A CAPTCHA (a backronym 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. CAPTCHAs are, by definition, fully automated, requiring little human maintenance or intervention to administer, producing benefits in cost and reliability. By definition, the algorithm used to create the CAPTCHA must be made public, though it may be covered by a patent. This is done to demonstrate that breaking it requires the solution to a difficult problem in the field of artificial intelligence (AI) rather than just the discovery of the (secret) algorithm, which could be obtained through reverse engineering or other means.

1. CAPTCHA
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2. CAPTCHA
Completely Automated Public Turing
test to tell
Computers and Humans Apart
It 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
Invented at CMU by Luis von Ahn, Manuel Blum, et.al.

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. WHAT DOES 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.

5. 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.

6. WHAT IS TURING TEST ?
• Proposed by Alan Turing.
• To test a machines level of intelligence.
• Human judge asks questions to two participants, one is
a machine & the other human.
• The judge doesn’t know which is which.
• After listening to the answer, if the judge fails to
recognize which one is the machine, then the machine
passes the test.

7. • CAPTCHA employs a Reverse Turing
Test
• Judge = CAPTCHA program,
participant = user
• If the user passes CAPTCHA, he is
human otherwise it is a machine.

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

9. FEATURES OF CAPTCHA
• Easy for your users to read, but not for automated programs
• Protects from spam bots remotely submitting spam to your sites.
• Protects against dictionary and brute force hacking, To protect
login forms and secure areas.
• Each unique CAPTCHA image is created on the fly with
configurable colours, noise and distortion levels
• Characters that cause confusion are not used, including (0 and O,
1, l and I, 9 and g, 4 and A, etc.)

10. COMMON USES OF CAPTCHA
• Protecting E-ticketing
• Web registration protection
• Protecting online polls
• Preventing spamming and worms.
• Preventing from Search Engine Bots

11. ISSUES WITH CAPTCHA
• However, there are methods out there used to bypass software’s
such as CAPTCHA to be aware of. It is difficult to protect online
content completely; CAPTCHA might stop less professional
attacks but is not that effective when bots or scrapers uses more
advanced methods.
• Most text based CAPTCHAs have been broken by software
1. OCR
2. Segmentation

12. CONCLUSION
 CAPTCHAS are any software that distinguishes human
and machine.
 Research in CAPTCHAS implies advancement in AI
making computers understand how human thinks.
• CAPTCHAs are an effective way to counter bots &
reduce spam.

13. THANK YOU!