Flickr-CAPTCHA v0.2 using PHP and the FlickrAPI for human recognition

Flickr-CAPTCHA is a captcha program that uses flickr images and tags
to decide whether someone is human. A random thing is selected (from a
list of things) and a random number of flickr images are chosen
a long with a set of decoys. Each decoy contains at least one other
thing in order to prevent images that are ambigious and to try and keep
it obvious they are decoys. The user must then select all images and
if they get 50% right (with minus marks for wrong answers) we consider
them definitely human. It should be quite difficult for a computer
to guess which images to select but fun and enjoyable for a human. It
requires only very basic english skills.


Image ID’s are hashed then proxied through a PHP page so that a bot
cannot simply lookup the image themselves through the flickr API. They
could potentially use the author and the title so an option exists to
hide the title as well if you are paranoid. You will also want to
adjust the acceptedRatio if you are really paranoid.


We are only selecting images that have attribution licenses, not those
that are all rights reserved. Every image being displayed is attributed
to the author with a link to their flickr page. I am not a lawyer, see
the LICENSE file for warranty (hint: there is none).


  • Fun and easy to use
  • Easy for human / hard for computer
  • Customizable


  • Slow to load all the images
  • Not secure

To get started check out the README file

Download Flickr-CAPTCHA v0.2

Flickr-CAPTCHA online tester

Older Releases: 0.1.

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