Re: [TLS] Distinguishing between external/resumption PSKs

Nikos Mavrogiannopoulos <nmav@redhat.com> Fri, 20 September 2019 11:32 UTC

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From: Nikos Mavrogiannopoulos <nmav@redhat.com>
Date: Fri, 20 Sep 2019 13:32:03 +0200
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To: Nico Williams <nico@cryptonector.com>
Cc: Richard Barnes <rlb@ipv.sx>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Distinguishing between external/resumption PSKs
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On Thu, Sep 19, 2019 at 11:49 PM Nico Williams <nico@cryptonector.com>; wrote:
>
> On Thu, Sep 19, 2019 at 04:57:17PM -0400, Richard Barnes wrote:
> > I don't think anyone's asking for these cases to be differentiable on the
> > wire.  The question is whether the *server* can differentiate, in
> > particular, the application running on the server.
>
> And the answer to that one is "yes", because the server has control over
> the PSK IDs.

You are making a lot of implicit assumptions for that. Consider a
"server" consisting of multiple front end implementations all
connected to a DB back-end. Does that "server" has control over PSK
IDs? A server can also be replaced on an upgrade with a different
implementation, or a different version of the implementation, does
that also guarantee that the server still has control over the PSK
IDs? It looks to me like a hard problem to tackle except for few
straightforward cases.

regards,
Nikos