Re: [DNSOP] Robert Wilton's No Objection on draft-ietf-dnsop-dns-zone-digest-12: (with COMMENT)

Donald Eastlake <d3e3e3@gmail.com> Sat, 10 October 2020 03:10 UTC

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From: Donald Eastlake <d3e3e3@gmail.com>
Date: Fri, 9 Oct 2020 23:10:14 -0400
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To: "Rob Wilton (rwilton)" <rwilton@cisco.com>
Cc: The IESG <iesg@ietf.org>, "draft-ietf-dnsop-dns-zone-digest@ietf.org" <draft-ietf-dnsop-dns-zone-digest@ietf.org>, Tim Wicinski <tjw.ietf@gmail.com>, "<dnsop@ietf.org>" <dnsop@ietf.org>, "dnsop-chairs@ietf.org" <dnsop-chairs@ietf.org>, Roman Danyliw <rdd@cert.org>
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Subject: Re: [DNSOP] Robert Wilton's No Objection on draft-ietf-dnsop-dns-zone-digest-12: (with COMMENT)
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Hi Rob,

I'm not aware of any precise analysis supporting the 12 byte minimum
size but I believe it is reasonable and in line with the lower end of
the range of hash sizes typically standardized by the IETF these days.

Thanks,
Donald
===============================
 Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
 2386 Panoramic Circle, Apopka, FL 32703 USA
 d3e3e3@gmail.com

On Fri, Oct 9, 2020 at 5:23 AM Rob Wilton (rwilton) <rwilton@cisco.com> wrote:
>
> Hi Donald,
>
> > -----Original Message-----
> > From: Donald Eastlake <d3e3e3@gmail.com>
> > Sent: 09 October 2020 00:47
> > To: Rob Wilton (rwilton) <rwilton@cisco.com>
> > Cc: The IESG <iesg@ietf.org>rg>; draft-ietf-dnsop-dns-zone-digest@ietf.org;
> > Tim Wicinski <tjw.ietf@gmail.com>om>; <dnsop@ietf.org> <dnsop@ietf.org>rg>;
> > dnsop-chairs@ietf.org
> > Subject: Re: [DNSOP] Robert Wilton's No Objection on draft-ietf-dnsop-dns-
> > zone-digest-12: (with COMMENT)
> >
> > On Thu, Oct 8, 2020 at 7:18 AM Robert Wilton via Datatracker
> > <noreply@ietf.org> wrote:
> > > Robert Wilton has entered the following ballot position for
> > > draft-ietf-dnsop-dns-zone-digest-12: No Objection
> > >
> > > ...
> > >
> > > ----------------------------------------------------------------------
> > > COMMENT:
> > > ----------------------------------------------------------------------
> > >
> > > ...
> > >
> > >     2.2.4.  The Digest Field
> > >
> > >        The Digest field MUST NOT be shorter than 12 octets.  Digests for
> > the
> > >        SHA384 and SHA512 hash algorithms specified herein are never
> > >        truncated.  Digests for future hash algorithms MAY be truncated,
> > but
> > >        MUST NOT be truncated to a length that results in less than 96-
> > bits
> > >        (12 octets) of equivalent strength.
> > >
> > > When I read this, I wonder why the limit of 12 bytes was chosen.
> > Possibly a
> > > sentence that justifies why this value was chosen might be useful,
> > noting that
> > > the two suggested algorithms have significantly longer digests.
> >
> > To me, the purpose of the limit is to establish a minimum strength
> > against brute force attacks. Of course, the hash algorithm also has to
> > be strong but the length of the Digest field puts a sharp limit on the
> > strength of a ZONEMD.
> [RW]
>
> I absolutely agree on specifying a minimum value.  My question is how was the minimum length of "12 bytes" chosen?  Is there some analysis performed that indicates that this is the right minimal value, or is this just a "12 bytes sounds like enough"?
>
> Regards,
> Rob
>
>
> >
> > Note that for the same reason there is a similar provision from 2006
> > in RFC 4635, Section 3.1, point 4, which sets a minimum size of 10
> > bytes for the hashes that appear in TSIG RRs.
> >
> > Thanks,
> > Donald
> > ===============================
> >  Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
> >  2386 Panoramic Circle, Apopka, FL 32703 USA
> >  d3e3e3@gmail.com
> >
> > >     ...
> > >
> > > Regards,
> > > Rob