[TLS] Additional warnings on 0-RTT data

Colm MacCárthaigh <colm@allcosts.net> Wed, 23 November 2016 17:49 UTC

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From: =?UTF-8?Q?Colm_MacC=C3=A1rthaigh?= <colm@allcosts.net>
Date: Wed, 23 Nov 2016 09:47:54 -0800
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Subject: [TLS] Additional warnings on 0-RTT data
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I've submitted a PR with an expanded warning on the dangers of 0-RTT data
for implementors:

https://github.com/tlswg/tls13-spec/pull/776/files

The text is there, and the TLDR summary is basically that time-based
defenses aren't sufficient to mitigate idempotency bugs, so applications
need to be aware of the sharp edges and have a plan.  For clarity, I've
outlined three example security issues that could arise due to realistic
and straightforward, but naive, use of 0-RTT. There's been some light
discussion of each in person and on the list.

In the PR I'm "MUST"ing that applications need to include /some/ mitigation
for issues of this class (or else they are obviously insecure). In my
experience this class of issue is so pernicious, and easy to overlook, that
I'm also "RECOMMEND"ing that applications using Zero-RTT data
*intentionally* replay 0-RTT data non-deterministically, so as to keep
themselves honest.

At a bare minimum I think it would be good to make clear that clients and
implementations MAY intentionally replay 0-RTT data; to keep servers on
their toes. For example a browser could infrequently tack on a dummy
connection with repeated 0-RTT data, or an implementation could
periodically spoof a 0-RTT section to the application. That should never be
considered a bug. but a feature. (And to be clear: I want to do this in our
implementation).


-- 
Colm