Re: [IPsec] Call for independent experts (IKEv2) for Stage 4 of the PAKE selection process

Dan Harkins <dharkins@lounge.org> Fri, 30 August 2019 07:11 UTC

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Date: Fri, 30 Aug 2019 00:11:28 -0700
From: Dan Harkins <dharkins@lounge.org>
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Subject: Re: [IPsec] Call for independent experts (IKEv2) for Stage 4 of the PAKE selection process
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On 8/29/19 5:11 PM, Tero Kivinen wrote:
> [removed cfrg from CC, as I do not think this issue really belongs
> there as we are discussing IKE signaling here].
>
> Dan Harkins writes:
>>    First of all this suggestion is for a particular PAKE and I'm not
>> suggesting that any of the other candidates would slide in so effortlessly.
>> In fact an augmented PAKE is, IMHO, not suitable for a protocol like IKE
>> where either side can initiate. The PAKE I'm describing here is SPEKE,
>> a balance PAKE.
>>
>>    SPEKE does a simple Diffie-Hellman but uses a secret generator that is
>> deterministically obtained from the password. This technique is basically
>> one of the hash-to-curve functions from the CFRG's hash-to-curve I-D
>> or a simple hashing and exponentiation for MODP groups. All this happens
>> at password provisioning time prior to IKE being run.
>>
>>    Then when IKE is run the secret generator for the negotiated group is
>> used to do the D-H, the IKE_SA_INIT exchange is basically SPEKE. The
>> result is, if they both have the same generator (which means they had the
>> same password), an authenticated shared secret. This secret is verified in
>> the IKE_AUTH exchange.
> How does the responder know which of the one million username password
> pairs to pick to generate the generator when calculating D-H in the
> IKE_SA_INIT? The actual identity of the user is only sent in the
> encrypted IKE_AUTH message.
>
> I.e., I think this has exactly same problem than IKEv1 has with
> pre-shared keys for main mode. You must know the initiator identity
> based on the IP-addresses, thus makes this completely unusable for
> non static VPN cases.

   HA! I did miss something :-)

   Yes, I guess a new payload is needed to express the username. This can
be added to the IKE_SA_INIT message. Or maybe it can be a special type of
NOTIFY, that's a six-of-one-and-a-half-dozen-of-the-other issue. But this
should not involve an extra Auth exchange (as the framework PAKEs do)
since SPEKE is just a Diffie-Hellman exchange which is exactly what IKE
is.

   regards,

   Dan.

>>    This would require a new Auth Method defined for SPEKE/PAKE to indicate
>> that the SPEKE shared secret is used. And that should be all that's needed.
>> It should be that simple. The protocol shouldn't have to change, no new
>> messages, no new payloads, no new nuthin. If I'm missing something please
>> let me know.