Section 3.7

Learning Goals

• IOC-2.B.5: Students will define Public Key Encryption and asymmetric encryption.

• Students will evaluate the effectiveness of public key encryption.

• Students will locate the public key on a website.

Objectives and General Description

A second encryption technique to safeguard our personal data is asymmetric encryption.  This type of encryption uses a key for encrypting and a different key for decrypting.  A widely used example of asymmetric encryption is Public Key Encryption.   In this lesson, students will watch a video about Public Key Encryption and will locate a public key on a website.  There is an optional whole class activity to further demonstrate asymmetric encryption.

Activities

Activity 3.7.1 (budget 15 minutes)

1. Watch The Internet:  Encryption and Public Keys and complete Guided Notes.    This can be completed in class or as a flipped assignment prior to coming to class.

2. Class discussion of content.

3. Challenge students to find the public key of a website and tell you how many bits it uses.  To find the public key...

   • Click on the lock icon to the left of the URL.

   • Select Certificate.

   • Click on Details.

   • Scroll down and click on Public Key.  You will see the number of bits used and in the bottom of the pop up box, you will see the actual key.

4. Using a snowball technique, pose this question for student discussion:  Is it likely that a public key encryption algorithm can be decrypted within a reasonable amount of time using a brute force attack?  Students may need computer access to define:

• 1. Brute force attack

  2. Reasonable time

5. Students may need help defining reasonable time.  The APCSP standards define it as:  

• 1. Reasonable time means that the number of steps the algorithm takes is less than or equal to a polynomial function (constant, linear, square, cube, etc.) of the size of the input. (AAP-4.A.7)

  2. Algorithms with exponential or factorial efficiencies are examples of algorithms that run in an unreasonable amount of time. (AAP-4.A.7)

6. At this time, public key encryption algorithms create such large numbers that a brute force attack can not break the code.  A solution does exist but our modern day computers do not have adequate memory storage or time to solve it.  Emphasize solvable, unsolvable and reasonable time terminology.

7. AAP-4.A.8 Some problems cannot be solved in a reasonable amount of time because there is no efficient algorithm for solving them. In these cases, approximate solutions are sought. 

8. AAP-4.A.9 A heuristic is an approach to a problem that produces a solution that is not guaranteed to be optimal but may be used when techniques that are guaranteed to always find an optimal solution are impractical. 

Activity 3.7.2 (budget 30 minutes)

This activity is optional and can be used if the teacher wants to further delve into Public Key Encryption.  Follow the lesson instructions for Kid Krypto from CSUnplugged.org

Resources

• 3.7.1: Video Guided Notes for Encryption (student worksheet)

• 3.7.1: Video Guided Notes for Encryption KEY (teacher resource)