Re: [TLS] Security review of TLS1.3 0-RTT

Ilari Liusvaara <ilariliusvaara@welho.com> Thu, 04 May 2017 09:34 UTC

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Date: Thu, 04 May 2017 12:34:30 +0300
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Colm MacCárthaigh <colm@allcosts.net>
Cc: "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Security review of TLS1.3 0-RTT
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On Tue, May 02, 2017 at 07:44:35AM -0700, Colm MacCárthaigh wrote:
> On Sunday at the TLS:DIV workshop I presented a summary of findings of a
> security review we did on TLS1.3 0-RTT, as part of implementing 1.3 in s2n.
> Thanks to feedback in the room I've now tightened up the findings from the
> review and posted them as an issue on the draft GitHub repo:
> 
> https://github.com/tlswg/tls13-spec/issues/1001

What I didn't see in the summary, but I think might be relevant in
relation to 0-RTT:

There is a thing called 0-RTT exporter, which are exporter values
available during 0-RTT transmission.

If the server uses 0-RTT exporter and doesn't enforce non-replay, the
value grossly fails to be "nonce", which means it is likely unsafe to
use for authentication.

Unfortunately, there are protocols that are already discussing the
use of TLS 1.3 0-RTT exporter, and switching to "full" exporter.
Unfortunately, the easiest way is not to switch, which means the
possibly weak 0-RTT exporter will be used for authenticating even
non-replayable data.



-Ilari