Services > Captcha Processing
What is a Captcha?
A CAPTCHA or Captcha (pronounced /'kæpt cha/) is a type of challenge-response test used in computing 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 unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human.
Thus, it 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. A common type of CAPTCHA requires that the user type letters or digits from a
distorted image that appears on the screen.
The term "CAPTCHA" (based upon the word capture) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J.
Hopper (all of Carnegie Mellon University), and John Langford (then of IBM). It is a contrived acronym for
"Completely Automated Public Turing test to tell Computers and Humans Apart." Carnegie Mellon University attempted
to trademark the term, but the trademark application was abandoned on 21 April 2008. Currently, CAPTCHA creators
recommend use of reCAPTCHA as the official implementation.
What does a Captcha do?
A CAPTCHA system 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.
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.)
Withholding of the algorithm can increase the integrity of a limited set of systems, as in the practice of
security through obscurity. The most important factor in deciding whether an algorithm should be made open or
restricted is the size of the system. Although an algorithm which survives scrutiny by security experts may be
assumed to be more conceptually secure than an unevaluated algorithm, an unevaluated algorithm specific to a very
limited set of systems is always of less interest to those engaging in automated abuse. Breaking a CAPTCHA
generally requires some effort specific to that particular CAPTCHA implementation, and an abuser may decide that
the benefit granted by automated bypass is negated by the effort required to engage in abuse of that system in the
first place.
How can a Captcha help me?
Preventing Comment Spam
Most people are familiar with programs that submit bogus comments,
usually for the purpose of raising search engine ranks of some website (e.g., "buy penny stocks here").
This is called comment spam. By using a CAPTCHA, only humans can enter comments in a form. 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 programs.
Search Engine Bots
It is sometimes desirable to keep webpages unindexed to prevent others from finding
them easily. There is an html tag to prevent search engine bots from reading web pages. The tag, however, doesn't
guarantee that bots won't read a web page; it only serves to say "no bots, please." Search engine bots, since they
usually belong to large companies, respect web pages that don't want to allow them in. However, in order to truly
guarantee that bots won't enter a web site, CAPTCHAs are needed.
Email Spam
CAPTCHAs also offer a plausible solution against email spam: "I will only accept an email
if I know there is a human behind the other computer." A few companies are already marketing this idea.
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