127 · LITTLE BROTHER
locked twice. The first lock — the boss’s public key — only comes off when combined with your boss’s private key. The second lock — your private key — only comes off with your public key. When your bosses receive the message, they unlock it with both keys and now they know for sure that: a) you wrote it and b) only they can read it. It’s very cool. The day I discovered it, Darryl and I immediately exchanged keys and spent months cackling and rubbing our hands as we exchanged our military-grade secret messages about where to meet after school and whether Van would ever notice him. But if you want to understand security, you need to consider the most paranoid possibilities. Like, what if I tricked you into thinking that my public key was your boss’s public key? You’d encrypt the message with your private key and my public key. I’d decrypt it, read it, re-encrypt it with your boss’s real public key and send it on. As far as your boss knows, no one but you could have written the message and no one but him could have read it. And I get to sit in the middle, like a fat spider in a web, and all your secrets belong to me. Now, the easiest way to fix this is to really widely advertise your public key. If it’s really easy for anyone to know what your real key is, man-in-the-middle gets harder and harder. But you know what? Making things well-known is just as hard as keeping them secret. Think about it — how many billions of dollars are spent on shampoo ads and other crap, just to make sure that as many people know about something that some advertiser wants them to know? There’s a cheaper way of fixing man-in-the-middle: the web of trust. Say that before you leave HQ, you and your bosses sit down over coffee and actually tell each other your keys. No more man-in-the-middle! You’re absolutely certain whose keys you have, because they were put into your own hands. So far, so good. But there’s a natural limit to this: how many people can you physically meet with and swap keys? How many hours in the day do you want to devote to the equivalent of writing your own phone book? How many of those people are willing to devote that kind of time to you? Thinking about this like a phonebook helps. The world was once a place with a lot of phonebooks, and when you needed a number, you could look it up in the book. But for many of the