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Analyzing CAPTCHAs

SUBMITTED BYVIPIN GARG

BTECH 7TH SEMCOMPUTER SCIENCE


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Overview

CAPTCHA stands for Completely AutomatedPublic Turing tests to tell Computers andHumans Apart.

A CAPTCHA is considered broken if a

computer algorithm can quickly solve thepuzzle at least four out of five times on average.


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AgendaWhat is CAPTCHA?

Types of CAPTCHA

Where to use CAPTCHAs?

Guidelines when making a CAPTCHA

Ways to break CAPTCHAs

Recaptcha

Human Computation Games


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WHAT IS CAPTCHA?Aprogram that can tell whether its user is a

human or a computer.

It uses a type ofchallenge-response test to determinethat the response is not generated by a computer.


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TURING TEST

Standard Interpretation"

player C, the interrogator,is tasked with trying todetermine which player

- A or B - is a computer andwhich is a human.


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REVERSE TURING TESTA CAPTCHA is sometimesdescribed as a reverseTuring test, because it is

administered by a machineand targeted to a human.


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TYPE:EARLY CAPTCHA Generated by the EZ-Gimpy program;

Used previously on Yahoo!


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IMPROVED CAPTCHA high contrast for human readability;

medium, per-character perturbation;

random fonts per character; low background noise;


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MODERN CAPTCHA rather than attempting to create a distorted

background and high levels of warping on the

text; focus on making segmentation difficult by adding

an angled line;


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MODERN CAPTCHA another way to make segmentation difficult is to

crowd symbols together;

this can be read by humans but cannot besegmented by bots;


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OTHER TYPE OF CAPTCHA

Animated CAPTCHAs

3D CAPTCHA

ASCII art


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WHERE TO USE CAPTCHAs? Data Collection

Worms and Spam

Preventing Comment Spam in Blogs. Prevent Brute Force Attacks


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WHERE TO USE CAPTCHAs? Preventing Comment Spam in Blogs.

Protecting Email Addresses From Scrapers.Mechanism to hide your email address, requireusers to solve a CAPTCHA before showing youremail address

Online Polls.You cannot trust the results of an

online roll because anybody could just write aprogram to vote for their favorite option thousandsof times.


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WHERE TO USE CAPTCHAs? Protecting Website Registration. (E-mail services:

Yahoo, Microsoft, Google)

Preventing Dictionary Attacks(in passwordsystems). Prevent a computer to iterate throughthe entire space of passwords by requiring it tosolve a CAPTCHA after a certain number ofunsuccessful logins.

Search Engine Bots. It is sometimes desirable tokeep webpages unindexed to prevent others fromfinding them easily.


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Variety of CAPTCHAs First CAPTCHA broken:

EZ-Gimpy

EZ-Gimpy CAPTCHA broken by Mori and Malikusing object recognition techniques and dictionarycrosschecking. Their program correctly interpretsthis CAPTCHA 93% of the time.


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Variety of CAPTCHAs

CAPTCHA used by General Electric

CAPTCHA used by Chicago Cubs


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Making a CAPTCHA

Making a

CAPTCHA

Pick random

string ofcharacters

(or words)

ifhkfp


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The Process

Generate CAPTCHA

Align CAPTCHA

Cut CAPTCHA

Transform CAPTCHA

Decode CAPTCHA


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Generate CAPTCHA

CAPTCHA generated with our Mathematica

code.


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Crop CAPTCHA

Crop CAPTCHA.


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Cut CAPTCHA

Cut CAPTCHA cut into 5 pieces.


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Decode CAPTCHA


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Other: Cognitive Puzzles

Other: Cognitive Puzzles


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Other: Mathematical CAPTCHA


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Future of CAPTCHA decoding

Gimpy-r CAPTCHA used by Yahoo! mail


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Future of CAPTCHA decoding


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Future of CAPTCHA decoding On Thursday, April 23, 2009, USA TODAY ran a cover

story, entitled Cracking the Code, about CAPTCHAdecoding methods currently being used.

As Captcha designers have made their work

increasingly distorted and camouflaged, captcha-breaking groups have turned to human captcha-solvers , employing humans and paying them centper decoded captcha.


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ReCAPTCHA Digitizing Books One Word at a Time

Goal of ReCAPTCHA project is to archive human

knowledge and to make information more accessibleto the world.

Uses Object Character Recognition to transform thephotographically scanned books into text.

Users are given two words to decipher one to whichthe answer is known and another that cannot be readcorrectly by OCR.


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Audio CAPTCHAUsers with poor vision cannot solve these problems, and

alternative called the Audio CAPTCHA has beendeveloped

In this kind of test, the user must listen to a garbledvoice and identify the numbers or words that are beingspoken

Audio CAPTCHAs are more difficult than visual

CAPTCHAs, especially for blind users who use screen-reader programs. These programs tend to talk over theaudio CAPTCHA as the user navigates between theaudio playback controls and answer box, frustrating

the user's ability to hear the CAPTCHA.


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New Technology in Audio CAPTCHA The following controls are available to the user:

Typing a period in this box will cause the captcha toplay, and pressing it again will pause playback

Typing a comma will rewind the captcha by 1 secondand then continue playing.

Typing a forward slash will fast forward the captchaby 1 second and then continue playing.

With this approach, there is no need for separateplayback controls.


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THANK YOU!!!

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