From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <9c3926d1a7517b520d07a0b749aa6d1c@vitanuova.com>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] ISP filtering - update
From: rog@vitanuova.com
In-Reply-To: <46016185506c42950346d712e2f1d96f@plan9.bell-labs.com>
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Date: Fri, 26 Sep 2003 14:01:25 +0100
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none of this works if spammers use dubious means
(e.g. viruses) to harness home machines (and by implication
the authentication info that allows the home user to send emails)
to send their spam for them.

doesn't this already happen?
or is it just for the DDOS attacks?

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-> random1
<- random2, hmac(random1, shared key)
-> hmac(random2, shared key)

This is incredibly weak authenitcation (susceptible to main in the middle)
but works if all you're trying to do is keep out spammers who ar just
injecting messages from their home machines.  They can't authenticate
without knowing the shared key.

The problem is always how many shared keys are there.  You could have
one per pair of communicating individuals but then you need an introduction
protocol.  You could have a trusted third party to automate that
introduction and you've essentially got plan 9's authentication protocol
(we mix with DES rather than a keyed hash but its pretty much the same).

Public keys have the advantage that the amount of cruft kept secret
is small, i.e., your own private key part and nothing else.  You don't
need cert's per se.  Just a place to get public keys that you trust.
That could be a place that just has to be kept secure, not secret.
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