While everybody is using public key encryption, it's not at all obvious how it works. The whole idea is counter-intuitive. Everyone can encrypt a message but only one person can decrypt it? Really? Are you kidding me?
However, when you try to gain some confidence in the system — if for nothing else, just to make sure that it's not a hoax played on you by the cryptographers — you are pointed to scientific papers: The system works like this: [complex math]. The system is secure because: [more complex math].
That being said, here's a simple game for kids that shows how asymmetric encryption works in principle, makes the fact that with only public key at your disposal encryption may be easy while decryption may be so hard as to be basically impossible, intuitive and gives everyone a hands-on experience with a simple asymmetric encryption system.
Here's how it works:
Buy a dictionary of some exotic language. The language being exotic makes it improbable that any of the kids involved in the game would understand it. Also, it makes cheating by using Google Translate impossible.
Let's say you've opted for Eskimo language. The story of the game can be located at the North Pole after all.
You should prefer a dictionary that comes in two bands: English-Eskimo dictionary and Eskimo-English dictionary. The former will play the role of public key and the latter the role of secret key. Obviously, if there's no two-band dictionary available, you'll have to cut a single-band one in two.
To distribute the public key to everyone involved in the game you can either buy multiple copies of English-Eskimo dictionary, which may be expensive, or you can simply place a single copy at a well-known location. In school library, at a local mom-and-pop shop or at a secret place known only to the game participants.
If a kid wants to send an encrypted message to the owner of the secret key, they just use the public key (English-Eskimo dictionary) to translate the message, word-by-word, from English to Eskimo. The owner of the secret key (Eskimo-English dictionary) can then easily decrypt the message by translating it back into English.
However, if the message gets intercepted by any other game participant, decrypting it would be an extremely time consuming activity. Each word of the message would have to be found in English-Eskimo dictionary, which would in turn mean scanning the whole dictionary in a page-by-page and word-by-word manner!
And that's it.
I recall that when I was 10 years old, we've used to, with no adult guidance or encouragement whatsoever, invent different secret alphabets and simple substitution ciphers. I am pretty sure that if we've got our hands on a true cryptographically secure cipher — and an asymmetric one for that matter — we would be simply blown away. I certainly intend to show the trick to my son when he gets to that age.
Martin Sústrik, Oct 24th, 2013