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 by Luis Von Ahn

1. Captcha

2. CONTENTS
 Definition
 Need for Turing Test
 Inventor
 Background
 Type of Captchas
 Text-Based Captcha
 Graphic Based Captcha
 Audio Based Captcha
 Guidelines
 Applications
 Breaking Captchas
 Captcha – A boredom
 Alternatives to Captcha
 Suggestions
 References

3. Definition
Completely Automated Public Turing test to tell Computers and
Humans Apart
A.K.A. Reverse Turing Test
3

4. Need for Turing Test

5. 5
Inventor: Luis von Ahn

6. Background
First used by Altavista in1997
◦ Reduced SPAM by over 95%
CMU/Yahoo!
◦ Automated the creating and grading of challenges
PARC
◦ Relies on document image degradation to prevent successful OCR
◦ Conducted user-focused studies to assess the effectiveness of CAPTCHAs
6

7. Types of CAPTCHAs
Text based
◦ Gimpy, ez-gimpy
◦ Gimpy-r, Google CAPTCHA
◦ Simard’s HIP (MSN)
Graphic based
◦ Bongo
◦ Pix
Audio based
7

8. Text Based CAPTCHAs
Gimpy, ez-gimpy
◦ Pick a word or words from a small dictionary
◦ Distort them and add noise and background
Gimpy-r, Google’s CAPTCHA
◦ Pick random letters
◦ Distort them, add noise and background
Simard’s HIP
◦ Pick random letters and numbers
◦ Distort them and add arcs
8

9. Text Based CAPTCHAs
9

10. Graphic Based CAPTCHAs
 Bongo
◦ Display two series of blocks
◦ User must find the characteristic that sets the two series apart
◦ User is asked to determine which series each of four single blocks belongs to
Difference? thick vs. thin lines
10

11. Graphic Based CAPTCHAs
PIX
◦ Create a large database of labeled images
◦ Pick a concrete object
◦ Pick four images of the object from the images database
◦ Distort the images
◦ Ask the user to pick the object for a list of words
11

12. Graphic Based CAPTCHAs
12
Dog
Pool

13. Audio Based CAPTCHAs
Pick a word or a sequence of numbers at random
Render them into an audio clip using a TTS software
Distort the audio clip
Ask the user to identify
and type the word or numbers
13

14. Guidelines
◦ Accessibility
◦ Image security
◦ Script security
◦ Security after widespread adoption
◦ Custom implementation or a general CAPTCHA?

15. Applications
◦ Preventing Comment Spam in Blogs
◦ Protecting Website Registration
◦ Protecting Email Addresses From Scrapers
◦ Online Polls
◦ Preventing Dictionary Attacks
◦ Worms and Spam

16. 16
Breaking CAPTCHAs
◦ Most text based CAPTCHAs have been broken by software
◦ OCR
◦ Segmentation
◦ Several other softwares such as
◦ CIntruder - pentesting tool to bypass captchas.

17. Captcha SweatShops
17
◦ SPAM COMPANIES HIRE HUMANS TO SOLVE
CAPTCHAS ALL DAY LONG
◦ 720 CAPTCHAS PER HOUR PER HUMAN

18. How Badly We Hate It(even the inventor)

19. Other Alternatives
reCaptchaMathematical
CAPTCHA
Distinguish
pictures
3D Object
CAPTCHA
You must enter them in the
exact sequence listed:
•The Head of the Walking Man,
•The Vase,
•The Back of the Chair.

20. News Splash!!!!!
Google has
cracked the
“ CAPTCHA”

21. Suggestions

22. References
1. en.wikipedia.org
2. “Protecting Websites with Reading Based CAPTCHAs” by Henry S. Baird and Mark Luk, California
3. “Telling Humans And Computers Apart Automatically OR How Lazy Cryptographers Do AI” by Luis
von Ahn, Manuel Blum, and JohnLangford.
4. “Visual Reverse Turing Tests: A False Sense of Security” by Miroslav Ponec in 7th Annual IEEE
Information Assurance Workshop, New York, 2006
5. “Recognizing Objects in Adversarial Clutter: Breaking a Visual CAPTCHA” by Jitendra Malik And
Greg Mori
6. “Enhanced CAPTCHAs: Using Animation to Tell Humans and Computers Apart” by Elias
Athanasopoulos and Spiros Antonatos.
7. areyouahuman.com

23. Thank You!!
Any Queries??