Re: [TLS] draft-dkg-tls-reject-static-dh

Peter Gutmann <pgut001@cs.auckland.ac.nz> Fri, 07 December 2018 07:14 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Nico Williams <nico@cryptonector.com>
CC: Daniel Kahn Gillmor <dkg@fifthhorseman.net>, Stephen Farrell <stephen.farrell@cs.tcd.ie>, "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] draft-dkg-tls-reject-static-dh
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Date: Fri, 7 Dec 2018 07:14:17 +0000
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Subject: Re: [TLS] draft-dkg-tls-reject-static-dh
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Nico Williams <nico@cryptonector.com>; writes:

>If it's different then that's costing the server the resources to generate
>it, which is precisely what its operator didn't want when they enabled eDH
>key reuse.

It depends on what those resources are, at one end you've got proper DHE with
a full modexp required, at the other end if you can fake it with something as
lightweight as a mod-add or similar it's essentially free while defeating DHE-
reuse detection.

I appreciate that people feel strongly about this, and I support the idea of
non-ephemeral DHE detection in principal [0] (along with many, many other
measures to strengthen TLS), but this draft reads a lot like the IETF blowing
raspberries at ETSI.  

Some years ago a draft was rejected by, of all places, PKIX, for being
"workgroup posturing", and that's what this seems to be.  The IETF could make
its point by releasing a statement saying they don't support what ETSI is
doing, but getting into an arms race you know you can't win seems like, well,
workgroup posturing.

Peter.

[0] "In principal" because there's a fair bit of SCADA gear that does this
    because it doesn't have the CPU power to generate new DHE values, as I 
    found out when I turned on non-DHE checking some years ago.