21-12-2012, 03:58 PM
CAPTCHA A STUDY AND IMPLEMENTATION
CAPTCHA.docx (Size: 129.08 KB / Downloads: 45)
Introduction
A CAPTCHA or Captcha is a type of challenge-response test used in computing as an attempt to ensure that the response is not generated by a computer. The process usually involves one computer (a server) asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are supposedly unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. A common type of CAPTCHA requires the user to type letters or digits from a distorted image that appears on the screen.
Standard Interpretation:
Player C an interrogator is tasked with trying to determine who among A and B is a human or a computer. A captcha is sometimes described as a reverse Turing test, because it is administered by a machine and targeted to a human, in contrast to the standard Turing test that is typically administered by a human and targeted to a machine.
Characteristics
A CAPTCHA is a means of automatically generating challenges which intends to:
• Provide a problem easy enough for all humans to solve.
• Prevent standard automated software from filling out a form.
This has the benefit of distinguishing humans from computers. It also creates incentive to further develop artificial intelligence of computers.
Types of CAPTCHAS
Text: By far the most common type of captcha involves the use of letters that are arranged randomly and are distorted in some way with various background colours or fonts.
Audio: Audio CAPTCHA’s for the visually impaired are probably the second most common type of CAPTCHA.
Miscellaneous: There are also other CAPTCHA’s that require you to solve a problem that should be easy for a person but very hard for a computer to solve such as choosing which item in a list is not a bird for example but the problem with this is that one needs to have a large number of questions before it really becomes effective.
Applications of CAPTCHAs
CAPTCHAs have several applications for practical security, including (but not limited to):
• Preventing Comment Spam in Blogs. Most bloggers are familiar with programs that submit bogus comments, usually for the purpose of raising search engine ranks of some website. This is called comment spam. By using a CAPTCHA, only humans can enter comments on a blog. There is no need to make users sign up before they enter a comment, and no legitimate comments are ever lost.
• Protecting Website Registration. Several companies (Yahoo!, Microsoft, etc.) offer free email services. Up until a few years ago, most of these services suffered from a specific type of attack: "bots" that would sign up for thousands of email accounts every minute. The solution to this problem was to use CAPTCHAs to ensure that only humans obtain free accounts. In general, free services should be protected with a CAPTCHA in order to prevent abuse by automated scripts.