Re: Re: [spf-help] SRS for Exim

On Wed, 28 Apr 2004, Stuart D. Gathman wrote:
>
> In fact, always rewriting is often called SES - and if receivers
> implement CBV, you don't need SPF.

You should distinguish between routine sender rewriting and only signing
the sender address at the source. Both of these are enough to detect
forgeries without assistance from SPF. However, SPF+SRS implements a
hop-by-hop security chain which is inherently weaker than the end-to-end
model.

It would be good to add a mechanism to SPF that allows a domain to state
that it signs its sender addresses; these addresses should not then be
rewritten when forwarded and should be verified by the recipient using
callouts or perhaps public key crypto (if that can be made to work).

> However, SPF is better because TCP connections are comparatively
> expensive.  With only SES, anyone could easily DOS your system by
> propagating lots of spam with your return address. Millions of recipient
> systems calling your system to check whether the sender is valid is not
> something you want to suffer through.

A nice way to solve this problem would be to use asymmetric cryptography
to sign sender addresses and publish the public key in the DNS.
Unfortunately there's only space for about 100-200 bits of signature in an
email address, which is too small for most asymmetric cryptosystems.

-- 
Tony Finch  <[email protected]>  http://dotat.at/