Subject: [Ipsec] Negotiating IKE_SAs

From: David Wierbowski <[email-masked]>
Date: Mon, 6 Nov 2006 19:57:05 -0500

In the IKE_SA_INIT exchange the initiator sends an SA payload listing
acceptable proposals.  The responder picks one and sends the chosen
proposal to the initiator.  The identities of the IKE endpoints are not
exchanged until the IKE_AUTH exchange.  If the identities are not exchanged
until the IKE_AUTH exchange, how does the responder know which of the
initiator's proposals are acceptable during the IKE_SA_INIT exchange?

RFC 4301 discusses the use of the SPD to find acceptable policy for the
creation of CHILD_SAs and it discusses the use of the PAD to authenticate
IKE endpoints.  It does not appear to define a construct to identify what
policy is acceptable for the creation of a of an IKE_SA with a specific IKE
peer.  Does this mean that RFCs 4301 and 4306 do support the definition of
peer specific policy for IKE_SAs?

RFC 4306 states, "All implementations of IKEv2 MUST include a management
facility that enables a user or system administrator to specify the suites
that are acceptable for use with IKE."  This seems to imply that peer
specific IKE_SA policy should not be defined and that the responder should
pick the most secure proposal that the responder supports.  Is that
correct?

Dave Wierbowski
z/OS Comm Server Developer

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From: Paul Hoffman <[email-masked]>
Date: Mon, 6 Nov 2006 21:26:58 -0800

At 7:57 PM -0500 11/6/06, David Wierbowski wrote:
>In the IKE_SA_INIT exchange the initiator sends an SA payload 
>listing acceptable proposals. The responder picks one and sends the 
>chosen proposal to the initiator. The identities of the IKE 
>endpoints are not exchanged until the IKE_AUTH exchange. If the 
>identities are not exchanged until the IKE_AUTH exchange, how does 
>the responder know which of the initiator's proposals are acceptable 
>during the IKE_SA_INIT exchange?

By looking at its own security policy.

>RFC 4301 discusses the use of the SPD to find acceptable policy for 
>the creation of CHILD_SAs and it discusses the use of the PAD to 
>authenticate IKE endpoints. It does not appear to define a construct 
>to identify what policy is acceptable for the creation of a of an 
>IKE_SA with a specific IKE peer. Does this mean that RFCs 4301 and 
>4306 do support the definition of peer specific policy for IKE_SAs?

No. They support definition of peer-specific policies for IPSEC_SAs.

>RFC 4306 states, "All implementations of IKEv2 MUST include a 
>management facility that enables a user or system administrator to 
>specify the suites that are acceptable for use with IKE." This seems 
>to imply that peer specific IKE_SA policy should not be defined and 
>that the responder should pick the most secure proposal that the 
>responder supports. Is that correct?

Yes, approximately.

--Paul Hoffman, Director
--VPN Consortium

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From: David Wierbowski <[email-masked]>
Date: Tue, 7 Nov 2006 11:37:54 -0500

>By looking at its own security policy.

Where is this policy defined?  Is this considered to be a matter
of "local policy" and outside the scope of the RFCs?

>No. They support definition of peer-specific policies for IPSEC_SAs.

Strangely enough I don't think the term IPSEC_SA appears in RFC 4301
or 4306.  By IPSEC_SA do you mean ESP and AH SAs?

>>responder supports. Is that correct?

Could you clarify why you said approximately?  To me approximately
means almost correct :>).

Thanks.

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From: Paul Hoffman <[email-masked]>
Date: Tue, 7 Nov 2006 09:47:13 -0800

At 11:37 AM -0500 11/7/06, David Wierbowski wrote:
>Where is this policy defined?  

In the application.

>Is this considered to be a matter  

Yes.

>or 4306.  By IPSEC_SA do you mean ESP and AH SAs?

Yes.

>means almost correct :>).

Yes and yes. Peer-specific IKE_SA should not be used because it is a 
bad security policy in many cases, but not all.

--Paul Hoffman, Director
--VPN Consortium

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From: Stephen Kent <[email-masked]>
Date: Tue, 7 Nov 2006 13:50:22 -0500

...
>4306 do support the definition of peer specific policy for IKE_SAs?

The PAD not only says how to authenticate an IKE peer, it also 
provides a way to restrict the range of IDs or addresses asserted by 
the peer when the SPD is searched for a matching policy. In that 
sense there is a notion of peer-specific policies, but for child SAs, 
not for IKE SAs.

Steve