Completely Automated Public Turing test to tell Computers and Humans Apart. Captcha word comes from capture. First developed by Alta Vista in 1997. The term coined in 2000 at CMU(Carnegie mellon university) by Luis von Ahn, Manuel Blum, Nicholas J. Hopper & John Langford.

1. CAPTCHA & its types

2. CAPTCHA
Completely Automated Public Turing test
to tell Computers and Humans Apart.
Captcha word comes from capture.
First developed by Alta Vista in 1997.
The term coined in 2000 at CMU(Carnegie
mellon university) by Luis von Ahn,
Manuel Blum, Nicholas J. Hopper & John
Langford.

3. What is CAPTCHA?
CAPTCHA is a program that protect
website against bots by generating &
grading test that
 Human can pass
but
 Computer program can’t
It is a computer program that can tell
whether its user is a human or a computer.

4. Continue.....
Captcha requires that the user types letters
or digits from distorted image appears on
the screen.
If correct solution is entered than system
assumes that response is generated
 by user.
else
 by bots & access denied.

5. Reverse Turing Test
CAPTCHA is also known as Reverse
Turing Test.
It is administrated by a Machine & targeted
to human whereas a Turing test is
administrated by human & targeted to a
Machine.
Turing test is used to test a machine’s level
of intelligence.

6. Types of CAPTCHA
Text based CAPTCHA
 Gimpy
 Gimpy-r, Google CAPTCHA
 Simard’s
Graphics based CAPTCHA
 Bongo
 Pix
Audio based CAPTCHA

7. Text based CAPTCHA
It is simple CAPTCHA type & carries
normal questions.
o What is the sum of three & thirty-five ?
o If today is Saturday, what is day after
tomorrow ?
Very effective, needs a large question
bank.
Cognitively challenged users find it hard.

8. Gimpy
Initially used by Yahoo. Yahoo used this
version in Messenger.
In this CAPTCHA two steps are followed as:
a)Pick random characters.
b)Distort them and add noise at background
Has only 1 random string of characters.

9. GIMPY-R
Modified version of Gimpy.
Picks up 10 random words from dictionary
& distorts, fills with noise.
User has to recognize at least 3 words.
If the user is correct, then he is admitted.

10. Simard’s
Here further advances made and arcs being
made into it.
Hence steps followed are as:
a)Pick random letters and numbers
b)Distort them and add arcs

11. Graphic based CAPTCHA: Bongo
 After M.M.Bongard, pattern recognition expert.
 User has to solve a pattern recognition problem.
 Has to tell the distinct characteristic between two
sets of figures.

12. Graphic based captcha:
Pix
Uses a large database of labelled images.
It shows a set of images, user has to
recognize the common feature among
those.

13. Audio based captcha
These are based on humans ability to depict
sounds that may be distorted, following
algorithm is followed in using it:
a) Pick a word or a sequence of numbers at
random
b) Render them into an audio clip using a TTS
software
c) Distort the audio clip
d) Ask the user to identify and type the word or
numbers

14. 3-D captcha
The 3-D CAPTCHA and modern
video games use the same basic
mechanism to generate unique
images.
Designing a computer vision program
that can recognize the objects within
the 3-D CAPTCHA images is
intrinsically difficult.

15. EXAMPLE OF 3-D CAPTCHA
We now have the code:
CKT

16. Applications of captcha
Prevent comment spam in blogs & spam e-
mails.
Prevent web registration abuse, protect
passwords from brute-force attack.
Protect Online polls.
Prevent emails from scrapers.
Preventing dictionary attacks.