Unless a security notice pops up on your computer screen warning you of an attempted hack or viral invasion, you’re seldom aware of the vicious guerrilla war in progress beneath your fingertips. But it’s constant, and with every escalation by the attackers, the measures taken to throw the enemy back escalates as well. As in every guerrilla war the offense has the advantage of knowing when and how it will strike, and it employs weapons of mass destruction in the form of bots to probe vulnerabilities, neutralize defenses, and overwhelm its victims.

Though your computer’s defensive team uses powerful programs of its own to thwart attacks, a surprisingly simple weapon has proven effective in holding the line against invaders. It’s called a captcha. New York Times reporter Anne Eisenberg, in New Puzzles That Tell Humans From Machines, describes them as “a set of distorted, squiggly letters and numbers that people can decipher and type correctly for admission, but that machines still can’t.” You’ve undoubtedly cooperated with requests to type in the word you see on the graphic, and, I suspect, you’ve done it with a tolerant sigh, wondering why you’re being asked to play this childish game.

The answer is that captchas are one of the most effective ways to thwart many forms of abuse. In addition to the wiggle-words, captchas employ pictures that are elementary to most nursery schoolers raised on Where’s Waldo? but make no sense to a crawling stealth bot seeking to penetrate the soft underbelly of your desktop or laptop.

“Captcha” is a splendid onomatopoeia, sounding like the task it performs. But it is also a clever acronym coined by the team at Carnegie Mellon University that worked on it: Completely Automated Public Turing Test to Tell Computers and Humans Apart.

Will evil hackers eventually gain the upper hand, like a flu virus recombining after losing out to a vaccine? Eisenberg thinks the good guys will stay ahead:

“Many people worry that as machines become smarter, the days of captcha protection will be numbered, whether the puzzles take the form of distorted text, audio snippets or rotated images. But Henry Baird, a professor in the department of computer science and engineering at Lehigh University, disagrees. Dr. Baird and colleagues have proposed a system for captchas that, like Google’s, can be woven into the theme of a Web site.

“’Machines’ abilities are slowly improving,’ he said, “’but I think there is still a huge gap between human inborn perceptual abilities and machine skills.’”

My curiosity about the technology took me to the official Captcha website, where I discovered that anyone can download a free implementation and plugins including audio tests for blind users. And if you want to pit your wits against Captcha’s computer system there’s a new website, GWAP.com, containing a host of “addictive games that help computers learn to think more like humans. You play the games, computers get smarter!” After I took the gender test I was informed that there was a 97% certainty I was female. Looks like I will now either have to straighten out the GWAP program or commence an extensive course of gender reorientation.

Captchas, the site informs us, “have several applications for practical security.” Among them are:

• Preventing Comment Spam in Blogs
• Protecting Website Registration
• Protecting Email Addresses From Scrapers
• Preventing Ballot stuffing for Online Polls
• Preventing “Dictionary” Password Attacks
• Thwarting Search Engine Bots
• Plausible solution against email worms and spam

Thanks to Captcha technology the playing field has tilted back to humanity after the humiliating defeat of the human race, represented by chess master Gary Kasparov, by the Deep Blue computer in 1997.

Richard Curtis