Re: [TLS] A la carte handshake negotiation

Martin Thomson <martin.thomson@gmail.com> Wed, 22 July 2015 09:16 UTC

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Date: Wed, 22 Jul 2015 02:16:43 -0700
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From: Martin Thomson <martin.thomson@gmail.com>
To: Kyle Rose <krose@krose.org>
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Subject: Re: [TLS] A la carte handshake negotiation
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On 22 July 2015 at 01:50, Kyle Rose <krose@krose.org>; wrote:
> I'd like to see the bits of the cipher suite associated entirely with
> ephemeral data tied together roughly by security margin

I've seen this argument several times, but there are reasons why you
might want a non-homogenous strength profile.

The argument for consistency is appealing, but given that the design
of TLS is historically[1] vulnerable to the weakest *supported*
algorithm as opposed to the weakest *used* algorithm, I am not
concerned about ensuring consistency.

[1] ... and likely in future, despite our best efforts

> The one thing I'm having trouble pinning down is PSK. I fear it's not
> a separate dimension, because it replaces both signature and KEX.

Yes, this is a problem.  I like to think of PSK as KEX with null auth.