this presentation contains basic information about captcha, its types, where it is used and how to break captcha

1. CAPTCHA
-By Bansari Rajani

2. 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

3. 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.

4. 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.

5. 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.

6. 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

7. So, CAPTCHA is...
A program that can generate and grade tests
that:
•Most human can pass;
•Current computer programs cannot pass

8. Making a CAPTCHA...
Pick a random
string of
characters
Renders it into a
distorted image
CAPTCHA

9. Making a CAPTCHA...
...and the program generates a test:
Type the characters that appear in the image

10. Type of CAPTCHA...
•Early CAPTCHAs
•Generated by EZ-gimpy program;
•Used previously in Yahoo!

11. Type of CAPTCHA...
•Improved CAPTCHAs:
• high contrast for human
readability;
• medium, per-character
perturbation;
• random fonts per character;

12. 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;

13. Other types of CAPTCHA...
•Animated CAPTCHAs
•3D CAPTCHAs
•ASCII Art

14. 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

15. 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

16. 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

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

18. 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.

19. 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;

21. 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;