Re: [TLS] 0-RTT and Anti-Replay

Eric Rescorla <ekr@rtfm.com> Mon, 23 March 2015 17:42 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Mon, 23 Mar 2015 12:41:33 -0500
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To: Nico Williams <nico@cryptonector.com>
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Subject: Re: [TLS] 0-RTT and Anti-Replay
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On Mon, Mar 23, 2015 at 12:19 PM, Nico Williams <nico@cryptonector.com>;
wrote:

> On Mon, Mar 23, 2015 at 08:11:59AM -0500, Eric Rescorla wrote:
> > On Mon, Mar 23, 2015 at 3:47 AM, Nico Williams <nico@cryptonector.com>;
> > wrote:
> > > This is an optimization.
> > >
> > > If the server loses its replay state then it must cause every 0-RTT
> > > connection to become a 1-RTT connection.  There's no question of
> "want".
> >
> > Maybe I wasn't clear. When I said "reject" I meant "hard-fail" rather
> than
> > make it 1-RTT.
>
> Sorry it wasn't clear to me.  Is there any reason that fallback to 1-RTT
> is not feasible?
>

Yes. The issue is that the client has no way of knowing whether the 0-RTT
message was duplicated and processed by a separate server instance, so
when it falls back to 1-RTT, resending the initial data may be a replay.

-Ekr



For context, I'm proposing similar fallbacks (but for other reasons) for
> Kerberos (which never had such a fallback).  This has proven desirable
> in the context of Kerberos, and so I suspect it will be for TLS.
>
> Obviously I'm missing some of the TLS 0-RTT context.
>
> Nico
> --
>