Good challenges for a crypto competition for teenagers I'm holding a cryptography workshop for teenagers (around 16 years old) at our university. As part of the workshop, I'm planning to run a crypto competition with prizes: there will be different tasks, each one will yield points, and the winners will get sweets.

However, for the amount of time I'm planning to kill with this game, I still need one or two more challenges. So far, what I have is:


Scytale: one band with letters on it and four potential objects - find out which is the right object so you can read the message and obtain the solution.
Caesar Cipher: Find out by how much the letters have been shifted to get the solution.
A transposition cipher (one where you write the text in a rectangle and then read the columns).
One with frequency analysis (they get the frequency table for the letters and a longer text which I chose to be pretty close to at least the most frequent values).


As you can see, these are all pretty basic stuff that can be done by hand. I don't really want to do anything that would require the kids to do any programming, although I suppose I could do something like install Cryptool2 on an old computer and have that as one station, if anyone has a nice idea that requires computer assistance.

I'm hoping that, among the community of crypto experts and enthusiasts here, some of you may have experience with running such competitions for kids and teenagers, and may be able to suggest some good challenges.  If you've organized something like this before, and know some suitable challenges that I haven't already thought of, please share them!
Perhaps you could do something with Visual Cryptography.
Maybe something like:


Gather a few low-resolution images (symbols or short text phrases), perhaps a few more images than you have kids
Use visual cryptography to split each image into 2 random-looking images, and print each random-looking image on its own piece of transparency paper
Shuffle the pile of transparency papers
somehow let the kids find matching pairs of transparencies that show the original symbol or phrase.


Is there some way to let kids produce Visual Cryptography image-pairs "by hand", without a computer?

Links and further reading:


"What is Visual Cryptography"
online visual cryptography generator
"Visual Cryptography Kit"
visual cryptography example
"Visual Cryptography and Bit-Plane Complexity Segmentation"
XOR with polarized film