Computers can do all sorts of amazing things, from searching the Web at an incredible rate to playing chess at a grandmaster level. Yet some tasks that are easy for people to perform remain remarkably difficult for computers. For example, computer programs have a hard time reading distorted text or deciphering images.
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In the last few years, computer scientists have worked out an ingenious security scheme that takes advantage of such a mismatch. The scheme relies on computer programs that can, without further human intervention, automatically generate and grade tests that the computer programs themselves can’t easily pass. Yet most people generally have no difficulty passing the same tests.