Captcha Final
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Transcript of Captcha Final
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SAURAV GAUTAM 3608146 CSE(2) IIIrd Year
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Agenda
Definition Background Types Process of CAPTCHAs Applications Constructing CAPTCHAs Conclusion
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Intro
CAPTCHA Completely Automated Public Turing test to tell Computers and Humans Apart
Invented at CMU by Luis von Ahn, Manuel Blum in 2000.
A program that is a challenge – response test to separate humans from computer programs
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Generic CAPTCHAs distort letters and numbers .
Distorted characters are presented to user.
User has to recognize the distorted letters.
If the guessed letters are correct, the user is inferred to be a human and allowed access .
Else, user is a bot and denied access.
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Humans can read the distorted and noisy text
Current OCRs cannot read them.
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Background
Why CAPTCHA was needed?
Sabotage of online polls
Spam emails
Abusing free online accounts
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Altavista first used a CAPTCHA in their sites.
Resulted in 95% spam reduction.
Yahoo partnered CMU to counter these threats in Messenger chat service.
Luis von Ahn and Manuel Blum of CMU trademarked CAPTCHA in 2000.
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What is a Turing test? Proposed by Alan Turing To test a machine’s level of intelligence Human judge asks questions to two
participants, one is a machine, he doesn’t know which is which
If judge can’t tell which is the machine, the machine passes the test
CAPTCHA employs a reverse Turing test,judge = CAPTCHA program,
participant = user if user passes CAPTCHA, he is human
if user fails, it is a machine
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Types of Captcha
Text based Captcha Gimpy Captcha E-Z Gimpy Captcha Pix Captcha Audio Captcha
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Text based:
Simple, normal language questions: What is sum of three and thirty-five? If today is Saturday, what is day after
tomorrow? Which of mango, table, water is a fruit?
Very effective, needs a large question bank
Cognitively challenged users find it hard
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Gimpy: Designed by Yahoo and CMU Picks up 10 random words from dictionary
and distorts, fills with noise User has to recognize at least 3 words If user is correct, he is admitted
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EZ-Gimpy: A modified version of Gimpy Yahoo used this version in Messenger Has only 1 random string of characters Not a dictionary word, so not prone to
dictionary attack Not a good implementation, already
broken by OCRs
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PIX: Uses a large database of labelled images It shows a set of images, user has to
recognize the common feature among those
E.g., Pick the common characteristic among the following four pictures-----”Aeroplane”
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Example of Pix
Dog Pool
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Audio CAPTCHAs: Consist of downloadable audio clip User listens and enters the spoken word Helps visually disabled users Below is the Google’s audio enabled
CAPTCHA Not popular
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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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Align CAPTCHA
Remove gridlines.
Undo angle
of rotation.
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Align CAPTCHA
Crop CAPTCHA.
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Cut CAPTCHA
Cut CAPTCHA cut into 5 pieces.
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Transform CAPTCHA
Perform the HWT on each of the 5 pieces.
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Decode CAPTCHA
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Constructing CAPTCHAs
Things to keep in mind: Don’t store CAPTCHA solution in Web
page’s metadata
A CAPTCHA is no good if it doesn't distort
Need a large database of different CAPTCHA questions
Avoid repetition of questions
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CAPTCHA Logic:
Generate the question
Persist the correct answer
Present the question to user
Evaluate answer, if incorrect, start again-- Generate a different CAPTCHA
If correct, allow access to user
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Applications
Preventing comment spam in blogs.
Prevent Web registration abuse, protect passwords from attack.
Online polls.
Preventing dictionary attack.
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Conclusion
CAPTCHAs prove a good security measure if they are strong enough and more accessible.
There is still room for improvement in the non-visual type of CAPTCHAs.
A CAPTCHA implies a win-win situation: either the CAPTCHA is not broken and there is a way to differentiate humans from computers, or the CAPTCHA is broken and a useful AI problem is solved.
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