Re: [TLS] Requiring that (EC)DHE public values be fresh

On Thu, Dec 29, 2016 at 10:15 AM, Stephen Farrell <stephen.farrell@cs.tcd.ie
> wrote:

>
> Hiya,
>
> On 29/12/16 17:37, Adam Langley wrote:
> > https://github.com/tlswg/tls13-spec/pull/840 is a pull request that
> > specifies that (EC)DH values must be fresh for both parties in TLS
> > 1.3.
> >
> > For clients, this is standard practice (as far as I'm aware) so should
> > make no difference. For servers, this is not always the case:
> >
> > Springall, Durumeric & Halderman note[1] that with TLS 1.2:
> >   ∙ 4.4% of the Alexa Top 1M reuse DHE values and 1.3% do so for more
> >     than a day.
> >   ∙ 14.4% of the Top 1M reuse ECDHE values, 3.4% for more than a day.
> ...
>
> As an individual, I'd be in favour of this change but reading
> over [1], section 5, I wondered if we'd analysed the effects of
> 0rtt/replayable-data with that kind of cross-domain re-use in mind?
> The situation being where session ID based caches or session ticket
> equivalents in tls1.3 are shared over multiple domains.
>
> I don't recall this being explicitly considered, but maybe that's
> just me forgetting. And hopefully the analysis is that such re-use
> doesn't enable broader replay of early data, but there may be
> something worth a mention in the tls1.3 spec, e.g. that there may
> be linkages between the duration for which entries are maintained
> in resumption and replay detection caches.
>

This question seems essentially orthogonal to the question of ECDHE key
reuse
because even if you use the same ECDHE key in perpetuity you get unique
traffic keying material for each connection.

-Ekr

> Cheers,
> S.
>
> >
> > [1] “Measuring the Security Harm of TLS Crypto Shortcuts”, IMC 2016,
> > pages 33–47, section 4.4. https://dl.acm.org/citation.cfm?id=2987480
> > [2] https://datatracker.ietf.org/doc/draft-green-tls-static-dh-in-tls13/
> >
> >
> > Cheers
> >
> > AGL
> >
>
>
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