Dang, Quynh wrote:
>My guess is that that would be an expensive operation because of many reasons.

Yes, and to make sure that the first two keys were not just before and just 
after a rekeying, you would need to look at three keys…

While any timing side channels in X25519MLKEM768 are hopefully mitigated by the 
hybrid construction, this is not true with standalone ML-KEM. For standalone 
ML-KEM, it is even more important to forbid reuse of encapsulation keys when 
communicating with several peers. Also, standalone ML-KEM is even faster than 
the already very fast X25519MLKEM768, so there is even less justification for 
any reuse.

Cheers,
John

From: Dang, Quynh H. (Fed) <quynh.dang=40nist....@dmarc.ietf.org>
Date: Monday, 10 March 2025 at 14:09
To: tls@ietf.org <tls@ietf.org>
Subject: [TLS] Re: FW: I-D Action: draft-kwiatkowski-tls-ecdhe-mlkem-03.txt
The server can detect a reused encapsulation key if it saves the keys which 
have been received and check the newly received key against the list of its 
saved keys. The server could just save the hashes of the keys or a "small" 
portion of the keys as the key IDs.  My guess is that that would be an 
expensive operation because of many reasons.

Regards,
Quynh.
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