CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a program designed to differentiate between human and computer users by requiring solutions to tasks that only humans can perform. It was first developed in 1997, with various types including text-based, graphic-based, and audio-based formats, each having unique challenges for users. Applications of CAPTCHA include preventing spam, protecting online accounts, and safeguarding web registration processes.

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.