Re: [TLS] Premaster/Master convention

Eric Rescorla <ekr@rtfm.com> Wed, 30 July 2014 16:00 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 30 Jul 2014 09:00:06 -0700
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To: Michael StJohns <msj@nthpermutation.com>
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Subject: Re: [TLS] Premaster/Master convention
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As discussed in the interim we are going to want to roll over the
MS periodically. Life will be a lot easier if it has a uniform structure.

Moreover, just because we aren't using key transport doesn't mean
there aren't other mechanisms with funky shared secrets, e.g.,
PSK.

-Ekr



On Wed, Jul 30, 2014 at 7:56 AM, Michael StJohns <msj@nthpermutation.com>
wrote:

> Given that TLS1.3 only does KeyAgreement, is there still any reason for
> the premaster -> master_secret derivation step?  We do (KA)->premaster and
> then premaster -> master and then master->(session keys).   We could
> probably do (KA)->master->(session keys) where the master secret is now the
> KA shared secret rather than premaster.
>
> 1) Is there any security reason for retaining the extra step given there
> is no longer a KeyTransport mechanism in TLS1.3?
> 2) Are there other *good* - non-security - reasons for retaining the extra
> step?
>
> Mike
>
>
>
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