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Versions: 00 01 02

6TiSCH Working Group                                       M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                         August 28, 2017
Expires: March 1, 2018


             Minimal Security rekeying mechanism for 6TiSCH
                draft-richardson-6tisch-minimal-rekey-02

Abstract

   This draft describes a mechanism to rekey the networks used by 6TISCH
   nodes.  It leverages the security association created during an
   enrollment protocol.  The rekey mechanism permits incremental
   deployment of new sets of keys, followed by a rollover to a new key.

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 http://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 March 1, 2018.

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
   (http://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.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Tree diagram notation . . . . . . . . . . . . . . . . . . . .   3
   4.  An approach to rekeying . . . . . . . . . . . . . . . . . . .   3
   5.  YANG models . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Tree diagram  . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  YANG model for keys . . . . . . . . . . . . . . . . . . .   5
     5.3.  YANG model for short-address  . . . . . . . . . . . . . .   8
   6.  Security of CoMI link . . . . . . . . . . . . . . . . . . . .  10
   7.  Rekey of master connection  . . . . . . . . . . . . . . . . .  10
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   6TiSCH networks of nodes often use a pair of keys, K1/K2 to
   authenticate beacons (K1), encrypt broadcast traffic (K1) and encrypt
   unicast traffic (K2).  These keys need to occasionally be refreshed
   for a number of reasons:

   o  cryptographic hygiene: the keys must be replaced before the ASN
      roles over or there could be repeated use of the same key.

   o  to remove nodes from the group: replacing the keys excludes any
      nodes that are suspect, or which are known to have left the
      network

   o  to recover short-addresses: if the JRC is running out of short
      (2-byte) addresses, it can rekey the network in order to garbage
      collect the set of addresses.

   This protocol uses the CoMI [I-D.ietf-core-comi] to present the set
   of 127 key pairs.

   In addition to providing for rekey, this protocol includes access to
   the allocated short-address.






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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
   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
   [I-D.ietf-anima-bootstrapping-keyinfra].

3.  Tree diagram notation

   A simplified graphical representation of the data models is used in
   this document.  The meaning of the symbols in these diagrams is as
   follows:

   o  Brackets "[" and "]" enclose list keys.

   o  Braces "{" and "}" enclose feature names, and indicate that the
      named feature must be present for the subtree to be present.

   o  Abbreviations before data node names: "rw" (read-write) represents
      configuration data and "ro" (read-only) represents state data.

   o  Symbols after data node names: "?" means an optional node, "!"
      means a presence container, and "*" denotes a list and leaf-list.

   o  Parentheses enclose choice and case nodes, and case nodes are also
      marked with a colon (":").

   o  Ellipsis ("...") stands for contents of subtrees that are not
      shown.

4.  An approach to rekeying

   Rekeying of the network requires that all nodes be updated with the
   new keys.  This can take time as the network is constrained, and this
   management traffic is not highest priority.

   The JRC must reach out to all nodes that it is aware of.  As the JRC
   has originally provided the keys via either zero-touch
   [I-D.ietf-6tisch-dtsecurity-secure-join] or
   [I-D.ietf-6tisch-minimal-security] protocol, and in each case, the
   JRC assigned the short-address to the node, so it knows about all the
   nodes.



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   The data model presented in this document provides for up to 127 K1/
   K2 keys, as each key requires a secKeyId, which is allocated from a
   255-element palette provides by [IEEE8021542015].  Keys are to be
   updated in pairs, and the pairs are associated in the following way:
   the K1 key is always the odd numbered key (1,3,5), and the K2 key is
   the even numbered key that follows (2,4,6).  A secKeyId value of 0 is
   invalid, and the secKeyId value of 255 is unused in this process.

   Nodes MAY support up to all 127 key pair slots, but MUST support a
   minimum of 6 keys (3 slot-pairs).  When fewer than 127 are supported,
   the node MUST support secKeyId values from 1 to 254 in a sparse array
   fashion.

   A particular key slot-pair is considered active, and this model
   provides a mechanism to query and also to explicitely set the active
   pair.

   Nodes decrypt any packets for which they have keys, but MUST continue
   to send using only the keypair which is considered active.  Receipt
   of a packet which is encrypted (or authenticated in the case of a
   broadcast) with a secKeyId larger (taking consideration that secKeyId
   wraps at 254) than the active slot-pair causes the node to change
   active slot pairs.

   This mechanism permits the JRC to provision new keys into all the
   nodes while the network continues to use the existing keys.  When the
   JRC is certain that all (or enough) nodes have been provisioned with
   the new keys, then the JRC causes a packet to be sent using the new
   key.  This can be the JRC sending the next Enhanced Beacon or unicast
   traffic using the new key if the JRC is also a regular member of the
   LLN.  In the likely case that the JRC has no direct connection to the
   LLN, then the JRC updates the active key to the new key pair using a
   CoMI message.

   The frame goes out with the new keys, and upon receipt (and
   decryption) of the new frame all receiving nodes will switch to the
   new active key.  Beacons or unicast traffic leaving those nodes will
   then update additional peers, and the network will switch over in a
   flood-fill fashion.

   ((EDNOTE: do we need an example?))

5.  YANG models








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5.1.  Tree diagram

   A diagram of the two YANG modules looks like:

   1700  module: ietf-6tisch-symmetric-keying
   1701      +--rw ietf6tischkeypairs* [counter]
   1702      |  +--rw counter           uint16
   1703      |  +--rw ietf6tischkey1
   1704      |  |  +--rw secKeyDescriptor
   1705      |  |  |  +--rw secKey?   binary
   1706      |  |  +--rw secKeyIndex?        uint8
   1707      |  +--rw ietf6tischkey2
   1708      |     +--rw secKeyDescriptor
   1709      |     |  +--rw secKey?   binary
   1710      |     +--rw secKeyIndex?        uint8
   1711      +--ro secKeyUsage
   1712      |  +--ro txPacketsSent?       uint32
   1713      |  +--ro rxPacketsSuccess?    uint32
   1714      |  +--ro rxPacketsReceived?   uint32
   1715      +--rw newKey?               binary

           rpcs:
   1716      +---x installNextKey

   1717  module: ietf-6tisch-short-address
   1718      +--ro ietf6shortaddresses
   1719         +--ro shortaddress    binary
   1720         +--ro validuntil      uint32
   1721         +--ro effectiveat?    uint32


               Figure 1: Tree diagrams of two rekey modules

5.2.  YANG model for keys

module ietf-6tisch-symmetric-keying {
  yang-version 1.1;

  namespace
    "urn:ietf:params:xml:ns:yang:ietf-6tisch-symmetric-keying";
  prefix "ietf6keys";

  //import ietf-yang-types { prefix yang; }
  //import ietf-inet-types { prefix inet; }

  organization
   "IETF 6tisch Working Group";




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  contact
   "WG Web:   <http://tools.ietf.org/wg/6tisch/>
    WG List:  <mailto:6tisch@ietf.org>
    Author:   Michael Richardson
              <mailto:mcr+ietf@sandelman.ca>";

  description
    "This module defines the format for a set of network-wide 802.15.4
    keys used in 6tisch networks.  There are 128 sets of key pairs,
    with one keypair (K1) used to authenticate (and sometimes encrypt)
    multicast traffic, and another keypair (K2) used to encrypt unicast
    traffic.  The 128 key pairs are numbered by the (lower) odd
    keyindex, which otherwise is a 0-255 value.  Keyindex 0 is
    not valid.  This module is a partial expression of the tables in
    https://mentor.ieee.org/802.15/dcn/15/15-15-0106-07-0mag-security-section-pictures.pdf.
To read and write the key pairs, a monotonically increasing counter is added. A new key pair must be added with current_counter = last_counter+1. The current specification allows overwriting of earlier key pairs. It is up to the server to remove old key pairs, such that only the last three (two) pairs are stored and visible to the client.";

  revision "2017-03-01" {
    description
     "Initial version";
    reference
     "RFC XXXX: 6tisch minimal security";
  }

  // list of key pairs
  list ietf6tischkeypairs {
  key counter;
  description
    "a list of key pairs with unique index: counter.";
  leaf  counter {
  type uint16{
     range "0..256";  // for the moment 256 items
    }
  mandatory "true";
  description
    "unique reference to the key pair for client access.";
  }  // counter

  // key descriptor for FIRST part of pair
  container ietf6tischkey1 {
    description
      "A voucher that can be used to assign one or more
       devices to an owner.";
// this container is pretty empty, a leaf would do the job.

    container secKeyDescriptor {
  // I assume this needs to be extended, why else a container?
      description



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        "This container describes the details of a
           specific cipher key";
      leaf secKey {
        type binary;
        description "The actual encryption key.
          This value is write only, and is not returned in a
          read, or returns all zeroes.";
      } // secKey
    }  // secKeyDescriptor

    // leaf secKeyIdMode is always 1, not described here.
    leaf secKeyIndex {
      type uint8;
      description
        "The keyIndex for this keySet.
            A number between 1 and 255.";
      reference
        "IEEE802.15.4";
    } // secKeyIndex
 } // ietf6tischkey1

// key descriptor for SECOND part of pair
  container ietf6tischkey2 {
    description
      "A voucher that can be used to assign one or more
       devices to an owner.";
    container secKeyDescriptor {
  // I assume this needs to be extended, why else a container?
      description
        "This container describes the details of a
           specific cipher key";
      leaf secKey {
        type binary;
        description "The actual encryption key.
          This value is write only, and is not returned in a
          read, or returns all zeroes.";
      } // secKey
    }  // secKeyDescriptor

    // leaf secKeyIdMode is always 1, not described here.
    leaf secKeyIndex {
      type uint8;
      description
        "The keyIndex for this keySet.
           A number between 1 and 255.";
      reference
        "IEEE802.15.4";
    } // secKeyIndex



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   } // ietf6tischkey2
 } //ietf6tischkeypairs

// the usage is over all pairs
    container secKeyUsage {
    config false; // cannot be set by client
    description
      "statistics of sent and received packets.";
      leaf txPacketsSent {
        type uint32;
        description "Number of packets sent with this key.";
      } // txPacketsSent
      leaf rxPacketsSuccess {
        type uint32;
        description "Number of packets received with this key that were
                     successfully decrypted and authenticated.";
      }// rxPacketsSuccess
      leaf rxPacketsReceived {
        type uint32;
        description "Number of packets received with this key, both
            successfully received, and unsuccessfully.";
      } // rxPacketsReceived

    } // secKeyUsage

 // setting new key, and validation of new key
     leaf newKey{
        type binary;
        description
        "new key value to be set by client.";
     }  // newKey
     rpc installNextKey{
         description
         "Client informs server that newKey is to be
         used as current key.";
     }  // installNextKey


  } // module ietf-6tisch-symmetric-keying

5.3.  YANG model for short-address

   module ietf-6tisch-short-address {
     yang-version 1.1;

     namespace
       "urn:ietf:params:xml:ns:yang:ietf-6tisch-short-address";
     prefix "ietf6shortaddr";



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     //import ietf-yang-types { prefix yang; }
     //import ietf-inet-types { prefix inet; }

     organization
      "IETF 6tisch Working Group";

     contact
      "WG Web:   <http://tools.ietf.org/wg/6tisch/>
       WG List:  <mailto:6tisch@ietf.org>
       Author:   Michael Richardson
                 <mailto:mcr+ietf@sandelman.ca>";

     description
      "This module defines an interface to set and interrogate
       the short (16-bit) layer-2 address used in 802.15.4
       TSCH mode networks.  The short addresses are used
       in L2 frames to save space.  A lifetime is included
       in terms of TSCH Absolute Slot Number, which acts
       as a monotonically increasing clock.  ";

     revision "2017-03-01" {
       description
        "Initial version";
       reference
        "RFC XXXX: 6tisch minimal security";
     }

     // top-level container
     container ietf6shortaddresses {
       config false;
       description
         "A 16-bit short address for use by the node.";

       leaf shortaddress {
         type binary{
            length 1..2;}
         mandatory true;
         description
           "The two byte short address to be set.";
       }
       leaf validuntil {
         type uint32;
         mandatory true;
         description "The Absolute Slot Number/256 at which
                      the address ceases to be valid.";
       }

       leaf effectiveat {



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         type uint32;
         description "The Absolute Slot Number/256 at which
                      time the address was originally set.
                      This is a read-only attribute that
                      records the ASN when the shortaddress
                      element was last written or updated.";
       }
     }
   }

6.  Security of CoMI link

   The CoMI resources presented here are protected by OSCOAP
   ([I-D.ietf-core-object-security]), secured using the EDHOC connection
   used for joining.  A unique application key is generated using an
   additional key generation process with the unique label "6tisch-
   rekey".

7.  Rekey of master connection

   Should the OSCOAP connection need to be rekeyed, a new EDHOC process
   will be necessary.  This will need access to trusted authentication
   keys, either the PSK used from a one-touch process, or the locally
   significant domain certificates installed during a zero-touch
   process.

8.  Privacy Considerations

   The rekey protocol itself runs over a network encrypted with the K2
   key.  The end to end protocol from JRC to node is also encrypted
   using OSCOAP, so the keys are not visible, nor is the keying traffic
   distinguished in anyway to an observer.

   As the secKeyId is not confidential in the underlying 802.15.4
   frames, an observer can determine what sets of keys are in use, and
   when a rekey is activated by observing the change in the secKeyId.

   The absolute value of the monitonically increasing secKeyId could
   provide some information as to the age of the network.

9.  Security Considerations

   This protocol permits the underlying network keys to be set.  Access
   to all of the portions of this interface MUST be restricted to an
   ultimately trusted peer, such as the JRC.

   An implementation SHOULD not permit reading the network keys.  Those
   fields should be write-only.



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   The OSCOAP security for this interface is initialized by a join
   mechanism, and so depends upon the initial credentials provided to
   the node.  The initial network keys would have been provided during
   the join process; this protocol permits them to be updated.

10.  IANA Considerations

   This document allocates a SID number for the YANG model.  There is no
   IANA action required for this document.

11.  Acknowledgments

12.  References

12.1.  Normative References

   [I-D.ietf-core-comi]
              Veillette, M., Stok, P., Pelov, A., and A. Bierman, "CoAP
              Management Interface", draft-ietf-core-comi-01 (work in
              progress), July 2017.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", draft-ietf-core-
              object-security-04 (work in progress), July 2017.

   [I-D.ietf-cose-msg]
              Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              draft-ietf-cose-msg-24 (work in progress), November 2016.

   [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>.

   [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>.








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12.2.  Informative References

   [I-D.ietf-6tisch-6top-protocol]
              Wang, Q., Vilajosana, X., and T. Watteyne, "6top Protocol
              (6P)", draft-ietf-6tisch-6top-protocol-07 (work in
              progress), June 2017.

   [I-D.ietf-6tisch-dtsecurity-secure-join]
              Richardson, M., "6tisch Secure Join protocol", draft-ietf-
              6tisch-dtsecurity-secure-join-01 (work in progress),
              February 2017.

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-03 (work in progress), June 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-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-07 (work in progress), July 2017.

   [IEEE8021542015]
              IEEE standard for Information Technology, ., "IEEE Std
              802.15.4-2015 Standard for Low-Rate Wireless Personal Area
              Networks (WPANs)", 2015.

Appendix A.  Example

   In the examples below, a new key value is set in the server
   example.com; followed by the setting of the new key value as the
   current key value.  The SID values of new Key and installNextKey are
   1715 and 1716 respectively.  The corresponding base64 values are: ez
   and e0 respectively.

   The setting of the new key value is done with the PUT request with
   the binary value 1234567890.







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   PUT coap://example.com/c/ez
      (Content-Format :application/yang-value+cbor)
   h'1234567890'

   RES: 2.01 Created

   Payload in CBOR:
   45            # bytes(5)
      1234567890 # "\x124Vx\x90"

   Consecutively, the RPC is invoked with a POST method to validate the
   new key value.

   POST coap://example.com/c/e0
      (Content-Format :application/yang-value+cbor)

   RES: 2.05 Content

   The client can query how many TX packets have been received.  The SID
   of secKeyUsage/txPacketsSent is 1712, corresponding with base64 ew.

   GET coap://example.com/c/ew

   RES: 2.05 Content (Content-Format :application/yang-value+cbor)
   3

   Payload in CBOR:
   03 # unsigned(3)

Author's Address

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca
















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