CAPTCHA Prabhakar Verma “08MC30”

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.

A 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.

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.

Proposed by Alan Turing. 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 Text Based CAPTCHAs Graphics Based CAPTCHAs 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: ts 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.

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 CATCHA 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.

Re-CAPTCHA Free CAPTCHA service that helps to digitize books, newspapers and old time radio shows. reCAPTCHA improves the process of digitizing books by sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher. Each word that cannot be read correctly by OCR is placed on an image and used as a CAPTCHA. This is possible because most OCR programs alert you when a word cannot be read correctly.

Working of reCAPTCHA: Two words are shown, one word is known as Control Word, and another one is known a questionable word. System assumes that if human types the control word correctly, the questionable word is also correct. The identification performed by each OCR program is given a value of 0.5 points, and each interpretation by a human is given a full point. Once a given identification hits 2.5 votes, the word is considered called.

Applications Preventing Comment Spam in Blogs Protecting Website Registration Protecting Email Addresses From Scrapers Online Polls Preventing Dictionary Attacks Search Engine Bots Worms and Spam

Advancing Artificial Intelligence Called Hard-AI problems. CAPTCHA tests are based on open problems in artificial intelligence (AI). A win-win scenario: either a CAPTCHA is not broken and there is a way to differentiate humans from computers. Or the CAPTCHA is broken and an AI problem is solved. 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

GUIDELINES: Accessibility Image security Script security Security after widespread adoption Custom implementation or a general CAPTCHA?

Breaking 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

Issues with CAPTCHAs Usability issues: W3C mandates Web to be accessible to all people 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 installed

Real World

Conclusion 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 improvements

PRABHAKAR VERMA