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Web Programming Week 11 Old Dominion University Department of Computer Science CS 418/518 Fall 2010 Martin Klein <[email protected]> 11/09/10 Image Resizing • PHP functions: – GetImageSize() 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 – 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=5 975815412653416464 • 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