Re: [dane] email canonicalization for SMIMEA owner names

Nico Williams <nico@cryptonector.com> Fri, 12 December 2014 18:38 UTC

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Date: Fri, 12 Dec 2014 12:38:28 -0600
From: Nico Williams <nico@cryptonector.com>
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Subject: Re: [dane] email canonicalization for SMIMEA owner names
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On Fri, Dec 12, 2014 at 05:52:42PM +0000, Viktor Dukhovni wrote:
> If queries are sent to an HTTPS service that is deployed with the
> (ultimate) inbound MTA for "example.com", then X.509 key lookup is
> rather similar to what the MTA already does to validate the inbound
> recipient so as not to be a backscatter source.

Yes.  It has to be HTTPS because that will go through firewalls.

Whereas my chaining through MSAs/MTAs idea is too burdensome on the
MSAs/MTAs.

For verification of sender signing certs this need not reveal anything
about valid local-parts.

For recipient encryption cert lookups... avoiding an oracle for
local-part validity is harder because the service would have to serve a
valid encryption cert (SMIMEA RRs, really) for every query.  For PK
algorithms where it's cheap to fake random public keys this is not a
problem at all.

> The main thing this would have to recommend itself is there is no
> encoding of the localpart into DNS labels, [...]

And thus no canonicalization concerns.

> The oracle can query LDAP, ... can make up fake replies for
> non-existent addresses to thwart directory harvesting attacks
> if desired, ...
> 
> HTTPS, allows the service to be reached from inside corporate
> environments that block most other outbound services (possibly
> including external DNS).  In some environments even HTTPS is subject
> to corporate MiTM (that the users are aware of with the HTTP proxy
> signing certs trusted by browsers, ...).  In such environments
> users don't get end-to-end email encryption, just like they don't
> get end-to-end HTTPS.  Their border email gateway might be able to
> play gateway-to-gateway SMIME with the destination.

Yeah, and then either the client gives up (hey, if it's in a corporate
network that seems fair) or we do the chaining-with-DNSSEC-proof thing I
proposed earlier.

Nico
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