[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Crypto algorithms for IKEv2



> > Come to think of it, I don't think we ever resolved the issue of what to
>do
> > when the initiator of a CREATE_CHILD_SA exchange doesn't propose PFS but
>the
> > responder requires it. This could be accomplished with a
> > NOTIFY_PFS_REQUIRED_ALWAYS or NOTIFY_PFS_REQUIRED_NEXT_SA message.
> >
>My reading of the current specification is that if the initiator doesn't
>propose PFS but the responder requires it, the proposal will be rejected
>with a NO_PROPOSAL_CHOSEN notification, just as would any other time there
>is no overlap between what the initiator proposes and what the responder is
>prepared to accept.


I guess the danger I was thinking of is that some people have been talking 
about PFS as a run-time parameter rather than a configuration parameter. 
I.e. you have a PFS interval for the original phase 1 key. Once that 
interval has elapsed, you delete SKEYSEED_D and require PFS for every 
subsequent CREATE_CHILD_SA (possibly reusing the exponent).

The trouble is that when you receive a NO_PROPOSAL_CHOSEN message, this 
normally represents a configuration error (where the administrator has to 
check that both sides are using the same ciphersuites). Now we have a case 
where the NO_PROPOSAL_CHOSEN message could also occur for an already 
functioning SA.

As I pointed out earlier, the UI suites in 
draft-hoffman-ipsec-algorithms-00.txt make no mention of PFS. So my guess is 
that half the implementations will have it on by default and half will have 
it off by default. This means that we either need a PFS on/off checkbox in 
the GUI (which defeats the purpose of a single configuration knob via UI 
suites) or we have to implement a routine to retry with PFS any time you 
receive a NO_PROPOSAL_CHOSEN message.

I suspect that most people will choose the former. (God forbid that anyone 
actually attempts the strategy where you rekey without PFS for the first N 
minutes and then with PFS for the remainder of the SA lifetime.)

Andrew
--------------------------------------
The odd thing about fairness is when
we strive so hard to be equitable
that we forget to be correct.

_________________________________________________________________
Protect your PC - get McAfee.com VirusScan Online  
http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963