[TLS] Re: What is the KDF of the combined classical and PQC keys in TLS 1.3 PQC hybrid?

Yes. This concatenation is then put through HKDF-Extract as the regular
part of a TLS 1.3 handshake. See Figure 1.

On Fri, Aug 7, 2026 at 3:28 PM Roger Grimes <roger=
[email protected]> wrote:

> I apologize in advance if this is the wrong forum for asking a technical
> question about the emerging standard.
>
>
>
> Regarding RFC 9954 : https://datatracker.ietf.org/doc/rfc9954/
>
>
>
> As I understand the process I’m interested in:
>
> The client sends X25519 ephemeral public key and MLKEM768 encapsulation
> key to the server.
>
> The server sends back its X25519 ephemeral public key and MLKEM768
> ciphertext to the client.
>
> They both, separately, generate both a shared classical key and a shared
> PQC key.
>
> These keys are concatenated together to create a single shared key.
>
>
>
> In the RFC, it states this:
>
>
>
> concatenated_shared_secret = MyECDH.shared_secret
>
>                                 || MyPQKEM.shared_secret
>
>
>
> Is that concatenation simply appending one string to the other?
>
>
>
> Roger
>
>
>
>
> ********************************************************************************************************************************************
>
> *Roger A. Grimes
>
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