Re: [TLS] TLS1.3 + PSK with multiple identities

Eric Rescorla <ekr@rtfm.com> Mon, 19 September 2016 21:14 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Mon, 19 Sep 2016 14:13:40 -0700
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To: David Woodhouse <dwmw2@infradead.org>
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Subject: Re: [TLS] TLS1.3 + PSK with multiple identities
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On Mon, Sep 19, 2016 at 1:35 PM, David Woodhouse <dwmw2@infradead.org>;
wrote:

> On Mon, 2016-09-19 at 09:53 -0700, Eric Rescorla wrote:
>
> > > > I would address this either by:
> > > >
> > > > 1. Registering a new extension which is used to indicate the right
> worker
> > > > process, but using existing TLS 1.2 PSK.
> > >
> > > OK... except this basically *is* the PSK identity. So the new extension
> > > we'd want to register, if we want to make it something that's useful in
> > > the general case rather than an application-specific hack, *is*
> > > basically draft-jay-tls-psk-identity-extension :)
> >
> > No, I don't think that's true. That extension also attempts to actually
> > negotiate the keys in that layer. I'm just talking about a hint.
>
> It's more than just a hint. It is saying precisely which client it is,
> which has a 1:1 correspondence with which PSK it's using.
>

Call it a promise, if you prefer. One that you fulfill with the CE.


It is purely a matter of software architecture — the initial incoming
> UDP packets reach a dispatcher that needs to hand the connection off to
> the appropriate worker process for that client.... and *really* wants
> to make that decision based on the ClientHello alone.
>
> If we *start* the handshake in the main dispatcher and get to the point
> of seeing the ClientKeyExchange, we have to hand over the partially-
> completed handshake (or keep going and then hand over a fully-completed
> handshake) to the appropriate worker. And in fact I don't even think
> the dispatcher *has* the actual keys; only the identities so that it
> knows where to dispatch connections to.
>

See above. The key advantage of what I am proposing here is that it has
exactly the same cryptographic properties as current TLS-PSK, with the
indicator just serving as a routing-ID.


>
> So I really do think that draft-jay-tls-psk-identity-extension was
> *exactly* what I wanted. I don't care about *negotiating* the PSK
> identity per se; I'm happy to support only one. It's purely about
> *telling* the server, in the ClientHello, which identity I'm using.
>

See above.

-Ekr