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Re: [TLS] Are we holding TLS wrong?

Juliusz Chroboczek <jch@irif.fr> Fri, 09 November 2018 16:11 UTC

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Date: Fri, 09 Nov 2018 17:10:54 +0100
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From: Juliusz Chroboczek <jch@irif.fr>
To: Martin Thomson <martin.thomson@gmail.com>
Cc: dschinazi.ietf@gmail.com, "<tls@ietf.org>" <tls@ietf.org>, draft-ietf-babel-dtls@ietf.org
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Subject: Re: [TLS] Are we holding TLS wrong?
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> I'm somewhat dismayed by the firm recommendation to use the HMAC
> mechanism,

Yeah, this could probably be loosened somewhat.

> which doesn't seem particularly robust.

It's designed to be fairly robust.  Of course, we may have done things
wrong.

> Offhand, it seems like replays are possible if you allow the possibility
> of the node crashing and dumping state.

Unless I've missed something -- they are not, assuming you have
a sufficiently strong random number generator.  The challenge mechanism
rebuilds the shared state in a secure manner, and the index mechanism
ensures that an (index, seqno) pair is never reused.

> The same applies during a rollover of the 32-bit counter.

You generate a new index when the counter overflows, and send a new
challenge.  First point of Section 4.2 of the HMAC draft.

-- Juliusz

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