\Proof-of-Work" Proves Not to Work

Abstract

A frequently proposed method of reducing unsolicited bulk email (\spam) is for senders to pay for each email they send. Proof-of- work schemes avoid charging real money by requiring senders to demon- strate that they have expended processing time in solving a crypto- graphic puzzle. We attempt to determine how di-cult that puzzle should be so as to be efiective in preventing spam. We analyse this both from an economic perspective, \how can we stop it being cost-efiective to send spam, and from a security perspective, \spammers can access insecure end-user machines and will steal processing cycles to solve puzzles. Both analyses lead to similar values of puzzle di-culty. Unfortunately, real- world data from a large ISP shows that these di-culty levels would mean that signiflcant numbers of senders of legitimate email would be unable to continue their current levels of activity. We conclude that proof-of-work will not be a solution to the problem of spam.