Re: [tcpinc] Resumption safety (was "Eric Rescorla's Discuss on draft-ietf-tcpinc-tcpcrypt-09: (with DISCUSS and COMMENT)")

Kyle Rose <krose@krose.org> Mon, 11 December 2017 19:29 UTC

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From: Kyle Rose <krose@krose.org>
Date: Mon, 11 Dec 2017 14:29:03 -0500
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To: "Black, David" <David.Black@dell.com>
Cc: Valery Smyslov <svanru@gmail.com>, Eric Rescorla <ekr@rtfm.com>, tcpinc <tcpinc@ietf.org>, "Mirja Kuehlewind (IETF)" <ietf@kuehlewind.net>
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Subject: Re: [tcpinc] Resumption safety (was "Eric Rescorla's Discuss on draft-ietf-tcpinc-tcpcrypt-09: (with DISCUSS and COMMENT)")
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I want to add:

On Mon, Dec 11, 2017 at 2:01 PM, Kyle Rose <krose@krose.org>; wrote:
> It's not clear that this will make its way into the entropy pool
> before use unless the hypervisor and guest kernel have some explicit
> interaction to re-seed the entropy pool before resumption.

...and unless applications pull new entropy from this pool for every
single PRNG iteration, something I think is not done by userspace
PRNGs more complicated than "read directly from /dev/urandom". The
application would need to be aware of the resumption, or add
expected-distinct input to the PRNG as a matter of course (e.g.,
timestamp). And even then, there's still a race condition depending on
the order of operations.

The more I look into this, the more convinced I am that SIV-like
constructions are the only way to entirely avoid catastrophic loss of
security. To otherwise deal completely with the problem of VM cloning
involves the invasive complexity of layer violation. That said, I
don't think we want to take that step at this late date, but we should
recognize the benefits and limitations of adding nonces and look into
adding GCM-SIV (and related) cipher modes as future TEPs.

Kyle