2. CAPTCHA
 A CAPTCHA is a type of challenge – response
 Test that response is generated by a human being or computer
CAPTCHA – Completely Automated Public Turing test to tell Computers &
Humans Apart.

3. Why captcha was needed?
 Online Polls
 Protecting Web Registration
 E–Ticketing
 Email Spam

4. Turing Test :-
 To test a machine’s level of intelligence
 CAPTCHA employs a reverse Turing test
judge = CAPTCHA program
participant = user
if user passes CAPTCHA, he is human
if user fails , it is a machine

5. Types of CAPTCHAs :-
Texts CAPTCHAs
Gimpy
Ez – Gimpy
MSNs Passport Service CAPTCHAs
Audio CAPTCHAs

6. Texts CAPTCHAs :-
Simple, normal questions like:
 What is twenty minus three?
 What is the third letter in UNIVERSITY?
 If yesterday was a Sunday, what is today?

7. Gimpy :-
 Designed by Yahoo & CMU(Carnegie Mellon University)
 Picks up 10 random words from dictionary, and displaying
them in a distorted and overlapped manner.
 User has to recognize at least 3 words.
 If the user is correct, then he is admitted.

8. Ez – Gimpy :-
 A modified version of Gimpy.
 Yahoo used this version in Messenger.
 Randomly picks a single word from a dictionary and applies
distortion to the text.

9. MSNs Passport Service CAPTCHAs :-
 Provided for Microsoft’s MSN services.
 Eight characters(upper case) and digits.
 Warping is used to distort.
 Very strong implementation, hasn’t been broken.
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10. Audio CAPTCHAs :-
 Consists of downloadable audio clip.
 User listens & enters the spoken word.
 Google’s audio enabled CAPTCHA

11. reCAPTCHA :-
Used in Google books project.
Two words are shown, the program knows the first word.
If the user enter the first word correctly, it assumes that the second unknown
word will also be entered correctly.

12. API Key for reCAPTCHA:-
 Login www.recaptcha.net with your Gmail account.
 Domain Name – localhost.com
 It will generate public key and private key
 Public Key: 6LfAIuASAABBBAl2TAmrFWlGUQyols8bqcVvI1c6
Use this in the JavaScript code that is served to your users
 Private Key: 6LfAIuASAABBBFHNHH--MitbQPqnezzg3XftrX5k
Use this when communicating between your server and our server. Be
sure to keep it a secret.
 https://developers.google.com/recaptcha/docs/display