Articles for kids about math in everyday life, written by Ivars Peterson for Muse magazine.

April 27, 2007

Twisted Security

You're at the Yahoo! or Google Web site signing up for a new email account. You fill out a form, typing in your name and other information. When you come to the end, you find you have to enter a code, which you get from a string of letters and numbers shown just below the entry form. The weird thing is that the letters and numbers are all twisted and angled, with squiggly lines running through them. You have to figure out what those characters are and type them in correctly before you can complete your account registration.

You'll often run into the same thing at other Web sites—when you're signing up to play a game, voting in an online poll, buying concert tickets, joining a group, and more.

So, what's that all about?

Computers can do all sorts of things, from searching the Web at lightning speed to playing championship chess. But they're not very good at, for example, figuring out what's in a photo or identifying a person's face—things that people, even little children, can do easily. So, one way to make sure that a person rather than a computer is performing some task to to ask for something that people do much better than computers do—like trying to decipher distorted numbers and letters.

Such test images are examples of little puzzles called captchas. The word captcha was invented by computer scientist Manuel Blum of Carnegie Mellon University. It stands for "Completely Automated Turing Test to Tell Computers and Humans Apart." Alan Turing was a famous mathematician who invented a test in which you ask questions of a hidden participant and see if you can tell whether the responses are coming from a person or an "intelligent" computer. In the case of captchas, computers can automatically generate the tests and judge whether a person types in the correct answer, but they can't themselves pass the test.

Blum and his coworkers invented captchas because companies like Yahoo! wanted to keep people from writing computer programs that would seek out Web sites and automatically sign up for large numbers of free email accounts in a very short time. They could then use these addresses to send out mass mailings, or spam.

Yahoo! started using captchas a few years ago to prevent computer programs from being written that would abuse their services. Now, you'll find captches all over the Internet. But there are some problems. Computers are getting better at figuring out what's in images, so some captchas can be solved by computers. And image captchas don't work for people who are blind, for example. As an alternative, Blum and others have come up with captchas that depend on unscrambling sounds, something else that computers have trouble doing. They've also created more complex captchas, for example, comnputer-generated images that contain several, overlapping words selected randomly from a dictionary and displayed against a colored pattern.

While some researchers work on creating tougher puzzles, others put a lot of effort into programming computers to solve those puzzles. The result is the computers keep getting better at all the different things that they do.

Muse, May/June 2007, p. 36.

You can learn more about captchas at

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