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by Ben Goren

There are many basic tools in the modern cryptographer’s toolkit; I’ve written a few papers describing some of them.

First up is the Playfair Cipher. It’s simple enough for back-of-the-napkin encryption, and probably perfect for kids wanting to trade secret messages. It was once state-of-the-art, but computers have long since rendered it obsolete. However, it works in exactle the same way as most modern ciphers, just with very weak (by today’s standards) math. Understand it and you’ll have a pretty good idea of how your bank transfers are kept from prying eyes.

The Playfair cipher and its modern cousins suffer from a crippling flaw: the strength of the cipher (theoretically) depends on the secrecy of a shared key. But if you have a way to share a key secretly, why not use that same way to send the message itself? RSA is the first popular implementation of public key cryptography, a system that allows parties who have never met to communicate securely. As a bonus, it also allows cryptographic signatures that can verify that a document came from the person it claims to have come from.

If you spend much time learning about cryptography, you’ll soon learn that there’s all kinds of snake oil out there. Most involve false claims of “unbreakable” encryption at super-fast speeds. In the real world, there’s only one form of encryption that is really, truly unbreakable: a One-Time Pad.

In the future, I plan on at least adding a paper about secure hash algorithms, and maybe even an example that ties all the other papers together.