A CAPTCHA is a type of challenge-response test used to determine if a user is human. It stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." CAPTCHAs were developed to protect websites from bots by displaying distorted text images that humans can read but current optical character recognition programs cannot. Common types of CAPTCHAs include text-based challenges, audio challenges, and reCAPTCHA which uses words from digitized texts that machines have not yet learned to read perfectly.

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.
XTNM5YRE L9D28229B

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