copy of captcha

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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