Lab 8: Substitution Cipher

Task Outline

• Due Date: Friday, November 22, 2019
• Total Points: 10
• Implement a Java program that encrypts a message using the substitution cipher.

Background Theory

• In a substitution cipher, we encrypt a message by replacing every letter in a plaintext message with some other letter.
• In order to perform this encryption, we make use of a key. In the case of the substitution cipher, the key is a mapping of each of the letters of the alphabet to the letter that it should correspond to, when we encrypt it.
• To decrypt the message, the receiver of the message would need to know the key, so that they could reverse the process.
• A key, for example, might be the string jtrekyavogdxpsncuizlfbmwhq. This 26-character key means that a(the first letter of the alphabet) should be converted into j(the first letter of the key). Similarly, the letter b(the second letter of the alphabet) should be converted into t(the second letter of the key) and so on.
• Therefore, a message like hello would be encrypted as vkxxn, replacing each of the letters according to the mapping as determined by the key.

Code Distribution

Description	File Size	File Name
Java Source Code for Substitution Cipher	6.0KB	lab08.zip

Contents of lab08.zip:

Lab08SubstitutionCipher/
├── package.bluej
├── README.TXT
├── SubstitutionCipher.class
├── SubstitutionCipher.ctxt
├── SubstitutionCipher.java
├── SubstitutionCipherJUnitTest.class
├── SubstitutionCipherJUnitTest.ctxt
└── SubstitutionCipherJUnitTest.java

Specification

• Write a Java program in the file SubstitutionCipher.java that encrypts a message using the substitution 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", "jtrekyavogdxpsncuizlfbmwhq") is executed, the output of the program should be: vkxxn

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 SubstitutionCipher.java to the Web-CAT automated grading platform.