From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <66da6c9b445553becc9e3195ef487bb0@proxima.alt.za>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] re: spam filtering fs
From: lucio@proxima.alt.za
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Date: Wed,  3 Sep 2003 11:13:59 +0200
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On Wed, Sep 03, 2003 at 10:24:54AM +0200, Fco.J.Ballesteros wrote:
>
> > not frightened off getting a certificate.  And some form of recourse
> > in the event of someone stealing the e-mail address, and that's the
> > hard part, sadly.
>
> Not just the sad, but also the common part. Most of the spam I get
> seems to use addresses from someone else.
>
> I'm afraid that certifying the from address would not work.
> I hope bayes is right.

This is the scenario I think would work:

My mail exchanger accepts mail that is "certified" and for which it
has the certificate public key.  Certified mail contains either a
signature in the body as with PGP or a header of some description,
encrypted with the sender's private key so it can be decrypted and
validated.  A preferable form of encryption would be at the SMTP
protocol level, but this is a different model.

The message may convey the public key in the headers as suggested by
the Privacy Enhanced Mail (PEM) RFCs, but then there has to be a CA in
the certificate hierarchy that validates the trust.

If trust cannot be validated, I suggest that a group of public
certificate servers, probably including the existing PGP public key
servers, should be queried for the certificate/public key.  If the
certification cannot be established in this fashion, then the
difficult procedure comes into action.

Here we expect the exchanger to submit a request to a preferred public
certificate server that causes the sender to be polled.  If the sender
replies with a valid certificate (or public key), it is stored in the
public server and forwarded to the exchanger, if not, then within some
time limit the exchanger is notified.

I hope I didn't abbreviate the above beyond usefulness, I'll be happy
to expand if I haven't been clear in any way.  And I will of course be
interested in flaws as well as improvements.

++L

PS: At the SMTP level, I would suggest an exchange between servers
that has contractual value.  In other words, the sending exchanger
ought to accept legal liability for mail it insists in forwarding.
Legislation to this effect would have to be enacted, naturally.