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

Colm MacCárthaigh <colm@allcosts.net> Tue, 02 May 2017 14:49 UTC

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From: =?UTF-8?Q?Colm_MacC=C3=A1rthaigh?= <colm@allcosts.net>
Date: Tue, 2 May 2017 07:44:35 -0700
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Subject: [TLS] Security review of TLS1.3 0-RTT
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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

I'll summarize the summary: Naturally the focus was on forward secrecy and
replay. On forward secrecy the main finding was that it's not necessary to
trade off Forward Secrecy and 0-RTT. A single-use session cache can provide
it, and with the modification that ekr has created in
https://github.com/tlswg/tls13-spec/pull/998 , such a cache works for both
pre-auth and post-auth tickets, and it allows clients to build up pools of
meaningfully distinct tickets.

There's also an observation there that it should really be that clients
"MUST" use tickets only once. Any re-use likely discloses the obfuscated
ticket age, which is intended to be secret. Right now it's a "SHOULD".

On replay, the main finding is that what's in the draft is not workably
secure, and the review includes 5 different attacks against 0-RTT data to
illustrate that. Attacks 1 and 2 show that the kind of replay permitted by
the draft is very different from the kind of replay permitted by dkg's
existing downgrade-and-retry attack. I also go over why it very very
difficult to many applications to achieve that idempotency, and why one
idempotency pattern actually relies on non-replayable messages.

Attack 3 shows that idempotency is not sufficient, applications must also
be free of measurable side-effects, which is not practical.  Attack 4 shows
that 0-RTT breaks a common security mechanism: spoofing-resistant
throttles. Attack 5 shows that 0-RTT replay-ability enables an additional
form of traffic analysis.

The recommendation in the review is that implementations "MUST" prevent
replays of 0-RTT section, with some additional discussion about why the
existing advice is unlikely to be followed, and why consistent
interoperability matters here.

Unfortunately, I wasn't aware until Friday that this review would be coming
so late in the TLD1.3 draft process, and my apologies for that. I am now
planning to attend the future WG in-person meetings and look forward to
seeing many of you there.

-- 
Colm