Luis Von Ahn here today, from Carnegie Mellon University.

His talk is on human computation.

Luis is a very new assistant professor in computer science

at the School of Computer Science at Carnegie Mellon

University.

He received his Ph.D. in 2005, and I'm told he was the

hottest new graduate on the market, with offers from just

about every university out there, including corporate

offers, too.

He received his B.S. from Duke University.

He received a Microsoft Research Fellowship Award.

His research interests include encouraging people to work for

free, as well as catching and thwarting cheaters in online

environments.

His work has appeared in over a hundred news publications

around the world.

New York Times, CNN, USA Today, BBC, and

the Discovery Channel.

Luis holds four patent applications and has licensed

technology to major internet companies.

Please join me in welcoming Luis Von Ahn.

[APPLAUSE]

LUIS VON AHN: Can you hear me now?

OK.

So, I want to start by asking a question to the people in

the audience.

How many of you have had to fill out a registration form

for something?

Like Yahoo, Hotmail, or Gmail, or some sort of web form where

you've been asked to read a distorted sequence of

characters or a distorted word such as this one?

How many of you found it annoying?

Awesome.

OK, well, that was part of my thesis.

That thing is called a CAPTCHA, and the reason it's

there is to make sure that you, the entity filling out

the web form, are actually a human, and not some sort of

computer program that was written to submit the form

millions and millions of times.

The reason it works is because humans--

at least non-visually impaired humans--

have no trouble reading distorted characters, whereas

computer programs simply can't do it as well yet.

More generally, a CAPTCHA is just a program that can tell

whether its user is a human or a computer.

OK, let me say that another way.

A CAPTCHA is a program that can generate and grade tests

that most humans can pass, but current computer

programs can not.

Notice the paradox here.

A CAPTCHA is a program that can generate and grade tests

that it itself cannot pass.

So in that way, CAPTCHAs are a lot like some professors.

[LAUGHTER]

Just to make things crystal clear, let me give you an

example of one of these programs that can generate and

grade tests that most humans can pass, but current computer

programs cannot.

Here's how the program works.

First, the program picks a random string of letters.

O-A-M-G, in this case.

Then the program renders the string into a randomly

distorted image, and then the program generates a test,

which consists of the randomly distorted image and the

question, "What are the characters in this image?"

CAPTCHAs are used all over the place, for all kinds of

things, and I could spend the next hour talking about all

the different applications of CAPTCHAs.

But since I don't want to do that, I want to illustrate one

of the applications through a little story.

So a few years ago, Slashdot--

which is a very popular website--

put up this poll in their site, asking which is the best

computer science graduate school in the United States?

This is a very dangerous question to ask over the web.

As with most online polls, IP addresses of voters were

recorded to make sure that each person could only vote,

at most, once.

However, as soon as the poll went up, students at CMU wrote

a program that voted for CMU thousands and

thousands of times.

The next day, students at MIT wrote their own program.

And a few days later, the poll had to be taken down with CMU

and MIT having, like, a gazillion votes and every

other school having less than 1,000.

I guess the poll worked in this case.

[LAUGHTER]

I'm just kidding.

But in general, this is a huge problem.

You simply cannot trust the results of an online poll,

because anybody could just write a program to vote for

their favorite option thousands and

thousands of times.

One solution is to use a CAPTCHA to make sure that only

humans can vote.

CAPTCHAs have many, many other applications.

Another one is in free email services.

For instance, there are several companies that offer

free email services--

Yahoo, Microsoft, Google--

and up until a few years ago, all of them were suffering

from a very specific type of attack.

It was people who wrote programs to obtain millions of

email accounts every day, and the people who wrote these

programs were usually spammers.

So if you're a spammer and you want to send spam from, say,

Yahoo, you run into the problem that each Yahoo

account only allows you to sound, like,

100 messages a day.

So if you want to send millions of messages a day

from Yahoo accounts, you have to own

millions of Yahoo accounts.

And this is why spammers wrote programs to obtain millions of

Yahoo accounts.

And the solution--

or one solution-- and this is what we originally suggested

to Yahoo-- was to use a CAPTCHA to make sure that only

humans can obtain free email accounts.

Now, since CAPTCHAs are used all over the place to stop

spammers from doing bad things, spammers have started

coming up with all kinds of dirty hacks to get around the

CAPTCHAs that are being used in practice.

So let me explain a couple of them.

Here's one.

I'm sure a lot of you have heard of this.

CAPTCHA sweatshops.

Spam companies actually are hiring people to solve

CAPTCHAs all day long.

And they are usually being hired in other countries where

the minimum wage is a lot lower, and this

is currently happening.

But there's at least two consolations.

First, it's at least costing them some.

So whereas before, they could get the accounts for free, now

it costs them a fraction of a cent per account, so they

can't get that many.

Second, CAPTCHAs are actually generating jobs in

underdeveloped countries.

[LAUGHTER]

So this is one dirty hack.

There's an even dirtier hack, and I'm sure a lot of you have

heard of it, and this is what some porn companies are

allegedly doing.

And I'm going to emphasize the word "allegedly." So, porn

companies also want to send spam.

They also want to break CAPTCHAs, and here's how they

are allegedly doing it.

They write a program the fills out the entire registration

form, say, at Yahoo.

And whenever the program gets to the CAPTCHA,

it can't solve it.

So what it does is it copies the CAPTCHA

back to the porn page.

Now, back at the porn page, there's a lot of people

looking at porn.

And suddenly, one of them gets this screen saying, "If you

want to see the next picture, you got to tell me what word

is in the box below." And you know what people do?

They type the word as fast as possible.

[LAUGHTER]

And by doing so, they are effectively solving the

CAPTCHA for the porn company bot.

That is, they're effectively obtaining a free

email account for them.

So pornographers, they're really, really smart.

So CAPTCHAs take advantage of human processing power in

order to differentiate humans from computers, and it turns

out that being able to do so has some very, very nice

applications in practice.

Now that I've told you about CAPTCHAs, now I can tell you

what this talk really is about.

This talk is not about CAPTCHAs.

This talk is about human computation.