Constrained Join Proxy for Bootstrapping Protocols

Constrained Join Proxy for Bootstrapping Protocols Sandelman Software Works

mcr+ietf@sandelman.ca

vanderstok consultancy

stokcons@bbhmail.nl

Cisco Systems

pkampana@cisco.com

Internet anima Working Group Internet-Draft This document extends the work of Bootstrapping Remote Secure Key Infrastructures (BRSKI) by replacing the Circuit-proxy between Pledge and Registrar by a stateless/stateful constrained Join Proxy. The constrained Join Proxy is a mesh neighbor of the Pledge and can relay a DTLS session originating from a Pledge with only link-local addresses to a Registrar which is not a mesh neighbor of the Pledge. This document defines a protocol to securely assign a Pledge to a domain, represented by a Registrar, using an intermediary node between Pledge and Registrar. This intermediary node is known as a “constrained Join Proxy”. An enrolled Pledge can act as a constrained Join Proxy.

The Bootstrapping Remote Secure Key Infrastructure (BRSKI) protocol described in provides a solution for a secure zero-touch (automated) bootstrap of new (unconfigured) devices. In the context of BRSKI, new devices, called “Pledges”, are equipped with a factory-installed Initial Device Identifier (IDevID) (see ), and are enrolled into a network. BRSKI makes use of Enrollment over Secure Transport (EST) with vouchers to securely enroll devices. A Registrar provides the security anchor of the network to which a Pledge enrolls. In this document, BRSKI is extended such that a Pledge connects to “Registrars” via a constrained Join Proxy. In particular, the underlying IP network is assumed to be a mesh newtork as described in , although other IP-over-foo networks are not excluded. A complete specification of the terminology is pointed at in . The specified solutions in and are based on POST or GET requests to the EST resources (/cacerts, /simpleenroll, /simplereenroll, /serverkeygen, and /csrattrs), and the brski resources (/requestvoucher, /voucher_status, and /enrollstatus). These requests use https and may be too large in terms of code space or bandwidth required for constrained devices. Constrained devices which may be part of constrained networks , typically implement the IPv6 over Low-Power Wireless personal Area Networks (6LoWPAN) and Constrained Application Protocol (CoAP) . CoAP can be run with the Datagram Transport Layer Security (DTLS) as a security protocol for authenticity and confidentiality of the messages. This is known as the “coaps” scheme. A constrained version of EST, using Coap and DTLS, is described in . The extends with BRSKI artifacts such as voucher, request voucher, and the protocol extensions for constrained Pledges. DTLS is a client-server protocol relying on the underlying IP layer to perform the routing between the DTLS Client and the DTLS Server. However, the Pledge will not be IP routable over the mesh network until it is authenticated to the mesh network. A new Pledge can only initially use a link-local IPv6 address to communicate with a mesh neighbor until it receives the necessary network configuration parameters. The Pledge receives these configuration parameters from the Registrar. When the Registrar is not a direct neighbor of the Registrar but several hops away, the Pledge discovers a neighbor constrained Join Proxy, which transmits the DTLS protected request coming from the Pledge to the Registrar. The constrained Join Proxy must be enrolled previously such that the message from constrained Join Proxy to Registrar can be routed over one or more hops. During enrollment, a DTLS connection is required between Pledge and Registrar. Once a Pledge is enrolled, it can act as constrained Join Proxy between other Pledges and the enrolling Registrar. This document specifies a new form of constrained Join Proxy and protocol to act as intermediary between Pledge and Registrar to relay DTLS messages between Pledge and Registrar. Two modes of the constrained Join Proxy are specified:

This document is very much inspired by text published earlier in . outlined the various options for building a constrained Join Proxy. adopted only the Circuit Proxy method (1), leaving the other methods as future work. The stateful and stateless modes differ in the way that they store the state required to forward the return packet to the pledge. Similar to the difference between storing and non_storing Modes of Operations (MOP) in RPL . In the stateful method, the return forward state is stored in the join proxy. In the stateless method, the return forward state is stored in the network.

The following terms are defined in , and are used identically as in that document: artifact, imprint, domain, Join Registrar/Coordinator (JRC), Pledge, and Voucher. In this document, the term “Registrar” is used throughout instead of “Join Registrar/Coordinator (JRC)”. The term “installation network” refers to all devices in the installation and the network connections between them. The term “installation IP_address” refers to an address out of the set of addresses which are routable over the whole installation network. The “Constrained Join Proxy” enables a pledge that is multiple hops away from the Registrar, to securely execute the BRSKI protocol over a secure channel. The term “join Proxy” is used interchangeably with the term “constrained Join Proxy” throughout this document.

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 BCP 14 when, and only when, they appear in all capitals, as shown here.

As depicted in the , the Pledge (P), in a Low-Power and Lossy Network (LLN) mesh can be more than one hop away from the Registrar (R) and not yet authenticated into the network. In this situation, the Pledge can only communicate one-hop to its nearest neighbor, the constrained Join Proxy (J) using their link-local IPv6 addresses. However, the Pledge (P) needs to communicate with end-to-end security with a Registrar to authenticate and get the relevant system/network parameters. If the Pledge (P), knowing the IP-address of the Registrar, initiates a DTLS connection to the Registrar, then the packets are dropped at the constrained Join Proxy (J) since the Pledge (P) is not yet admitted to the network or there is no IP routability to Pledge (P) for any returned messages from the Registrar.

Without routing the Pledge (P) cannot establish a secure connection to the Registrar (R) over multiple hops in the network. Furthermore, the Pledge (P) cannot discover the IP address of the Registrar (R) over multiple hops to initiate a DTLS connection and perform authentication. To overcome the problems with non-routability of DTLS packets and/or discovery of the destination address of the Registrar, the constrained Join Proxy is introduced. This constrained Join Proxy functionality is configured into all authenticated devices in the network which may act as a constrained Join Proxy for Pledges. The constrained Join Proxy allows for routing of the packets from the Pledge using IP routing to the intended Registrar. An authenticated constrained Join Proxy can discover the routable IP address of the Registrar over multiple hops. The following specifies the two constrained Join Proxy modes. A comparison is presented in . When a mesh network is set up, it consists of a Registrar and a set of connected pledges. No constrained Join Proxies are present. The wanted end-state is a network with a Registrar and a set of enrolled devices. Some of these enrolled devices can act as constrained Join Proxies. Pledges can only employ link-local communication untill they are enrolled. A Pledge will regularly try to discover a constrained Join Proxy or a Registrar with link-local discovery requests. The Pledges which are neigbors of the Registrar will discover the Registrar and be enrolled following the BRSKI protocol. An enrolled device can act as constrained Join Proxy. The Pledges which are not a neighbor of the Registrar will eventually discover a constrained Join Proxy and follow the BRSKI protocol to be enrolled. While this goes on, more and more constrained Join Proxies with a larger hop distance to the Registrar will emerge. The network should be configured such that at the end of the enrollment process, all pledges have discovered a neigboring constrained Join Proxy or the Registrar, and all “legal” Pledges are enrolled.

A Join Proxy can operate in two modes: Stateful mode Stateless mode A Join Proxy MAY implement both. A mechanism to switch between modes is out of scope of this document. It is recommended that a Join Proxy uses only one of these modes at any given moment during an installation lifetime.

In stateful mode, the Join Proxy forwards the DTLS messages to the Registrar. Assume that the Pledge does not know the IP address of the Registrar it needs to contact. The Join Proxy has been enrolled via the Registrar and learns the IP address and port of the Registrar, for example by using the discovery mechanism described in . The Pledge first discovers (see ) and selects the most appropriate Join Proxy. (Discovery can also be based upon section 4.1). For service discovery via DNS-SD , this document specifies the service names in . The Pledge initiates its request as if the Join Proxy is the intended Registrar. The Join Proxy receives the message at a discoverable join-port. The Join Proxy constructs an IP packet by copying the DTLS payload from the message received from the Pledge, and provides source and destination addresses to forward the message to the intended Registrar. The Join Proxy stores the 4-tuple array of the messages received from the Registrar and copies it back to the header of the message returned to the Pledge. In the various steps of the message flow are shown, with 5684 being the standard coaps port:

The stateless Join Proxy aims to minimize the requirements on the constrained Join Proxy device. Stateless operation requires no memory in the Join Proxy device, but may also reduce the CPU impact as the device does not need to search through a state table. If an untrusted Pledge that can only use link-local addressing wants to contact a trusted Registrar, and the Registrar is more than one hop away, it sends its DTLS messages to the Join Proxy. When a Pledge attempts a DTLS connection to the Join Proxy, it uses its link-local IP address as its IP source address. This message is transmitted one-hop to a neighboring (Join Proxy) node. Under normal circumstances, this message would be dropped at the neighbor node since the Pledge is not yet IP routable or is not yet authenticated to send messages through the network. However, if the neighbor device has the Join Proxy functionality enabled; it routes the DTLS message to its Registrar of choice. The Join Proxy transforms the DTLS message to a JPY message which includes the DTLS data as payload, and sends the JPY message to the join-port of the Registrar. The JPY message payload consists of two parts: Header (H) field: consisting of the source link-local address and port of the Pledge (P), and Contents (C) field: containing the original DTLS payload. On receiving the JPY message, the Registrar (or proxy) retrieves the two parts. The Registrar transiently stores the Header field information. The Registrar uses the Contents field to execute the Registrar functionality. However, when the Registrar replies, it also extends its DTLS message with the header field in a JPY message and sends it back to the Join Proxy. The Registrar SHOULD NOT assume that it can decode the Header Field, it should simply repeat it when responding. The Header contains the original source link-local address and port of the Pledge from the transient state stored earlier and the Contents field contains the DTLS payload. On receiving the JPY message, the Join Proxy retrieves the two parts. It uses the Header field to route the DTLS message containing the DTLS payload retrieved from the Contents field to the Pledge. In this scenario, both the Registrar and the Join Proxy use discoverable join-ports, for the Join Proxy this may be a default CoAP port. The depicts the message flow diagram:

The JPY message is constructed as a payload with media-type application/cbor Header and Contents fields together are one CBOR array of 5 elements: header field: containing a CBOR array with the Pledge IPv6 Link Local address as a CBOR byte string, the Pledge’s UDP port number as a CBOR integer, the IP address family (IPv4/IPv6) as a CBOR integer, and the proxy’s ifindex or other identifier for the physical port as CBOR integer. The header field is not DTLS encrypted. Content field: containing the DTLS payload as a CBOR byte string. The address family integer is defined in with:

The Join Proxy cannot decrypt the DTLS payload and has no knowledge of the transported media type.

The contents are DTLS encrypted. In CBOR diagnostic notation the payload JPY[H(IP_P:p_P)], will look like:

On reception by the Registrar, the Registrar MUST verify that the number of array elements is larger than or equal to 5, and reject the message when the number of array elements is smaller than 5. After replacing the 5th “content” element with the DTLS payload of the response message and leaving all other array elements unchanged, the Registrar returns the response message. Examples are shown in . The header field is completely opaque to the receiver. A Registrar MUST copy the header and return it unmodified in the return message.

It is assumed that Join Proxy seamlessly provides a coaps connection between Pledge and Registrar. In particular this section extends section 4.1 of for the constrained case. The discovery follows two steps with two alternatives for step 1: Step 1. Two alternatives exist (near and remote): Near: the Pledge is one hop away from the Registrar. The Pledge discovers the link-local address of the Registrar as described in . From then on, it follows the BRSKI process as described in and , using link-local addresses. Remote: the Pledge is more than one hop away from a relevant Registrar, and discovers the link-local address and join-port of a Join Proxy. The Pledge then follows the BRSKI procedure using the link-local address of the Join Proxy. Step 2. The enrolled Join Proxy discovers the join-port of the Registrar. The order in which the two alternatives of step 1 are tried is installation dependent. The trigger for discovery in Step 2 is implementation dependent. Once a Pledge is enrolled, it may function as Join Proxy. The Join Proxy functions are advertised as described below. In principle, the Join Proxy functions are offered via a join-port, and not the standard coaps port. Also, the Registrar offers a join-port to which the stateless Join Proxy sends the JPY message. The Join Proxy and Registrar show the extra join-port number when responding to a /.well-known/core discovery request addressed to the standard coap/coaps port. Three discovery cases are discussed: Join Proxy discovers Registrar, Pledge discovers Registrar, and Pledge discovers Join Proxy. Each discovery case considers three alternatives: CoAP based discovery, GRASP Based discovery, and 6tisch based discovery. The choice of discovery mechanism depends on the type of installation, and manufacturers can provide the pledge/Join Proxy with support for more than one discovery mechanism. The pledge/Join Proxy can be designed to dynamically try different discovery mechanisms until a successful discovery mechanism is found, or the choice of discovery mechanism could be configured during device installation.

In this section, the Join Proxy and Registrar are assumed to communicate via Link-Local addresses. This section describes the discovery of the Registrar by the Join Proxy.

The discovery of the coaps Registrar, using coap discovery, by the Join Proxy follows sections 6.3 and 6.5.1 of . The stateless Join Proxy can discover the join-port of the Registrar by sending a GET request to “/.well-known/core” including a resource type (rt) parameter with the value “brski.rjp” . Upon success, the return payload will contain the join-port of the Registrar.

; rt="brski.rjp" ]]> The discoverable port numbers are usually returned for Join Proxy resources in the <URI-Reference> of the payload (see section 5.1 of ).

This section is normative for uses with an ANIMA ACP. In the context of autonomic networks, the Join Proxy uses the DULL GRASP M_FLOOD mechanism to announce itself. Section 4.1.1 of discusses this in more detail. The Registrar announces itself using ACP instance of GRASP using M_FLOOD messages. Autonomic Network Join Proxies MUST support GRASP discovery of Registrar as described in section 4.3 of .

The discovery of the Registrar by the Join Proxy uses the enhanced beacons as discussed in .

In this section, the Pledge and Registrar are assumed to communicate via Link-Local addresses. This section describes the discovery of the Registrar by the Pledge.

The discovery of the coaps Registrar, using coap discovery, by the Pledge follows sections 6.3 and 6.5.1 of .

This section is normative for uses with an ANIMA ACP. In the context of autonomic networks, the Pledge uses the DULL GRASP M_FLOOD mechanism to announce itself. Section 4.1.1 of discusses this in more detail. The Registrar announces itself using ACP instance of GRASP using M_FLOOD messages. Autonomic Network Join Proxies MUST support GRASP discovery of Registrar as described in section 4.3 of .

The discovery of Registrar by the Pledge uses the enhanced beacons as discussed in .

In this section, the Pledge and Join Proxy are assumed to communicate via Link-Local addresses. This section describes the discovery of the Join Proxy by the Pledge.

In the context of a coap network without Autonomic Network support, discovery follows the standard coap policy. The Pledge can discover a Join Proxy by sending a link-local multicast message to ALL CoAP Nodes with address FF02::FD. Multiple or no nodes may respond. The handling of multiple responses and the absence of responses follow section 4 of . The join-port of the Join Proxy is discovered by sending a GET request to “/.well-known/core” including a resource type (rt) parameter with the value “brski.jp” . Upon success, the return payload will contain the join-port. The example below shows the discovery of the join-port of the Join Proxy.

; rt="brski.jp" ]]> Port numbers are assumed to be the default numbers 5683 and 5684 for coap and coaps respectively (sections 12.6 and 12.7 of ) when not shown in the response. Discoverable port numbers are usually returned for Join Proxy resources in the <URI-Reference> of the payload (see section 5.1 of ).

This section is normative for uses with an ANIMA ACP. The Pledge MUST listen for GRASP M_FLOOD announcements of the objective: “AN_Proxy”. See section 4.1.1 for the details of the objective.

The discovery of the Join Proxy by the Pledge uses the enhanced beacons as discussed in .

The stateful and stateless mode of operation for the Join Proxy have their advantages and disadvantages. This section should enable to make a choice between the two modes based on the available device resources and network bandwidth.

All the concerns in section 4.1 apply. The Pledge can be deceived by malicious Join Proxy announcements. The Pledge will only join a network to which it receives a valid voucher . Once the Pledge joined, the payload between Pledge and Registrar is protected by DTLS. A malicious constrained Join Proxy has a number of routing possibilities: It sends the message on to a malicious Registrar. This is the same case as the presence of a malicious Registrar discussed in RFC 8995. It does not send on the request or does not return the response from the Registrar. This is the case of the not responding or crashing Registrar discussed in RFC 8995. It uses the returned response of the Registrar to enroll itself in the network. With very low probability it can decrypt the response. Successful enrollment is deemed too unlikely. It uses the request from the pledge to appropriate the pledge certificate, but then it still needs to acquire the private key of the pledge. Also this is assumed to be highly unlikely. A malicious node can construct an invalid Join Proxy message. Suppose, the destination port is the coaps port. In that case, a Join Proxy can accept the message and add the routing addresses without checking the payload. The Join Proxy then routes it to the Registrar. In all cases, the Registrar needs to receive the message at the join-port, checks that the message consists of two parts and uses the DTLS payload to start the BRSKI procedure. It is highly unlikely that this malicious payload will lead to node acceptance. A malicious node can sniff the messages routed by the constrained Join Proxy. It is very unlikely that the malicious node can decrypt the DTLS payload. A malicious node can read the header field of the message sent by the stateless Join Proxy. This ability does not yield much more information than the visible addresses transported in the network packets. It should be noted here that the contents of the CBOR array used to convey return address information is not DTLS protected. When the communication between JOIN Proxy and Registrar passes over an unsecure network, an attacker can change the CBOR array, causing the Registrar to deviate traffic from the intended Pledge. These concerns are also expressed in . It is also pointed out that the encryption in the source is a local matter. Similarly to , the use of AES-CCM with a 64-bit tag is recommended, combined with a sequence number and a replay window. If such scenario needs to be avoided, the constrained Join Proxy MUST encrypt the CBOR array using a locally generated symmetric key. The Registrar is not able to examine the encrypted result, but does not need to. The Registrar stores the encrypted header in the return packet without modifications. The constrained Join Proxy can decrypt the contents to route the message to the right destination. In some installations, layer 2 protection is provided between all member pairs of the mesh. In such an enviroment encryption of the CBOR array is unnecessay because the layer 2 protection already provide it.

This specification registers two new Resource Type (rt=) Link Target Attributes in the “Resource Type (rt=) Link Target Attribute Values” subregistry under the “Constrained RESTful Environments (CoRE) Parameters” registry per the procedure.

This specification registers two service names under the “Service Name and Transport Protocol Port Number” registry.

Contact: IESG Description: Bootstrapping Remote Secure Key Infrastructure constrained Join Proxy Reference: [this document] Service Name: brski-rjp Transport Protocol(s): udp Assignee: IESG Contact: IESG Description: Bootstrapping Remote Secure Key Infrastructure Registrar join-port used by stateless constrained Join Proxy Reference: [this document] ]]>

Many thanks for the comments by Cartsen, Bormann, Brian Carpenter, Esko Dijk, Toerless Eckert, Russ Housley, Ines Robles, Jürgen Schönwälder, Mališa Vučinić, and Rob Wilton.

Sandeep Kumar, Sye loong Keoh, and Oscar Garcia-Morchon are the co-authors of the draft-kumar-dice-dtls-relay-02. Their draft has served as a basis for this document. Much text from their draft is copied over to this draft.

Discovery of Join Proxy and Registrar ports

Registrar used throughout instead of EST server Emphasized additional Join Proxy port for Join Proxy and Registrar updated discovery accordingly updated stateless Join Proxy JPY header JPY header described with CDDL Example simplified and corrected

copied from vanderstok-anima-constrained-join-proxy-05

&RFC6347; &RFC8366; &RFC8995; &I-D.ietf-ace-coap-est; &I-D.ietf-anima-constrained-voucher; &RFC8949; &RFC8990; IEEE 802.1AR Secure Device Identifier Address Family Numbers &RFC2119; &RFC8174; &RFC6763; &I-D.richardson-anima-state-for-joinrouter; &I-D.ietf-6tisch-enrollment-enhanced-beacon; &RFC6690; &RFC7030; &RFC7102; &RFC7228; &I-D.kumar-dice-dtls-relay; &RFC4944; &RFC3610; &RFC7252; &RFC6775; &RFC8974; &RFC6550;

The examples show the request “GET coaps://192.168.1.200:5965/est/crts” to a Registrar. The header generated between Join Proxy and Registrar and from Registrar to Join Proxy are shown in detail. The DTLS payload is not shown. The request from Join Proxy to Registrar looks like:

]]> In CBOR Diagnostic:

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