hashcash spam prevention & firewalls

[phelix at vallnet.com]

On 22 Dec 1997 14:15:31 -0600, Adam Back <aba at dcs.ex.ac.uk> wrote:

>The more thorny problem to solve is that posed by Robert Costner: what
>do you do about nyms.  You (the sender) can't include postage for nym
>reply blocks because you don't know (and mustn't know) the remailer
>chain pointed to by the reply block.

Could you throw hashcash into the reply block?  This would, of course, make
reply blocks non-reusable. 

Perhaps all remailers could agree to accept hashcash made out to a generic
name like "remailer".  This way, the sender can just generate X coins
without worrying about which remailers the message would go through.  The
only problem is that the sender would have to know how many coins to
generate.

Actually, if the message never went through the same remailer twice, only
one coin would needed.  Set up the remailers so that they don't strip away
a coin made out to "remailer".  There could be problems with tracking the
message though.


>
>> Mailing lists are still hard, and perhaps best handled by the user's
>> software (or some fancy variant like user-selectable filters at the
>> ISP mailbox.)
>
>I think it's simplest to have the user explicitly allow the mailing
>list.
>
>You could possibly auto detect the pattern of a user subscribing to a
>mailing list at the mail filter level also.
>

Each user will probably have a list of names that he will accept coins for.
Mail sent to a listserver could have a coin made out to
"listserver at foo.bar".  The listserver could, after checking the coin,
simply pass that coin along with the message to everyone subscribed on the
list.  The end user will receive a coin made out to "listserver at foo.bar".
If that name is on his list, the coin will be checked; otherwise the
message is automatically dropped/bounced/whatever.

This way, the listserver is never burdened with generating hashcash; but
those who send messages to the list are (but they only have to generate one
coin for each message).

-- Phelix