I think this is an excellent idea. Crypto is hard to learn because (a) proving that it's hard to crack requires complex formal mathematical constructions, and (b) any real crypto uses numbers too big to write down, let alone think about.
Re big numbers: I think it really open things up for kids that we have big integers easily available now. Even though we're doing all the same ops, there's a psychological bridge that gets crossed when the numbers start taking a couple paragraphs or even pages to write down. For one thing, you really start to appreciate that we have computing machinery, and that opens up the space for a discussion of how that came to be (another thread in Cryptonomicon -- because of course Turing was involved in the war effort to break German and Japanese codes, and it was this push which led to the faster evolution of digital computing). Java also has a BigNumber class, and an op for spitting out large numbers with a percentage change that said numbers are prime (you approach a probability of 1 depending on how computationally intensive you want to get -- I haven't studied the source for this, am curious what algorithms are used (would like to see them re-expressed in Python, just for reference)). Probably the Java methods are inherited from some well-known C library and the code is in some numerical recipes book I haven't studied yet.
A clear implementation of various crypto algorithms in Python would be a great thing to have around. If we can take its bit-length down to, oh, 9, then the numbers are numbers high-schoolers can crunch in a class period with a calculator, but the computer can take care of a lot of the calculations automatically.
Plus just consider those really simple substitution codes where you make A = J, B = Z -- whatever random assignment. Also, you want to parse your messages into 5 letter chunks: ALSOY OUWAN TOPAR SEYOU RMESSA GESIN TO5LE TTERC HUNKS. You find this simple "club house" codes in books for little kids, with titles like 'I, Spy' and stuff. Of course this is nothing sophisticated, but it's a great opportunity to use Python string ops to: make the 5 letter uppercase chunks out of whatever plaintext input (could be interactive and first, then convert to file i/o); use the dictionary data structure to do the lookup/substition.
There are some *excellent* opportunities for assignments in there, too. And they're virtually impossible to cheat on :)
Dustin
The basic strategy here is to make Python a fun "toy" (in the sense that kids will want to pick it up and play with it spontaneously, not because some teacher is standing over them with a whip). The way to do this is just provide enough interactive experience to suggest a vocabulary of "tinker toys" (components), which kids while then use in synergetic ways to assemble whatever. What we want to avoid is the intimidating sense that you're not allowed to start small. Too many educational experiences include some older kids (e.g. teachers) suggesting that your work is nothing special, not interesting, because you don't know "all" of what's to know about. Crypto is a case in point. Regarding crypto, there are lots of segues to random numbers (the Standard Library book re Python keeps warning us that the randoms are "pseudo" and hence potentially an Achilles heal if you're using them as a basis for some encryption algorithm). This whole concept of random numbers generated by machines is especially rich, philosophically. We have to be clear what we mean by "random" -- what are the tests such numbers must pass? Knuth has done a lot of work in this area, and computer science has gotten a big boost from this thread -- which thinking we can recapitulate in curriculum writing geared for those relatively new to the subject. I get into random numbers some in my "Random walks through the matrix" piece, wherein a turtle swallows an n-sided die and then hops in a spatial lattice defined by n degrees of freedom at each turn to play (the so-called isotropic vector matrix features 12 spokes from every hub, to the surrounding closest packed spheres at the corners of a cuboctahedron). Kirby