Re: [ANNOUNCE] contrib/hashcash.el spam fighter

[Simon Josefsson <jas@extundo.com>]

Stainless Steel Rat <ratinox@peorth.gweep.net> writes:

> X-Hashcash is intended for sending mail to big mail-to-news gateways,
> mailing list, and such.

Not only, it works in peer 2 peer as well.  Frankly, hashcash has some
problems in big mailing lists (do you mint the coin to the mailing
list or to each member?  The latter probably requires the mailserver
to do it, which is impractical, the former require some additional
intelligence in clients (it knows which email lists it is a member
of)).

> It requires that the recipient -- the gateway or list -- publish a
> challenge phrase and number of collisions bits that are required for
> sending messages through it.  These published challenges take the
> place of the randomly generated numbers -- session keys -- that MTA
> hashcash uses.
>
> The system is not so useful for end users simply because many (most?) have
> no means of publishing challenges or informing legitimate senders that they
> even exist.  The whole thing falls apart if recipients cannot challenge
> senders.

hashcash.el uses the recipients email address as a challenge.  If you
know the recipients email address, you know the hashcash challenge, so
this should work.  A spammer could compute these, but he could only
use it once.