6.6. Cryptography Basics

After introducing students to cryptography in the preceding lesson using the Caesar Cipher app, this lesson explores cryptography in depth through the study of additional ciphers.

Information sent over the Internet is visible to others unless encrypted — but then it must also be decrypted so the recipient can view it. Cryptography has ancient roots, increasing in complexity from the time of Caesar to today's advanced systems. In this lesson students learn some basic principles of cryptography by exploring some of the historically important cipher systems through several hands-on activities.

The overall goal of this and the follow-up lesson on cryptography is to reinforce the enduring understanding that cybersecurity is an important concern for the Internet. An important principle to be drawn from this lesson is that the story of cryptography is a never ending struggle between those who create ciphers to protect information and those who want to break the ciphers to gain access to secret information. This battle is playing out today -- think of the debate between the FBI and Apple over iPhone encryption. This lesson presents that back-and-forth story through historical examples of actual ciphers.

Several encryption algorithms are presented as well as several strategies (attacks) for breaking ciphers. The several activities provide students a hands-on way to experience these. However, students are not expected to memorize the algorithms or to be able to apply them on an exam. The College Board guideline states that the specific mathematical functions used in cryptography are beyond the scope of this course.

Professional Development

The Student Lesson: Complete the activities for Mobile CSP Unit 6: Lesson 6.6 Cryptography Basics.


  • Slides: This is a long slide deck (77 slides) but should be broken into several parts and interspersed with activities, as described below.
  • There are four short video clips in this lesson: Video 1, Substitution Cipher (1:52), Video 2, Frequency Analysis (3:28), Video 3, Vigenere cipher (3:38), and Video 4, Key exchange problem (6:07).
  • CS Unplugged Information Hiding Activity - requires pad of paper and pen/pencil
  • Computer lab with projection system
  • Optional: Caesar Cipher Wheel printouts and scissors and paper fasteners to make them.

6.6.1. Learning Activities

Estimated Length: 90 minutes

  • Hook/Motivation (10 minutes): Complete the CS Unplugged Information Hiding exercise.
  • Experiences and Explorations (70 minutes):
    • Part 1 - Simple Substitution Cipher: Using the slides (14-17) or Video 1 (1:52) discuss the fact that Caesar cipher is a fairly trivial example of a substitution cipher -- i.e., very easy to crack. Describe in general how a simple substitution cipher works. Have the students practice with the interactive exercise.
    • Part 2 - Frequency Analysis: Using the slides (18-31) or Video 2 (3:28) discuss how frequency analysis can be used to break encrypted text and how transposition ciphers work. Have the students practice with the interactive exercise and the question listed after. (Hint: they can paste the text into the interactive question to see a histogram. Examine the occurrences of common letters such as vowels and s, t, etc. to help you figure out which is which.)
    • Part 3 - Vigenere Cipher: Using the slides (32-55) or Video 3 (3:38) discuss how a Vigenere cipher works. Have the students practice with the interactive exercise.
    • Part 4 - Perfect Secrecy and the Key Exchange Problem (10 minutes): Using the slides (56-77) or Video 4 (6:07) describe how the Vigenere Cipher can be broken and some strategies to prevent it. This topic will be continued in a later lesson that looks at a more sophisticated technique for encryption.
  • Rethink, Reflect and/or Revise (10 minutes): Have students complete the interactive exercises and portfolio reflections

AP Classroom

The College Board's AP Classroom provides a question bank and Topic Questions. You may create a formative assessment quiz in AP Classroom, assign the quiz (a set of questions), and then review the results in class to identify and address any student misunderstandings.The following are suggested topic questions that you could assign once students have completed this lesson.

Suggested Topic Questions:

Assessment Opportunities and Solutions

Solutions Note: Solutions are only available to verified educators who have joined the Teaching Mobile CSP Google group/forum in Unit 1.

Assessment Opportunities

You can examine students’ work on the interactive exercise and their reflection portfolio entries to assess their progress on the following learning objectives. If students are able to do what is listed there, they are ready to move on to the next lesson.

  • Interactive Exercises:
  • Portfolio Reflections:
    LO X.X.X - Students should be able to ...
  • In the XXX App, look for:

Differentiation: More Practice

If students are struggling with lesson concepts, have them review the following resources:

Differentiation: Enrichment

  • You can find more cryptography challenges at CryptoClub.org.
  • Here is a crypto challenge game at Khan Academy.
  • The Code Book by Simon Singh is an in-depth history of encryption and its relationship to computer science.

6.6.2. Professional Development Reflection

Discuss the following questions with other teachers in your professional development program.

  • While encryption techniques used with the Internet will be introduced in a later lesson, this lesson points out common concerns with any encryption technique. What are they and how can you make the connection to information on the Internet?
    I am confident I can teach this lesson to my students.
  • 1. Strongly Agree
  • 2. Agree
  • 3. Neutral
  • 4. Disagree
  • 5. Strongly Disagree