Basic Crypto Systems II

Theresa C. McGoldrick (

Substitution Cipher

The Problem
In WWII, the Germans used the Enigma machine, which they thought was an unbreakable code machine, to send messages to each other. The Enigma is a more elaborate application of the substitution cipher. The students will explore the substitution cipher and variations of it. The students will look at the advantages and disadvantages of this cipher.

code sheet, substitution cipher sheet

Lesson 1-Basic Crypto Systems

Activity Description

Guided Exploration

1) Put on the board an encrypted message and ask the students to decrypt it.
EX: Umlv fn vmq bkhhkufrd akjd?
(What is the following code?)

2) As the students try to decipher the code the teacher should pose the following questions?
a) Is there a pattern?
b) Do the size of the letters help you?
c) Do same letters next to one another help you?
d) Which letters appear most and can this help you?

3) Have the students use their Mod 26 to try to help them. After a few minutes ask them is the sheet is of any help?

4) Ask the students if they see any formal pattern?

5) Introduce the term Substitution Cipher to the students. Remind the students of the Enigma machine. (A lesson from history class)

6) Have the students encrypt the following message using the information known from above.
The quick brown fox jumped over the lazy dog.

7) Ask the students if it is possible and why?

8) Hand out the sheet with the substitution cipher on it. Now have the students encrpyt the above message.


1) Have students write a message using the code and try to have a classmate decipher it.

2) Have students write two advantages and disadvantages of the substitution cipher.

Concluding the Exploration

By now the students have had time to explore and discuss the substitution cipher. Have students work in groups of three to try to think of a way to keep the code an absolute secret.

Further Activities

For homework the students will complete the attached worksheet.