Building Cryptosystems – Secret Messages


 

A replica of a Jefferson cylinder cipher at the National Cryptologic Museum. Photo from Wikimedia Commons.

ENGINEERING – The French passenger ship Lamoricie crossed the Mediterranean from Algiers to France when she sank near the Balearic Isles on January 9, 1942. The weather took a turn for the worse and the ship altered course to assist a freighter in distress, the SS Jumieges. Unfortunately, the ship foundered in heavy seas before the Lamoriciere could help. The captain attempted to take shelter behind the island of Menorca, but the ship could not cross the wind. Finally the boilers shut down. The ship lost power as water began pouring in through the coal hatches, causing it to list heavily and begin to sink. (Because of wartime shortages, the ship’s power source changed from diesel oil to coal.)

301 passengers and crew died in the shipwreck, but 93 survived. One of those who perished was Jerzy Rozycki, one of the three Polish cryptologists who worked on cracking the German Enigma code in 1932. Rozycki and his team had travelled from France to Algiers in late 1941 to work on the Enigma codes and was returning on the Lamoricie when disaster struck. Two other members of the code breaking team, Jan Gralinski and Piotr Smalenski also died.

OBJECTIVES:

  • Students will learn ways to create and send secret messages using three methods of increasing sophistication in this highly interactive introduction to the field of cryptography. Julius Caesar used these same methods in ancient times and Thomas Jefferson (third president of the U.S.) in modern times to send sensitive messages.
  • Students will also ‘crack’ the secret messages sent by others in the classroom.  Cryptography can provide motivating examples for subjects including statistics, probability, computer science, and others.
  • This class is designed for the junior high and high school level, but could probably work in classes with children as young as 11. Classroom discussion and assignments can be tailored to be age appropriate.

Watch the video portion of this lesson here at MIT Blossoms.

Pre Class Preparation:

Some materials are required. These include:

• 3 small cups per person. These cups must have a lip that is visible when cups are stacked. Small Styrofoam cups work great for this purpose.

• One sheet of paper per pair that contains 7 columns of evenly spaced boxes for each character in your alphabet. These strips will be cut out and wrapped around the outside of the lip of the cups. Creating this sheet is the most complicated portion of pre-class preparation. Follow the instructions carefully. Download alphabet strips here.

• Scissors

• Tape or glue. Tape allows to more easily correct mistakes.

• 2-3 sheet of paper for writing messages, encrypting them, and doing scratch work to decrypt messages from others.

• Pencil, pen, marker: Must be able to write on paper and also make markings on the cups.

Instructions:

Part 1: Introduce what the student will do and make sure they have the necessary supplies. Students should work in pairs. Assign each pair  a team number. Each person within that pair should have a designation such as ‘red’ or ‘blue.’

Part 2: Show the students how to use the materials to create the devices they will need. Be sure that when the cups are being assembled that the strips are tight around the lip and are on securely. It is extremely important that everyone in the classroom holds the cup using the same hand when taping the strips on. If everyone in the class stacks their cups, the alphabet strips must all be right side up. Make sure all the cups in the class have the same orientation or the last exercise will fail. You can correct this if you use tape.

Part 3: This section demonstrates how to create and send a message using a cipher that Julius Caesar used. The video shows students how to use their cups to do this. Students should create messages and then decrypt their partner’s message when this section of the video ends. These messages can contain multiple words, but they probably should not be too long. Download a worksheet that visually represents the encryption/decryption process. Alternatively, students can simply do their work on blank paper.

Part 4: Introduce a competition in the classroom. Students should trade encrypted messages with other teams and then attempt to crack other students’ messages. There are a variety of other things you could do in this break. You can write a message on the board and have all of the students crack it alone, in pairs, or as a class. The fastest way for the class to crack the message is to have each student trying to use different keys in parallel. Parallelizing the process makes it much simpler. You can also discuss features of your language that might make it easier to attack a message. This includes the placement and frequency of different letters and the patterns that exist that can help guide their intuition. Cryptography experts use many statistical patterns in a language to guide their attacks.

Part 5: This section has the students encrypt another message using a slightly more sophisticated technique using random sequence strips created in part 2. This should be relatively easy for the students. A discussion you may want to have in the class is about the importance of random numbers in probability, statistics, and computer science. Attackers broke many cryptography systems in the past because of a flawed process used to generate randomness.

Part 6: Students will split into two large groups and send messages using Thomas Jefferson’s cryptosystem. Each device will have as many cups as there were teams in the classroom. The messages these teams send can be large. If the message exceeds the length of the column of cups, have the teams repeat the encryption process repeatedly until they encode the entire message. You may want to have multiple classes contribute to the same stack of cups so that you eventually have a very large device. In order to do this, assign team numbers that continue from the numbers used in the previous class.

Additional Resources:

Learn about Thomas Jefferson’s Wheel Cipher, created encode and decode messages during the American Revolution.

Read this article that provides a comprehensive history of the British cryptoanalysts who broke the Enigma Machine code during World War II.

Click here for more on the Enigma Machine.

Watch video clips of famous code breakers throughout history.

Education Standards

CSTA K-12 Computer Science Standards

AP – Algorithms & Programming

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