Robert Costner
At 12:59 AM 12/13/97 GMT, Adam Back wrote:
(Hashcash is a way of proving that the sender has consumed a tunable amount of CPU time. The verification process consumes negligible CPU time. This allows us to require the would be spammer to spend say 20 seconds per mail, which will slow him down considerably, over his current tactics of 1000 long Bcc lists allowing him to hand off spamming tasks to mail servers.)
So a remailer, such as Cracker, that might send out an average of 3,000 emails per day would be required to use up 3,000 * 20 secs = 17 hours of cpu time per day. Since a portion of these emails are to multiple recipients, then let's add 1/3 extra hashcash CPU time, or a total of about 22 hours of CPU time per day.
Remailers require a different strategy. With remailers you are trying to discourage spammers from using the remailer, with email you are also trying to discourage spammers, but you have to do it in ways which is easy for neophytes to cope with. With remailers people already have to get new software, and all of the source is available, so it is relatively easy for us to add requirements for hashcash postage into type I remailers, in to mixmaster, and newnym. Hashcash was originally designed for this. There is an implementation on http://www.dcs.ex.ac.uk/~aba/hashcash/ Also Andy Dustman, who is the guy who administers the efga remailer (right?) wrote a python interface to hashcash, and is keen on python hacking. I suspect efga remailer may even be written in python if Andy had much to do with it :-) So with remailers you insist that the sender generate the hashcash with all of their remailer requests, you bounce it or silently trash it if there is no valid hashcash. Also I would highly recommend that you use Ian Goldbergs exit man script for remailer delivery -- it delivers via HotMail and other such free web based email systems, via publically accessible proxies and ensures that your remailer address doesn't appear in the headers where you are the exit remailer.
Of course the Cracker mail system also runs various mail lists for EFGA.
Mailing lists you have to realistically I think not use hashcash for. The majordomo process would become overloaded if it had to generate hashcash for each recipient of each message. Similarly the poster would be over loaded if he had to generate hashcash for each recipient of the message. (I would not like to have to generate 1000 (or whatever the current readership is ) of 20 second hashcash stamps to post this message to cypherpunks for instance). The way to cope with this in hashcash filters is to exempt mailing lists from hascash requirement via a user administered allow list. (You could possibly auto detect mailing lists at the hashcash filter agent -- the subscription process could be detected -- or more risky in that spammers might work out a way to abuse this -- by recognising the pattern of mails -- lots of mails with the same Sender, and different From fields.
Of course, since we might be able to delete the need for hashcash among people who know each other, we could have Cracker build a database of people who like to have privacy
I tend to argue against this include list -- remailers become less useful if you can not send mail to anyone who can receive SMTP mail. I prefer the exit man via HotMail approach to the send notification that there is anonymous email ready to be picked up approach.
ISP's in general could handle the hashcash generation at the SMTP level by keeping databases of who sends email to whom.
Yes. This can work reasonably well because people do not reply to spammers, and most email has a to and fro pattern.
It is of course far easier to do a single database lookup than to generate the 20 seconds of hashcash.
I don't know. I just don't understand the plan fully. I'll have to think about it some more.
Read the stuff on http://www.dcs.ex.ac.uk/~aba/hashcash/ it talks more about remailers. Adam