1. Fighting the WebBots
A webbot is a program that visits web sites for all kinds of purposes.
For example, Google webbots make copies of all web sites for their search engines.
The challenge is to stop malicious webbots
2/17/2019
ITS102-12, Third Class

2. Webbots and Spam
Spammers send webbots to get accounts from sites that offer them for free.
How can you tell that someone who asks for an account is a person or a webbot?
2/17/2019
ITS102-12, Third Class

3. Are you a person or a bot?
We know that there are certain things that computers cannot do.
Ask the “applicant” to do something that computers cannot do.
Cook a meal?
Read something impossible for computers to read!
2/17/2019
ITS102-12, Third Class

4. CAPTCHA
Completely Automated Public Turing test to tell Computers and Humans Apart
2/17/2019
ITS102-12, Third Class

5. CAPTCHA
CAPTCHA does not have to be text, but “computer unreadable” text is convenient.
Alternatives include pictures.
For example, ask if a person in a pictures is smiling or not. What is wrong with such a CAPTCHA
2/17/2019
ITS102-12, Third Class

6. How Computers Read Optical Character Recognition (OCR)
Step 1: Separate print (usually dark) from background (usually light).
Step 2: Pick up individual characters (group of dark pixels)
Step 3: Identify their shape by looking for strokes, loops, corners, etc
Step 4: Use rules to classify. For example, an H has two vertical strokes and a short horizontal strokes.
2/17/2019
ITS102-12, Third Class

7. Frustrating OCR 1 Use messy background. 2
Separate background from print
Use messy background. 2
Pick up individual characters
Have them blend with each other. 3
Find strokes, loops, etc
Make the letters “wiggly” 4
Apply classification rules
It should be hopeless by this point.
2/17/2019
ITS102-12, Third Class

8. Make your own CAPTCHA www.codeproject.com/KB/aspnet/ CaptchaImage.aspx
A web site that offers you the means:
CaptchaImage.aspx
For a general tutorial see:
captcha/tutorial.htm
2/17/2019
ITS102-12, Third Class

9. From Yahoo’s Briefcase
Some Weak CAPTCHAs
From Paypal
From Yahoo’s Briefcase
2/17/2019
ITS102-12, Third Class

10. Some CAPTCHAs that may be too hard for people
From Yahoo:
From Passport:
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ITS102-12, Third Class

11. But Human Vision is Amazing
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ITS102-12, Third Class

12. But Human Vision is Amazing
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ITS102-12, Third Class

13. Non Text CAPTCHAs Use pictures as CAPTCHAs
Plus: There are very tough to break
Minus:
Need to label a huge number of pictures.
If we use few pictures the webbot can just keep guessing.
2/17/2019
ITS102-12, Third Class

14. Synthetic Pictures (an idea by M. Kaplan)
Please click on or enter each letter corresponding to the following list in the field below. You must enter them in the exact sequence listed. C K
2/17/2019
ITS102-12, Third Class