Subject: [IPsec] Question about D-H transform and PFS

From: "Jonathan Childs" <[email-masked]>
Date: Fri, 12 Dec 2008 11:20:52 -0500

Hi all,

We are having an interoperability problem and I couldn't seem to find
the answer in any of the RFCs, so I was hoping someone on the list could
give some input.  Both sides are configured to do PFS but the problem
appears to be that one side expects the D-H transform in every message
(including the IKE_AUTH message) and the other only expects it only in
the CREATE_CHILD_SA messages.  So does anyone know the correct procedure
here?  Are the D-H transforms usually included in every message or just
for IPSEC SAs after the 1st one that comes as part of the IKE
negotiations?

Thanks

Jon

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From: Yoav Nir <[email-masked]>
Date: Sat, 13 Dec 2008 23:06:02 +0200

Hi Jon

PFS never applies to the child SA created in the IKE_AUTH exchange.

See Appendix A.2 and A.3 in RFC 4718.

Also, section 1.3 of RFC 4306 has the following:

    In the CHILD_SA created as part of the initial exchange, a second KE
    payload and nonce MUST NOT be sent.  The nonces from the initial
    exchange are used in computing the keys for the CHILD_SA.

On Dec 12, 2008, at 6:20 PM, Jonathan Childs wrote:

> Hi all,
>
> We are having an interoperability problem and I couldn’t seem to  
> find the answer in any of the RFCs, so I was hoping someone on the  
> list could give some input.  Both sides are configured to do PFS but  
> the problem appears to be that one side expects the D-H transform in  
> every message (including the IKE_AUTH message) and the other only  
> expects it only in  the CREATE_CHILD_SA messages.  So does anyone  
> know the correct procedure here?  Are the D-H transforms usually  
> included in every message or just for IPSEC SAs after the 1st one  
> that comes as part of the IKE negotiations?
>
> Thanks
> Jon

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