> My main concern with Adam Back's scheme (the only one I've seen mentioned)
> is that it's purely a cost to the sender, there's no benefit to the
> recipient. Ideally it should be a transfer of value, but I can't think of
> too many calculations that the average person would care much about.
Adam's hashcash scheme is described at http://www.dcs.ex.ac.uk/~aba/hashcash/. An academic paper with a similar (but more complex) idea is available from: http://www.wisdom.weizmann.ac.il/Papers/CSreports/reps95/95-20.ps.Z. That paper concludes with a comment like yours:
: Finally, the evaluation of the pricing function serves no useful purpose, : except serving as a deterrent. It would be exciting to come up with a : scheme in which evaluating the pricing function serves some additional : purpose.
There is a problem, though, if the costly function evaluation actually produces something valuable. If it would be valuable to anyone (say, some kind of useful simulation program output) then the spammers could sell the output to make up for the cost of the computers to produce it, and the bulk mail would no longer be costly to them.
If it were some kind of digital cash, then it requires encryption of the data, which means that everyone has to know everyone else's keys. This requires a large public key infrastructure and an electronic payment infrastructure, and so it becomes much more complicated.
Performing a wasteful calculation may seem unaesthetic, but there is actually considerable precedent. Robin Hanson recommened a book last summer called "The Handicap Principle, A Missing Piece of Darwin's Puzzle", by Amotz Zahavi and Avishag Zahavi. This describes applications in biology of this same phenomenon, costly and wasteful expenditures of effort in order to send a signal of quality and fitness. Apparently the same phenomenon has been observed in many areas of economics as well. It may seem counter-intuitive, but sometimes being intentionally wasteful is the best way to communicate.