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8/7/2019 Copy of CAPTCHA
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CONTENTS
Introduction
Process
History
Guidelines
Applications
How to defeat captchas
More accessible captchas
Image recognition captchas
summary
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FULL FORM
It is a contrived acronym for completely
Automated Public Turing test to tell
Computers and Humans Apart.
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INTRODUCTION
A PROGRAM THAT CAN TELL WHETHER ITS
USER IS A HUMAN OR A COMPUTER
A CAPTCHA or Captcha is a type ofchallenge-response test used in computing to ensurethat the response is not generated by a
computer.
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EXAMPLE
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A PROGRAM THAT CAN GENERATE AND GRADETESTS THAT:
A. MOST HUMANS CAN PASS
B. CURRENT COMPUTER PROGRAMS CANNOT
PASS
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REVERSETURINGTEST
It is sometimes desribed os a Reverse Turing
Test because it is a administered by a
machine and targeted to a human, incontrast to the standard Turing test that is
typically administered by a human and
targeted to a machine.
A common type of CAPTCHA requires thatthe user type letters or digit from a distoredimage that appears on the screen.
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PROCESS
The process usually involves one computer(a
server) asking a user to complete a simle test
which the computer is able to generate andgrade.
Because other computers are unable to solve
the CAPTCHA any user entering a correct
solution is presumed to be human.
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Picks a random renders it to a
string of letters: distorted image:-
o m a g
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......and generates a test
TYPE THE CHARACTERS
THAT APPEAR IN
THE IMAGE
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HISTORY
Moni Naor was the first person to theorize a
list of ways to verify that a request comes
from a human and not a bot.[4] Primitive CAPTCHAs seem to have been
developed in 1997 by Andrei Broder, Martin
Abadi, Krishna Bharat, and Mark Lillibridge to
prevent bots from adding URLs to theirsearch engine.
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Early CAPTCHAs such as these,generated by
the EZ-Gimpy program, were used on Yahoo
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However technology was developed to read
this type of CAPTCHA
A modern CAPTCHA, rather than attempting
to create a distored background and highjlevels of warping on the text, might focus onmaking segmentation difficult by adding an
angled line
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Another way to make segmentation difficult
is to crowd symbols together. This is yahoos
current CAPTCHA format. This might bedifficult for some people to read, as seen in
the leftmost example
Is it klopshorkbpsh?.
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GUIDELINES
Accessibility
Image security
Script security Security even after wide-spread adoption.
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APPLICATIONS
Preventing comment spam in blogs.
Protecting website Registration.
Online polls Preventing dictionary attacks.
Search engine bots.
Worms and spam.
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HOWTODEFEATCAPTCHAS
There are few approaches to defeatingCAPTCHAs.
1.Exploiting bugs in the implementation that allowthe attacker to completely bypass the CAPTCHA,
2.Improving character recognition software.
1.pre-processing
2.segmentation
3.classification
3.Using cheap human labor to process the tests.
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ATTEMPTSATMOREACCESSIBLE
CAPTCHAs
Even an audio visual CAPTCHA will require
manual intervantion for some users, such as
those who have visual disabilities and also aredeaf.there have been various attempts at
creating CAPTCHAs that are more accessible.
Attempts include the use of javascript,
mathematical questions(what is 1+1)orcommon sense question(what color isthe sky on a clear day).
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IMAGERECOGNITIONCAPTCHAs
phpBB forum software have implemented in
an open source image recognition captcha
system which in its deafult form presents aquestion requiring the user to select a stated
type of animal from an array of thumbnail
images of assorted animals.
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PIX
WHAT ARE THESE PICTURES OF?
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SUMMARY
Completely automated public turing test to
tell computers and humans apart
is a captcha
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THANKYOU
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