Re: [dane] Digest Algorithm Agility discussion

Mark Andrews <marka@isc.org> Sun, 23 March 2014 19:26 UTC

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To: Peter Palfrader <peter@palfrader.org>
From: Mark Andrews <marka@isc.org>
References: <20140315051704.GY21390@mournblade.imrryr.org> <0l4n2sa5a0.fsf@wjh.hardakers.net> <20140322074737.GA5739@anguilla.noreply.org> <20140323174205.63C6111B2111@rock.dv.isc.org> <20140323182106.GX24183@mournblade.imrryr.org> <20140323185718.7A84711B2CB8@rock.dv.isc.org> <20140323191037.GA1469@anguilla.noreply.org>
In-reply-to: Your message of "Sun, 23 Mar 2014 20:10:37 +0100." <20140323191037.GA1469@anguilla.noreply.org>
Date: Mon, 24 Mar 2014 06:25:57 +1100
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Cc: dane@ietf.org
Subject: Re: [dane] Digest Algorithm Agility discussion
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In message <20140323191037.GA1469@anguilla.noreply.org>;, Peter Palfrader writes
:
> On Mon, 24 Mar 2014, Mark Andrews wrote:
> 
> > If you don't trust a algorithm you should not be using it.  Period.
> > This fall back to this untrusted/broken algorithm is bad engingeering
> > and bad security practice.
> > 
> > If the site you want to email only has broken TLSA records, get
> > them on the phone to fix the problem.
> 
> Assume we may have reason to believe that SHA1 is within reach of well
> funded adversaries, and assume it had a code-point in DANE.
> 
> Site A only publishes SHA1 entries.  Would rather do unauthenticated TLS
> than trust SHA1?

You left out - report and refuse to send until fixed.
 
> Site B publishes both SHA2-512 and SHA1 entries.  Would you still want
> to trust SHA1?

Once you decide SHA1 is not acceptable you ignore the records with SHA1
hashes.

Publishing new hashes is trivial and will remain trivial.

Once a algorithm has reached the state where you don't trust it for a
purpose you don't use it for thar purpose.

Mark
-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka@isc.org