Lab 5: Enigma | CS 2113 Software Engineering - Spring 2022

Lab 5: Enigma

View the videos for this lab on youtube


Accept this assignment here:

Development Environment

This lab can be developed locally on your machine using any java installation or on replit.

Running/Compile your program

You should compile your program using javac and run it with java. The specific command line arguments are provided below.

Testing your lab

We have provided you with a file to help you test your program.

Enigma Machines (simplified model)

The Enigma machine was used by the Germans in WWII to send encoded messages. At the time, it was a breakthrough in cryptography, and was essentially an extremely advanced substitution cipher. The Enigma machine is also famous for not just being a very advanced cipher, but also because it was broken by none other than Alan Turning, whom many consider the founder of computer science.

In this lab, we will model a simplified version of the Enigma machine. If you’re interested in learning more, Prof. Gavin Taylor (USNA) has a comprehensive write up on the topic. (More details and an image are found at the end of this lab.)

What you need to know for this lab

What you need to know for this lab: Enigma machines used interchangeable rotors that could be placed in different orientations to obtain different substitution patterns. More significantly, the rotors rotated after each character was encoded, changing the substitution pattern and making the code very difficult to break. The behavior of the rotating rotors can be modeled, in a simplified form, by a device consisting of labeled, concentric rings. For example, the picture here has three rings labeled with the letters of the alphabet and ‘#’ (representing a space).

To encrypt a character using this model, find the character on the inner rotor (i.e., the inside ring) and note the character aligned with it on the outer rotor (i.e., the outside ring), then find that character on the middle rotor (i.e., the middle ring) and output the one aligned with it on the outer rotor. After a character is encrypted, turn the inner rotor clockwise one step. Whenever the inner rotor returns to its original orientation, the middle rotor turns once in lock-step, just like the odometer in a car.

For example, in this configuration the character ‘A’ would be encrypted as ‘N’, since ‘A’ on the inner rotor is aligned with ‘H’ on the outer rotor, and ‘H’ on the middle rotor is aligned with ‘N’ on the outer rotor. After performing this encryption, the inner rotor is rotated clockwise, so the letter ‘A’ would next be encrypted as ‘D’.

Note that decrypting a message requires following the same steps, only in reverse: Find the character on the outer rotor, note the character aligned with it on the middle rotor, find that character on the outer rotor, then output the character aligned with it on the inner rotor. Don’t forget to rotate the rotors after each letter is decrypted.

Task Requirements

The Task

You will define a class Rotor to simulate the workings of a single rotor, and the class Enigma to simulate the workings of an Enigma machine using Rotor instances. You will be provided a class Comms (with a main method) as part of the initial material. You should read that file to see how Enigma instances are suppose to be used.

You may not alter in any way.

Your task is to write both Enigma and Rotor using proper OOP design with class constructors, information hiding, and encapsulation.

The Rotor Class

The Rotor class represents a rotor, including the values of the characters in the rotor and its current orientation (which character is currently on top of the rotor). A good strategy for representing this would be to store the characters in a String of length 27 (# indicating space) – index 0 is the top most character. Note that Rotors rotate! And after a full rotation, the next outer rotate rotates (like a odometer in a car). That means you’ll need to remember where the rotor started. All of this can lead to some good OOP! :)

For example, your rotor should be able to do the following

  1. Be constructed, requiring a String that defines the rotor and a single character defining the symbol that should be initially at the top of the rotor. Note: in the constructor, you can call other methods, like the method to rotate!
  2. Rotate one click clockwise. This should involve changing the String.
  3. Return the index in the String at which a given character appears.
  4. Return the character at a given index.

An example of a rotor String is #GNUAHOVBIPWCJQXDKRYELSZFMT … which you are to interpret circularly, so that the last character loops around to the first. If you imagine that the first positition indicates the top spot on the rotor, then:


The Enigma Class

This we leave partly up to you. We expect your Engima to have 5 possible rotors, and when your Enigma class is created, it chooses which 3 to use along with their rotor starting symbols. You must hardcode the 5 possible rotors in your class as the following Strings:


You must also have encrypt and decrypt methods for encrypting and decrypting strings. These must be compatible with the file that we give you. The behavior in these methods must follow the enigma procedure described above in “Our Simple Model of the Enigma”.

The Comms Class

Note you should not edit the Comms class, but you do need to know how it works.

This is the program’s main class, and it is provided for you. The program takes as input (from the command line) the three rotors and their starting characters. A correct call to the program Comms will provide all the information needed to setup enigma for that encryption/decryption session on the command-line, and the actual string to encrypt/decrypt should be input from standard in.

                  ,-- inner rotor initially positioned so X is on top
                  |,-- middle rotor initially positioned so # is on top
                  ||  ,-- outer rotor initially positioned so Y is on top
                  || /
java Comms 4 2 3 "X#Y" encrypt
           | | |
           | | `-- outer rotor is rotor 3
           | `-- middle rotor is rotor 2
           `-- inner rotor is rotor 4

A couple example runs are here. You must also make your own tests and fully test your code:

~/$ java Comms 1 2 3 "###" encrypt
~/$ java Comms 3 1 2 "SAT" encrypt
~/$ java Comms 5 2 4 "EST" decrypt


You must submit

  1. : Enigma class
  2. : Rotor class
  3. : Answer questions and describe your work in this lab

Your and must meet the specifications above.

This lab is adopted from IC211 (spring 2019) at USNA.