Hashcash - Advantages and Disadvantages

Advantages and Disadvantages

The Hashcash system has the advantage over micropayment proposals applying to legitimate email that no real money is involved. Neither the sender nor recipient need pay, thus the administrative issues involved with all micropayment systems are entirely avoided.

On the other hand, as Hashcash requires significant computational resources to be expended on each e-mail being sent, it is impractical to use with low-end embedded systems without the help of an external server.

Hashcash is also fairly simple to implement in mail user agents and spam filters. No central server is needed. Hashcash can be incrementally deployed—the extra Hashcash header is ignored when it is received by mail clients that do not understand it.

Some plausible estimates come to the conclusion that you can only have one of these: Either good e-mail will get stuck due to lack of processing power of the sender, or bad e-mail is bound to still get through. The reasons for this are botnets or cluster farms with which spammers can increase their processing power enormously, or centralized e-mail-topologies like mailing lists, in which some server is to send an enormous amount of legitimate e-mails.

Most of these issues may be addressed. E.g., botnets may expire faster because users notice the high CPU load and take counter-measures, and mailing list servers can be registered in white lists on the subscribers' hosts and thus be relieved from the hashcash challenges. But they represent serious obstacles to hashcash deployment that need to be addressed somehow.

Another projected problem is that computers continue to get faster according to Moore's law. So the difficulty of the calculations required must be increased over time. However, developing countries can be expected to use older hardware, which means that they will find it increasingly difficult to participate in the email system. This also applies to lower-income individuals in developed countries who cannot afford the latest hardware.