Introduction about Captcha and its types

CAPTCHA

-By Bansari Rajani

Contents...•What is CAPTCHA?•Making a CAPTCHA•Type of CAPTCHA•Where to use CAPTCHA?•Guidelines when making a CAPTCHA•Ways to break CAPTCHA•reCAPTCHA•Time spent

CAPTCHA means...

Completely Automated Public Turing test to tell Computers and Humans Apart

Created in 2000 for Yahoo to prevent automated e-mail account registration.

Cont...A program that can tell whether its user is a human or a computer.

It uses a type of challenge-response test to determine that the response is not generated by a computer.

Turing test:“Standard Interpretation”

Player C, the interrogator is asked with trying to determine which player A or B is a computer and which is a human.

Reverse Turing Test:A CAPTCHA is sometimes described as a Reverse Turing Test, because it is...

Administrate by a machine and targeted to a human

So, CAPTCHA is...

A program that can generate and grade tests that:

•Most human can pass;•Current computer programs cannot pass

Making a CAPTCHA...

Pick a random string of characters

Renders it into a distorted image

CAPTCHA

Making a CAPTCHA...

...and the program generates a test:

Type the characters that appear in the image

Type of CAPTCHA...

•Early CAPTCHAs

•Generated by EZ-gimpy program;•Used previously in Yahoo!

Type of CAPTCHA...•Improved CAPTCHAs:

• high contrast for human readability;

• medium, per-character perturbation;

• random fonts per character;

Type of CAPTCHA...•A Morden CAPTCHAs:

•rather than attempting to create a distorted background and high levels of warping on the text;•focus on making segmentation difficult by adding an angled line;

Other types of CAPTCHA...

•Animated CAPTCHAs

•3D CAPTCHAs

•ASCII Art

Where to use CAPTCHAs???

•Data connection•Worms and spam•Protecting Email address from scrappers•Online polls•Protecting website registration•Preventing dictionary attacks•Search engine bots

Guidelines when making a CAPTCHA...

•Image security : Image of text distorted randomly•Script level security : Systems where a solution to the same CAPTCHA can be

used multiple times

Guidelines when making a CAPTCHA...Accessibility : It prevent visually impaired users from accessing the protected resource

•They use screen reader, so when you reached an image, all it can do is to read the caption of that image•Solution: permitting users to opt for an audio or sound CAPTCHA

Ways to break CAPTCHAs...•Using cheap human labor to process the tests•Improving Character Recognition software (OCR – Optical Character Recognition )

Break: character recognition...•Programs that have the following functions:• Extraction of the image from the web page • Removal of background clutter, for example with color filters and detection of thin lines • Segmentation, i.e. splitting the image into regions each containing a single letter;• Identifying the letter for each region.

reCAPTCHA...•New form of CAPTCHA that also helps digitize books;•The words displayed to the user come directly from old books that are being digitized; •Words that OCR could not identify;

Time spent...•Roughly 60 million CAPTCHAs are solved each day;

•Medium 10 seconds to solve a captcha;

•People around the world waste more than 150,000 hours on solving CAPTCHAs;