Human or Computer? Take This TestBy SARA ROBINSON
 s chief
scientist of the Internet portal Yahoo, Dr. Udi Manber had a profound problem:
how to differentiate human intelligence from that of a machine. His
concern was more than academic. Rogue computer programs masquerading as teenagers
were infiltrating Yahoo chat rooms, collecting personal information or posting
links to Web sites promoting company products. Spam companies were creating
havoc by writing programs that swiftly registered for hundreds of free Yahoo
e-mail accounts then used them for bulk mailings. Advertisement
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"What we needed," said Dr. Manber, "was a simple way of telling a human user from a computer program."
So, in a September 2000 conference call, Dr. Manber discussed the problem
with a group of computer science researchers at Carnegie Mellon University.
The result was a long-term project that is just now beginning to bear fruit.
The roots of Dr. Manber's philosophical conundrum lay in a paper written
50 years earlier by the mathematician Dr. Alan Turing, who imagined a game
in which a human interrogator was connected electronically to a human and
a computer in the next room. The interrogator's task was to pose a series
of questions that determined which of the other participants was the human.
The human helped him, while the computer did its best to thwart him.
Dr. Turing suggested that a machine could be said to think if the human interrogator
could not distinguish it from the other human. He went on to predict that
by 2000, computers would be able to fool the average interrogator over five
minutes of questioning at least 30 percent of the time. Although the
Turing test, as it is now called, spawned a vibrant field of research known
as artificial intelligence, his prediction has proved false. Today's computers
are capable of feats Dr. Turing never imagined, yet in many simple tasks,
a typical 5-year-old can outperform the most powerful computers. Indeed,
the abilities that require much of what is usually described as intelligence,
like medical diagnosis or playing chess, have proved far easier for computers
than seemingly simpler abilities: those requiring vision, hearing, language
or motor control. "Abilities like vision are the result of billions
of years of evolution and difficult for us to understand by introspection,
whereas abilities like multiplying two numbers are things we were explicitly
taught and can readily express in a computer program," said Dr. Jitendra
Malik, a professor specializing in computer vision at the University of California
at Berkeley. Dr. Manuel Blum, a professor of computer science at Carnegie
Mellon who took part in the Yahoo conference, realized that the failures
of artificial intelligence might provide exactly the solution Yahoo needed.
Why not devise a new sort of Turing test, he suggested, that would be simple
for humans but would baffle sophisticated computer programs. Dr. Manber
liked the idea, so with his Ph.D. student Luis von Ahn and others Dr. Blum
devised a collection of cognitive puzzles based on the challenging problems
of artificial intelligence. The puzzles have the property that computers
can generate and grade the tests even though they cannot pass them. The researchers
decided to call their puzzles Captchas, an acronym for Completely Automated
Public Turing Test to Tell Computers and Humans Apart (on the Web at www.captcha.net).
One puzzle, called Gimpy, consists of a display of seven distorted, overlapping
words chosen at random from a dictionary of simple words. Solving the puzzle
requires identifying three of the seven words and typing them into the box
provided. The Carnegie Mellon group also created a simplified version of
Gimpy — a single distorted word displayed against a complicated background.
It is now part of Yahoo's registration process. Another Captcha, called
Sounds, consists of a distorted, computer-generated sound clip containing
a word or sequence of numbers. To solve the puzzle, a user must listen to
the clip and type the word or numbers into the box provided. The idea
of using puzzles to prevent automated registrations was not new. Other e-commerce
sites, including the AltaVista search engine and eBay's PayPal service, were
experiencing problems like Yahoo's and independently came up with Captcha-like
puzzles. Through its acquisitions, Hewlett-Packard holds a patent on text-based
Captchas. Still, researchers credit Dr. Blum for the breadth of his
vision. Dr. Blum "did a great thing by recognizing that this problem is much
more than solving a nuisance for Yahoo and AltaVista," said Dr. Andrei Broder,
who helped develop the AltaVista puzzle and is now at I.B.M. As a
cryptographer, Dr. Blum was familiar with the constant efforts of cryptographic
researchers to advance the field by cracking codes to discover their weaknesses.
He hoped to start a similar dynamic for Captchas, spurring researchers to
try to create better Captchas while building computer programs that crack
existing ones. "Captchas are useful for companies like Yahoo, but
if they're broken it's even more useful for researchers," Dr. Blum said.
"It's like there are two lollipops and no matter what you get one of them."
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