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RE: [IPsec] CHILD_SA and PFS

From: <Pasi.Eronen_at_nospam>
Date: Tue Nov 20 2007 - 11:07:39 GMT
To: <cnguyen@certicom.com>

Chinh Nguyen wrote:

> So my query is this. There is a statement in the RFC that since
> there is no KE payload (and nonces) in the IKE_AUTH, this implies
> that the DH group should be NONE or omitted. I don't understand
> logically why the latter follows from the former.
>
> I thought an SA proposal is to enumerate supported/acceptable
> transforms (and one possible usage is the inclusion of a DH group
> in the CHILD_SA SA proposals to signal PFS in rekey).

The SA proposal here enumerates the transforms to be used for *this* CHILD_SA creation (not future CHILD_SAs; those exchanges have their own SA payloads, which may contain different things). Since no Diffie-Hellman calculation is done when this CHILD_SA is created, you can't get negotiate the Diffie-Hellman transform here.

Best regards,
Pasi

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