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[TLS] Re: WG Last Call: draft-ietf-tls-mlkem-05 (Ends 2025-11-26)

Viktor Dukhovni <ietf-dane@dukhovni.org> Sat, 29 November 2025 05:01 UTC

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Date: Sat, 29 Nov 2025 16:01:35 +1100
From: Viktor Dukhovni <ietf-dane@dukhovni.org>
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Subject: [TLS] Re: WG Last Call: draft-ietf-tls-mlkem-05 (Ends 2025-11-26)
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On Sat, Nov 29, 2025 at 05:16:12AM +0100, Bas Westerbaan wrote:

> John, I'm curious: how well is the HelloRetryRequest flow supported in your
> environment? That is: advertise support for X25519MLKEM768 but don't send
> it, and then have the server ask for it using HelloRetryRequest. In our
> experiments to origins, we didn't see any issues with this flow and enabled
> it by default.

FWIW, I haven't encountered, or read reports of, any issues with
X25519MLKEM768 after HRR in SMTP STARTTLS

The default supported groups setting in the upcoming Postfix 3.11 (when
compiled against OpenSSL 3.5 or later) is:

    tls_eecdh_auto_curves = ?X25519MLKEM768:DEFAULT

which amounts to a small tweak to the OpenSSL default (which has clients
send both X25519MLKEM768 and X25519 keyshares):

    ?*X25519MLKEM768 / ?*X25519:?secp256r1 / ?X448:?secp384r1:?secp521r1 / ?ffdhe2048:?ffdhe3072

as a result of which a keyshare for X25519MLKEM768 is sent only in
response to HRR from a server that prefers it over the remaining non-PQ
kexes.

-- 
    Viktor.  🇺🇦 Слава Україні!

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