Navigate Captcha codes to access protected areas of the internet

It’s possible to navigate the web without ever having to log in to particular sites or pages or otherwise supply any information.

However, to do so is to miss out on many interesting aspects of the online world. Many internet forums, for example, require visitors to register a user account before they are allowed to take part in the conversation.

But to check that visitors are human – as opposed to a ‘robot’ program that wants to log in to forums in order to harvest email addresses to sell to spammers – many websites present little word or number puzzles.

These puzzles are known as Captchas and they’re designed to make web surfers prove that they’re human beings. In this Back To Basics, we’ll explain how they work and why they’re important.

What are Captchas?
Depending on who you ask, Captcha may or may not stand for ‘Completely Automated Public Turing test to tell Computers and Humans Apart’. Regardless, the system was invented by a group of professors at Carnegie Mellon University in the US and first used around 10 years ago, by Yahoo for its email service.

Basically, a Captcha is a challenge-and-response test designed to help you prove you are a human by asking a question that’s difficult for machines to decipher.

Most Captchas are images that show one or two words bent out of shape, sometimes with a line running through them – and all you have to do is type what you see into the empty box underneath them.

Letters are often squeezed together or have ‘noise’ scattered behind them because this makes it harder for web robots to decode the image with software that can recognise letters by their shape.

Some Captchas ask a general-knowledge-style question or serve up a maths problem; Microsoft Research even has a picture-based one called Asirra that asks visitors to select all the cats from a collection of cat and dog photographs.

Dealing with Captchas
If you’ve never seen or had to use a Captcha, the easiest thing to do is try one out. For example, go to the Computeractive website, click the Forums link in the navigation bar at the top.

At the next screen, click the Register link and then agree to the Forum rules by ticking the box. Then click Register. On the following screen, choose a Username for yourself and then pick a password. Add and confirm your email address and then fill in the Captcha form at the bottom.

The Captcha will ask for two words and they won’t necessarily be easy to read instantly. This is deliberate: if the words were clear then the Captcha would be easier for robots to solve. Type the two words into the box below, finish the rest of the form and then click the Complete registration button at the bottom.

If you have entered either of the words incorrectly, the form will reset and you will have to try again (you will have to retype the password twice, as a security measure).

Well-designed Captchas accommodate visually impaired visitors who use screen-reading software by offering the option to pick out and enter a sequence of words played back over some background noise.