Re: [TLS] draft-ietf-tls-dnssec-chain-extensions security considerations

Paul Wouters <paul@nohats.ca> Thu, 05 July 2018 03:59 UTC

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Date: Wed, 4 Jul 2018 23:59:40 -0400 (EDT)
From: Paul Wouters <paul@nohats.ca>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] draft-ietf-tls-dnssec-chain-extensions security considerations
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On Wed, 4 Jul 2018, Eric Rescorla wrote:

> In any case, as Martin Thomson says, we have a perfectly good
> extension mechanism which can be used to add pinning later without creating any placeholder here.

The IETF should not publish security protocols that are trivially downgraded.

The work _should_ really be completed within this document to address
all downgrade attacks. As a _compromise_ there have been suggestions
done to let this document be published while commiting to a method for
downgrade protection. That was not an invitation to punt it to infinity
or an invitation to have a bis replace this entire document just to be
ignored because some people only care about the DOH case. Besides, we
would end up with identical specs apart from two bytes and the same
discussion about these two bytes would happen about whether the new
document Obsoletes: this one. It won't resolve the issue at all and
meanwhile some people have implemented and deployed a protocol that can
be trivially downgraded, or bothered to just not implement because it
offers no real security benefit.

If you don't like TLS extension pinning, come up with another mechanism
that resolves the downgrade attack. But note that none of the comments
against using two bytes for hour scale extension pinning withstood
scrutiny and half of them were appeals to authority or based on people's
gut feelings.

So far, I've only heard incorrect comparisons with other pinning mechanism
problems that ignore the fact that the source of this extension material
is in DNSSEC and that the extension data is a "live snapshot" of active
DNSSEC data and thus never stale/old/bad/brokeb.

> That would first require there being consensus to do pinning.

Again, why do you think it would be even valid to have consensus to
deploy a trivially downgradable security mechanism? I would certainly
expect the IESG to send this back to the drawing board, even if they
missed it on their first evaluation round.

Paul