Telling Humans and Computers Apart (Automatically) Or How Lazy Cryptographers do AI Luis von Ahn The Aladdin Center Carnegie Mellon University

A program that can tell whether its user is a human or a computer. CAPTCHA

A program that can generate and grade tests that: 1. Most humans can pass 2. Current computer programs cannot pass CAPTCHA

Example Renders the words into a distorted image Picks 7 words out of a dictionary shoe drop blade ball hair kettle mouth

Type three words appearing in the image Example And generates a TEST:

P stands for Public All code and data used by a CAPTCHA should be public. (We allow a small amount of private randomness)

Completely Automatic Public Turing Test to Tell Computers and Humans Apart

Example What are these pictures of? QUESTION How do we distort images so that a computer cannot tell they are related?

Bongo Visual Analogy Problems (Bongard 1951)

Bongo Visual Analogy Problems (Bongard 1951)

Bongo Visual Analogy Problems (Bongard 1951) four

Sound Oriented CAPTCHA (Written by Nancy Chan in Hong Kong) Humans are better than computers at understanding spoken language. Question: which English digits are being said?

OPEN PROBLEM Create a TEXT-based CAPTCHA

The Enemy An expert programmer with: 1.Complete knowledge of the code and data used by the CAPTCHA 2. Full access to the web 3. A super computer 4. Lots of free time It should be really hard for The Enemy to write a program that can pass the tests generated by a CAPTCHA

Proving Hardness Reductions to AI-unsolved problems: Any program that passes the tests generated by a CAPTCHA can be used as an oracle to solve a hard unsolved AI problem. What is a hard AI problem? OPEN PROBLEM Prove this!

P stands for Public. Why? 1.The Enemy can learn a lot about the way a CAPTCHA works and all the data it uses. 2.If the Enemy cannot write a program to pass the tests even when the code and data are publicly available, then we are really showing a CAPTCHA is hard to beat. 3.We want our CAPTCHAs to be broken!

Applications QUICKVOTE Which is the best Computer Science Grad School in the US? CMU MIT vote Berkeley StanfordCornell Princeton (from

Applications -Free Services -Chat Rooms -Search Engine Bots -Shopping Agents

Stealing Cycles From Humans Every time a shopping agent encounters a CAPTCHA test, it can simply ask the human behind the computer to solve it! Free Porn Sites: before seeing a free picture the user must solve a CAPTCHA test. OPEN PROBLEM How do we get around this?

Stealing Cycles From Humans How can we use humans as oracles to expand our computational abilities? HotOrNot Find a bug in the code

Stealing Cycles From Humans The whole idea of a CAPTCHA can be seen under this light. Malicious, intelligent programmers can be put to work on unsolved AI problems!

A Challenge to the AI Community CAPTCHAs are well specified challenges to the AI community