What physical examples can I use for public key/asymmetric encryption? I'm going to be teaching a lesson about asymmetric cryptography to US high school students who have no background in Computer Science, specifically the way that public key cryptography is used while browsing the web. The typical example that you'd see when reading a book about public key crypto is a padlock. In this classic example, one person has a lock and gives out keys to the lock. Then, anyone with a key can unlock/verify it, but only the person who has the locks can lock/sign document.

However, this doesn't work for a hands-on example, because someone can use the key to close the lock again, and lock/sign different information. Is there some similar tool I can use for a hands-on, physical (not on a computer) example of public key/asymmetric encryption?
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 want to "communicate" with. 

To access the information, the person presents his/her half of the paper and the guard matches it with the guard's own half. If they match up the guard will unlock the door and give access to the "information". 

Depending on the protocol the guard might re-lock the door or destroy the "information". 

Locking the "information" in the room is encryption. The receiver and sender "keys" are the two halves of the piece of paper. Presenting a torn piece of paper to the guard is the attempt to decrypt. 

Note that neither half of the "key" is shared with anyone but the sender and receiver of the message, so duplication isn't an issue. This is fine for a one time message or even repeated messages between the same to entities. 

But this isn't sufficient for the public-private asymmetry that you seek. In that scheme you want to publish one half of the "key". So modify the scheme as follows. 

If someone wants to communicate with you, you control the "keys" and the room. As before, you rip a piece of paper and keep one part, though it shouldn't be a standardized size of paper. The piece torn off, which will be made public, shouldn't  be enough to reveal the overall shape of the paper, perhaps a hole torn out of it or a corner torn off. One piece (the public key) is duplicated and published. Anyone who gets a copy of the public key can communicate with the you, key owner. You keep the other "private piece" without revealing the true overall shape of the original paper. 

Anyone with a copy of the (other person's) public key can put something in the key owner's room, guarded by the key owner's guard. Only the key owner can remove things from the room as controlled by the guard. 

To communicate with you, the key owner, the person presents their information and a copy of the public key (piece of paper) that you published, to the guard who locks up the information and keeps the key (public key half) that you presented. The room is re-locked. When you, the key owner, wants access to the information you present the private key to the guard who will then unlock the room for you after matching the two pieces of paper and verifying that the overall shape is correct. 

Only the key owner can remove things from the room, since the overall shape of the "key" is unknown and "can't" (we hope) be duplicated. Anyone can put things in the room but it will be readable only if the public key held by the guard matches the overall key. 





A related key pair can be created by taking a private drawing (that you produce yourself) and using a part of it as the public key. This might be easier to reproduce than the torn paper. However, it must not be easy to deduce the entire drawing from the fragment. Infeasible, if not impossible.