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Captchas are tests that distinguish human from software botsin an online environment, used to stop bots abusing services intended for humans. Paypal and Yahoo have captchas in their new user registration process, but there are programs toovercome them. Investigate proposals for new kinds of captchas. Captchas are used to prevent bots from using various types of computing services. Applications include preventing bots from taking part in online polls, registering for free email accounts (which may then be used to send spam), and, more recently, preventing bot-generated spam by requiring that the (unrecognized) sender pass a captcha test before the email message is delivered.
Captchas are by definition fully automated, requiring little human maintenance or intervention in administering the test. This has obvious benefits in cost and reliability.
The algorithm used to create the captcha is often made public, though it may be covered by a patent. This is done to demonstrate that breaking it requires the solution of a hard problem in the field of artificial intelligence (AI) rather than just the discovery of the (secret) algorithm, which could be obtained through reverse engineering or other means.
They are programs or tests that help to differentiate human from software bots in the internet environment.. Widely used by Paypal and Yahoo in their new user registration process, captchas prevents the software bots from misusing the various services provided to the human.
They are programs or tests that help to differentiate human from software bots in the internet environment.. Widely used by Paypal and Yahoo in their new user registration process, captchas prevents the software bots from misusing the various services provided to the human.
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A CAPTCHA or Captcha is a type of challenge-response test used in computing to ensure that the response is not generated by a computer. It is a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart."
Characteristics
The challenges for the creation of captcha are:
* Current software is unable to solve accurately.
* Most humans can solve
* Does not rely on the type of CAPTCHA being new to the attacker.
CAPTCHAs rely on difficult problems in artificial intelligence
Applications
CAPTCHAs are used to prevent automated software from performing actions which degrade the quality of service of a given system. CAPTCHAs can be deployed to protect systems vulnerable to e-mail spam. in stopping automated posting to blogs, forums and wikis, whether as a result of commercial promotion, or harassment and vandalism, they are very much handy. a CAPTCHA can enforce automated usage policies as set by the administrator when certain usage metrics exceed a given threshold.
Accessibility:
sites implementing CAPTCHAs may provide an audio version of the CAPTCHA in addition to the visual method for accessibility reasons..
For more details, visit this link:
http://en.wikipediawiki/Captcha
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Captchas
INTRODUCTION
1.1 Overview:
You're trying to sign up for a free email service offered by Gmail or Yahoo. Before you can submit your application, you first have to pass a test. It's not a hard test -- in fact, that's the point. For you, the test should be simple and straightforward. But for a computer, the test should be almost impossible to solve.
This sort of test is a CAPTCHA. They're also known as a type of Human Interaction Proof (HIP). You've probably seen CAPTCHA tests on lots of Web sites. The most common form of CAPTCHA is an image of several distorted letters. It's your job to type the correct series of letters into a form. If your letters match the ones in the distorted image, you pass the test.
CAPTCHAs are short for Completely Automated Public Turing test to tell Computers and Humans Apart. The term "CAPTCHA" was coined in 2000 by Luis Von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University, and John Langford (then of IBM). They are challenge-response tests to ensure that the users are indeed human. The purpose of a CAPTCHA is to block form submissions from spam bots – automated scripts that harvest email addresses from publicly available web forms. A common kind of CAPTCHA used on most websites requires the users to enter the string of characters that appear in a distorted form on the screen.
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For more details about Captchas with full report,please go through:
http://studentbank.in/report-captcha-usi...ull-report
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[attachment=8883]
INTRODUCTION
1.1 Overview:
You're trying to sign up for a free email service offered by Gmail or Yahoo. Before you can submit your application, you first have to pass a test. It's not a hard test -- in fact, that's the point. For you, the test should be simple and straightforward. But for a computer, the test should be almost impossible to solve.
This sort of test is a CAPTCHA. They're also known as a type of Human Interaction Proof (HIP). You've probably seen CAPTCHA tests on lots of Web sites. The most common form of CAPTCHA is an image of several distorted letters. It's your job to type the correct series of letters into a form. If your letters match the ones in the distorted image, you pass the test.
CAPTCHAs are short for Completely Automated Public Turing test to tell Computers and Humans Apart. The term "CAPTCHA" was coined in 2000 by Luis Von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University, and John Langford (then of IBM). They are challenge-response tests to ensure that the users are indeed human. The purpose of a CAPTCHA is to block form submissions from spam bots – automated scripts that harvest email addresses from publicly available web forms. A common kind of CAPTCHA used on most websites requires the users to enter the string of characters that appear in a distorted form on the screen.
CAPTCHAs are used because of the fact that it is difficult for the computers to extract the text from such a distorted image, whereas it is relatively easy for a human to understand the text hidden behind the distortions. Therefore, the correct response to a CAPTCHA challenge is assumed to come from a human and the user is permitted into the website.
Why would anyone need to create a test that can tell humans and computers apart? It's because of people trying to game the system -- they want to exploit weaknesses in the computers running the site. While these individuals probably make up a minority of all the people on the Internet, their actions can affect millions of users and Web sites. For example, a free e-mail service might find itself bombarded by account requests from an automated program. That automated program could be part of a larger attempt to send out spam mail to millions of people. The CAPTCHA test helps identify which users are real human beings and which ones are computer programs.
Spammers are constantly trying to build algorithms that read the distorted text correctly. So strong CAPTCHAs have to be designed and built so that the efforts of the spammers are thwarted.
1.2 Motivation:
The proliferation of the publicly available services on the Web is a boon for the community at large. But unfortunately it has invited new and novel abuses. Programs (bots and spiders) are being created to steal services and to conduct fraudulent transactions. Some examples:
• Free online accounts are being registered automatically many times and are being used to distribute stolen or copyrighted material.
• Recommendation systems are vulnerable to artificial inflation or deflation of rankings. For example, EBay, a famous auction website allows users to rate a product. Abusers can easily create bots that could increase or decrease the rating of a specific product, possibly changing people’s perception towards the product.
• Spammers register themselves with free email accounts such as those provided by Gmail or Hotmail and use their bots to send unsolicited mails to other users of that email service.
• Online polls are attacked by bots and are susceptible to ballot stuffing. This gives unfair mileage to those that benefit from it.
In light of the above listed abuses and much more, a need was felt for a facility that checks users and allows access to services to only human users. It was in this direction that such a tool like CAPTCHA was created.
1.3 Background:
The need for CAPTCHAs rose to keep out the website/search engine abuse by bots. In 1997, AltaVista sought ways to block and discourage the automatic submissions of URLs into their search engines. Andrei Broder, Chief Scientist of AltaVista, and his colleagues developed a filter. Their method was to generate a printed text randomly that only humans could read and not machine readers. Their approach was so effective that in an year, “spam-add-ons’” were reduced by 95% and a patent was issued in 2001.
In 2000, Yahoo’s popular Messenger chat service was hit by bots which pointed advertising links to annoying human users of chat rooms. Yahoo, along with Carnegie Mellon University, developed a CAPTCHA called EZ-GIMPY, which chose a dictionary word randomly and distorted it with a wide variety of image occlusions and asked the user to input the distorted word.
In November 1999, slashdot.com released a poll to vote for the best CS college in the US. Students from the Carnegie Mellon University and the Massachusetts Institute of Technology created bots that repeatedly voted for their respective colleges. This incident created the urge to use CAPTCHAs for such online polls to ensure that only human users are able to take part in the polls.
1.4 CAPTCHAs and the Turing Test:
CAPTCHA technology has its foundation in an experiment called the Turing Test. Alan Turing, sometimes called the father of modern computing, proposed the test as a way to examine whether or not machines can think -- or appear to think -- like humans. The classic test is a game of imitation. In this game, an interrogator asks two participants a series of questions. One of the participants is a machine and the other is a human. The interrogator can't see or hear the participants and has no way of knowing which is which. If the interrogator is unable to figure out which participant is a machine based on the responses, the machine passes the Turing Test.
Of course, with a CAPTCHA, the goal is to create a test that humans can pass easily but machines can't. It's also important that the CAPTCHA application is able to present different CAPTCHAs to different users. If a visual CAPTCHA presented a static image that was the same for every user, it wouldn't take long before a spammer spotted the form, deciphered the letters, and programmed an application to type in the correct answer automatically.
Most, but not all, CAPTCHAs rely on a visual test. Computers lack the sophistication that human beings have when it comes to processing visual data. We can look at an image and pick out patterns more easily than a computer. The human mind sometimes perceives patterns even when none exist, a quirk we call pareidolia. Ever see a shape in the clouds or a face on the moon? That's your brain trying to associate random information into patterns and shapes.
But not all CAPTCHAs rely on visual patterns. In fact, it's important to have an alternative to a visual CAPTCHA. Otherwise, the Web site administrator runs the risk of disenfranchising any Web user who has a visual impairment. One alternative to a visual test is an audible one. An audio CAPTCHA usually presents the user with a series of spoken letters or numbers. It's not unusual for the program to distort the speaker's voice, and it's also common for the program to include background noise in the recording. This helps thwart voice recognition programs.
Another option is to create a CAPTCHA that asks the reader to interpret a short passage of text. A contextual CAPTCHA quizzes the reader and tests comprehension skills. While computer programs can pick out key words in text passages, they aren't very good at understanding what those words actually mean.
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presented by:
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[attachment=9313]
A Seminar Report On CAPTCHA full report
ABSTRACT
Short for completely automated public turing test to tell computers and humans apart, a technique used by a computer to tell if it is interacting with a human or another computer. Because computing is becoming pervasive, and computerized tasks and services are commonplace, the need for increased levels of security has led to the development of this way for computers to ensure that they are dealing with humans in situations where human interaction is essential to security. Activities such as online commerce transactions, search engine submissions, Web polls, Web registrations, free e-mail service registration and other automated services are subject to software programs, or bots, that mimic the behavior of humans in order to skew the results of the automated task or perform malicious activities, such as gathering e-mail addresses for spamming or ordering hundreds of tickets to a concert.
In order to validate the digital transaction, using the CAPTCHA system the user is presented with a distorted word typically placed on top of a distorted background. The user must type the word into a field in order to complete the process. Computers have a difficult time decoding the distorted words while humans can easily decipher the text. Some CAPTCHAs now use pictures instead of words where the user is presented with a series of pictures and asked what is the common element among all of the pictures. By entering that common element, the user validates the transaction and the computer knows it is dealing with a human and not a bot
INTRODUCTION
A CAPTCHA is a program that protects websites against bots by generating and grading tests that humans can pass but current computer programs cannot. For example, humans can read distorted text as the one shown below, but current computer programs can't:
A CAPTCHA or Captcha 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.
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. 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 their search engine. In order to make the images resistant to OCR (Optical Character Recognition), the team simulated situations that scanner manuals claimed resulted in bad OCR.
In 2000, Luis von Ahn and Manuel Blum coined the term 'CAPTCHA', improved and publicized the notion, which included any program that can distinguish humans from computers. They invented multiple examples of CAPTCHAs, including the first CAPTCHAs to be widely used, which were those adopted by Yahoo!.
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Presented by:
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[attachment=11311]
Introduction
CAPTCHAà Completely Automated Public Turing test to tell
Computers and Humans Apart
A CAPTCHA is a program that protects websites against bots by generating and grading tests that humans can pass but current computer programs cannot.
A CAPTCHA or Captcha is a type of challenge-response test used in computing to ensure that the response is not generated by a computer.
CAPTCHA requires that the user type letters or digits from a distorted image that appears on the screen.
Any user entering a correct solution is presumed to be human else user is bot and denied access.
It is sometimes described as a reverse Turing test.
OCRs(Optical Character Recognition) are not able to read CAPTCHAs.
Characteristics
A CAPTCHA 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.
CAPTCHAs rely on difficult problems in artificial intelligence
Origin
First developed by Alta Vista in 1997.
The term coined in 2000 by Luis von Ahn , Manuel Blum and Nicholas J. Hopper of Carnegie Mellon University and John Langford of IBM.
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 their search engine.
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
Types of CAPTCHAs
1. Text Based CAPTCHAs
2. Graphics Based CAPTCHAs
3. Audio or Sound Based CAPTCHAs
Text Based CAPTCHAs
Typically relay on sophisticated distortion of text images rendering them unrecognizable to the state of the art of the pattern recognition programs but recognizable by humans.
Examples:
Simple, normal language questions:
What is sum of three and thirty-five?
If today is Saturday, what is day after tomorrow?
Very effective, needs a large question bank
Cognitively challenged users find it hard .
Gimpy:
Originally designed by Yahoo and CMU.
Based on human ability to read heavily distorted and corrupted text.
works by choosing a certain number of words from a dictionary, and then displaying them corrupted and distorted in an image; after that Gimpy asks the user to type the words displayed in that image.
EZ-Gimpy:
A modified version of Gimpy.
Used in Yahoo Messenger Service.
It contains only one random character string.
The word is random and not picked from the dictionary.
Its not a good implementation of CAPTCHA, and already broken OCRs.
MSN Passport service CAPTCHAs:
its provided for Microsoft MSN services.
uses 8 characters.
Warping is used to distort.
Its very strongly implemented and hasn’t been broken.
Graphics Based CAPTCHAs
Requires user to perform image recognition test.
Examples:
IMAGINATION:
CAPTCHA that requires two steps to be passed.
first step visitor clicks elsewhere on the picture that composed of a few images and selects in this way a single image.
second step the selected image is loaded. It is enlarged but very distorted. Also variants of the answer are loaded on the client side. The visitor should select a correct answer from the set of the proposed words BONGO:
After M.M.Bongard, pattern recognition expert.
User has to solve a pattern recognition problem
ASSIRA:
Animal Species Image Recognition for Restricting Access.
It’s a HIP that works by asking users to identify photographs of cats and dogs.
Difficult for computers but humans can accomplish it very quickly and accurately.
Audio CAPTCHAs
Require user to solve a speech recognition test.
In this version of captcha letters are read aloud instead of being displayed in an image.
Helps visually disabled users
Below is the Google’s audio enabled CAPTCHA.
3D CAPTCHA
3DCaptcha is the "captcha nice to humans, bad to machines".
It is written in PHP.
A new approach to captchas, using human's spatial cognition abilities to differentiate humans from machines.
It uses a markov-chain to generate words that resemble human language and are easy to type, yet avoid dictionary lookups.
It filters profane language.
It's easy to deploy.
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[attachment=12159]
A CAPTCHA is a type of challenge-response test used in computing as an attempt 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 supposedly 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 the user to type letters or digits from a distorted image that appears on the screen.
The term "CAPTCHA" was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford (all of Carnegie Mellon University). It is a contrived acronym based on the word "capture" and standing 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.
A CAPTCHA is a means of automatically generating challenges which intends to:
Provide a problem easy enough for all humans to solve.
Prevent standard automated software from filling out a form, unless it is specially designed to circumvent specific CAPTCHA systems.
A check box in a form that reads "check this box please" is the simplest (and perhaps least effective) form of a CAPTCHA. CAPTCHAs do not have to rely on difficult problems in artificial intelligence, although they can.
This has the benefit of distinguishing humans from computers. It also creates incentive to further develop artificial intelligence of computers.
Vulnerabilities
HTTP does not distinguish between human & machine users.
HTTP & SSL do not guarantee client software or user is benign.
Malicious bots can be anonymous and distributed.
Benign bots spider for searches, etc.
Threats to Web
Content Theft-- stealing paid data
Copyright Infringement-- “scraping” content from one site to display on another, “out of context”
Unwanted spidering-- search engines may ignore robots.txt or “nofollow” tags
Poll Stuffing-- MIT vs. CMU on /. [1]
Web Spam-- unsolicited commenting, abusing free email, scraping addresses
Web Spam
• Web comments, discussions, guest books, Wikis, many public forms are open to spam messages.
• More eyeballs per message than e-mail
• E-mail spam is illegal, but most Web spam is legal.
• Bots collect email addresses on Web.
TYPES OF CAPTCHA
1.TEXT BASED:
• Images of distorted text.
• Frequently cracked and improved.
• In current version, 5 pairs of overlapped words. User identifies 3 words.
• Random placement, font, distortion, background pattern
• Overlapping words need no noise.
2.Visual puzzle
• Computer can generate & display, but not solve.
• If too many choices, humans get it wrong.
• If not enough choices, computers can be effective with random guess.
3. Photo Recognition
• Need large image DB
• Images need keywords
• Four images with same keyword shown
• Random subset of keywords as choices
• Poor implementations easy to crack (color of top left pixel unique, etc.)
4.SPEECH CAPTCHA
• Usually spells out one-time-password in synthesized or recorded voices
• Voice recognition cracks simple case.
• Applied audio filters risk human misunderstanding.
• Used with image CAPTCHA for increased accessibility.
• If both use same OTP, easier to crack.
5. 3D CAPTCHA
• Renders OTP in 3D space to image
• Reputedly the most difficult to crack
• Server needs good graphics card to be practical (rare)
• Can be combined with other methods
• Not yet common
• Might see more in future
VIDEO CAPTCHA
Unfortunately, many users find existing character-recognition based CAPTCHAs frustrating and attack success rates as high as 60% have been reported for Microsoft’s
Hotmail CAPTCHA . To address these problems, we present a first attempt at using content-based video labeling (‘tagging’) as a CAPTCHA task. We define correct responses using tags provided by the individual that posts a video to a public database (YouTube.com), along with tags on videos designated as being ‘related’ in the database.In an experiment involving 184 human participants, we were able to increase human pass rates on our video CAPTCHAs from roughly 70% to 90% while keeping the success of a frequency-based attack fixed at around 13%. Through a different parameterization of the challenge generation and tag matching algorithms, we were able to reduce the success rate of the same attack to 2%, while still increasing the human pass rate to 75% .The frequency-based attack we consider is simple but logical for this type of CAPTCHA: the computer submits the three tags with the highest estimated frequencies below the rejection threshold, on the assumption that the tag frequency estimates used in creating the CAPTCHAs are publicly available. A screenshot of our video-based CAPTCHA is shown in Figure 1. To pass the challenge, a user provides three words (‘tags’) describing the video. If one of the submitted tags belongs to the automatically generated ground truth tag set, the challenge is passed. This task is similar to the ESP game of von Ahn et al. in which online users are randomly paired and presented with an image that they then submit tags to describe. Players cannot see each other’s submitted tags until they agree on a common tag, at which point the round of the game ends. Our video CAPTCHA is similar to a game of ESP in which one player is online, while the other player’s responses (the ground truth tags) are computed automatically.
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PRESENTED BY:
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CAPTCHA
[attachment=13781]
INTRODUCTION
CAPTCHA Completely Automated Public Turing Test to tell Computers and Humans Apart
The term "CAPTCHA" was coined in the year 2000 by Luis Von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University(CMU)), and JohnLangford (then of IBM).
CAPTCHA is a program that is a challenge –response test to separate humans from computer programs
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
Humans can read the distorted and noisy text.
Background
Why CAPTCHA was needed?
To prevent the following
Sabotage of online polls
Worms and Spam e-mails
Search engine BOTs
Preventing dictionary attacks(password cracking)
Tampering with rankings on recommendation systems (e.g. Ebay,Amazon etc)
Altavista first used a crude CAPTCHA in their sites
That resulted in 95% spam reduction
Yahoo partnered CMU to counter the threats in Messenger chat service
Luis von Ahn and Manuel Blum of
CMU trademarked CAPTCHA in 2000
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 and other is a human the judge doesn’t know which is machine or which is human.
If judge can’t tell which is the machine,
The machine passes the test
CAPTCHA employs a Reverse Turing Test(RTT),
judge = CAPTCHA program, participant = user if user passes CAPTCHA, he is human if user fails, it is a machine
Types of CAPTCHAs
Text based
Gimpy, ez-gimpy
Gimpy-r, Google CAPTCHA
Simard’s HIP (MSN)
Graphic based
Bongo
Pix
Audio based
Text Based CAPTCHAs
Gimpy, ez-gimpy
Pick a word or words from a small dictionary
Distort them and add noise and background
Gimpy-r, Google’s CAPTCHA
Pick random letters not from dictionary.
Distort them, add noise and background
Simard’s HIP(MSN’s CAPTCHA)
Pick random letters and numbers
Distort them and add arcs
Some Text Based CAPTCHAs
Graphic based CAPTCHAs:
BONGO:
User has to solve a pattern recognition
problem
Has to tell the distinct characteristic
between two sets of figure
Then to which set the given figure belongs to
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
“Airplane”
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
APPLICATION
Protect online polls
Prevent Web registration abuse,protect passwords from brute-force attack
Prevent comment spam and spam e-mail
E-ticketing , prevent scalping
Help advance Artificial Intelligence knowledge
CAPTCHAs are called Hard-AI problems
A win-win scenario:
If CAPTCHAs are broken by a bot, a Hard-
AI problem is solved
If its not yet broken, then current implementation is able to withstand Attacks
Thus AI knowledge is advanced if CAPTCHAs are broken
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
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
Embeddable CAPTCHAs:
Available freely, just embed code into
Web page’s HTML, from e.g.,recaptcha.net
No maintenance
Custom CAPTCHAs:
Fits to the theme of the page
Better protected from spammers
Can be written in any language– Perl,
.NET, ASP, JavaScript
BREAKING CAPTCHAs
Exploiting bugs in the implementation that allows attacker to bypass CAPTCHAs.
Cracking CAPTCHAs through programs :
Convert CAPTCHA into greyscale
Detect patterns in the image
corresponding to Characters
Or, read session files of that user and
know the CAPTCHA word
Solution:
Only store a hash of the
CAPTCHA word in session files
Greg Mori and Jitendra Malik have broken text CAPTCHAs, e.g., Ez- Gimpy
To break this CAPTCHA )
Segmentation: Locate possible
letters in the image------->
Construct graph of consistent
letters ----------------------------->
Find out plausible words from
the graph, use scores to rank
roll=11.94, profit=9.42 (better match)
Social engineering to break CAPTCHAs:
Spammer encounters a CAPTCHA
That CAPTCHA is copied to another site
Humans are baited, e.g., free MP3s
To get those MP3s, users are told to solve the copied CAPTCHA
Solution is routed to the spammer
Solution: Fix a time-to-live period for a Question.
CAPTCHA cracking as a business:
Firms offer CAPTCHA cracking service in
exchange for money
ISSUEs with CAPTCHAs
Usability issues:
Some CAPTCHAs are inaccessible to
visually impaired, cognitively challenged
People
Compatibility issues:
JavaScript may need to be activated in
browsers
Some may need Adobe Flash plugin
SUMMARY
CAPTCHAs are an effective way to
counter bots and reduce spam
They serve dual purpose– help
advance AI knowledge
Applications are varied– from
stopping bots to character
recognition & pattern matching
Some issues with current
implementations represent
challenges for future improvement
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