[cryptography] CAPTCHA as a Security System?

Jack Lloyd lloyd at randombit.net
Mon Jan 2 14:30:41 EST 2012

On Mon, Jan 02, 2012 at 08:03:07PM +0100, Marcus Brinkmann wrote:

> Computer programs today are limited by attention of experts (programmers,
> researchers).  What does "hard for computer programs" actually mean then?  Is
> there a theoretical boundary that limits the abilities of computer programs to
> recognize captures, or is Ahn just exploiting a temporary lack of economic
> incentive to realize the full capabilities of computer systems for these kind
> of problems?

That was a pretty explicit aspect to the whole proposal. It adds
incentives to solve supposedly difficult AI problems. (Or incentives
to build very efficient mechanical turk systems, which is of course
what mostly happened because that's cheaper and more reliable than
funding AI research). Quoting from the paper

"Much like research in cryptography has had a positive impact on
algorithms for factoring and discrete log, we hope that the use of
hard AI problems for security purposes allows us to advance the field
of Artificial Intelligence. We introduce two families of AI problems
that can be used to construct captchas and we show that solutions to
such problems can be used for steganographic communication. captchas
based on these AI problem families, then, imply a win-win situation:
either the problems remain unsolved and there is a way to
differentiate humans from computers, or the problems are solved and
there is a way to communicate covertly on some channels."


"A primary goal of the captcha project is to serve as a challenge to
the Artificial Intelligence community."


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