CAPTCHA

Jeff Yan, a computer scientist at Newcastle University, is one of many researchers interested in cracking CAPTCHAs.

The idea of negative CAPTCHAs is not to ask a user to proof that he is human, but reveal that a spam robot is a bot.

Dr von Ahn thinks it will be possible for software to break text CAPTCHAs most of the time within five years.

This means designers of CAPTCHAs cannot simply increase the amount of distortion to foil attackers.

Keep in mind that things like Captchas are barriers to user interaction and will likely degrade the user experience.

The completed captchas help spammers open new online accounts to send junk e-mail and carry out other mischief.

Scrapers sometimes fight back with software that deciphers captchas.

ActsAsSnook is a simple and elegant method of handling comment spam that doesn't rely on CAPTCHAS or Javascript or external web services.

The faster that software evolves, and the harder it gets to distinguish between people and computers, the faster CAPTCHAS have to change.

Mr Hughes, along with many others in the field, thinks the lifespan of text-based CAPTCHAs is limited.

So negative CAPTCHAs might not be good to protect login forms.

"We modify our CAPTCHAs when we detect new abuse trends, " says Macduff Hughes, engineering director at Google.

Additionally, some visually impaired users have found certain kinds of distorted CAPTCHAs difficult to read.

Can we take this understanding to create a new breed of CAPTCHAs?

The problem with CAPTCHAs is, they are annoying.

But how secure are CAPTCHAs?

You can also get around CAPTCHAS by being clever.

Teams of people based in developing countries can be hired online for $3 per 1, 000 CAPTCHAs solved.