Thomas Dowling <[log in to unmask]> wrote:
> Does anyone know anything concrete about "cognitive" captchas?  I've run 
> into anecdotal support for things like:
>    Enter the word "orange" <input name="foo">
[...]
> Are these known to work?  Or are they just clever guesses about what 
> bots might not be able to figure out?

There are mostly anecdotes because this stuff is hard to test
properly.  I found they worked a little, but are just clever guesses.

"3.1 Logic puzzles

The goal of visual verification is to separate human from machine. One
reasonable way to do this is to test for logic. Simple mathematical
word puzzles, trivia, and the like may raise the bar for robots, at
least to the point where using them is more attractive elsewhere.

Problems: Users with cognitive disabilities may still have trouble.
Answers may need to be handled flexibly, if they require free-form
text. A system would have to maintain a vast number of questions, or
shift them around programmatically, in order to keep spiders from
capturing them all. This approach is also subject to defeat by human
operators."

Source: http://www.w3.org/TR/turingtest/#logic


As that last phrase hints, bots are not the only problem.  See
http://www.schneier.com/blog/archives/2007/11/spammers_using.html
for example.


Hope that helps,
-- 
MJ Ray (slef)
Webmaster for hire, statistician and online shop builder for a small
worker cooperative http://www.ttllp.co.uk/ http://mjr.towers.org.uk/
(Notice http://mjr.towers.org.uk/email.html) tel:+44-844-4437-237