pbSigning up for a webmail account or social networking site usually requires the user to decipher and re-type some scrambled letters first./b/pp/ppThese text systems are called Captchas (Completely Automated Public Turing Test to tell Computers and Humans Apart). /ppUsers are asked to fill out a captcha form to confirm they are a real person rather than a computer program. /ppTheir purpose is to stop hackers creating programs that could automatically sign for thousands of accounts. /ppScammers and spammers would then exploit these to send out more effective junk mail. /ppbSpreading malware/b/ppA lot of spam messages are automatically caught and blocked by spam filters because the sender's address does not exist. /ppMessages sent from a real email account appear more genuine, and tend to get through spam filters. /ppimg src="http://newsimg.bbc.co.uk/media/images/45842000/jpg/_45842499_wood_203.jpg" align="left" width="203" height="152" alt="Paul Wood from web security firm MessageLabs" border="0" vspace="4" hspace="4"/ppBut fake accounts on social networking sites open up even more possibilities. /ppFor instance, hackers exploit the ability to share multimedia content to spread malware, said Paul Wood from web security firm MessageLabs. /ppquot;If you can encourage someone to click on a link, and that site is within a social networking environment, it's very difficult for the person to identify if it's genuine or not,quot; he explained. /ppSome techniques are being used to make captchas harder, so software is less likely to identify the characters or remove any background noise. /ppquot;The most important technique to make a captcha difficult to crack is to make those characters either overlapped or connected,quot; said Jeff Yan, a lecturer on computer security at Newcastle University. /ppquot;The second most important technique is to use all sorts of distortion techniques,quot; he added. /ppbTrickier tests/b/ppBut crafty hackers are employing automated speech recognition software to break the audio alternative to captchas, which are meant for visually impaired users. /ppHowever, researchers are also working on alternatives to come up with increasingly tricky tests. /ppimg src="http://newsimg.bbc.co.uk/media/images/45842000/jpg/_45842500_scramble_203.jpg" align="left" width="203" height="152" alt="Moving monogram when signing up for an Opera mail account" border="0" vspace="4" hspace="4"/ppFor example, anyone registering for an Opera mail account has to make sense of a moving mangled monogram. /ppA test scheme from Microsoft known as a href="http://research.microsoft.com/en-us/um/redmond/projects/asirra/ "Asirra/a (Animal Species Image Recognition for Restricting Access) has swapped the text for pictures. /ppUsers are asked to identify whether animal pictures randomly selected from a pet adoption site contain cats or dogs. /ppA similar idea from a href="http://www.cmu.edu/index.shtml"Carnegie Mellon University/a requires the user to identify and trace a specific object from a selection. /ppDespite this proving much harder for computers and easier for humans, these kinds of tests have been criticised for not being proper captchas. /ppAlso, the design of every single test needs the involvement of a person to decide on the right answer. /ppbCaptcha breaking/b/ppThe human mind, as the ultimate deciphering tool, is available to hackers who employ people to break captchas. /ppquot;In India, for example, there are a number of businesses that specialise in this activity. They have 24/7 coverage, enable people to work from home, flexible working hours, and all they're doing is data processing. But that data processing is of course captcha breaking,quot; said Mr Wood./ppimg src="http://newsimg.bbc.co.uk/media/images/44150000/jpg/_44150846_captcha203.jpg" align="left" width="203" height="152" alt="Recaptcha" border="0" vspace="4" hspace="4"/ppHe added that many others do a hacker's work for them without being aware. /ppquot;For example, they may be trying to get access to an adult website, and in order to gain that access they have to solve a captcha - on behalf of the bad guys. /ppquot;Another technique, more recently, is where some malware may be infecting a machine, and it will pop up and say that if you don't solve this captcha within three minutes then your machine will shut down,quot; he said. /ppCarnegie Mellon University in Pittsburgh has come up with the a href="http://recaptcha.net/learnmore.html"Recaptcha/a project that puts solved text to good use. /ppThe project takes words from old books and newspapers that optical character reading software has marked as unreadable by computers. /ppBy deciphering these words, users are helping to complete the conversion of old texts to digital form. /phrpThis article is from the a href="http://news.bbc.co.uk"BBC News website/a. #169; British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites./pdiv class="feedflare"
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