Re: hashcash spam prevention & firewalls

[Adam Back <aba@dcs.ex.ac.uk>]

Phelix <phelix@vallnet.com> writes:
> 
> >The Right Way to do it perhaps is as an SMTP extension, however I
> >consider this impractical in the short term because as far as I know
> >there is no SMTP extension plug-in mechanism [...]
> 
> How does PGP do it with their policy enforcer?

Their controversial GAK ready policy enforcer works as a a local SMTP
proxy.  You set up your normal SMTP server on another machine, and
configure the policy enforcer to forward mails to the normal SMTP
server.  Or presumably if you can configure your SMTP server to accept
connections on a port other than 25 you could have the SMTP enforcer
on the same machine.

The same thing could work for a hashcash filter, and it is one
feasible solution.

> >Secondly the proxy approach prevents some of the SMTP server functions
> >from operating properly because the process on the other end of the
> >socket is our hashcash proxy on localhost rather than the remote mail
> >hub (modern sendmails can be configured to perform reverse name
> >lookups on IP addresses, call ident (ident sucks anyway IMO), or block
> >based on IP address or domain, etc.)  Is this kind of thing likely to
> >be a big problem?
> 
> I don't see why.  Just have the proxy work both ways.  Isn't it possible
> for the proxy to keep track of which message came from which address and
> relay server requests back to the right user?  (I'm not familiary with how
> sendmail works, so I'm probably missing something)

The functionality which is lost is the possibility for the sendmail to
be configured to do a reverse DNS lookup on the IP address which is
connecting to it, and check that that domain is the same as the domain
in the From field.  Also ident lookups don't work anymore either.
However I am not sure how reliable either of these mechanisms are, and
I'm not sure that many people are using them because they would be
unlikely to be reliable in the general case.

So probably this is not a problem.

> >This still leaves open the question of the user generating their own
> >hashcash postage.  Again this could be problematic for neophytes.  One
> >solution is to include a URL for a web page including a javascript
> >hashcash generator -- this means that no new software must be
> >installed, the user cut and pastes the generated hashcash into their
> >message.
> 
> How many of the popular email pacakges have support for plug-ins?
> Netscape Communicator is the only package (that neophytes will use) I know
> of that doesn't support email plugins.  Perhaps in this case a small proxy
> could be installed on the user's machine.  The only thing it would have to
> do is generate hashcash for outgoing messages.  

A local SMTP proxy to add hashcash on the way out should work fine.

> >require valid hashcash on all mail, _until_ the
> >recipient replies to a mail.  This is good because people rarely reply
> >to spammers.
> >
> >Then you add some hashcash accounting so that users who overspend
> >(consuming more than say 1 minutes CPU consumption on the server in a
> >24 hour period have their email bounced with explanation of how to
> >generate their own hashcash as a heavy user).
> 
> What's the difference between this and simply keeping track of how many
> messages each user sends in a 24 hour period and blocking people who are
> obviously spamming?

The difference is that it allows the ISP which is hit by spam attacks
to install a hashcash filter.

With simple resource metering at the spammers ISP side, the spammers
typically abuse other peoples open SMTP agents to forward their spams
for them.

Their ISPs can do little about this.

Really it's best if the sender is left to generate his own hashcash,
the motivation for working out ways to have the originators ISP's
outgoing SMTP hub generate hashcash for them is that it is simpler to
install in the short term.

Adam
-- 
Now officially an EAR violation...
Have *you* exported RSA today? --> http://www.dcs.ex.ac.uk/~aba/rsa/

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