1. Web Programming Week 11 Old Dominion University
Department of Computer Science
CS 418/518 Fall 2010
Martin Klein
11/09/10

2. Image Resizing PHP functions: GetImageSize() ImageCreateTrueColor()
list($width,$height,$type,$attr) = GetImageSize($srcfile);
ImageCreateTrueColor()
$dst = ImageCreateTrueColor($new_width, $new_height);
ImageCreateFromJpeg()
$src = ImageCreateFromJpeg($srcfile);
ImageCopyResized()
ImageCopyResized($dst, $src, 0, 0, 0, 0, $new_width, new_height, $width, $height);
Examples:
Also see:

3. CAPTCHA
Completely Automated Public Turing test to tell Computers and Humans Apart
Challenge – response test provided by server
User solves problem and is (by server) considered human (and not a machine)
Goal:
Ensure that interaction is with user
Prevent spam of all kinds e.g. mass account creation, posts, etc.

4. CAPTCHA How to? Distorted text
Make it hard for Optical Character Recognition (OCR)
Difficult to distinguish between background and text (color, shape)
Character overlap
Out of alignment

5. CAPTCHA How to? Distorted images Somewhat hybrid:
Provide array of images and have user select a few (airplane, boat, bird)
Have user point to “top” or “bottom” of image
Difficult for machines to determine if no indicators (sky, water, horizon) given
“Chose the item that does not fit here!”
Somewhat hybrid:
trivial (math) problems to solve
picture taken from:

6. CAPTCHA Problem solved?
Vulnerable to relay attacks
Relay captcha to human when encountered
Capture and re-use successful session ID
Dictionary attacks
“Iron out” images and use ORC, dictionaries

7. CAPTCHA Problem solved?
How about accessibility?
Blind users?
possible solution: audio stream
Voice recognition software!
Deaf-blind users?
???

8. reCAPTCHA Originates from CMU, bought by Google in 2009
Help needed to digitize books (using OCR)
“Wisdom of the Crowds” (can be used against us)
Captcha contains
1 term not recognized by OCR
1 term well known
Assumption: if user gets known term right, she also gets unknown term right
To be confirmed by 2, 3, … others
Digitization project benefits!!!

9. reCAPTCHA Examples http://www.google.com/addurl/
Implementation: