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[TLS] Re: Disallowing reuse of ephemeral keys

"Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com> Fri, 13 December 2024 17:38 UTC

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From: "Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com>
To: "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] Re: Disallowing reuse of ephemeral keys
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Date: Fri, 13 Dec 2024 17:38:13 +0000
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Subject: [TLS] Re: Disallowing reuse of ephemeral keys
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Open questions about ephemeral key reuse (and I don't know the answers; that's why they're open questions) - the answers to these questions may help us guide us as to whether to forbid it or not:

- To what extent do the proofs of security for TLS 1.3 depend on the non-reuse of key shares (either (EC)DH or KEM or hybrid)?  I asked this question about 5 years ago (at a NIST conference, not on this list), and I believe the answer was "yes", at the time, but the proofs may have advanced (or I might have misunderstood the answer).

- To what extent was we concerned about ultralow power devices (battery powered)?  After all, reusing previous keys would use less power than creating new ones - not a huge amount of power (both ML-KEM and ECDH are fairly power efficient), but I could see someone making the case.  Would we take that case seriously?  (One could make a similar case about performance, but given the overhead of doing a TLS exchange, that's a lesser concern, at least IMHO).

> -----Original Message-----
> From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
> Sent: Friday, December 13, 2024 7:20 AM
> To: tls@ietf.org
> Subject: [TLS] Re: Disallowing reuse of ephemeral keys
> 
> 
> Hiya,
> 
> On 12/12/2024 17:59, Richard Barnes wrote:
> > My preference order would be 3 > 1 >> 2.
> 
> I agree with the above for reasons already stated on the list.
> 
> Cheers,
> S.

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