'15-'16 General Discussion for Lesson 4.7


Use this thread to discuss your questions and comments about how to run the lesson.

General Discussion for Lesson 2.18

I would consider combining this with Lesson 2.19…I think both lessons can be done in a single day, particularly if students have experience with modulo arithmetic.


In the Teaching Guide, in the Getting Started section, it states:

Prompt: Let’s do a thought experiment: […snip…] Can you figure out a way for Bob to send Alice an encrypted message that only Alice can read?

Have small groups or pairs discuss this problem for a few minutes and then ask if there are any solutions.
Teacher’s Note: There is no way to do this. […snip…]

One of my students came up with the answer of “One-time Pad” and I couldn’t come up with a reasonable argument why his answer isn’t valid. Can someone elaborate as to whether or not a one-time pad is a valid answer to the posed question? Thanks…


Great question! A one time pad is an excellent way to encrypt information but it depends on Alice and Bob having met ahead of time to establish the contents of the one time pad. In the scenario described all communication between Alice and Bob takes place through Eve, so they have no way of establishing the one time pad without Eve knowing it as well.

Hope that helps!


This lesson went fairly well. I think you could also do a whole-class demo of this activity if you wanted to combine it with lesson 19. I have also seen this activity done with paint and I think next year I might do the demo with glasses of colored water and eyedroppers because I think that gets at the idea of public key and two separate private keys a bit better. I might do that as my warm-up for lesson 19 to practice using the vocabulary introduced in this lesson.