Re: [6tsch] Feedback on draft-ohba-6tsch-security-00

<> Sat, 06 July 2013 01:28 UTC

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Thread-Topic: [6tsch] Feedback on draft-ohba-6tsch-security-00
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Subject: Re: [6tsch] Feedback on draft-ohba-6tsch-security-00
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Hi Rene,

Now I understand your comments better.  Please see my comments below.

From: [] On Behalf Of Rene Struik
Sent: Saturday, July 06, 2013 4:28 AM
To: ohba yoshihiro
Subject: Re: [6tsch] Feedback on draft-ohba-6tsch-security-00

Hi Yoshi:

Please see feedback below.

Best regards, Rene

On 7/5/2013 2:05 PM,<> wrote:
Hi Rene,

Thank you very much for your feedback.  Please see my comments below.

From:<> [] On Behalf Of Rene Struik
Sent: Friday, July 05, 2013 11:57 PM
To: ohba yoshihiro
Subject: [6tsch] Feedback on draft-ohba-6tsch-security-00

Hi Yoshi:

I had a look at your draft and reflected somewhat on the requirements mentioned.

Initial feedback:

General remark:
1) The draft text seems almost independent of 802.15.4e behavior. While the security considerations make reference to the need to authenticate time synch and channel hopping info in beacon frames and refer to potential nonce "overflow" issues, it is not that easy to see how the mechanisms described in the draft address these 802.15.4e-specific considerations.
[YO] We have identified issue with Enhanced Beacon frame security in Security Considerations section.  Once we agree on a solution framework to address the EB security issue we will describe it.
2) The draft seems to aim at providing both (a) key establishment functionality (setting up a secure peer-to-peer channel); (c) key distribution (handing out link keys, muulticast keys, network keys). It would be useful to organize the draft so as to separate these functionalities more clearly (e.g., Phase-II KMP vs. Phase-II Key Distribution). What about key updates?
[YO] We will describe two functionalities in Phase-2 description.  Key updates will use key distribution.
Security Framework (Section 3):
a) The security framework (Phase-III) seems to require incremental roll-out/planning of the network, witness Fig. 2. If so, it would be good to indicate how devices transgress through the different "phases" defined and to what degree this requires planning/intervention by an operator.
[YO] I am not sure if I understand your comment above, but Fig.2 shows how a mesh network is formed after devices are placed where they are expected to operate.
It seems very well possible that secure channels can be established between devices in a less hierarchical fashion than suggested in Fig. 2. As an example, B and D may set-up a secure channel locally ("Phase II"), without B first having to perform a protocol with A ("Phase I"). This ties into my question re the security model, where (in my mind, at least) Phase-I may very well be superfluous, e.g., when devices already have certs on board. In the model 3-phase model described in the draft, however, every device first has to talk with the Mothership node A (authentication server), thus forcing a hierarchical model. In short: your picture seems to reflect a hierarchical trust model, which seems to be a choice, rather than a necessity. If devices already have some credentials on board, one seems to be able to collapse (1) and (2) and remove (3); moreover, one could swap the order of the (1)-(2) tandem and (4) then (at least from a secure channel set-up perspective).
[YO] While it is possible to swap the order from a secure channel set-up perspective, the swapping will cause higher-layer connectivity problem and is no good from  networking perspective.

b) The bootstrapping phase (Phase-I) suggests inline interaction of a device with an authentication server (which is assumed to mostly be the coordinator of the mesh network). Since bootstrapping only results in issuance of credentials to be used by devices for Link Establishment (Phase-II), I can imagine this step to be often organized differently than described, e.g., by embedding a device certificate with a device at device manufacturing. In that case, no inline interaction with an authentication server seems required, at least not for *authentication* purposes.
[YO] Yes, that is why R2-2 says “Phase-2 KMP authentication credentials MAY be pre-provisioned or MAY be obtained via Phase-1 KMP.”
If a device already has a cert on board, Phase-I seems superfluous. If you agree, I would suggest adding some language to that effect (right now, the Phase-I, II, III sequence comes up all over the place in the draft).
[YO] OK, we will add some language to indicate that Phase-I can be skipped if a device already has Phase-2 KMP credentials.

KMP Requirements (Section 4):
R1-1: The requirement for mutual authentication seems to rule out "resurrecting duckling" policy models. Is this correct?
[YO] Not necessarily, because we plan to add Phase-0 where implanting bootstrap credential is expected to happen.
My comment was on the current draft, so perhaps a future planned Phase-0 step would allow resurrecting duckling type configuration. That would be good from a trust policy lifecycle management perspective.

Moreover, this rules out one/two pass protocols.
[YO] I am not sure what you meant by “one/two pass protocols”..
Mutual authentication (I presumed entity authentication) assumes each party to provide cryptographic evidence based on an input received from the other party, so requires more than two passes. If you meant mutual authentication without aliveness guarantees, one may end up with replay attacks.
[YO] Now I understand.  Yes, mutual authentication  rules out one/two pass protocols (and nothing is wrong with it).

R1-2: The requirement for stateless relay may not work within the 802.15.4e context: if a device A talks with an authentication server T via its neighbor R, where R and T operate in a secured network (i.e., one where all communications are cryptographically secured), R needs to "remember" that it should initially communicate back to A without security (since A and R do not have a shared key yet). Thus, R needs to maintain state that communications to A have to be treated differently (in 802.15.4e speak: A has "exempt status").
[YO] We meant “stateless” in R1-2 is about state maintained above MAC layer.  If 802.15.4e requires to maintain state for unauthenticated nodes at MAC layer to communicate with them, then we should not try to change it in IETF.
The requirement for stateless behavior has much diminish meaning, if one needs to maintain state at the MAC anyway (thus, should perhaps be dropped). On a more general note, I would suggest checking the incoming and outgoing frame processing sections of the 802.15.4 specification, to see what state needs to be maintained for processing to work.
[YO] Even if MAC layer needs to maintain state, being stateless in authentication messaging over MAC layer is an important property for constrained devices.  Also seeing 802.15.4 specification, one could implement the 802.15.4 behavior around Exempt element in a stateless manner.

Or, is the idea that R somehow encodes this exempt status into its relay messages, so that it can pass this "hot potato" along and receive this back via T?

R2-2: I can imagine other scenarios than pre-provisioning of credentials or passing these along via Phase-I KMP: wouldn't piggy-backing this info along communication flows of Phase-II KMP also be an option?

[YO] I don’t understand how piggy-backing (i.e., use of P2 KMP to provide P2 KMP credentials) can work.
Something along these lines has been suggested in the context of 802.11 join protocols (between STA and AP). We can discuss offline.

Candidate KMPs (Section 5):
a) PANA. With bootstrapping during manufacturing, Phase-I may be organized differently. It would be good to show how PANA can fit 802.15.4e needs in a stateless fashion.
[YO] Please see my comment above.
As suggested before, Phase-I as described in the draft is not the only way of doing things (and arguably an expensive way). One additonal note on PANA: RFC 6786 uses a different nonce format when encrypting attributes than 802.15.4 uses. Depending on hardware/software boundaries of CCM impementations, this
could cause a problem.

b) HIP-DEX. The draft suggests this protocol (an expired draft), but at the same time points out that it is deficient, since does not support carrying certificates. This is somewhat puzzling.
[YO] Please see Rafa’s comment on this.
See my separate email note on HIP-DEX to Rafa.

Yoshihiro Ohba

Yoshihiro Ohba

On 7/5/2013 1:20 AM, Thomas Watteyne wrote:


Below is the proposed agenda for the 6TSCH call tomorrow:
*         Approval minutes last call [1min]
*         draft-ohba-6tsch-security-00 [10min]
*         Simulator [10min]
*         Description of PCE [10min]
*         Preparing for the BOF [25min]
*         Re-organization Bitbucket [2min]
*         AOB [1min]

As usual, feel free to propose any changes to the agenda, also at the beginning of the call.

Remember that this call will be recorded.

Pascal & Thomas

Topic: 6TSCH Weekly
Time: 8:00 am, Pacific Daylight Time (San Francisco, GMT-07:00)
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