TMDA Re: [Gllug] New worm doing the rounds?

David Damerell damerell at
Wed Feb 18 12:08:58 UTC 2004

On Wednesday, 18 Feb 2004, Doug Winter wrote:
>You may think it's perfectly reasonable for DNS ownser to control which
>servers can send email when their addresses are in their domains - it's
>not an unreasonable position.  However, it does change one of the basic
>freedoms that we have, to send email from any address we choose.

We can send email from any address we choose whether or not we have
SPF. The recipient may reject it on the basis of the originating IP
address whether or not we have SPF. What's different?

I don't like SPF very much, but this is bunk.

I would propose a scheme with public keys in the DNS. An organisation
can publish a key for all users, or individual users'
keys which take precedence. Some property of the message not
ordinarily mangled in transmission [1] is signed. Roaming users
publish their own keys [2] and can send mail from anywhere; the
organisation's private key is squirreled away on their main
MTAs. Computation is kept to a minimum by using short key lengths and
changing them periodically; sure, the bad guys can crack a short key,
but not in a week.

[The previous key has to stay available for about a week, to handle
delays in transit. Computation could also be reduced if a receiving
MTA, after getting _n_ correctly signed messages from a given IP
address and key, only periodically checks in future until a
non-matching message shows up.]

[1] What if your mail system does mangle it? Then you can't use this
scheme until you fix your mail system. Tough.

[2] Not really, obviously; the organisation issues them a fresh
keypair when necessary - in some automated fashion, I hope.

David Damerell <damerell at> Kill the tomato!
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