Problem Set 11: Vigenere Cipher

Task Outline

• Due Date: Monday, December 2, 2019
• Total Points: 10
• Implement a Java program that encrypts a message using the Vigenere Cipher.

Background Theory

• 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:

• 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 $p$.

Example Case

• 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.

Hints

• 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.
• 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 Java, we must use the Math class to use the ceiling function:

int num = Math.ceil(x)

Code Distribution

Contents of pset11.zip:

PSet11VigenereCipher/
├── package.bluej
├── README.TXT
├── VigenereCipher.class
├── VigenereCipher.ctxt
├── VigenereCipher.java
├── VigenereCipherJUnitTest.class
├── VigenereCipherJUnitTest.ctxt
└── VigenereCipherJUnitTest.java

Specification

• Write a Java program in the file VigenereCipher.java that encrypts a message using the Vigenere Cipher.
• You will write your solution in a function called encrypt(String message, String 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

Testing

• Click on the Run Tests button to execute the JUnit test bench. JUnit indicates a successful test with a green bar, and an unsuccessful test with a red bar.

Submission

• Upload the file VigenereCipher.java to the Web-CAT automated grading platform.