Telling Humans and Computers Apart Automatically or How Lazy Cryptographers do AI
If you try to get a new email account at Yahoo, you’ll be asked to prove that you’re a human and not a computer. Why? Because a single computer program can get thousands of free email accounts per second. And that’s bad for Yahoo. But how do you prove to a computer that you’re a human? Proving that you’re a human to another human can be done using an idea from the 1950s: the Turing Test [11]. A human judge asks you a bunch of questions and decides, depending on your answers, whether he’s talking to a human or a computer. Proving that you’re a human to a computer is another matter. It requires a test (or a set of tests) that computers can grade, humans can pass, but paradoxically, computers can’t pass. In our lingo, it requires a captcha.