This document provides an overview of CAPTCHA systems. It begins with defining CAPTCHA as a test used to differentiate humans from bots. It then discusses the different types of CAPTCHAs, including text, image, audio, video, and puzzle-based CAPTCHAs. The document outlines the design principles, process flow, generation process and working of CAPTCHAs. It also discusses the algorithmic process, applications, drawbacks and issues in designing CAPTCHAs. In conclusion, it emphasizes that security is important for web applications and CAPTCHAs provide a potential method for securing websites against bot attacks.

1. CAPTCHA System
Presenting by: Kanchan Dixit
M.Tech. (CS)

2. Contents:
• Keywords
• Motivation
• Introduction
• Properties of CAPTCHA
• Different Types of CAPTCHA
• Design Principles
• Process Flow
• Working of CAPTCHA
• Generation Process of CAPTCHA
• Algorithmic Process
• Drawbacks
• Applications
• Working of OCR
• Issues in designing of CAPTCHA
• Conclusion
• REFERENCES

3. Keywords:
Turing test
Bot
HIP
OCR

4. Motivation:
 The original motivation of CAPTCHA came from an
online poll.
Basically, The CAPTCHA is used to prevent a bot from
accessing a website.
To prevent the automated access of a website.
To block spammers and bots that try to:
 Automatically sign up.
 Submitting forms.
 Creating accounts.
 use of websites, blogs or forums.
To provides security against the attacks performed by all the
malicious threats.

5. CAPTCHA

6. Introduction:
CAPTCHA is an acronym for Completely Automated Public
Turing test to tell Computer and Human Apart.[2]
A CAPTCHA is a challenge response test used in computing.
CAPTCHA is a program that can generate and grade test that
human can pass but bot can not.[4]
The first CAPTCHA was invented at Carnegie Mellon
University by John Langford, Nicholas J. Hooper and Luis
Von Ahn [1]

7. CAPTCHA technology based on graphical passwords.
CAPTCHA are also known as a type of Human Interaction
Proof (HIP).
CAPTCHA is an HIP system that is widely used to secure the
internet based applications.
CAPTCHA is one such shield which can be used as a
protection from these malicious programs.

8. Properties of CAPTCHA:
There are some basic properties of CAPTCHAs :
 Easy for human user pass.
 Hard enough for a bot to pass.
 Flexible enough so that a tester machine easily generate and
grade it.
 Fully automated system.
 Require little human maintenance or interaction in
administering the test.

9. Different types of CAPTCHA:
Some types of CAPTCHAs are given below:
 Text based CAPTCHAs
 Image based CAPTCHAs
 Audio based CAPTCHAs
 Video based CAPTCHAs
 Puzzle based CAPTCHAs

10. (1) Text based CAPTCHA:
Text based CAPTCHA are the most widely used in web
applications. [1,2]
Very effective and requires a large question bank.
The Number of classes of characters and digits are very small.
Figure 1: Text based CAPTCHA using by Facebook

11. Sr. No. Text Based CAPTCHA Website name
1. Wikipedia
2. Yahoo
3. Gmail
4. Facebook
5. Microsoft
6. Hotmail

12.  Image based CAPTCHAs are challenge tests in which the
users have to guess the similar images.[1]
 The user is required to identify image.
 Some types of Image based CAPTCHAs are :
(2) Image based CAPTCHA:
 ESP Pix CAPTCHA
 Dynamic Image
based CAPTCHA
 Multi model
CAPTCHA
 IdentiPic CAPTCHA
Figure 2: Image based CAPTCHA using by Google

13. Examples:
Figure 3: ESP Pix CAPTCHA [4] Figure 4: Dynamic Image based CAPTCHA [4]
Figure 5: Multi model CAPTCHA [4] Figure 6: IdentiPic CAPTCHA [4]

14. (3) Audio based CAPTCHA:
Developed for visually disabled users.
Audio-based CAPTCHAs are based on the sound-based
systems. [2]
It contains downloadable audio-clips.[3]
In this type of CAPTCHA,
First the user listens and after that submits the spoken
word.

15. Figure 8: Audio based CAPTCHA using by JimThatcher.com
Figure 7: Audio based CAPTCHA using by mollom

16. (4) Video based CAPTCHA:
• Video CAPTCHA is a newer and less commonly used
CAPTCHA system.
• In video-based CAPTCHAs, three words (tags) are
provided to the user which describes a video. [2]
• The user’s tag must match to a set of automatically
generated ground truth tags then only the test is said to be
passed. [4]
• CAPTCHAs contain big downloadable files.

17. Figure 9: Video based CAPTCHA using by olzaonsecurity.com

18. (5) Puzzle based CAPTCHA:
• Usually in Puzzle based CAPTCHA a given picture is divided
to chunks [2].
• A user is supposed to combine these chunks to form the
complete picture same as the original one.[3]

19. Figure 10: Puzzle based CAPTCHAs using by mozenda and TYNAX

20. Design Principles:
The principles behind CAPTCHA are as follows:
 Accessibility must be High.
 Image security by distortion of image.
 User friendly.
Must take less time.
Processing must be fast.

21. Process Flow:
The process behind CAPTCHA are as follows:
The user is presented with a garbled image on which some text
is displayed. This image is generated by the server using
random text.
The user must enter the same letters in the text into a text field
that is displayed on the form to protect.
When the form is submitted, the server checks if the text
entered by the user matches the initial generated text.
If it does, the transaction continues. Otherwise, an
error message is displayed and the user has to enter a
new code.

22. Working of CAPTCHA:
Figure 11: Working of the fusion of Image based CAPTCHA and Text based CAPTCHA

23. Generation Process of CAPTCHA :
Figure 12: Generation process of the fusion of Image based CAPTCHA and Text based CAPTCHA

24. Algorithmic Process:
b
Splitting and Rotating the CAPTCHA images
[6]
Step 1. Start the session.
Step 2. Generate n letters random string from the string
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmn
opqrstuvwxyz0123456789".
Step 3. Create the copy for the random text and put it into the
session.
Step 4. Create transparent CAPTCHA image with w by h image
size and add CAPTCHA text over it.
Step 5. Set the initial X-position and Y-position of captcha image
to 0.

25. Step 6. Split the captcha into k by l Matrix shape by dividing
the captcha width into k parts and the height into l.
Step 7. Start a loop from 0 to k*l
If (i+1) Mod k+1 = 0 Then
{
Set initial X-Position to 0 and
initial Y-Position to Split Height
(Image Height / l)
}
End If
• Step 7.1. Create an array to put the split parts and put the
split images into array.
• Step 7.2. To pass to another split in one row, increase
the initial X-Position by Split Width (Image Width
/ k)in each loop step.

26. • Step 7.3. Randomize integer between -d and d to give
random rotation to the splits. Rotate the splits with
randomized variable that is random between -d and d.
• Step 7.4. End Loop.
Step 8. Combine the splits to create new CAPTCHA with
split and rotation.
Step 9. Add background to transparent new Captcha image
object with randomly drawn lines and special effects
Step 10. Export the final CAPTCHA image as a JPEG
file in the name of ‘captcha.jpg’.
Step 11. Destroy the final CAPTCHA image object to be
refreshed in each session.

27. Drawbacks:
1. Text based
CAPTCHA
1. In text images, user has some problem to identify the correct text or
characters.
i. Multiple fonts. ii. Font size.
iii. Blurred Letters iv. Wave Motion.
2. It can be easily identified by OCR techniques.
2. Image based
CAPTCHA
Some users face problem of image identification who have low vision or due
to blurring of images. [4]
3. Audio based
CAPTCHA
1. It is available only in English therefore end user must have a
comprehensive English vocabulary.
2. Character that have similar sound.
4. Video based
CAPTCHA
Due to large size of file, users face problem to download video and find
correct captcha.[2]
5. Puzzle based
CAPTCHA
It is not easy for users because this captcha take more time to solve the
puzzle and to identify actual arrangement of puzzles.[3]

28. Applications:
Protecting Web Registration: There are many sites on
the Internet which provide free registration to avail their services.
But they are susceptible to web bots.
Online polling sites: These sites takes user’s response or
feedback in the form of questionnaires. To ensure that only
human makes the response they make use of CAPTCHA.[2]
To avoid web crawling: If a site doesn’t want to get
indexed by a search engine then they can make use of
CAPTCHA.[3]

29. Contd…
Preventing Dictionary Attacks: Dictionary attack is a
technique of defeating the authentication mechanism by trying
to determine a decryption key.
• Online Games: Another application of CAPTCHA is online
games where it is preventing web Robots from playing games.
E-mail spam.
Verification of digitized books.
E-Ticketing.

30. Working of OCR:
Figure 12: Working of an OCR System against the Text based CAPTCHA

31. Issues in Designing of CAPTCHA:
 Security Issues
 Securing CAPTCHA against attacks
 Usability Issues
 Improving Usability Issues

32. Conclusion:
A brief survey has been done on the CAPTCHA and list out the
applications and drawbacks of different CAPTCHA based on
Text, Images, Audio, Video and Puzzles.
Security is one of the most important factors of web based
applications now a days. As we have discussed an introduction
and overview of the CAPTCHA based security system as a
potential method of securing web based application.
CAPTCHA’s are mainly produced from Asynchronous Java-
script And XML (AJAX) & using a bit of Hypertext Pre-
Processor (PHP) technology, various algorithms are present.

33. Future Work:
In near future, main focus will be on the developing of a
CAPTCHA that provide the ease of access to the user and
highest level of security by preventing the BOT attacks.
We encourage security researchers to create captchas based on
different AI problems, hopefully they will become more user-
friendly to people with disabilities (visual/mental).
The researches are ongoing for a CAPTCHA that can be able
to authenticate a user as well as preventing the site from the
malicious attacks.

34. QUERIES??

35. References:
[1] P. Gutmann, D. Naccache and C. C. Palme, “CAPTCHAs: Humans vs. Bots”,
IEEE Security & Privacy, 2013, pp. 68-70.
[2] Ved Prakash Singh et al, “Survey of Different Types of CAPTCHA”, (IJCSIT)
International Journal of Computer Science and Information Technologies, Vol. 5
(2) , 2014, 2242-2245
[3] Baljit Singh Saini and Anju Bala “A Review of Bot Protection using CAPTCHA
for Web Security”, IOSR Journal of Computer Engineering, 2013, pp. 36-42.
[4] Rizwan Ur Rahman “ SURVEY ON CAPTCHA SYSTEMS”, JGRCS Journal
of Global Research in Computer Science, Volume 3, No. 6, June 2012, pp.45-53
[5] C. Pope and K. Kaur, “Is It Human or Computer? Defending E-Commerce
with Captchas”, IEEE IT Professional, Vol. 7, No. 2, 2005, 43-49.
[6] http://www.captcha.net

36. [6] Ibrahim Furkan Ince, Ilker Yengin, Yucel Batu Salman , “ DESIGNING
CAPTCHA ALGORITHM: SPLITTING AND ROTATING THE IMAGES
AGAINST OCRs”, IEEE, Innovations in Information Technology, Dubai,
2012,pp. 1-5.
[7] M. Tariq Banday, N. A. Shah, “ A Study of CAPTCHAs for Securing Web
Services ”, IJSDIA International Journal of Secure Digital Information Age, Vol.
1. No. 2, December 2009

37. THANK YOU