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3

A Vigenère cypher can be unbreakable if, and only if: the key is random, the key is at least as long as the message being encrypted the key is used only once. In that case a Vigenère cypher is mathematically equivalent to a One Time Pad, and the same mathematical proof of unbreakability applies. You proposal is closer to being unbreakable than standard ...


2

Since you encrypt just a single letter, there are $26^2$ combinations of $p$ and $c$ where $c=E(p)$. This is because there are $26$ possible shift keys in the key space, an therefore each $p$ can be mapped to one of $26$ letters in the code space. Now, assuming that the key is distributed uniformly in the key space, each of those combinations of $(p,c)$ has ...


5

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 ...


-2

Give every kid a deck of cards (2 decks of cards?) and a short (unique) text message on paper. Each kid encodes the message -- by arranging the cards in some order; writing on the cards is cheating. Then every kid decodes a deck of cards (arranged by some other kid) and write the message on paper. Challenge the kids to come up with a better algorithm for ...


4

You could challenge them to devise low-tech, physical zero-knowledge proofs (of knowledge) for games like "Where's Waldo?" and Sudoku, then show them some methods that really work and why. I've done this before with high school CS students and they seemed to really like it. For "Where's Waldo?" one can prepare a large sheet of paper (at least twice as big ...



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