Applications of CAPTCHA

  • CAPTCHA is a powerful tool
  • Qingkun Ramteen Talib
  • A CAPTCHA or Captcha is a type of challenge-response test used in to computing o 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
  • Characteristeics

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 Vistain 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.
  • Turning 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
  • 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
  • Graphic Based CAPTCHAs
  • Requires user to perform image recognition test.
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  • 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.
  • 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.
  • 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.
  • 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 animage and used as a CAPTCHA.
  • This is possible because most OCR programs alert you when a wordcannot be read correctly.
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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.


  • Preventing Comment Spam in Blogs
  • Protecting Website Registration
  • Protecting Email Address 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.
  • V/S
  • Constructing CAPTCHAs

Things to keep in mind:

  • Don’t store CAPTCHA solution in Web page’s meta data
  • 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
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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


  • CAPTCHAs are an effective way to counter bots and reduce spam
  • They serve dual purpose²help advance AI knowledge
  • Applications are varied²from stopping bots to character recognition & pattern matching
  • Some issues with current implementations represent challenges for future improvements
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