Re: Microsoft takes over British Telecom

On 28/10/2011 19:28, Douglas Otis wrote:
> There are many methods that might be used to authenticate outbound 
> MTAs, such as SMTP Auth.  Ideally SMTP would include DANE extensions 
> used in conjunction with Kerberos.  If it were not for DKIM ignoring 
> prepended headers, it would have merit as an anti-phishing strategy.  
> Since DKIM does not verify who sent what to whom, it can only identify 
> domains considered "too big to block" as well.

Excuse me for being thick, I'm trying to understand your thoughts.

I can sort of see how you could AUTHENTICATE outbound MTAs, however 
that's not the problem as far as I can see. You can reliably 
authenticate an outbound MTA based on its IP address at the moment 
(although that would probably be less useful than authenticating a 
'group' of MTAs based on the owner, eg Yahoo, and will be less useful 
with IPv6). Some of the technicalities of authentication based on 
cryptographic means are a bit unclear, but I can see that it could 
probably be made to work.

However, once you have authenticated the MTA, you then need to decide 
whether to authorise it,and what to authorise it for. Are you talking 
about just authorising it to be able to send mail from a particular 
domain? (If so, I'm not sure how this fixes the 'forwarding problem'), 
or authorising it be able to send mail to me at all? (in which case, who 
would decide on that authorisation?)

(AFAICS Something like SRS should probably be able to fix the forwarding 
problem if the rest of the system is setup correctly)

You say "Ideally SMTP would include DANE extensions used in conjunction 
with Kerberos" - now, I'm not very familiar with DANE or Kerberos at the 
moment, but I think DANE lets you associate a certificate with a domain 
name using DNS - yes?. Since it wouldn't be possible for a sending MTA 
to authenticate using such a certificate based on the sender's email 
domain, I presume you mean it will use the certificate based on the 
MTA's reverse DNS entry? So, how would that link the sending MTA with 
the sender's email address?
And would Kerberos need a separate server? If so, where would that be, 
the sending or receiving end? (I'm not sure I like the idea of having to 
use Kerberos)

If you are just going to authenticate based on the MTA's reverse DNS, 
then why not just mandate TLS and authenticate the client certificate 
using DANE?

Could you not change SMTP to go something like:
EHLO sender
220 ENVSIGN CHALLENGE=RandomText

MAIL FROM: <[email protected]>
220 OK
RCPT TO: <[email protected]>
220 OK
RCPT TO:<[email protected]>
220 OK
ENVSIGN: <PKI signature of 
[email protected][email protected][email protected]>
220 OK

The sender signs the envelope data using the private key; the public key 
is exposed using DNS or something (DANE?) . Kerberus isn't needed, and 
the sending MTA can send from multiple different domains in a single 
session.

(Still doesn't solve forwarding without return path rewriting, or the 
general authorisation problem, but authenticates the sender MTA 
effectively, AFAICS)