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

Nico Williams <nico@cryptonector.com> Mon, 23 March 2015 17:47 UTC

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Date: Mon, 23 Mar 2015 12:47:31 -0500
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From: Nico Williams <nico@cryptonector.com>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] 0-RTT and Anti-Replay
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On Mon, Mar 23, 2015 at 12:41 PM, Eric Rescorla <ekr@rtfm.com> wrote:
>> > 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.

The server can certainly indicate what, if any 0-RTT, it received (by
sending a hash back).  If it did not receive the 0-RTT data then the
client can fail (TLS over TCP) or retransmit (DTLS over UDP or
similar).  If the server received the wrong 0-RTT data then the client
can fail and/or sound the alarms, or even retransmit if the data was
inconsequential (e.g., a SASL mechanism negotiation, which can then be
redone).

Nico
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