Re: [dane] Digest Algorithm Agility discussion

[Mark Andrews <[e-mail]>]

In message <[e-mail]>, Viktor Dukhovni writ
es:
> On Sun, Mar 23, 2014 at 09:00:08PM +0100, Peter Palfrader wrote:
> 
> > On Mon, 24 Mar 2014, Mark Andrews wrote:
> > 
> > > > Site A only publishes SHA1 entries.  Would rather do unauthenticated TL
> S
> > > > than trust SHA1?
> > > 
> > > You left out - report and refuse to send until fixed.
> > 
> > No, that's not what the SMTP draft suggests.  When DANE is not there,
> > then servers just fall back to not authenticating a peer's cert, as they
> > do nowadays.

The SMTP draft says how to securely go from a email domain to a
CERT using DNSSEC and TLSA.  It does not say whether one should use
a non secure connection or not.  That is seperate policy.

> Indeed if one simply considers (again hypothetically) SHA1 to be
> "unusable", then with no "usable" TLSA records, the connection
> would fall back to unauthenticated TLS.

It might or it might not.  That is a seperate policy decision.
 
> To do what Mark suggests, we'd have to treat SHA1 as usable, but
> always fails.  That is new code to make SHA1 never match.  

	One will have something like

		if (!supported(match))
			skip record;


>  And
> still I don't see anyone shooting themselves in the foot with
> self-imposed flag days for a long time after an algorithm becomes
> suspect.
> 
> I sees that, the unstated objection must be a belief that SHA2-256
> will never fail, and thus we're wasting time designing solutions
> to a non-problem.  While I don't believe in eternal unbounded
> progress, and (barring a P=NP revolution) it is likely that at some
> point we'll have algorithms that never need replacement, it is
> perhaps premature to declare mission-complete with SHA2.

I don't assume that it will never be broken.  I also don't think we
need to "if has_alg(a) then {} else {}".
 
> For if we are to take the threat of gradual degradation of our
> confidence in SHA2 seriously, we need usable approaches for phasing
> it out.  Flag days don't look like usable approaches to me.
> 
> -- 
> 	Viktor.
> 
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Mark Andrews, ISC
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