Lab 6: Caesar Cipher

Task Outline

• Due Date: Friday, November 8, 2019
• Total Points: 10
• Implement a Java program that encrypts a message using the caesar cipher.

Background Theory

• The main idea behind the Caesar Cipher is to shift each letter in a secret message by a fixed number of positions. If this shifting behaviour goes further than the end of the alphabet, then it wraps around to the beginning, and continues from there.
• The security of this crypto-system relies on having only the sender and the recipient know the secret key, which is the number of places by which the letters have been shifted.

Hints

• Unencrypted text is generally called plaintext, and encrypted text is generally known as ciphertext. The quantity by which the letters have been shifted is called a key.
• In general, the Caesar Cipher encrypts messages by rotating each letter by key positions. More formally, if p is the alphabet index of a plaintext letter, and key is the amount by which that letter is shifted, then the alphabet index of the corresponding letter in the ciphertext c, is computed by the following equation:

• You may assume that all of the characters in the plaintext messages are in lowercase.
• There will be no punctuation present in any of the plaintext messages, with the exception of the space character. You should design your program so that any spaces in the plaintext message are transferred into the encrypted ciphertext.
• Note that the complete lowercase alphabet has been provided for you:

String alphabet = "abcdefghijklmnopqrstuvwxyz";

• This means that "a" corresponds to index 0, "b" corresponds to index 1, etc.

Code Distribution

Contents of lab06.zip:

Lab06CaesarCipher/
├── CaesarCipher.class
├── CaesarCipher.ctxt
├── CaesarCipher.java
├── CaesarCipherJUnitTest.class
├── CaesarCipherJUnitTest.ctxt
├── CaesarCipherJUnitTest.java
├── package.bluej
└── README.TXT

Specification

• Write a Java program in the file CaesarCipher.java that encrypts a message using the caesar cipher.
• You will write your solution in a function called encrypt(String message, int key) right below the place where it says: YOUR CODE HERE
• When the function call encrypt("hello", 1) is executed, the output of the program should be: ifmmp

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