Applications of CAPTCHA

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.
CAPTCHAÂ 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

A CAPTCHA is a means of automatically generating new challenges which:

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.

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 .

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.

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.

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.

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 animage and used as a CAPTCHA.
This is possible because most OCR programs alert you when a wordcannot 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

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.

Things to keep in mind:

Usability issues

W3C mandates
Web to be accessible to all people
Some CAPTCHAs are inaccessible to visually impaired, cognitively challenged people

Compatibility issues

Conclusion

CAPTCHAs are an effective way to counter bots and reduce spam
They serve dual purposeÂ²help advance AI knowledge
Applications are variedÂ²from stopping bots to character recognition & pattern matching
Some issues with current implementations represent challenges for future improvements

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