Web Programming Week 11 Old Dominion University Department of Computer Science CS 418/518 Fall 2010 Martin Klein <mklein@cs.odu.edu> 11/09/10

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: http://mln-web.cs.odu.edu/~mklein/code/code/ch11/resize.html Also see: http://us2.php.net/getimagesize

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.

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

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: http://en.wikipedia.org/wiki/Captcha

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

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

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!!!

reCAPTCHA Examples http://www.google.com/addurl/ https://www.blogger.com/comment.g?blogID=25215770&postID=5975815412653416464 http://www.johnmwillis.com/other/top-10-worst-captchas/ Implementation: http://captchas.net/ http://mln-web.cs.odu.edu/~mklein/php/recaptcha/query.php http://www.google.com/recaptcha http://mln-web.cs.odu.edu/~mklein/php/recaptcha/one.php