I have some kids that like encryption schemes such as the Caesar cipher and the Vigenère cipher. I would like to teach them something that's not easily breakable by todays maths and computers, but I want them to be able to use it just using pen and paper.
RSA probably isn't the way to go, since specially the prime finding part is quite intensive.
A symetric cipher like AES would also be fine, but the current AES standard requires lots of rounds of calculations to be effective.
One-time pads are nice, but not really a proper cipher.
Are there any cute modern schemes that are really cheap, but still have a reasonable amount of security?