Van shoved me hard with both hands. She was strong from school athletics -- fencing, lacrosse, field hockey, all the girls-school sports -- and I ended up on my ass on the disgusting San Francisco sidewalk. She took off and I didn't follow.

#

> The important thing about security systems isn't how they work, it's how they fail.

That was the first line of my first blog post on Open Revolt, my Xnet site. I was writing as M1k3y, and I was ready to go to war.

> Maybe all the automatic screening is supposed to catch terrorists. Maybe it will catch a terrorist sooner or later. The problem is that it catches us too, even though we're not doing anything wrong.

> The more people it catches, the more brittle it gets. If it catches too many people, it dies.

> Get the idea?

I pasted in my HOWTO for building a arphid cloner, and some tips for getting close enough to people to read and write their tags. I put my own cloner in the pocket of my vintage black leather motocross jacket with the armored pockets and left for school. I managed to clone six tags between home and Chavez High.

It was war they wanted. It was war they'd get.

#

If you ever decide to do something as stupid as build an automatic terrorism detector, here's a math lesson you need to learn first. It's called "the paradox of the false positive," and it's a doozy.

Say you have a new disease, called Super-AIDS. Only one in a million people gets Super-AIDS. You develop a test for Super-AIDS that's 99 percent accurate. I mean, 99 percent of the time, it gives the correct result -- true if the subject is infected, and false if the subject is healthy. You give the test to a million people.

One in a million people have Super-AIDS. One in a hundred people that you test will generate a "false positive" -- the test will say he has Super-AIDS even though he doesn't. That's what "99 percent accurate" means: one percent wrong.

What's one percent of one million?

1,000,000/100 = 10,000

One in a million people has Super-AIDS. If you test a million random people, you'll probably only find one case of real Super-AIDS. But your test won't identify one person as having Super-AIDS. It will identify 10,000 people as having it.

Your 99 percent accurate test will perform with 99.99 percent inaccuracy.

That's the paradox of the false positive. When you try to find something really rare, your test's accuracy has to match the rarity of the thing you're looking for. If you're trying to point at a single pixel on your screen, a sharp pencil is a good pointer: the pencil-tip is a lot smaller (more accurate) than the pixels. But a pencil-tip is no good at pointing at a single atom in your screen. For that, you need a pointer -- a test -- that's one atom wide or less at the tip.