CAPTCHA or Captcha (pronounced as cap-ch-uh) which stands for “Completely Automated Public Turing test to tell Computers and Humans Apart” is a type of challenge-response test to ensure that the response is only generated by humans and not by a computer.
In simple CAPTCHA is a word verification test to tell that you are a human and not any BOTS.You can see CAPTCHA test all most all SING-UP forms,FORUM,and also some times requsted while you submit COMMENTS and etc….,
What Purpose does CAPTCHA Exactly?
The purpose of CAPTCHA is mainly to stop activitys form BOTS(automated softwares)…
Thus you have to prove that you are a human before you SING-UP or etc….,
WHY SHOULD THEY STOP BOTS(automated softwares)?
As it is said BOTS are AUTOMATED SOFTWARES which will send unlimited requests(for sing-up or comments or etc…,) to the destined server and crash the server,Thus the real users cannot not get the access to that SITE or FORUM.Thus CAPTCHAs can be deployed to protect systems that are vulnerable to email spam, such as the services from Gmail, Yahoo and Hotmail…
A CAPTCHA is a means of automatically generating new challenges which:
- Current software is unable to solve accurately.
- Most humans can solve
- Does not rely on the type of CAPTCHA being new to the attacker.CAPTCHAs are implemented by presenting users with an image which contains distorted or randomly stretched characters which only humans should be able to identify. Sometimes characters are striked out or presented with a noisy background to make it even more harder for computers to figure out the patterns.
Although a checkbox “check here if you are not a bot” might serve to distinguish between humans and computers, it is not a CAPTCHA because it relies on the fact that an attacker has not spent effort to break that specific form. (Such ‘check here’ methods are very easy to defeat.) Instead, CAPTCHAs rely on difficult problems in artificial intelligence. In the short term, this has the benefit of distinguishing humans from computers.
As it is said CAPTCHAs are not only visual test,in other words there are many methods like
Asking an mathematical equation like 1+1=?
OR
A user is presented with 4 images in which 3 contains picture of LIONS and one contain a CHEETAH. The user is asked to select only those images which contain animals in them. This Turing test can easily be solved by any human, but almost impossible for a computer.
OR
Asking some “common sense” questions (“what color is the sky on a clear day”) and given some options below which may scramble the options each time.
Breaking the CAPTCHA
However there is no universal algorithm that could pass through and break any CAPTCHA system and hence each CAPTCHA algorithm must have to be tackled individually. It might not work 100 percent of the time, but it can work often enough to be worthwhile to spammers.
There are a few approaches to defeating CAPTCHAs:
- exploiting bugs in the implementation that allow the attacker to completely bypass the CAPTCHA,
- improving character recognition software, or
- using cheap human labor to process the test.
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