do u know Captcha ?

What is a CAPTCHA?
If you have ever registered yourself on any site (mail, social networking etc) there’s a good chance you’ve seen a CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart). It is a simple test to prove your humanity to the site you are visiting. In this post I will be discussing CAPTCHAs in general, and experimenting with some new designs that might further thwart spammers.
Why is it needed?
The problem stems from how HTML forms work – they’re very simple to fill out and submit. Using a specially coded software, a spammer could  create thousands of email account in a matter of seconds, these could be used for sending email spam. (Because the email sending page is essentially another HTML form.) Spamming guestbooks is also a popular SEO technique to drive up a particular sites PageRank with both Google and other search engines, which employ similar mechanisms.
CAPTCHA systems were invented to stop spammers. A CAPTCHA is usually a generated image that shows some sort of distorted text, which the user can read and input properly. A lot of work go into crafting CAPTCHAs that are simple for the user to read and understand, but difficult for OCR software to comprehend properly. This is usually done by adding distortion of some sort. But as OCR software improves and some people actually write custom code that target specific CAPTCHA systems, more obfuscation is needed, and so unreadable examples as shown in the image below are not uncommon.

An excellent example of a horrible CAPTCHA

There has been a few ones including identification of cats and dogs, although these are also flawed because of their limited databases, which are subject to brute force attacks.
Here’s another one,

An excellent example of an excellent CAPTCHA

This simple substitution CAPTCHA is good because it is unbreakable by standard OCR and requires a special algorithm. This involves someone programming it, which involves manpower, which involves money, which is a rare commodity – therefore, unless this kind of CAPTCHA is run on a high-profile site it would be too much hassle breaking it, and so this kind of alternative implementations works well for smaller sites.

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