>> VON AHN: Hi, everyone. I'm sure most of you
have seen those squiggly characters that you have to type all over the Internet. For example,
when you're getting a Gmail account or trying to buy tickets online. That thing is called
the CAPTCHA. And the reason it's there is to make sure that you, the entity filling
out the form are actually a human and not some sort of computer program that was written
to submit the form millions of millions of times. The reason it works is because humans,
at least, non-visually impaired humans have no trouble reading these distorted squiggly
characters whereas computer programs simply can't do it as well yet. So for example, the
reason you have to type a CAPTCHA when buying tickets online for a show is to prevent scalpers
from writing programs that try to buy millions of tickets too at a time. Now, CAPTCHAs are
a very effective tool against spammy behavior. Chances are that if you allow any type of
user interaction on your site, you are susceptible to automated attacks by so-called bots. And
CAPTCHAs can help you mitigate this problem. For example, in comment forms, if you have
a blog that allows comments, you are susceptible to so-called comments spam. So, spammers actually
write programs that search the whole Web for unprotected blogs in which they can send basically
comments that advertise their products. One possible solution against this is to use a
CAPTCHA to make sure that only humans can comment on your blogs. There are many other
cases in which CAPTCHAs are necessary to prevent automated abuse such as account sign offs,
forms in which you can--people can contact you, etcetera. Now, because we here at Google
hate spam as much as you do, we provide a free CAPTCHA service called reCAPTCHA. Here,
I'm going to explain a few things about why reCAPTCHA is so cool and why it is so successful
at stopping automated abuse. First, I should say that reCAPTCHA is fundamentally different
from every other CAPTCHA out there. And to explain why, it's [INDISTINCT] how we originally
got the idea for reCAPTCHA? Okay, so basically, about three years ago, we did this calculation,
we calculated, we wondered how many CAPTCHAs are typed every day by people around the world.
And after we're doing a simple calculation, it turns out that approximately 200 million
CAPTCHAs are typed every day by people around the world. Okay, I type a CAPTCHA roughly
once every two or three days because I use the Internet very heavily and it turns out
there's a lot of people like me. And so, if you add it little up, approximately 200 million
CAPTCHAs are typed every day by people around the world. Now, each time you type a CAPTCHA,
essentially you waste 10 seconds of your time, takes about 10 seconds to type a CAPTCHA.
And if you multiply that by 200 million, you get the humanity as a whole who's wasting
approximately 500,000 hours every day typing CAPTCHAs. Now, of course, we can't just get
rid of CAPTCHAs because of the security of the Web server depends on them. So, the idea
of reCAPTCHA was born by wondering is there any way in which we can use these 500,000
hours for something that's good for humanity? Okay, another way of putting it is, is there
some way in which you can use those 10 seconds while you're typing a CAPTCHA for something
that's useful? Okay, so think about it, while you're typing a CAPTCHA during those 10 seconds,
your brain is doing something amazing. Your brain is doing something that computers cannot
yet do. So, is there any way in which we can use this effort for something good? And it
turns out, the answer is yes. And this is what we're doing with reCAPTCHAs, so with
reCAPTCHA, not only are you authenticating yourself as a human, but in addition, you're
helping to digitize books and newspapers, okay? And let me explain how. There's many
projects out there trying to digitize old books and newspapers so that anybody on the
Web can act system, okay? And the way book digitization works is that first, you start
with a book in physical form, okay? And then, you scan this book. Now, scanning a book is
like taking a digital photograph of every page of the book. You see an image for every
page of the book, okay? The next step in the process is that the computer needs to be able
to decipher all of the characters that are in this image, okay? And the reason for that
is so that they can search through the book. This is done using a technology called OCR
for Optical Character Recognition, okay? So, OCR keeps an image with text in it and tries
to decipher what the text is and that is in it, okay? Now, the problem with OCR is that
it doesn't always work very well. For things that were written a long time ago because
the ink has faded and the pages have turned yellow, OCR cannot recognize many of the characters.
So, in this example, on the screen, if you run OCR in this and if you'll try to get the
computer to decipher all of these characters, here's what you get, okay? So basically, that
you can see all mistakes that this makes. So, what we're doing now is we're taking all
the words that the OCR cannot recognize, we're taking all the words the computer cannot recognize
in the book digitization process. And we're using CAPTCHAs so that the answers that people
enter CAPTCHAs are being used to help us correct these mistakes. Okay, so that's the fundamental
idea of reCAPTCHA is that the answers that people enter are used to correct the mistakes
in the book digitization process. So, let me explain the exact process of how this works.
So, first, we start with a scan of an old book. Then we take all the words that the
computer cannot recognize. It turns out the computer actually tells us when it cannot
recognize a word. So, we're going to take all the words the computer cannot recognize.
And we're going to use these as the basis for reCAPTCHA. Notice these are perfect for
CAPTCHA because these are, by definition, words that the computer cannot recognize.
And so, we take this word, we distort it even further to really make sure that the computer
cannot recognize it. And then we use this as a CAPTCHA. Now, you may be wondering, how
can we use this word as a CAPTCHA if the system doesn't know the answer for it? This is a
word that the system just took out of a book for which it didn't know the answer. It should
be the case that if we use it as a CAPTCHA somewhere, when you're buying tickets online
for example, we should only let you through if you type the word correctly. But how can
the system do this when it doesn't know the answer? The solution to this problem is that
whenever we gift a user a word for which the system doesn't know the answer, we actually
give it to them along with another word. One for which we do know the answer, one for which
the system does know the answer. So, we actually give the users two words, one for which the
system knows the answer, one for which it doesn't. And, we ask them to type both words.
Well, of course, we don't tell the users which word is which. And if the user enters the
correct answer to the one for which the system already knows the answer, we assume that the
user is a human. And we also get some confidence that they enter the correct one for the other
word. And if we repeat this process multiple times giving the new word to different people,
and they all typed the word "between" then, we get with overwhelmingly high probability
that this word really is the word "between." So, it's the basic idea of how reCAPTCHA works.
Now, we put a lot of effort in making reCAPTCHA the best CAPTCHA out there. For example, reCAPTCHA
is also accessible to visually impaired users because it has an audio alternative. It is
very important that if you use a CAPTCHA on your site, you provide an audio alternative,
because blind people cannot navigate around image-based CAPTCHAs. And you see, blind people
surf the web using screen readers, programs that read the entire screen to them out loud.
Now, whenever a screen reader gets to a CAPTCHA, to an image of a CAPTCHA, it can't read it
because, by definition, the screen reader is itself a program. So, if your CAPTCHA doesn't
provide an audio alternative, blind people are locked out of your site. And now with
reCAPTCHA, we provide an audio alternative. Hundreds of thousands of sites use reCAPTCHA
on the Web because also, we spend a lot of effort balancing security and usability. We
want reCAPTCHA to be impossible to read for computers but also easy to read for humans.
One of the things we do to achieve this balance between usability and security is that we
have multiple levels of difficulty of the distorted images of reCAPTCHA. For most users,
the first CAPTCHA that they see is relatively easy to read. But if they attempt to solve
too many CAPTCHAs in a small period of time, with reCAPTCHA, we start giving them harder
and harder CAPTCHAs, in case, it's somebody trying to attack your site. Also, reCAPTCHA
is a Web service meaning that the images for reCAPTCHAs that are served to your users actually
come directly from our servers. This is good for many reasons. First, reCAPTCHA doesn't
require you to download a program and uses your server's resources to generate the distorted
images. We do that for you. Second, and most importantly, if we ever notice that automated
bots are able to read reCAPTCHA images because we watch our logs very carefully, we can immediately
change the distortions to the images without you having to download an update. Okay, so
this ensures that reCAPTCHA is effective at stopping spam at all times. Well, hopefully
I've conveyed our excitement for our product. ReCAPTCHA is very easy to install. So, if
you need a CAPTCHA on your site, you should use a reCAPTCHA especially because it's free.