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Re: [TLS] on sharing PSKs between TLS 1.2 and TLS 1.3

Eric Rescorla <ekr@rtfm.com> Fri, 20 July 2018 11:06 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 20 Jul 2018 04:05:30 -0700
Message-ID: <CABcZeBOkfaMTVa9G6-A=BRKfzqu628wVPHGEFEHbC9LfaTQWow@mail.gmail.com>
To: Ilari Liusvaara <ilariliusvaara@welho.com>
Cc: Matt Caswell <matt@openssl.org>, "<tls@ietf.org>" <tls@ietf.org>
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Subject: Re: [TLS] on sharing PSKs between TLS 1.2 and TLS 1.3
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On Fri, Jul 20, 2018 at 3:43 AM, Ilari Liusvaara <ilariliusvaara@welho.com>
wrote:

> On Fri, Jul 20, 2018 at 10:43:48AM +0100, Matt Caswell wrote:
> >
> >
> > On 20/07/18 10:38, Eric Rescorla wrote:
> > > The issue is not cross-protocol attacks; it's the reuse of PSKs with
> > > different KDFs, which we don't have any analysis for and which the TLS
> > > 1.3 document prohibits.
> >
> > Can you supply the reference for that prohibition?
> >
>
> Section 'Pre-Shared Key Extension'. In practicular, the paragraph
> starting "Each PSK is associated with a single Hash algorithm." That
> one prohibits using PSK associated with a hash with another hash.
> However, I did not find prohibition of using PSKs from prior versions
> of TLS using SHA-256 as the associated hash (as that is the default).
>

Right. There's no such text. However, the TLS 1.2 SHA-256 KDF isn't HKDF
(and of course the TLS 1.0 KDF isn't even SHA-256), so we're kind of in an
unclear area.

-Ekr


>
> -Ilari
>
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