Re: [TLS] RFC 7919 on Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)

Geoffrey Keating <geoffk@geoffk.org> Fri, 19 August 2016 18:35 UTC

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To: Peter Gutmann <pgut001@cs.auckland.ac.nz>
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From: Geoffrey Keating <geoffk@geoffk.org>
Date: 19 Aug 2016 11:35:45 -0700
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Subject: Re: [TLS] RFC 7919 on Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)
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Peter Gutmann <pgut001@cs.auckland.ac.nz> writes:

> The problem is that 7919 doesn't say "I want to do DHE, if possible
> with these parameters", it says "I will only accept DHE if you use
> these parameters, otherwise you cannot use DHE but must drop back to
> RSA".  Talk about cutting off your nose to spite your face, you'd
> have to have rocks in your head to want to break your implementation
> like that.

Actually, my problem with the RFC is the reverse.  If you have an
implementation which won't accept certain DH groups, and those are
extremely common in legacy software (1024-bit, for example), there's
no way to stop those legacy implementations ignoring the extension and
choosing DH, which you will then have to reject, doubling connection
establishment time.

So Apple's client still has DH off.

However I don't see a helpful solution to this; the obvious thing is
to have a new ciphersuite, but it's hard to see how that would be
worth the effort if it requires new implementations and ECDHE is
already standardised and already implemented in many places.