Re: [TLS] Distinguishing between external/resumption PSKs

Jonathan Hoyland <jonathan.hoyland@gmail.com> Thu, 19 September 2019 13:31 UTC

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From: Jonathan Hoyland <jonathan.hoyland@gmail.com>
Date: Thu, 19 Sep 2019 14:31:36 +0100
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To: "Owen Friel (ofriel)" <ofriel@cisco.com>
Cc: Martin Thomson <mt@lowentropy.net>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Distinguishing between external/resumption PSKs
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Hi Owen,

If I understand your question correctly the distinguishing is done
implicitly (not explicitly) through the key schedule.
If the client and server do not agree on whether the PSK is a resumption or
an OOB PSK then the `binder_key` will not match, and the handshake will
fail.

See pp. 93-94 of the spec.

Does that answer your question?

Regards,

Jonathan

On Thu, 19 Sep 2019 at 11:52, Owen Friel (ofriel) <ofriel@cisco.com>; wrote:

>
> > -----Original Message-----
> > From: TLS <tls-bounces@ietf.org>; On Behalf Of Martin Thomson
> > Sent: 04 September 2019 02:46
> > To: tls@ietf.org
> > Subject: Re: [TLS] Binder key labels for imported PSKs
> >
> >
> > When we built the ext/res distinction, there was a clear problem
> expressed.
> > We had the potential for both to be used by the same servers at the same
> > time (though not for the same connection) and distinguishing between them
> > was important
>
> Martin, maybe I am missing something in the threads on this. Is there
> anything explicit planned in ClientHello PreSharedKeyExtension or
> PskKeyExchangeModes to explicitly distinguish between ext/res PSKs? Or is
> it up to server implementation and how the server handles the opaque
> PskIdentity.identity? e.g. ImportedIdentity.external_identity fields could
> be stored in one DB table, and (ignoring
> https://tools.ietf.org/html/draft-ietf-tls-external-psk-importer-00#section-9
> for now) the server on receipt of a ClientHello searches for
> PskIdentity.identity in its ImportedIdentity.external_identity  table and
> if that lookup fails, then try to parse PskIdentity.identity  as a
> NewSessionTicket.ticket? And the order of those two operations is of course
> implementation specific too.
>
>
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