The problem with coming up with effective CAPTCHA's is that the dataset should
not rely on obscurity or secrecy to work. Anyone can come up with hard
questions that can consistently trip up a computer, but how effective would
those questions be if the adversary had access to your question/answer
database? Many ideas I hear for CAPTCHA's rely solely upon the secrecy of the
data set. And that is security that relies upon obscurity.
Try coming up with a CAPTCHA where the code is public, the dataset is public,
and the only secret is the randomness generated for each individual test, and
you will find that it is quite difficult. The problem, in part, is that we want
a machine to generate a test that another machine with the same data cannot
solve, but that a human can. We can bypass that by having humans come up with
the questions, but that means they also need to store the answers for
verification, again bringing us back to the problem that we are relying upon
the secrecy of the data.
Another mistake that people often make with CAPTCHA's are questions with
multiple choice answers. If you asked a question like "Which of these
strawberries is most rotten?" you would have to provide enough pictures to
reduce the significance of a luck guess. Even if you had 10 possible answers to
select from, that might not be effective in stopping a spammer from setting up
massive free e-mail accounts. It might statistically take them 10 times as long
but they can still do it. However, if you provide too many answers, the chances
of several good answers increases, making it less effective. How many times
have you taken a multiple choice test and there are two answers that, in your
opinion, would work? Especially in the case of a subjective question such as
which strawberry is most rotten.
It is definitely a good challenge and it will be cool seeing someone someday
solve this problem.
Mark Burnett
On Thu, 25 Aug 2005 08:40:40 -0700, Jayson Anderson wrote:
That was an interesting article, I definetely got caught up clicking
thru for awhile.. One has to wonder, why hasn't a more effective system
been placed into production let alone conceptualized and largely
accepted as a solid approach for the future ? More specifically, the
claim that CAPTCHA as it stands now is not a Turing machine. I'm not
sure if that's entirely true as symbols pre-date their interpretation by
machine.=20
Regardless, like one gentleman mentioned in an article, a much more
clear method to differentiate man vs. machine would be to ask abstract
questions. Barring the cultural, linguistic and socioeconomic
implications, why not ask things like "which one is a pachyderm?". Or
"which texture most resembles stipple?". Or "Which of these strawberries
is most rotten?". Or "Which person is taller?" with same-sized figures,
but one the same sized as the car she stands next to, the other only
half. etc. etc. Ya know ? Sure it would take a significant multi-faceted
approach utilizing an amazingly heterogeneous set of contributors, but
that's where open source comes in. Pool a huge bank of acceptable
abstracts based on image size, obscurity and all the other standards
(which do NOT need to be complex at all), then refine that, seed the
array and answer presentations with some decent entropy, use yet more
entropy to randomize the units by which answers are delineated,
"a,b,c,d", "circle[~],eye{=3D],carrot[%],money[E]" each different each
time, and all the hundreds of other variables i've not thought of. It
seems like it is workable to me. Keep the project always living so that
submissions and refined objects are always being added to an update-able
system..... SOMETHING is going to have to be done that is superior to
"crazytext", as ultimately it will be rendered nothing worse than a
speedbump. I think CAPTCHA still qualifies as Turing, just not an
effective one in it's environment. Seems that machine-proofing should
use anything BUT that which is found in almost every machine that would
be used to circumvent it :)=20
Sorry for the chatter but I've ALWAYS felt that crazytext(tm) was an
amazingly poor way to differentiate machine from man, and these articles
just prove what I and so many others I'm sure had always felt.....
Jayson
-
On Wed, 2005-08-24 at 14:29 -0400, robert@webappsec.org wrote:
This was linked off of slashdot
(http://it.slashdot.org/article.pl?sid=05/08/24/1629213&tid=172&tid=95)
and explains some of the ways people are breaking CAPTCHA
(http://en.wikipedia.org/wiki/Captcha) based systems.
http://sam.zoy.org/pwntcha/
- Robert
robert_at_webappsec.org
http://www.cgisecurity.com
