| Network Working Group | M. Richardson |
| Internet-Draft | SSW |
| Intended status: Informational | July 21, 2014 |
| Expires: January 22, 2015 |
6tisch secure join using 6top
draft-richardson-6tisch--security-6top-02
This document details a security architecture that permits a new 6tisch compliant node to join an 802.15.4e network. The process bootstraps the new node authenticating the node to the network, and the network to the node, and configuring the new node with the required 6tisch schedule. Any resemblance to WirelessHART/IEC62591 is entirely intentional.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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A challenging part with constructing an LLN with nodes from multiple vendors is providing enough security context to each node such that the network communication can form and remain secure. Most LLNs are small and have no operator interfaces at all, and even if they have debug interfaces (such as JTAG) with personnel trained to use that, doing any kind of interaction involving electrical connections in a dirty environment such as a factory or refinery is hopeless.
It is necessary to have a way to introduce new nodes into a 6tisch network that does not involve any direct manipulation of the nodes themselves. This act has been called "zero-touch" provisioning, and it does not occur by chance, but requires coordination between the manufacturer of the node, the service operator running the LLN, and the installers actually taking the devices out of the shipping boxes.
For the process described in this document to work, some assumptions about available infrastructure are made. These are perhaps more than assumptions, but rather architectural requirements; the exact operation of said infrastructure to be defined in a subsequent document.
In the diagrams and text that follows entities are named (and defined in the terminology section). Unless otherwise stated these are roles, not actual machines/systems. The roles are seperated by network protocols in order that they roles can be performed by different systems, not because they have to be. Different deployments will have different scaling requirements for those entities. Smaller deployments might co-located many roles together into a single ruggedized platform, while other deployments might operate all of the roles on distinct, multiply-redundant server classes located in a fully equipped datacentre.
Most terminology should be taken from [I-D.ietf-6tisch-architecture] and from [I-D.ietf-6tisch-6top-interface] and [I-D.wang-6tisch-6top-sublayer]. As well, many terms are taken from [RFC6775].
The following roles/things are defined:
The following terms are used in this document and come from other documents:
This section works from the ultimate goal, and goes backwards to prerequisit actions. Section 6 presents the protocol from beginning to end order.
The ultimate goal of the join protocol is to provide a new node with enough locally significant security credentials that it is able to take part in the network directly. The credentials may vary by deployment. They can be:
Given one of the the above, there are a number of possible protocols that can be used to generate layer-2 sessions keys for the node, including:
The intermediate goal of the join protocol is to enable a Join Coordination Entity (JCE) to reach out to the new node, and install the credentials detailed above. The JCE must authenticate itself to the joining node so that the joining node will know that it has joined the correct network, and the joining node must authenticate itself to the JCE so that the JCE will know that this node belongs in the network. This two way authentication occurs in the 6top/CoAP/DTLS session that is established between the JCE and the joining node.
[I-D.ietf-6tisch-6top-interface] presents a way to interface to a 6top information model (defined in YANG). [I-D.ietf-6tisch-coap] explains how to access that information model using CoAP. That model is to be extended to include security attributes for the network. The JCE would therefore reach out to the joining node and simply provision appropriate security properties into the joining node, much like the PCE will provision schedules.
This 6top-based secure join protocol has defined a push model for security provisioning by the JCE. This has been done for three reasons:
In order for a 6top/DTLS/CoAP connection to occur between the JCE and the joining node, there needs to be end-to-end IPv6 connectivity between those two entities. The joining node will not participate in the route-over RPL mesh, but rather will be seen by the network as being a 6lowpan only leaf-node.
There are some alternatives to having full end to end connectivity which are discussed in the security considerations section.
The specific mechanism to enable end to end connectivity with the JCE are still open but will consist of one of:
Of these mechanisms, the only one which does not require additional state on the Join Assistant (which is also a constrained device) is (1) and (2). Mechanism (2) additionally requires no specific state on the Join Assistant. Mechanism (2), in a non-storing DODAG requires additional state on the DODAG root (6LBR) only; while mechanism (1) requires a similar amount of state on the JCE. For deployments where the JCE is part of the 6LBR, the amount of state is similar, but in any case, the 6LBR is assumed to be a non-constrained node.
As long as the Join Assistant does not do any kind of stateful firewalling, the IPIP tunnel and the DAO (2) method can be done by the Join Assistant statelessly. Upward traffic from the Join Network must be restricted to a 6tisch slotframe(s) to which join traffic is welcome, no tunnelling is necessary as the upwards routes are all in place. A destination address ACL on traffic from the Join Network restricts the Joining Nodes to sending traffic only to the address of the JCE. (If JCE and 6LBR are colocated, then this is the address in the ABRO, if they are not colocated, then this address needs to have been provisioning in the Join Assistant when it joined, or could be carried in a new RA option)
When using option (2), networks that have storing mode DODAGs will consume routing resources on all intermediate nodes between the Join Assistant and the DODAG root. This resource will be depleted without any authentication, and this threat is detailed below.
Continuing to work backwards, in order the JCE reach out to provision the Joining Node, it needs to know that the new node is present. This is done by taking advantage of the 6lowPAN Address Resolution Option (ARO) (section 4.1 [RFC6775]). The ARO causes the new address to also be sent up to the 6LBR for duplicate detection using the DAR/DAC mechanism. The 6LBR simply needs to tell the JCE about this using a protocol that needs to be defined, but could be either DAR or NS.
In addition to needing to know the joining devices address from the DAR/NS, the JCE also needs to know the joining node' IDevID. If the serialNumber attribute of the IDevID is less than 64 bits, then it is possible that it could be placed into the EUI-64 option of the ARO, or the OUI of the [I-D.thubert-6lo-rfc6775-update-reqs] EARO. The JCE needs to know the joining node's serialNumber to know if this is device that it should even attempt to provision; and if so, it may need to retrieve an appropriate certificate chain (see [I-D.richardson-6tisch-idevid-cert]) from the Factory in order for the JCE to prove it is the legitimate owner of the joining node.
Neither 802.1AR nor [RFC5280] provide any structure for the serialNumber, except that they are positive integers of up-to 20 octets in size (numbers up to 2^160). This specification would require that the serialNumber encoded in the IDevID is the same as the EUI-64 used by the device. Some consideration needs to be given as to whether there are privacy considerations to doing this: any observer that can see the join traffic, can also see the source MAC address of the node as well.
Prior to being able to announce itself in a NS, the joining node needs to find the Join Network. This is done by listening to an extended beacon which are broadcast in designated slotframes by Join Assistants. The Extended Beacon provides a way for the Joining Node to synchronize itself to the overall timeslot schedule and provides an Aloha period in which the Joining Node can send a Router Soliticiation, and receive an appropriate Router Advertisement giving the Joining Node a prefix and default route to which to send join traffic.
It may be possible to eliminate a message exchange if space for a Router Advertisement can be found as part of the Join Network Extended Beacon. This Enhanced Beacon would be distinct to the Join Network, and would be encrypted with the well-known Join Network key.
What prefix would the joining node for communication? There are three options:
There are three kinds of threats that a join process must deal with: threats to the joining node, threats to the resources of the network, and threats to other joining nodes.
A node may be taken out of it's box by a malicious entity and powered on. This could happen during shipping, while being stored in a warehouse. The device may be subject to physical theft, or the goal of the attacker may be to turn the device into a trojan horse of some kind. Physical protection of the device is out of scope for this document; this document will henceforth assume that the device is sealed in some tamper-evident way and this document deals with attacks over the network.
An attacker may attempt to convince the joining node that it is the legitimate Production Network; this is done by putting up a legitimate looking Join Network, and following the protocol as described in this document. The Joining Node can not know if it has the corrrect Production Network until steps 11-13, when it attempts to validate the ClientCertificate provided by the JCE.
When the joining node determines that this is the incorrect network, it must remember the PANID of the network that it has attempted to join, and then look for another network to try. It SHOULD have some limit as the number of times it will try before going back to sleep, or shutting down, and it SHOULD take care not to consume more than some specified percentage of any battery it might have.
Should a malicious production network be present at the same time/place as the legitimate production network, a the malicious agent could intercept and replay various packets from the proper join network, but ultimately this either results in a jamming-like denial of service, and/or the the ClientCertificate will not validate.
It is a legitimate situation for there to be multiple possible join networks, and the joining node may have to try each one before it finds the network that it the right one for it. The incorrect, but non-malicious networks will not attempt the 6top provisioning step, and SHOULD return a negative result in steps 8/9, refusing the node's NS. Those incorrect networks will be recognize that the node does not belong to them, because they will be able to see the Joining Node's IDevID in the ARO of step 4.
The production network has two important resources that may be attacked by malicious Joining nodes: 1) energy/bandwidth, 2) memory for routing entries.
A malicious joining node could send many NS messages to the Join Assistant (from many made up addresses), which would send many NS/DAR messages to the 6LBR, and this would consume bandwidth, and therefore energy from the members of the mesh along the path to the 6LBR. This can be mitigated by limited the total bandwidth available for joining.
A malicious joining node could send many NS messages, and if the 6LBR agreed to accept the new node (by IDevID), then the Join Assistant would MAY inject routing information into mesh for the Joining node. Non-storing DODAGs store are routing information in the DODAG Root (probably the 6LBR), which is generally not a constrained node. Storing DODAGs store routing entries at all nodes up to the DODAG, and those are constrained nodes. Using a separate Join DODAG, and having that DODAG be non-storing will reduce any impact on intermediate nodes, but it does cause resources to be used for the second DODAG, and it may have a code impact if the nodes otherwise would not implement non-storing RPL.
A joining node (or the nodes of a malicious network, co-located near the legitimate production network) may mount attacks on legitimate nodes which have not yet joined.
The malicious nodes may attempt to perform 6top operations against the joining node to keep it from being able to respond to the legitimate 6top session from the legitimate JCE. During the Join phase, the Joining node MUST have all other resources and protocols turned off, even if they would normally be accessible as read-only unauthenticated CoAP resources.
Malicious nodes could use the Join Network to mount various DTLS based attacks against the joining node, such as sending very long certificate chains to validate. One might think to limit the length of such chains, but as shown in [I-D.richardson-6tisch-idevid-cert] the chain may as a long as the supplier chain, plus may include additional certificates due to resales of plants/equipment/etc. Validating from a trusted certificate down to the specific certificate which proves ownership would eliminate random certificate chains, but the attacker could just feed the joining node legitimate chains that it observed (and replayed) from the legitimate JCE. This does no good; the Joining node finds that the DTLS connection is invalid, but it may significantly run batteries down.
(storage of security material, computational cost)
other communication impacts of security protocol mechanics
dependencies on centralized or external functionality, inline and offline
+-----+ +------+ +----------+ +-----------+
| | | | | JOIN | | Joining |
| JCE | | 6LBR | | Assistant| | Node |
+-----+ +------+ | (proxy) | | |
| | +----------+ +-----------+
| | | |
| | |-------BEACON (1)---------->|
| | | |
| | |<----Router Solicitation----|
| | |---Router Advertisement---->|
| | | |
| | |<------CERT CACHE-----------|
| | | LOAD (2) |
| | | |
| | | |
| | |-------CERT CACHE---------->|
| | | RESPONSE (multiple) |
| | | (packets) |
| | | |
| | |<-----JOIN REQUEST (4) -----|
| | | (NS w/ARO ) |
| | | |
| |<---NS (DAR) (5)-----| |
|<--??(6)-| | |
| | | |
|--??(7)->| | |
| | | |
| |----NS (DAC)-(8)---->| |
| | +------+ | |
| |<DAO-| mesh |<--DAO--| |
| |-DAO-| node |--DACK->| |
| | ACK +------+ | |
| | |-------JOIN ACK (9)-------->|
| | | |
| | | |
|================(10)==========>|----------6top----(11)----->|
| | | DTLS |
|<===============(13)===========|<---------CoAP----(12)------|
| | | (many packets) |
Figure 1: Message sequence for JOIN message
A 6tisch join/synchronization beacon is broadcast periodically, and is authenticated with a symmetric “beacon key”:
The purpose of this key is not to provide a high level of assurance, but rather to filter out 6tisch traffic from another random traffic that may be sharing the same radio frequencies.
These beacons are used for JOIN purpose only, and are not related to the Enhanced Beacons used in the rest of 6tisch.
The joining node sends a router solicitation during the Aloha period of the beacon.
The joining node receives a router advertisement from the Join Assistant. It could include 6CO options to help compress packets, and should contain a prefix appropriate for join traffic.
At step 10, the JCE will need to present a certificate chain anchored at a trusted CA built into the joining node. It has been speculated that a significant amount of traffic could be avoided at step (10) if the common parts of the certificate chains could be cached in the join assistant.
This optional step involves the joining node asking for certificates from the join assistant.
the proxy neighbour sends requested cached certificates to the joining node
A regular Neighbour Solicitation is sent. This should contain an ARO (or EARO) option containing the Joining Nodes' IDevID. The ARO/EARO will be proxied by the Join Assistant as part of normal 6LowPAN processing for leaf nodes (non-RPL nodes) upwards to the 6LBR
The JCE could reply in the negative, and this would cause a DAC failure, TBD
The double lines indicate that an IPIP tunnel operation may be required. If a straight DAO or seperate Join DODAG is used, then this is just a straight forwarding root to leaf node forwarding operation, and involves either using source routes (non-storing), or just forwarding for storing DODAGs.
A specific bandwidth allocation would be used for this join traffic
The production network encryption keys would be used for the join traffic
The JOIN Assistant would forward traffic to the Joining Node. Recognizing that this traffic the JOIN Network, the JOIN Assistant would use the JOIN Network key.
The joining node replies, using JOIN Network key.
The JOIN Assistant, recognizing that the traffic came from the JOIN Network, restricts the destination that can be reached to the the JCE only. It can do this in a stateless way, and it does NOT need to track the traffic at (10) to open pinhole, etc.
Recognizing that the traffic came from the JOIN Network, the traffic would be placed into a bandwidth allocation (track?) that allows such traffic.
and number of frames needed to contain it.
An end to end IPv6 CoAP/DTLS connection is created between the JCE and the Joining Node. This connection carries 6top commands to update security parameters. This results in either deployment of a single-level, locally relevant certificate (LDevID), or deployment of a network-wide symmetric "Master Key"
The JCE authenticates the joining node using a certificate chain provided inline during the DTLS negotiation. The certificate chain is rooted in a vendor certificate that the JCE must have preloaded, and is a statement as to the node's 802.1AR IDevID. The joining node authenticates the
Note: RPL Root authentication is a chartered item
The EUI-64 of the Joining node is transmitted using a Well Known layer-2 encryption key. Within the ARO/EARO of the Neighbour Solicitation is an OUI, which may be identical to the EUI-64 of the Joining node, or it might be an unrelated IDevID.
An eavesdropper can therefore learn something about the manufacturer of every device as it joins.
how is this communicated in the (extended) beacon.
(allocation of slotframes after join, network statistics, neighboetc.)
lifecycle (key management, trust management)
what prevents a node from transmitting when it is not their turn (part one: jamming)
can a node successfully communicate with a peer at a time when not supposed to, may be tied to link layer security, or will it be policed by receiver?
security architecture and fit of e.g. join protocol and provisioning into this
(SACM related work)
| [RFC5280] | Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. |
| [RFC7252] | Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, June 2014. |
| [RFC6347] | Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012. |
| [I-D.thubert-6lowpan-backbone-router] | Thubert, P., "6LoWPAN Backbone Router", Internet-Draft draft-thubert-6lowpan-backbone-router-03, February 2013. |
| [I-D.ietf-netconf-zerotouch] | Watsen, K., Hanna, S., Clarke, J. and M. Abrahamsson, "Zero Touch Provisioning for NETCONF Call Home (ZeroTouch)", Internet-Draft draft-ietf-netconf-zerotouch-00, July 2014. |
| [I-D.kelsey-intarea-mesh-link-establishment] | Kelsey, R., "Mesh Link Establishment", Internet-Draft draft-kelsey-intarea-mesh-link-establishment-06, May 2014. |
| [I-D.ohba-6tisch-security] | Chasko, S., Das, S., Lopez, R., Ohba, Y., Thubert, P. and A. Yegin, "Security Framework and Key Management Protocol Requirements for 6TiSCH", Internet-Draft draft-ohba-6tisch-security-01, March 2014. |
| [I-D.piro-6tisch-security-issues] | Piro, G., Boggia, G. and L. Grieco, "Layer-2 security aspects for the IEEE 802.15.4e MAC", Internet-Draft draft-piro-6tisch-security-issues-02, June 2014. |