere yourself, only vigenere.

Correctness

check50 cs50/problems/2019/x/sentimental/vigenere

Style

style50 vigenere.py

Staff’s Solution

~cs50/2019/x/pset6/vigenere

How to Submit

Execute the below, logging in with your GitHub username and password when prompted. For security, you’ll see asterisks (*) instead of the actual characters in your password.

submit50 cs50/problems/2019/x/sentimental/vigenere

You can then go to https://cs50.me/cs50x to view your current scores!

Hints

Not sure where to begin? As luck would have it, this program’s pretty similar to caesar! Only this time, you need to decide which character in k to use as you iterate from character to character in p.