CAPTCHA is a type of challenge-response test used to determine if a user is human. It was created in 2000 to prevent bots from spamming websites. There are different types of CAPTCHAs including distorted words, picture identification, puzzle solving, 3D images, and reCAPTCHAs which help digitize books while determining if a user is human. The document discusses the history and applications of CAPTCHAs, providing examples of how they are used to prevent comment spam, protect website registrations, and block dictionary attacks from non-human users.

1. Demystifying CAPTCHA
Udit Gupta
SDE @ Commonfloor.com

2. Introduction
What is 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.
● It is actually an inverted Turing test.

3. A Brief History
● Finds its origin within the hacking community
● Leetspeak (Leet or 1337)
● The term Captcha was coined in 2000 by Luis von Ahn, Manuel Blum,
Nicholas J. Hopper of Carnegie Mellon University and John Langford of
IBM.

4. The 3 Ws of CAPTCHA
● Why? - To prevent your system against malicious threats such as
“bots”.
● When? - You are under spam attack.
● Where? - Internet based applications.

5. Applications of CAPTCHA
● Preventing comment spam in blogs.
● Protecting website registration.
● Protecting email addresses from scrapers.
● Online Polls.
● Preventing Dictionary Attacks.
● Search Engine Bots.
● Worms and Spam.

6. Types of CAPTCHA
1. Standard distorted word

7. Types of CAPTCHA
2. Picture identification

8. Types of CAPTCHA
3. Puzzle solving

9. Types of CAPTCHA
4. 3-D Captcha

10. Types of CAPTCHA
5. reCAPTCHA
● Stop spam and help digitize books at the same time.
● Easy for people, hard for bots.
● Not just distorted text.
● Annotate images
● Build Machine Learning datasets.

11. Questions?

12. Thank You