Re: [TLS] FYI: new TLS HandshakeType allocation, from draft-ietf-perc-srtp-ekt-diet

Stephen Farrell <stephen.farrell@cs.tcd.ie> Tue, 03 September 2019 21:00 UTC

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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Tue, 3 Sep 2019 21:59:52 +0100
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Subject: Re: [TLS] FYI: new TLS HandshakeType allocation, from draft-ietf-perc-srtp-ekt-diet
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Hiya,

On 03/09/2019 18:58, Richard Barnes wrote:
> Finally, to Stephen's question of analysis -- This doesn't affect the DTLS
> analysis.  All that's being done here is that another value is being
> encrypted with the keys from the DTLS handshake.  In that regard, it's like
> any other application protocol.  In the current approach, where we use a
> different handshake type, it is true that the EKTKey messages are fed into
> the DTLS transcript hash, which could affect future keys.  But this
> shouldn't allow any undue influence on those future keys, because unless
> the hash function has a preimage attack, an attacker can't choose a
> malicious input that leads to a particular value.

Thanks. However, we're previously seen unexpected comments
from people who do such analyses (e.g. "if you did it <this
other way> it'd be easier to analyse"), so personally I'd
prefer we still had that kind of analyst do that kind of
analysis.

I think there's real value in not lowering the bar for
TLS changes like this, and this does still smell like that
to me.

Cheers,
S.