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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?

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I also found this thread: crypto.stackexchange.com/questions/844/… –  Thomas Ahle Oct 17 '12 at 20:17
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up vote 7 down vote accepted

You might get a kick out of the solitaire cypher that appeared in Neal Stephenson's novel, Cryptonomicon. The novel itself might be heavy going for kids, but the cypher is cool, since it's not only pretty robust but, even better, uses an ordinary deck of playing cards.

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So it's sort of like Vigenère with strong key generation, but the number of different keys are really just 28 if people know the algorithm. Interesting. –  Thomas Ahle Aug 7 '12 at 16:34
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@ThomasAhle -- no, there are as many keys as there are ways to shuffle the deck of cards. That's about $2^{237}$ different keys. And of course people know the algorithm: it's described fully in a published (and rather well selling) novel. –  Henning Makholm Aug 7 '12 at 17:11
    
Could one also use rc4 reduced to a size 56 (deck of cards) state? –  Thomas Ahle Oct 17 '12 at 20:18
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