This document provides an overview of CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart). It defines CAPTCHAs as challenges used to determine if a user is human or computer. It describes how CAPTCHAs work by presenting distorted text or images that are easy for humans but difficult for computers to decipher. The document outlines different types of CAPTCHAs, how they can be broken, their applications in preventing automated activities, and concludes that CAPTCHAs advance both security and artificial intelligence.

1. www.studymafia.org
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www.studymafia.org www.studymafia.org
Seminar
On
CAPTCHA

2. CONTENT
 Introduction
 Definition
 Bascilly what a ‘Captcha do ?
 How does Captcha work?
 Types of Captchas
 Breaking Captchas
 Characteristics
 Applications
 Conclusion

3. INTRODUCTION
 Computers and Humans are apart , a technique used
by a computer to tell if it is interacting with human or
some other computer.
 It is needed because activities such as online
commerce transactions, search engine submissions,
Web polls, Web registrations, free e-mail service
registration and other automated services are
subject to software programs, or bots, that mimic
the behavior of humans in order to skew the results
of the automated task or perform malicious
activities, such as gathering e-mail addresses for
spamming.

4. DEFINITION
 A CAPTCHA (an acronym for "completely automated public
turing test to tell computers and humans apart") is a type of
challenge-response test used in computing to determine
whether or not the user is human.
A CAPTCHA is a program that can generate and grade tests
that:
 Most humans can pass, but
 Current computer programs can't pass

5. BASCILLY WHAT A ‘CAPTCHA DO ?
 In order to validate the digital transaction, using the
CAPTCHA system the user is presented with a distorted
word typically placed on top of a distorted background.
The user must type the word into a field in order to
complete the process.
 Computers have a difficult time decoding the distorted
words while humans can easily decipher the text.
 Some CAPTCHAs now use pictures instead of words
where the user is presented with a series of pictures and
asked what the common element among all of the
pictures is.
 By entering that common element, the user validates
the transaction and the computer knows it is dealing
with a human and not a bot.

6. HOW DOES A CAPTCHA WORKS?
 A CAPTCHA image shows a random string which the user has to
type to submit a form.
 This is a simple problem for humans, but a very hard problem
for computers which have to use character recognition,
especially, because the displayed string is alienated in a way,
which makes it very hard for a computer to decode.

7. TYPES OF CAPTCHA
 Text based
• Gimpy, ez-gimpy
• Gimpy-r, Google CAPTCHA
• Simard’s HIP (MSN)
 Graphic based
• Bongo
• Pix
 Audio based

8. Text based CAPTCHAs
 Gimpy, ez-gimpy
• Pick a word or words from a small dictionary
• Distort them and add noise and background
 Gimpy-r, Google’s CAPTCHA
• Pick random letters
• Distort them, add noise and background
 Simard’s HIP
• Pick random letters and numbers
• Distort them and add arcs

9. Graphic based CAPTCHAs
 Bongo
• Display two series of blocks
• User must find the
characteristic that sets the
two series apart
• User is asked to determine
which series each of four
single blocks belongs to
• Difference? thick vs. thin
lines

10. Graphic based CAPTCHAs
 PIX
• Create a large database of
labeled images
• Pick a concrete object
• Pick four images of the object
from the images database
• Distort the images
• Ask the user to pick the object
for a list of words

11. Audio based CAPTCHAs
 Pick a word or a sequence of numbers at random
 Render them into an audio clip using a TTS
software
 Distort the audio clip
 Ask the user to identify and type the word or
numbers

12. BREAKING CAPTCHAS
Most text based CAPTCHAs have been broken
by software
• OCR
• Segmentation
Other CAPTCHAs were broken by streaming the
tests for unsuspecting users to solve.

13. CHARACTERISTICS
 They are completely automated. This avoids the
necessity for human maintenance or intervention in
the test, with obvious benefits in cost and reliability.
 The algorithm used is made public, though it may be
encumbered by a patent. This is stipulated so as to
require that breaking a captcha requires the solution
of a hard problem in the field of artificial intelligence
(AI) rather than just the discovery of the (secret)
algorithm, which could be obtained through other
techniques.

14. APPLICATIONS
Online polls
Search engine bots
Worms and spams
Search Engine Bots
Prevent dictionary attacks

15. CONCLUSION
 CAPTCHAS are any software that distinguishes human and
machine.
 Research in CAPTCHAS implies advancement in AI making
computers understand how human thinks.
 Internet companies are making billions of dollars every year,
their security and services quality matters and so does the
advancement in CAPTCHA technology.

16. REFERENCE
www.google.com
www.wikipedia.com
www.studymafia.org

17. THANK YOU!

18. QUERIES
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