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The best example I have encountered comes from mathoverflow.net. Here it is for you convenience. Imagine your friend is color-blind. You have two billiard balls; one is red, one is green, but they are otherwise identical. To your friend they seem completely identical, and he is skeptical that they are actually distinguishable. You want to prove to him (I ...


4

If all you want is a simple hash function you could do something like this. First take your original message and apply something like ROT 13 to it. This gives a reversible cipher, of course. Then give each letter a number, say N = 1, O = 2, etc, wrapping after Z. Next, just add up all of those values. It is no longer a reversible cipher, but a hash. To get ...


2

Since cryptography is a bit elaborate, this will be too. Basic encryption is like the following: Take the "information" you want to transmit and put it into a locked room. A guard sits at the room with an actual key. Take a piece of paper and rip it in half, the more jagged the rip the better. Give half to the guard and the other half to the person you ...


2

A simple hash algorithm is modulo or remainder (they are the same for positive numbers). You only need be able to count. To calculate $n$ modulo $m$, take $m$ empty pots, and $n$ marbles. Place one marble in front of each pot, then put the marbles in the pots, and repeat until you do not have enough marble for each pot, count the marbles that are in-front ...


1

High school students seem to have one thing in common in any country with even moderate technology: cell phones. So, use that to explain the concept in a manner that fits with what they know. The how and why of cryptography is buried in the math that's outside the bounds of what they are likely to know. If, however, the math is accepted as valid, and the ...


1

I used to know a guy who used to explain public-key encryption while showing off a small box that was fitted with a mechanical door lock. The lock had a single keyway, and it came with two different brass keys; One key would only turn to the left (or was it, to the right?) It could only be used to lock the box, not to open it. The other key would only turn ...


1

Pick your first word from the sentence to hash. Look it up in a dictionary and write down the first word of the description. Pick the next word and look it up. Write down the second word from the description. Do this repeatedly and wrapping around from start to finish of the description as many times as necessary depending on the position of the word. A ...


1

Have you considered a practical example like teaching PGP email encryption? Keybase.io has a browser based crypto solution for doing encryption / decryption and while I don't recommend for "real" crypto, it is a great convenience tool for showing how public/private key encryption works without the need to address pgp tools installed on a machine or access ...


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I thoroughly recommend Cryptool2. It's an open-source GUI where you can drag and drop different algorithms, inputs and outputs and see the results in real time. It can be used to show very basic caesar cypers to customizing advanced encryption and hashing techniques.


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There are some engaging lessons on encryption in Unit 4 of the AP Computer Science Principles curriculum from Code.org. Lessons 5, 6 & 7 take students from the concept of encryption using a caesar cypher, to a vigenere cypher, and then through the concepts of public key encryption. Students usually have fun with the widgets that interactively ...


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