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May 10, 2006

CAPTCHAs (1): Technical screens are vulnerable to technical progress

One of the most wildly successful technical screening mechanisms for blocking pollution in recent years is the CAPTCHA (Complete Automated Public Turing Test to Tell Computers and Humans Apart). The idea is ingenious, and respects basic incentive-centered design principles necessary for a screen to be successful. However, it suffers from a common flaw: purely technical screens often are not very durable because technology advances. I think it may be important to include human-behavior incentive features in screening mechanisms.

The basic idea behind a CAPTCHA is beautifully simple: present a graphically distorted image of a word to a subject. A computer will not be able to recognize the word, but a human will, so a correct answer identifies a human.

Of course, as we know from screening theory, for a CAPTCHA to work, the cost for the computer to successfully recognize the word has to be substantially higher than for humans. And, since the test is generally dissipative (wasteful of time, at least for the human user), the system will be more efficient (user satisfaction will be higher) the lower is the screening cost for the humans. So, the CAPTCHA should be very easy for humans, but hard to impossible for computers.

With rapidly advancing technology (not just hardware, but especially machine vision algorithms), the cost of decoding any particular family of CAPTCHAs will decline rapidly. Once the decoding cost is low enough, the CAPTCHA no longer screens effectively: we get a pooling equilibrium rather than a separating equilibrium (the test can't tell computers and humans apart). The creators of CAPTCHAs (Ahn, Blum, Hopper and Langford) note, reasonably enough, that this isn't all bad: developing an algorithm that has a high success rate against a particular family of CAPTCHAs is solving an outstanding artificial intelligence problem. But, while good for science, that probably isn't much comfort to people who are relying on CAPTCHAs to secure various open access systems from automated polluting agents.

The vulnerability of CAPTCHAs to rapid technological advance is now clear. A recent paper shows that computers can now beat humans at single character CAPTCHA recognition. The CAPTCHA project documents successful efforts to break two CAPTCHA families (ez-gimpy and gimpy-r).

Posted by jmm at May 10, 2006 12:12 AM

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