Problem Set 11: Vigenere Cipher

Web-CAT: Submit Python programs to this automated grading platform.

Task Outline

The Vigenere Cipher improves upon the Caesar Cipher by encrypting a message using a sequence of keys, also known as a keyword.
A keyword is an alphabetical string, where a represents 0, b represents 1, c represents 2, and so on, all the way up to z which represents 25.
In other words, if $p$ is some plaintext, and $k$ is a keyword, then each letter $c_i$ in the ciphertext $c$ is computed as:

$c_i = (p_i + k_j)\, \mbox{mod}\, 26$

Note that the Vigenere Cipher uses $k_j$ in constrast to the Caesar Cipher, which just uses $k$ . Also, if $k$ is shorter than the plaintext message $p$ (which is usually the case), then the letters in $k$ must be reused cyclically as many times as it takes to encrypt $p$ .

Consider the case where the plaintext message is hello, and the keyword is abc. With this keyword, the h would be encrypted with a key of 0, the e with a key of 1, and the first l with a key of 2.
At this point, we have exhausted all of the letters in the keyword, so we must resuse part of it to encrypt the rest of the plaintext message. The second l would be encrypted with a key of 0, and the o would be encrypted with a key of 1.
After the encryption is complete, the result would be the ciphertext hfnlp.

Encryption shift

In order to solve this problem set, a clever strategy may be to figure out how many times you need to repeat the keyword so that it matches up with the message. One way to do this is to divide the length of the message by the length of the keyword. Remember to multiply by 1.0 when performing this division, because Web-CAT uses Python 2.7.
Once you have this comparison value, you want to determine the next largest integer value. An effective way of doing this is to use the mathematical ceiling function. For example, ceiling applied to 5.279 will produce a value of 6. In Python, we must import the math library to use the ceiling function:

num = math.ceil(x)

Description	File Size	File Name
`Python` Source Code for Vigenere Cipher	1.6KB	pset11.zip

Contents of pset11.zip:

PSet11VigenereCipher/
├── testvigenerecipher.py
└── vigenerecipher.py

Write a Python program in the file vigenerecipher.py that encrypts a message using the Vigenere Cipher.
You will write your solution in a function called encrypt(message, key) right below the place where it says: YOUR CODE HERE
When the function call encrypt("hello", "abc") is executed, the output of the program should be: hfnlp

Run the file testvigenerecipher.py to execute the PyUnit test bench. PyUnit indicates a successful test with an OK output, and an unsuccessful test with a FAIL output.