One of the greatest inventions in the history of computer security is Hashcash. Internet blights like spam and denial-of-service attacks are what economists call “tragedy of the commons” problems. They exploit the fact that it’s free to send email and make web requests. At zero cost, you can have a profitable business even at extremely low success rates.
One way to fix these problems is to impose tariffs that hurt bad actors without hurting good actors. For example, you could impose “postage fees” on every email and web request. Unfortunately, in practice, this is impossible, because you’d have to set up billing relationships between every computer that wants to communicate.
The brilliant idea behind Hashcash is to replace a monetary postage fee with a computational postage fee. In order to send an email, the sender first has to solve a math problem. Legitimate activities suffer an indiscernible delay, but illegitimate activities that require massive volume are hobbled. MORE