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Re: [DNSOP] One Chair's comments on draft-wessels-dns-zone-digest

"John Levine" <johnl@taugh.com> Thu, 02 August 2018 19:41 UTC

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From: John Levine <johnl@taugh.com>
To: dnsop@ietf.org
Cc: paul@nohats.ca
In-Reply-To: <alpine.LRH.2.21.1808021512160.1415@bofh.nohats.ca>
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Archived-At: <https://mailarchive.ietf.org/arch/msg/dnsop/tbCSOvrY8tyHESi_K2MPAlsPNXA>
Subject: Re: [DNSOP] One Chair's comments on draft-wessels-dns-zone-digest
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In article <alpine.LRH.2.21.1808021512160.1415@bofh.nohats.ca> you write:
>On Tue, 31 Jul 2018, Matt Larson wrote:
>
>> For all those reasons, I think a checksum in the zone file itself that can be verified with DNSSEC is the best option for this use case, and I like the
>ZONEMD solution.
>
>Note that the checksum in this case must be at least as
>cryptographically strong as the signature algorithm used
>in the individual RRSIGs/DNSKEYs. This would have to be
>enforced by software/RFC to prevent a downgrade attack.

As someone else pointed out, this would be a second-preimage attack.
As far as I know, even the cruddy old hashes like MD5 and SHA-1 aren't
subject to it.  Could you explain in more detail what sort of
downgrade attack you're thinking of?

R's,
John

PS: I have no objection to making a list of hash functions for ZONEMD
that currently only includes SHA-256.  I mean, why not?

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