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Re: [TLS] Closing on 0-RTT

Ilari Liusvaara <ilariliusvaara@welho.com> Tue, 27 June 2017 18:49 UTC

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Date: Tue, 27 Jun 2017 21:49:07 +0300
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Eric Rescorla <ekr@rtfm.com>
Cc: Colm MacCárthaigh <colm@allcosts.net>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Closing on 0-RTT
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On Mon, Jun 26, 2017 at 11:16:02AM -0700, Eric Rescorla wrote:
> OK, I'll move this out of the "if you can do a lot of replays" section
> 

Another thing:

The PR briefly mentions to be careful with 0-RTT exporters, but nothing
concrete-looking.


If 0-RTT data is replayed and the replay accepted, all replays share the
same 0-RTT exporter values.  This causes two kinds of problems:

1) If 0-RTT exporters are used for authentication, then an attacker
in possession of resumption secret and DHE key (if any) can replay the
generated tokens to another connection with replayed 0-RTT, even
without the better-protected authentication key.

2) If 0-RTT exporters are used for key material for to-client
direction, then the replays will have the same keying material, which
is highly dangerous with many encryption algorithms.



-Ilari

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