Re: MTS transparency and anonymity

> the usual trick is to try to arrange things so that those who do upgrade
> their systems will see an immediate benefit - i.e. to put the incentives
> in the right place.  I think this would be true for RP authentication -
> MTAs today see a substantial load from boucing forged mail.

We've found that SMTP call-back return path verification is quite
effective, though it has very nasty behaviour for the victims of joe jobs.
A more lightweight and scalable alternative is needed for Internet-wide
deployment.

Detecting backscatter by inserting a cookie in the return path does have
the right incentives: it can be implemented unilaterally by a site and
should be effective without co-operation from the rest of the Internet
(though we need to study its interoperability in more detail).

If you couple this with a means for recipients to find out whether to
expect a cookie, and to verify one if it is present, then you have a
general return path authentication system. However this idea has a number
of security holes in the area of replay attacks and message modification
attacks, so it is not a panacea. It's also skating very close to the
territory of DomainKeys and IIM so working on it might be a pointless
duplication of effort.

I've written more on this topic elsewhere:
http://www.cus.cam.ac.uk/~fanf2/hermes/doc/talks/2005-02-ukuug/
http://www.cus.cam.ac.uk/~fanf2/hermes/doc/antiforgery/cam.txt

Tony.
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