25 March 2004, 16:23 UTCA new sort of HashCash?
One of the many schemes that have been proposed to fight junk e-mail is
HashCash.
The basic idea is that a HashCash token is a string which a particular property that make is hard to create, but easy to check if that property is present.
The way you use it is to require that anyone who sends you Email, and who isn't on your whitelist, must include a new HashCash token in the mail item. This proves that they have done a measurable amount of work for the privilege of sending you mail.
The value of this is that spammers, who tend to need to send tens of thousands of emails just to get a few responses, would not be able to afford to generate these tokens and so wouldn't be able to get their mail through to you. On the other hand, individually who want to send you one mail (and hopefully then get onto your whitelist) would have no problem with the cost of generating a hashcash token for you.
One problem with the scheme as proposed is that you need to keep a list of all hashcash tokens that you have received so that a spammer cannot generate just one and use it over and over again.
My idea is to overcome that problem by making hashcash tokens safely reusable.
