[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-vucinic-6tisch-minimal-security) 00 01 02 03 04

6TiSCH Working Group                                     M. Vucinic, Ed.
Internet-Draft                                  University of Montenegro
Intended status: Standards Track                                J. Simon
Expires: May 3, 2018                                      Analog Devices
                                                               K. Pister
                                       University of California Berkeley
                                                           M. Richardson
                                                Sandelman Software Works
                                                        October 30, 2017


                 Minimal Security Framework for 6TiSCH
                 draft-ietf-6tisch-minimal-security-04

Abstract

   This document describes the minimal configuration required for a new
   device, called "pledge", to securely join a 6TiSCH (IPv6 over the
   TSCH mode of IEEE 802.15.4e) network.  The entities involved use CoAP
   (Constrained Application Protocol) and OSCORE (Object Security for
   Constrained RESTful Environments).  The configuration requires that
   the pledge and the JRC (join registrar/coordinator, a central
   entity), share a symmetric key.  How this key is provisioned is out
   of scope of this document.  The result of the joining process is that
   the JRC configures the pledge with link-layer keying material and a
   short link-layer address.  This specification also defines a new
   Stateless-Proxy CoAP option.  Additional security mechanisms may be
   added on top of this minimal framework.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 3, 2018.






Vucinic, et al.            Expires May 3, 2018                  [Page 1]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  One-Touch Assumption  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Pre-Shared Key  . . . . . . . . . . . . . . . . . . . . .   4
   4.  Join Overview . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Step 1 - Enhanced Beacon  . . . . . . . . . . . . . . . .   6
     4.2.  Step 2 - Neighbor Discovery . . . . . . . . . . . . . . .   7
     4.3.  Step 3 - Join Request . . . . . . . . . . . . . . . . . .   7
     4.4.  Step 4 - Join Response  . . . . . . . . . . . . . . . . .   8
   5.  Architectural Overview and Communication through Join Proxy .   8
     5.1.  Stateless-Proxy CoAP Option . . . . . . . . . . . . . . .   9
   6.  OSCORE Security Context . . . . . . . . . . . . . . . . . . .  10
     6.1.  Persistency . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Specification of Join Request . . . . . . . . . . . . . . . .  11
   8.  Specification of Join Response  . . . . . . . . . . . . . . .  11
     8.1.  Link-layer Keys Transported in COSE Key Set . . . . . . .  12
     8.2.  Short Address . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Error Handling and Retransmission . . . . . . . . . . . . . .  13
   10. Parameters  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   11. Mandatory to Implement Algorithms . . . . . . . . . . . . . .  14
   12. Link-layer Requirements . . . . . . . . . . . . . . . . . . .  14
   13. Rekeying and Rejoin . . . . . . . . . . . . . . . . . . . . .  15
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   15. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  16
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     16.1.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  16
   17. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     18.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .  19



Vucinic, et al.            Expires May 3, 2018                  [Page 2]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   This document presumes a 6TiSCH network as described by [RFC7554],
   [RFC8180], [I-D.ietf-6tisch-6top-protocol], and
   [I-D.ietf-6tisch-terminology].  By design, nodes in a 6TiSCH network
   [RFC7554] have their radio turned off most of the time, to conserve
   energy.  As a consequence, the link used by a new device for joining
   the network has limited bandwidth [RFC8180].  The secure join
   solution defined in this document therefore keeps the number of over-
   the-air exchanges for join purposes to a minimum.

   The micro-controllers at the heart of 6TiSCH nodes have a small
   amount of code memory.  It is therefore paramount to reuse existing
   protocols available as part of the 6TiSCH stack.  At the application
   layer, the 6TiSCH stack already relies on CoAP [RFC7252] for web
   transfer, and on OSCORE [I-D.ietf-core-object-security] for its end-
   to-end security.  The secure join solution defined in this document
   therefore reuses those two protocols as its building blocks.

   This document defines a secure join solution for a new device, called
   "pledge", to securely join a 6TiSCH network.  The specification
   configures different layers of the 6TiSCH protocol stack and also
   defines a new CoAP option.  It assumes the presence of a JRC (join
   registrar/coordinator), a central entity.  It further assumes that
   the pledge and the JRC share a symmetric key, called PSK (pre-shared
   key).  How the PSK is installed is out of scope of this document.

   When the pledge seeks admission to a 6TiSCH network, it first
   synchronizes to it, by initiating the passive scan defined in
   [IEEE802.15.4-2015].  The pledge then exchanges messages with the
   JRC; these messages can be forwarded by nodes already part of the
   6TiSCH network.  The messages exchanged allow the JRC and the pledge
   to mutually authenticate, based on the PSK.  They also allow the JRC
   to configure the pledge with link-layer keying material and a short
   link-layer address.  After this secure joining process successfully
   completes, the joined node can establish an end-to-end secure session
   with an Internet host.  The joined node can also interact with its
   neighbors to request additional bandwidth using the 6top Protocol
   [I-D.ietf-6tisch-6top-protocol].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  These




Vucinic, et al.            Expires May 3, 2018                  [Page 3]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   words may also appear in this document in lowercase, absent their
   normative meanings.

   The reader is expected to be familiar with the terms and concepts
   defined in [I-D.ietf-6tisch-terminology], [RFC7252],
   [I-D.ietf-core-object-security], and [RFC8152].

   The specification also includes a set of informative examples using
   the CBOR diagnostic notation [I-D.ietf-cbor-cddl].

   The following terms are used throughout this document:

   pledge:  The new device that wishes to join a 6TiSCH network.

   joined node:  The new device, after having completed the join
      process, often just called a node.

   join proxy (JP):  A node already part of the 6TiSCH network that
      serves as a relay to provide connectivity between the pledge and
      the JRC.

   join registrar/coordinator (JRC):  A central entity responsible for
      the authentication, authorization and configuration of the pledge.

3.  One-Touch Assumption

   This document assumes a one-touch scenario.  The pledge is
   provisioned with a PSK before attempting to join the network, and the
   same PSK (as well as the uniquer identifier of the pledge) is
   provisioned on the JRC.

   There are many ways by which this provisioning can be done.
   Physically, the PSK can be written into the pledge using a number of
   mechanisms, such as a JTAG interface, a serial (craft) console
   interface, pushing buttons simultaneously on different devices, over-
   the-air configuration in a Faraday cage, etc.  The provisioning can
   be done by the vendor, the manufacturer, the integrator, etc.

   Details of how this provisioning is done is out of scope of this
   document.  What is assumed is that there can be a secure, private
   conversation between the JRC and the pledge, and that the two devices
   can exchange the PSK.

3.1.  Pre-Shared Key

   The PSK SHOULD be at least 128 bits in length, generated uniformly at
   random.  It is RECOMMENDED to generate the PSK with a




Vucinic, et al.            Expires May 3, 2018                  [Page 4]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   cryptographically secure pseudorandom number generator.  Each pledge
   SHOULD be provisioned with a unique PSK.

4.  Join Overview

   This section describes the steps taken by a pledge in a 6TiSCH
   network.  When a pledge seeks admission to a 6TiSCH network, the
   following exchange occurs:

   1.  The pledge listens for an Enhanced Beacon (EB) frame
       [IEEE802.15.4-2015].  This frame provides network synchronization
       information, and tells the device when it can send a frame to the
       node sending the beacons, which plays the role of join proxy (JP)
       for the pledge, and when it can expect to receive a frame.

   2.  The pledge configures its link-local IPv6 address and advertises
       it to the join proxy (JP).

   3.  The pledge sends a Join Request to JP in order to securely
       identify itself to the network.  The Join Request is directed to
       the JRC, which may be co-located on the JP or another device.

   4.  In case of successful processing of the request, the pledge
       receives a join response from JRC (via the JP) that sets up one
       or more link-layer keys used to authenticate and encrypt
       subsequent transmissions to peers, and a short link-layer address
       for the pledge.

   From the pledge's perspective, minimal joining is a local phenomenon
   - the pledge only interacts with the JP, and it need not know how far
   it is from the 6LBR, or how to route to the JRC.  Only after
   establishing one or more link-layer keys does it need to know about
   the particulars of a 6TiSCH network.

   The process is shown as a transaction diagram in Figure 1:
















Vucinic, et al.            Expires May 3, 2018                  [Page 5]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


      +--------+                 +-------+                 +--------+
      | pledge |                 |  JP   |                 |  JRC   |
      |        |                 |       |                 |        |
      +--------+                 +-------+                 +--------+
         |                          |                          |
         |<---Enhanced Beacon (1)---|                          |
         |                          |                          |
         |<-Neighbor Discovery (2)->|                          |
         |                          |                          |
         |-----Join Request (3)-----|------Join Request (3a)-->|
         |                          |                          |
         |<---Join Response (4)-----|-----Join Response (4a)---|
         |                          |                          |

             Figure 1: Overview of a successful join process.

   The details of each step are described in the following sections.

4.1.  Step 1 - Enhanced Beacon

   The pledge synchronizes to the network by listening for, and
   receiving, an Enhanced Beacon (EB) sent by a node already in the
   network.  This process is entirely defined by [IEEE802.15.4-2015],
   and described in [RFC7554].

   Once the pledge hears an EB, it synchronizes to the joining schedule
   using the cells contained in the EB.  The pledge can hear multiple
   EBs; the selection of which EB to use is out of the scope for this
   document, and is discussed in [RFC7554].  Implementers SHOULD make
   use of information such as: what Personal Area Network Identifier
   (PAN ID) [IEEE802.15.4-2015] the EB contains, whether the source
   link-layer address of the EB has been tried before, what signal
   strength the different EBs were received at, etc.  In addition, the
   pledge may be pre-configured to search for EBs with a specific PAN
   ID.

   Once the pledge selects the EB, it synchronizes to it and transitions
   into a low-power mode.  It deeply duty cycles its radio, switching
   the radio on when the provided schedule indicates slots which the
   pledge may use for the join process.  During the remainder of the
   join process, the node that has sent the EB to the pledge plays the
   role of JP.

   At this point, the pledge may proceed to step 2, or continue to
   listen for additional EBs.






Vucinic, et al.            Expires May 3, 2018                  [Page 6]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


4.2.  Step 2 - Neighbor Discovery

   The pledge forms its link-local IPv6 address based on EUI-64, as per
   [RFC4944].  The Neighbor Discovery exchange shown in Figure 1 refers
   to a single round trip Neighbor Solicitation / Neighbor Advertisement
   exchange between the pledge and the JP (Section 5.5.1 of [RFC6775]).
   The pledge uses the link-local IPv6 address for all subsequent
   communication with the JP during the join process.

   Note that ND exchanges at this point are not protected with link-
   layer security as the pledge is not in possession of the keys.  How
   JP accepts these unprotected frames is discussed in Section 12.

   The pledge and the JP SHOULD keep a separate neighbor cache for
   untrusted entries and use it to store each other's information during
   the join process.  Mixing neighbor entries belonging to pledges and
   nodes that are part of the network opens up the JP to a DoS attack.
   How the pledge and JP decide to transition each other from untrusted
   to trusted cache, once the join process completes, is out of scope.
   One implementation technique is to use the information whether the
   incoming frames are secured at the link layer.

4.3.  Step 3 - Join Request

   The Join Request is a message sent from the pledge to the JP using
   the shared slot as described in the EB, and which the JP forwards to
   the JRC.  The JP forwards the Join Request to the JRC on the existing
   6TiSCH network.  How exactly this happens is out of scope of this
   document; some networks may wish to dedicate specific slots for this
   join traffic.

   The Join Request is authenticated/encrypted end-to-end using an AEAD
   algorithm from [RFC8152] and a key derived from the PSK, the pledge's
   EUI-64 and a request-specific constant value.  Algorithms which MUST
   be implemented are specified in Section 11.

   The nonce used when securing the Join Request is derived from the
   PSK, the pledge's EUI-64 and a monotonically increasing counter
   initialized to 0 when first starting.

   Join Request construction is specified in Section 7, while the
   details on processing can be found in Section 7 of
   [I-D.ietf-core-object-security].








Vucinic, et al.            Expires May 3, 2018                  [Page 7]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


4.4.  Step 4 - Join Response

   The Join Response is sent by the JRC to the pledge, and is forwarded
   through the JP as it serves as a stateless relay.  The packet
   containing the Join Response travels from the JRC to JP using the
   operating routes in the 6TiSCH network.  The JP delivers it to the
   pledge using the slot information it has indicated in the EB it sent.
   The JP operates as the application-layer proxy, and does not keep any
   state to relay the message.  It uses information sent in the clear
   within the Join Response to decide where to forward to.

   The Join Response is authenticated/encrypted end-to-end using an AEAD
   algorithm from [RFC8152].  The key used to protect the response is
   different from the one used to protect the request (both are derived
   from the PSK, as explained in Section 6).  The response is protected
   using the same nonce as in the request.

   The Join Response contains one or more link-layer key(s) that the
   pledge will use for subsequent communication.  Each key that is
   provided by the JRC is associated with an 802.15.4 key identifier.
   In other link-layer technologies, a different identifier may be
   substituted.  The Join Response also contains an IEEE 802.15.4 short
   address [IEEE802.15.4-2015] assigned by the JRC to the pledge, and
   optionally the IPv6 address of the JRC.

   Join Response construction is specified in Section 8, while the
   details on processing can be found in Section 7 of
   [I-D.ietf-core-object-security].

5.  Architectural Overview and Communication through Join Proxy

   The Join Request/Join Response exchange in Figure 1 is carried over
   CoAP [RFC7252] and secured using OSCORE
   [I-D.ietf-core-object-security].  The pledge plays the role of a CoAP
   client; the JRC plays the role of a CoAP server.  The JP implements
   CoAP forward proxy functionality [RFC7252].  Because the JP can also
   be a constrained device, it cannot implement a cache.  Rather, the JP
   processes forwarding-related CoAP options and makes requests on
   behalf of the pledge, in a stateless manner.

   The pledge communicates with a JP over link-local IPv6 addresses.
   The pledge designates a JP as a proxy by including the Proxy-Scheme
   option with value "coap" (CoAP-to-CoAP proxy) in CoAP requests it
   sends to the JP.  The pledge MUST include the Uri-Host option with
   its value set to the well-known JRC's alias "6tisch.arpa".  This
   allows the pledge to join without knowing the IPv6 address of the
   JRC.  The pledge learns the actual IPv6 address of the JRC from the
   Join Response; it uses it once joined in order to operate as a JP.



Vucinic, et al.            Expires May 3, 2018                  [Page 8]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   The JRC can be co-located on the 6LBR.  Before the 6TiSCH network is
   started, the 6LBR MUST be provisioned with the IPv6 address of the
   JRC.

5.1.  Stateless-Proxy CoAP Option

   The CoAP proxy defined in [RFC7252] keeps per-client state
   information in order to forward the response towards the originator
   of the request.  This state information includes at least the CoAP
   token, the IPv6 address of the host, and the UDP source port number.
   If the JP used the stateful CoAP proxy defined in [RFC7252], it would
   be prone to Denial-of-Service (DoS) attacks, due to its limited
   memory.

   The Stateless-Proxy CoAP option Figure 2 allows the JP to be entirely
   stateless.  This option inserts, in the request, the state
   information needed for relaying the response back to the client.  The
   proxy still keeps some general state (e.g. for congestion control or
   request retransmission), but no per-client state.

   The Stateless-Proxy CoAP option is critical, Safe-to-Forward, not
   part of the cache key, not repeatable and opaque.  When processed by
   OSCORE, the Stateless-Proxy option is neither encrypted nor integrity
   protected.

        +-----+---+---+---+---+-----------------+--------+--------+
        | No. | C | U | N | R | Name            | Format | Length |
        +-----+---+---+---+---+-----------------+--------+--------|
        | TBD | x |   | x |   | Stateless-Proxy | opaque | 1-255  |
        +-----+---+---+---+---+-----------------+--------+--------+
             C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

                   Figure 2: Stateless-Proxy CoAP Option

   Upon reception of a Stateless-Proxy option, the CoAP server MUST echo
   it in the response.  The value of the Stateless-Proxy option is
   internal proxy state that is opaque to the server.  Example state
   information includes the IPv6 address of the client, its UDP source
   port, and the CoAP token.  For security reasons, the state
   information MUST be authenticated, MUST include a freshness indicator
   (e.g. a sequence number or timestamp) and MAY be encrypted.  The
   proxy may use an appropriate COSE structure [RFC8152] to wrap the
   state information as the value of the Stateless-Proxy option.  The
   key used for encryption/authentication of the state information may
   be known only to the proxy.

   Once the proxy has received the CoAP response with Stateless-Proxy
   option present, it decrypts/authenticates it, checks the freshness



Vucinic, et al.            Expires May 3, 2018                  [Page 9]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   indicator and constructs the response for the client, based on the
   information present in the option value.

   Note that a CoAP proxy using the Stateless-Proxy option is not able
   to return a 5.04 Gateway Timeout Response Code in case the request to
   the server times out.  Likewise, if the response to the proxy's
   request does not contain the Stateless-Proxy option, for example when
   the option is not supported by the server, the proxy is not able to
   return the response to the client.

6.  OSCORE Security Context

   The OSCORE security context MUST be derived at the pledge and the JRC
   as per Section 3 of [I-D.ietf-core-object-security].

   o  the Master Secret MUST be the PSK.

   o  the Master Salt MUST be pledge's EUI-64.

   o  the Sender ID of the pledge MUST be set to byte string 0x00.

   o  the Recipient ID (ID of the JRC) MUST be set to byte string 0x01.

   o  the Algorithm MUST be set to the value from [RFC8152], agreed out-
      of-band by the same mechanism used to provision the PSK.  The
      default is AES-CCM-16-64-128.

   o  the Key derivation function MUST be agreed out-of-band.  Default
      is HKDF SHA-256.

   The derivation in [I-D.ietf-core-object-security] results in traffic
   keys and a common IV for each side of the conversation.  Nonces are
   constructed by XOR'ing the common IV with the current sequence number
   and sender identifier.  For details on nonce construction, refer to
   [I-D.ietf-core-object-security].

   It is RECOMMENDED that a PAN ID be provisioned to the pledge out-of-
   band by the same mechanism used to provision the PSK.  This prevents
   the pledge from attempting to join a wrong network.  If the pledge is
   not provisioned with the PAN ID, it SHOULD attempt to join one
   network at a time.  In that case, implementations MUST ensure that
   multiple CoAP requests to different JRCs result in the use of the
   same OSCORE context so that sequence numbers are properly incremented
   for each request.







Vucinic, et al.            Expires May 3, 2018                 [Page 10]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


6.1.  Persistency

   Implementations MUST ensure that mutable OSCORE context parameters
   (Sender Sequence Number, Replay Window) are stored in persistent
   memory.  A technique that prevents reuse of sequence numbers,
   detailed in Section 6.5.1 of [I-D.ietf-core-object-security], MUST be
   implemented.  Each update of the OSCORE Replay Window MUST be written
   to persistent memory.

   This is an important security requirement in order to guarantee nonce
   uniqueness and resistance to replay attacks across reboots and
   rejoins.  Traffic between the pledge and the JRC is rare, making
   security outweigh the cost of writing to persistent memory.

7.  Specification of Join Request

   The Join Request the pledge sends SHALL be mapped to a CoAP request:

   o  The request method is POST.

   o  The type is Non-confirmable (NON).

   o  The Proxy-Scheme option is set to "coap".

   o  The Uri-Host option is set to "6tisch.arpa".

   o  The Uri-Path option is set to "j".

   o  The Object-Security option SHALL be set according to
      [I-D.ietf-core-object-security].  The OSCORE Context Hint SHALL be
      set to pledge's EUI-64.  The OSCORE Context Hint allows the JRC to
      retrieve the security context for a given pledge.

   o  The payload is empty.

8.  Specification of Join Response

   If the JRC successfully processes the Join Request using OSCORE, and
   if the pledge is authorized to join the network, the Join Response
   the JRC sends back to the pledge SHALL be mapped to a CoAP response:

   o  The response Code is 2.04 (Changed).

   o  The payload is a CBOR [RFC7049] array containing, in order:

      *  the COSE Key Set, specified in [RFC8152], containing one or
         more link-layer keys.  The mapping of individual keys to
         802.15.4-specific parameters is described in Section 8.1.



Vucinic, et al.            Expires May 3, 2018                 [Page 11]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


      *  the link-layer short address to be used by the pledge.  The
         format of the short address follows Section 8.2.

      *  optionally, the IPv6 address of the JRC transported as a byte
         string.  If the IPv6 address of the JRC is not present in the
         Join Response, this indicates the JRC is co-located with 6LBR,
         and has the same IPv6 address as the 6LBR.  The address of the
         6LBR can then be learned from DODAGID field in RPL DIOs
         [RFC6550].

   response_payload = [
       COSE_KeySet,
       short_address,
       ? JRC_address : bstr,
   ]

8.1.  Link-layer Keys Transported in COSE Key Set

   Each key in the COSE Key Set [RFC8152] SHALL be a symmetric key.  If
   the "kid" parameter of the COSE Key structure is present, the
   corresponding keys SHALL belong to an IEEE 802.15.4 KeyIdMode 0x01
   class.  In that case, parameter "kid" of the COSE Key structure SHALL
   be used to carry the IEEE 802.15.4 KeyIndex value.  If the "kid"
   parameter is not present in the transported key, the application
   SHALL consider the key to be an IEEE 802.15.4 KeyIdMode 0x00
   (implicit) key.  This document does not support IEEE 802.15.4
   KeyIdMode 0x02 and 0x03 class keys.

8.2.  Short Address

   The "short_address" structure transported as part of the join
   response payload represents the IEEE 802.15.4 short address assigned
   to the pledge.  It is encoded as a CBOR array object, containing, in
   order:

   o  Byte string, containing the 16-bit address.

   o  Optionally, the lease time parameter, "lease_asn".  The value of
      the "lease_asn" parameter is the 5-byte Absolute Slot Number (ASN)
      corresponding to its expiration, carried as a byte string in
      network byte order.

   short_address = [
       address : bstr,
       ? lease_asn : bstr,
   ]





Vucinic, et al.            Expires May 3, 2018                 [Page 12]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   It is up to the joined node to request a new short address before the
   expiry of its previous address.  The mechanism by which the node
   requests renewal is the same as during join procedure, as described
   in Section 13.  The assigned short address is used for configuring
   both link-layer short address and IPv6 addresses.

9.  Error Handling and Retransmission

   Since the Join Request is mapped to a Non-confirmable CoAP message,
   OSCORE processing at JRC will silently drop the request in case of a
   failure.  This may happen for a number of reasons, including failed
   lookup of an appropriate security context, failed decryption,
   positive replay window lookup, formatting errors possibly due to
   malicious alterations in transit.  Silent drop at JRC prevents a DoS
   attack where an attacker could force the pledge to attempt joining
   one network at a time, until all networks have been tried.

   Using Non-confirmable CoAP message to transport Join Request also
   helps minimize the required CoAP state at the pledge and the Join
   Proxy, keeping it to a minimum typically needed to perform CoAP
   congestion control.  It does, however, introduce complexity at the
   application layer, as the pledge needs to implement a retransmission
   mechanism.

   The following binary exponential back-off algorithm is inspired by
   the one described in [RFC7252].  For each Join Request the pledge
   sends while waiting for a Join Response, the pledge MUST keep track
   of a timeout and a retransmission counter.  For a new Join Request,
   the timeout is set to a random value between TIMEOUT and (TIMEOUT *
   TIMEOUT_RANDOM_FACTOR), and the retransmission counter is set to 0.
   When the timeout is triggered and the retransmission counter is less
   than MAX_RETRANSMIT, the Join Request is retransmitted, the
   retransmission counter is incremented, and the timeout is doubled.
   Note that the retransmitted Join Request passes new OSCORE
   processing, such that the sequence number in the OSCORE context is
   properly incremented.  If the retransmission counter reaches
   MAX_RETRANSMIT on a timeout, the pledge SHOULD attempt to join the
   next advertised 6TiSCH network.  If the pledge receives a Join
   Response that successfully passed OSCORE processing, it cancels the
   pending timeout and processes the response.  The pledge MUST silently
   discard any response not protected with OSCORE, including error
   codes.  For default values of retransmission parameters, see
   Section 10.

   If all join attempts to advertised networks have failed, the pledge
   SHOULD signal to the user the presence of an error condition, through
   some out-of-band mechanism.




Vucinic, et al.            Expires May 3, 2018                 [Page 13]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


10.  Parameters

   This specification uses the following parameters:

                +-----------------------+----------------+
                | Name                  | Default Value  |
                +-----------------------+----------------+
                | TIMEOUT               | 10 s           |
                +-----------------------+----------------+
                | TIMEOUT_RANDOM_FACTOR | 1.5            |
                +-----------------------+----------------+
                | MAX_RETRANSMIT        | 4              |
                +----------------------------------------+

   The values of TIMEOUT, TIMEOUT_RANDOM_FACTOR, MAX_RETRANSMIT may be
   configured to values specific to the deployment.  The default values
   have been chosen to accommodate a wide range of deployments, taking
   into account dense networks.

11.  Mandatory to Implement Algorithms

   The mandatory to implement AEAD algorithm for use with OSCORE is AES-
   CCM-16-64-128 from [RFC8152].  This is the algorithm used for
   securing 802.15.4 frames, and hardware acceleration for it is present
   in virtually all compliant radio chips.  With this choice, CoAP
   messages are protected with an 8-byte CCM authentication tag, and the
   algorithm uses 13-byte long nonces.

   The mandatory to implement hash algorithm is SHA-256 [RFC4231].

12.  Link-layer Requirements

   In an operational 6TiSCH network, all frames MUST use link-layer
   frame security [RFC8180].  The frame security options MUST include
   frame authentication, and MAY include frame encryption.

   The pledge does not initially do any authentication of the EB frames,
   as it does not know the K1 key [RFC8180].  When sending frames, the
   pledge sends unencrypted and unauthenticated frames.  The JP accepts
   these frames (using the "exempt mode" in 802.15.4) for the duration
   of the join process.  How the JP learns whether the join process is
   ongoing is out of scope of this specification.

   As the EB itself cannot be authenticated by the pledge, an attacker
   may craft a frame that appears to be a valid EB, since the pledge can
   neither know the ASN a priori nor verify the address of the JP.  This
   opens up a possibility of DoS attack, as discussed in Section 14.
   Beacon authentication keys are discussed in [RFC8180].



Vucinic, et al.            Expires May 3, 2018                 [Page 14]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


13.  Rekeying and Rejoin

   This specification handles initial keying of the pledge.  For reasons
   such as rejoining after a long sleep, expiry of the short address, or
   node-initiated rekeying, the joined node MAY send a new Join Request
   using the already-established OSCORE security context.  The JRC then
   responds with up-to-date keys and a (possibly new) short address.
   How the joined node decides when to rekey is out of scope of this
   document.  Mechanisms for rekeying the network are defined in
   companion specifications, such as
   [I-D.richardson-6tisch-minimal-rekey].

14.  Security Considerations

   This document recommends that the pledge and JRC are provisioned with
   unique PSKs.  The request nonce and the response nonce are the same,
   but used under a different key.  The design differentiates between
   keys derived for requests and keys derived for responses by different
   sender identifiers (0x00 for pledge and 0x01 for JRC).  Note that the
   address of the JRC does not take part in nonce or key construction.
   Even in case of a misconfiguration in which the same PSK is used for
   several nodes, the keys used to protect the requests/responses from/
   towards different pledges are different, as they are derived using
   the pledge's EUI-64 as Master Salt.  The PSK is still important for
   mutual authentication of the pledge and JRC.  Should an attacker come
   to know the PSK, then a man-in-the-middle attack is possible.  The
   well-known problem with Bluetooth headsets with a "0000" pin applies
   here.

   Being a stateless relay, the JP blindly forwards the join traffic
   into the network.  While the exchange between pledge and JP takes
   place over a shared 6TiSCH cell, join traffic is forwarded using
   dedicated cells on the JP to JRC multi-hop path.  In case of
   distributed scheduling, the join traffic may therefore cause
   intermediate nodes to request additional bandwidth.  Because the
   relay operation of the JP is implemented at the application layer,
   the JP is the only hop on the JP-6LBR path that can distinguish join
   traffic from regular IP traffic in the network.  It is therefore
   recommended to implement stateless rate limiting at JP; a simple
   bandwidth cap would be appropriate.

   The shared nature of the "minimal" cell used for the join traffic
   makes the network prone to DoS attacks by congesting the JP with
   bogus radio traffic.  As such an attacker is limited by its emitted
   radio power, the redundancy in the number of deployed JPs alleviates
   the issue and also gives the pledge a possibility to use the best
   available link for joining.  How a network node decides to become a
   JP is out of scope of this specification.



Vucinic, et al.            Expires May 3, 2018                 [Page 15]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   At the beginning of the join process, the pledge has no means of
   verifying the content in the EB, and has to accept it at "face
   value".  In case the pledge tries to join an attacker's network, the
   Join Response message will either fail the security check or time
   out.  The pledge may implement a blacklist in order to filter out
   undesired EBs and try to join using the next seemingly valid EB.
   This blacklist alleviates the issue, but is effectively limited by
   the node's available memory.  Bogus beacons prolong the join time of
   the pledge, and so the time spent in "minimal" [RFC8180] duty cycle
   mode.

15.  Privacy Considerations

   This specification relies on the uniqueness of the node's EUI-64 that
   is transferred in clear as an OSCORE Context Hint.  Privacy
   implications of using such long-term identifier are discussed in
   [RFC7721] and comprise correlation of activities over time, location
   tracking, address scanning and device-specific vulnerability
   exploitation.  Since the join protocol is executed rarely compared to
   the network lifetime, long-term threats that arise from using EUI-64
   are minimal.  In addition, the Join Response message contains a short
   address which is assigned by JRC to the pledge.  The assigned short
   address SHOULD be uncorrelated with the long-term EUI-64 identifier.
   The short address is encrypted in the response.  Use of short
   addresses once the join protocol completes mitigates the
   aforementioned privacy risks.

16.  IANA Considerations

   Note to RFC Editor: Please replace all occurrences of "[[this
   document]]" with the RFC number of this specification.

   This document allocates a well-known name under the .arpa name space
   according to the rules given in: [RFC3172].  The name "6tisch.arpa"
   is requested.  No subdomains are expected.  No A, AAAA or PTR record
   is requested.

16.1.  CoAP Option Numbers Registry

   The Stateless-Proxy option is added to the CoAP Option Numbers
   registry:

             +--------+-----------------+-------------------+
             | Number | Name            | Reference         |
             +--------+-----------------+-------------------+
             |  TBD   | Stateless-Proxy | [[this document]] |
             +--------+-----------------+-------------------+




Vucinic, et al.            Expires May 3, 2018                 [Page 16]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


17.  Acknowledgments

   The work on this document has been partially supported by the
   European Union's H2020 Programme for research, technological
   development and demonstration under grant agreement No 644852,
   project ARMOUR.

   The authors are grateful to Thomas Watteyne and Goeran Selander for
   reviewing, and to Klaus Hartke for providing input on the Stateless-
   Proxy CoAP option.  The authors would also like to thank Francesca
   Palombini, Ludwig Seitz and John Mattsson for participating in the
   discussions that have helped shape the document.

18.  References

18.1.  Normative References

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-06 (work in
              progress), October 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3172]  Huston, G., Ed., "Management Guidelines & Operational
              Requirements for the Address and Routing Parameter Area
              Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
              September 2001, <https://www.rfc-editor.org/info/rfc3172>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.






Vucinic, et al.            Expires May 3, 2018                 [Page 17]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


18.2.  Informative References

   [I-D.ietf-6tisch-6top-protocol]
              Wang, Q., Vilajosana, X., and T. Watteyne, "6top Protocol
              (6P)", draft-ietf-6tisch-6top-protocol-09 (work in
              progress), October 2017.

   [I-D.ietf-6tisch-terminology]
              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terminology in IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-terminology-09 (work in
              progress), June 2017.

   [I-D.ietf-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "Concise data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-ietf-cbor-cddl-00
              (work in progress), July 2017.

   [I-D.richardson-6tisch-minimal-rekey]
              Richardson, M., "Minimal Security rekeying mechanism for
              6TiSCH", draft-richardson-6tisch-minimal-rekey-02 (work in
              progress), August 2017.

   [IEEE802.15.4-2015]
              IEEE standard for Information Technology, ., "IEEE Std
              802.15.4-2015 Standard for Low-Rate Wireless Personal Area
              Networks (WPANs)", 2015.

   [RFC4231]  Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
              224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
              RFC 4231, DOI 10.17487/RFC4231, December 2005,
              <https://www.rfc-editor.org/info/rfc4231>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.






Vucinic, et al.            Expires May 3, 2018                 [Page 18]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015,
              <https://www.rfc-editor.org/info/rfc7554>.

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <https://www.rfc-editor.org/info/rfc7721>.

   [RFC8180]  Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
              IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
              Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
              May 2017, <https://www.rfc-editor.org/info/rfc8180>.

Appendix A.  Example

   Figure 3 illustrates a successful join protocol exchange.  The pledge
   instantiates the OSCORE context and derives the traffic keys and
   nonces from the PSK.  It uses the instantiated context to protect the
   Join Request addressed with a Proxy-Scheme option, the well-known
   host name of the JRC in the Uri-Host option, and its EUI-64
   identifier as OSCORE Context Hint.  Triggered by the presence of
   Proxy-Scheme option, the JP forwards the request to the JRC and adds
   the Stateless-Proxy option with value set to the internally needed
   state, authentication tag, and a freshness indicator.  The JP learned
   the IPv6 address of JRC when it acted as a pledge and joined the
   network.  Once the JRC receives the request, it looks up the correct
   context based on the Context Hint parameter.  It reconstructs
   OSCORE's external Additional Authenticated Data (AAD) needed for
   verification based on:

   o  the Version of the received CoAP header.

   o  the Algorithm value agreed out-of-band, default being AES-CCM-
      16-64-128 from [RFC8152].

   o  the Request ID being set to the value of the "kid" field of the
      received COSE object.





Vucinic, et al.            Expires May 3, 2018                 [Page 19]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   o  the Join Request sequence number set to the value of "Partial IV"
      field of the received COSE object.

   o  Integrity-protected options received as part of the request.

   Replay protection is ensured by OSCORE and the tracking of sequence
   numbers at each side.  Once the JP receives the Join Response, it
   authenticates the Stateless-Proxy option before deciding where to
   forward.  The JP sets its internal state to that found in the
   Stateless-Proxy option, and forwards the Join Response to the correct
   pledge.  Note that the JP does not possess the key to decrypt the
   COSE object (join_response) present in the payload.  The Join
   Response is matched to the Join Request and verified for replay
   protection at the pledge using OSCORE processing rules.  In this
   example, the Join Response does not contain the IPv6 address of the
   JRC, the pledge hence understands the JRC is co-located with the
   6LBR.


































Vucinic, et al.            Expires May 3, 2018                 [Page 20]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


       <---E2E OSCORE-->
     Client   Proxy  Server
     Pledge    JP     JRC
       |       |       |
       +------>|       |            Code: { 0.02 } (POST)
       | GET   |       |           Token: 0x8c
       |       |       |    Proxy-Scheme: [ coap ]
       |       |       |        Uri-Host: [ 6tisch.arpa ]
       |       |       | Object-Security: [ kid: 0 ]
       |       |       |         Payload: Context-Hint: EUI-64
       |       |       |                  [ Partial IV: 1,
       |       |       |                    { Uri-Path:"j" },
       |       |       |                    <Tag> ]
       |       |       |
       |       +------>|            Code: { 0.01 } (GET)
       |       | GET   |           Token: 0x7b
       |       |       |        Uri-Host: [ 6tisch.arpa ]
       |       |       | Object-Security: [ kid: 0 ]
       |       |       | Stateless-Proxy: opaque state
       |       |       |         Payload: Context-Hint: EUI-64
       |       |       |                  [ Partial IV: 1,
       |       |       |                   { Uri-Path:"j" },
       |       |       |                   <Tag> ]
       |       |       |
       |       |<------+            Code: { 2.05 } (Content)
       |       | 2.05  |           Token: 0x7b
       |       |       | Object-Security: -
       |       |       | Stateless-Proxy: opaque state
       |       |       |         Payload: [ { join_response }, <Tag> ]
       |       |       |
       |<------+       |            Code: { 2.05 } (Content)
       | 2.05  |       |           Token: 0x8c
       |       |       | Object-Security: -
       |       |       |         Payload: [ { join_response }, <Tag> ]
       |       |       |

     Figure 3: Example of a successful join protocol exchange. { ... }
          denotes encryption and authentication, [ ... ] denotes
                              authentication.

   Where join_response is as follows.










Vucinic, et al.            Expires May 3, 2018                 [Page 21]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   join_response:
   [
       [   / COSE Key Set array with a single key /
           {
                1 : 4, / key type symmetric /
                2 : h'01', / key id /
               -1 : h'e6bf4287c2d7618d6a9687445ffd33e6' / key value /
           }
       ],
       [
           h'af93' / assigned short address /
       ]
   ]

   Encodes to
   h'8281a301040241012050e6bf4287c2d7618d6a9687445ffd33e68142af93' with
   a size of 30 bytes.

Authors' Addresses

   Malisa Vucinic (editor)
   University of Montenegro
   Dzordza Vasingtona bb
   Podgorica  81000
   Montenegro

   Email: malisav@ac.me


   Jonathan Simon
   Analog Devices
   32990 Alvarado-Niles Road, Suite 910
   Union City, CA  94587
   USA

   Email: jonathan.simon@analog.com


   Kris Pister
   University of California Berkeley
   512 Cory Hall
   Berkeley, CA  94720
   USA

   Email: pister@eecs.berkeley.edu






Vucinic, et al.            Expires May 3, 2018                 [Page 22]


Internet-Draft    Minimal Security Framework for 6TiSCH     October 2017


   Michael Richardson
   Sandelman Software Works
   470 Dawson Avenue
   Ottawa, ON  K1Z5V7
   Canada

   Email: mcr+ietf@sandelman.ca












































Vucinic, et al.            Expires May 3, 2018                 [Page 23]


Html markup produced by rfcmarkup 1.124, available from https://tools.ietf.org/tools/rfcmarkup/