The Kindness of Strangers

By Bruce Schneier
The Wall Street Journal
March 12, 2009

When I was growing up, children were commonly taught: "don't talk to strangers." Strangers might be bad, we were told, so it's prudent to steer clear of them.

And yet most people are honest, kind, and generous, especially when someone asks them for help. If a small child is in trouble, the smartest thing he can do is find a nice-looking stranger and talk to him.

These two pieces of advice may seem to contradict each other, but they don't. The difference is that in the second instance, the child is choosing which stranger to talk to. Given that the overwhelming majority of people will help, the child is likely to get help if he chooses a random stranger. But if a stranger comes up to a child and talks to him or her, it's not a random choice. It's more likely, although still unlikely, that the stranger is up to no good.

As a species, we tend help each other, and a surprising amount of our security and safety comes from the kindness of strangers. During disasters: floods, earthquakes, hurricanes, bridge collapses. In times of personal tragedy. And even in normal times.

If you're sitting in a cafe working on your laptop and need to get up for a minute, ask the person sitting next to you to watch your stuff. He's very unlikely to steal anything. Or, if you're nervous about that, ask the three people sitting around you. Those three people don't know each other, and will not only watch your stuff, but they'll also watch each other to make sure no one steals anything.

Again, this works because you're selecting the people. If three people walk up to you in the café and offer to watch your computer while you go to the bathroom, don't take them up on that offer. Your odds of getting three honest people are much lower.

Some computer systems rely on the kindness of strangers, too. The Internet works because nodes benevolently forward packets to each other without any recompense from either the sender or receiver of those packets. Wikipedia works because strangers are willing to write for, and edit, an encyclopedia – with no recompense.

Collaborative spam filtering is another example. Basically, once someone notices a particular e-mail is spam, he marks it, and everyone else in the network is alerted that it's spam. Marking the e-mail is a completely altruistic task; the person doing it gets no benefit from the action. But he receives benefit from everyone else doing it for other e-mails.

Tor is a system for anonymous Web browsing. The details are complicated, but basically, a network of Tor servers passes Web traffic among each other in such a way as to anonymize where it came from. Think of it as a giant shell game. As a Web surfer, I put my Web query inside a shell and send it to a random Tor server. That server knows who I am but not what I am doing. It passes that shell to another Tor server, which passes it to a third. That third server -- which knows what I am doing but not who I am -- processes the Web query. When the Web page comes back to that third server, the process reverses itself and I get my Web page. Assuming enough Web surfers are sending enough shells through the system, even someone eavesdropping on the entire network can't figure out what I'm doing.

It's a very clever system, and it protects a lot of people, including journalists, human rights activists, whistleblowers, and ordinary people living in repressive regimes around the world. But it only works because of the kindness of strangers. No one gets any benefit from being a Tor server; it uses up bandwidth to forward other people's packets around. It's more efficient to be a Tor client and use the forwarding capabilities of others. But if there are no Tor servers, then there's no Tor. Tor works because people are willing to set themselves up as servers, at no benefit to them.

Alibi clubs work along similar lines. You can find them on the Internet, and they're loose collections of people willing to help each other out with alibis. Sign up, and you're in. You can ask someone to pretend to be your doctor and call your boss. Or someone to pretend to be your boss and call your spouse. Or maybe someone to pretend to be your spouse and call your boss. Whatever you want, just ask and some anonymous stranger will come to your rescue. And because your accomplice is an anonymous stranger, it's safer than asking a friend to participate in your ruse.

There are risks in these sorts of systems. Regularly, marketers and other people with agendas try to manipulate Wikipedia entries to suit their interests. Intelligence agencies can, and almost certainly have, set themselves up as Tor servers to better eavesdrop on traffic. And a do-gooder could join an alibi club just to expose other members. But for the most part, strangers are willing to help each other, and systems that harvest this kindness work very well on the Internet.

earlier essay: Privacy in the Age of Persistence
later essay: Blaming The User Is Easy -- But It's Better to Bypass Them Altogether
categories: Psychology of Security
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Photo of Bruce Schneier by Per Ervland.

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