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6TiSCH                                                           G. Piro
INTERNET-DRAFT                                     (Politecnico di Bari)
Intended Status: Informational                                 G. Boggia
Expires: April 21, 2014                            (Politecnico di Bari)
                                                            L. A. Grieco
                                                   (Politecnico di Bari)
                                                        October 18, 2013


A standard compliant security framework for Low-power and Lossy Networks
                  draft-piro-6tisch-security-issues-00


Abstract

   The aim of this Internet Draft is to define a standard compliant
   security framework for Low-power and Lossy Networks, in order to
   enable the possibility to encrypt and authenticate messages exchanged
   by nodes at the MAC layer. The framework is fully compatible with
   both IEEE 802.15.4 and IEEE 802.15.4e standards and offers a wide
   range of security features to network architectures developed within
   the 6TiSCH Working Group. In particular, it offers: (i) different
   kinds of security architectures; (ii) an efficient mechanism for
   initializing a secure IEEE 802.15.4 domain; and (iii) a lightweight
   mechanism to negotiate link keys among devices.


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html




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Copyright and License Notice

   Copyright (c) 2013 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.



Table of Contents

   1  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3  Security in IEEE 802.15.4 . . . . . . . . . . . . . . . . . . .  5
   4  Definition of the secured domain  . . . . . . . . . . . . . . .  8
   5  Security Configurations . . . . . . . . . . . . . . . . . . . .  8
     5.1  Minimum security requirements . . . . . . . . . . . . . . .  9
   6  Initialization of a secured IEEE 802.15.4 domain  . . . . . . . 11
     6.1  Setting-up phase  . . . . . . . . . . . . . . . . . . . . . 11
     6.2  Bootstrap phase for a FFD device  . . . . . . . . . . . . . 13
     6.3  Bootstrap phase for a RFD device in a Beacon-enabled
          network . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.4  Bootstrap phase for a RFD device in a not-Beacon-enabled
          network . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.5  Key negotiation phase . . . . . . . . . . . . . . . . . . . 16
       6.5.1  Key exchange mechanism based on DH  . . . . . . . . . . 18
       6.5.2  Generation of the LinkKeys  . . . . . . . . . . . . . . 21
       6.5.3  Update of MAC security attribute for the FFD node
              after the generation of the LinkKey . . . . . . . . . . 21
       6.5.4  Update of MAC security attribute for the RFD node
              after the generation of the LinkKey . . . . . . . . . . 23
   7  Additional features . . . . . . . . . . . . . . . . . . . . . . 23
   8  Security Considerations . . . . . . . . . . . . . . . . . . . . 25
   9  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 25
   10  References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     10.1  Normative References . . . . . . . . . . . . . . . . . . . 25
     10.2  Informative References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25





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1  Acronyms

   The following acronyms are used in this document:

   DH Diffie Hellman

   DEX HIP Diet EXchange

   DTLS Datagram Transport Layer

   LLN Low-power and Lossy Network

   LBR LLN Border Routers

   FFD Full Function Device

   HIP Host Identity Protocol

   MAC Medium Access Control

   PAN Personal Area Network

   PANA Protocol for Carrying Authentication for Network Access

   PHY Physical

   RFD Reduced Function Device

   RSA Rivest-Shamir-Adleman

   TSCH Time-slotted Channel Hopping


2  Introduction

   The IEEE 802.15.4 standard [IEEE802154] is widely recognized as one
   of the most successful enabling technologies for short-range low-rate
   wireless communications. It covers all the details related to the
   Medium Access Control (MAC) and Physical (PHY) layers of the protocol
   stack and supports the possibility to protect MAC packets by means of
   symmetric-key cryptography techniques with several security options.

   This standard relies on upper layers to orchestrate, enable,
   configure, and negotiate security services, as well as to handle the
   creation and exchange of encryption keys. But it leaves open some
   aspects, such as the initialization of a secure IEEE 802.15.4 domain,
   the generation and the exchange of keys, the management of joining
   operations in a secure 802.15.4 network already configured in the



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   past.

   The IEEE 802.15.4e standard introduces some amendments to the IEEE
   802.15.4 standard. The most important one is the Time-slotted Channel
   Hopping (TSCH), i.e., a novel MAC protocol, which better supports
   multi-hop communications in emerging industrial applications.

   Since the IEEE 802.15.4e amendment focuses only on link-layer
   aspects, the 6TiSCH Working Group was born to suggest the adoption of
   IPv6 over the TSCH, thus covering all facets related to the schedule
   of network communications in complex (and eventually distributed)
   Low-Power and Lossy Networks (LLNs).

   In industrial environments, security issues are very important.
   Hence, a complete framework, which supports a wide range of security
   features, is highly required.

   In this context, the security in LLNs has been firstly addressed
   within ZigBee IP specifications, i.e., a suite of high level
   communication protocols sitting on top of the IEEE 802.15.4 MAC
   [ZIGBEEIP]. ZigBee IP supports end-to-end and link-layer security and
   a public key infrastructure based on X.509 certificates. It imposes
   the adoption of the same link-key (shared among all nodes) to protect
   packets belonging to any kind of services (i.e., only a single
   security level is allowed). This makes the network highly sensible to
   the presence of compromised devices. Moreover, the cost needed to
   update the key within the network could be high, especially for heavy
   loaded systems.

   Security aspects have been also faced in [6TSCH-SEC] and [GARCIA-SEC-
   IOT]. The work [6TSCH-SEC] introduces a simple security framework
   based on four consecutive phases (i.e., implanting, bootstrapping,
   link establishment, and operational phases) which allow the
   initialization of a secured IEEE 802.15.4 network. It exploits well-
   known approaches, like PANA [RFC5191], HIP DEX [HIPDEX], and DTLS
   [RFC6347], to authenticate nodes and to negotiate link keys. Instead,
   [GARCIA-SEC-IOT] defines a set of security profiles to guarantee in
   different scenarios (e.g., network without security requirements,
   home, managed home, industrial, and advanced industrial). For each
   scenario, it identifies a number of security threats and suggests how
   fixing them through well-known protocols and algorithms.

   Proposals presented in [6TSCH-SEC] and [GARCIA-SEC-IOT] do not
   provide a complete definition of a security framework for LLNs, which
   offers, at the same time, an accurate procedure for the network
   initialization, the support for different security configurations,
   and a fully compatibility with the standard. In addition, they do not
   guarantee the real possibility to implement conceived proposals in



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   devices, i.e., sensors with very limited computational, energy, and
   storage capabilities.

   This Internet Draft proposes a complete and standard compliant
   framework offering a number of security features at the MAC layer of
   LLNs. All aspects have been designed in order to ensure an easy and
   effectively implementation on real devices.

   In particular, the security framework described in this document
   introduces all the details required to enable the security for both
   IEEE 802.15.4 and IEEE 802.15.4e networks, as well as the possibility
   to encrypt and authenticate any kind of communications devised by the
   6TiSCH Working Group.

   In summary, the security framework covers: (1) five different kinds
   of security configurations; (2) an efficient mechanism for
   initializing a secure IEEE 802.15.4 (or IEEE 802.15.4e) domain; (3) a
   lightweight mechanism to negotiate link keys among devices; (4) a
   very simple technique adopted to update link keys during the time.




3  Security in IEEE 802.15.4


   This section summarizes security features defined within the standard
   [IEEE802154], thus simplifying the comprehension of the remaining
   part of this Internet Draft.

   The IEEE 802.15.4 standard defines eight security levels to protect
   MAC frames, as summarized in Fig. 1.


   +----------+-------------+-----------+----------------+
   | Security | Security    | Data      | Data           |
   | level    | attribute   | Integrity | Confidentiality|
   +----------+-------------+-----------+----------------+
   | 0        | None        | No        | No             |
   +----------+-------------+-----------+----------------+
   | 1        | MIC-32      | Yes       | No             |
   +----------+-------------+-----------+----------------+
   | 2        | MIC-64      | Yes       | No             |
   +----------+-------------+-----------+----------------+
   | 3        | MIC-128     | Yes       | No             |
   +----------+-------------+-----------+----------------+
   | 4        | ENC         | No        | Yes            |
   +----------+-------------+-----------+----------------+



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   | 5        | ENC-MIC-32  | Yes       | Yes            |
   +----------+-------------+-----------+----------------+
   | 6        | ENC-MIC-64  | Yes       | Yes            |
   +----------+-------------+-----------+----------------+
   | 7        | ENC-MIC-128 | Yes       | Yes            |
   +----------+-------------+-----------+----------------+
   Figure 1. Security levels available for a IEEE 802.15.4 network.


   At the MAC layer, encryption and decryption functionalities are
   implemented within the "outgoing frame security" and the "incoming
   frame security" procedures, respectively. They exploits a number of
   security attributes, summarized in what follows:

      - macKeyTable: it is composed by a set of KeyDescriptor elements.
      A specific KeyDescriptor element is created for each key, composed
      by (see Tab. 61 of the IEEE 802.15.4 standard for more details
      [IEEE802154]):

            - The KeyIdLookupList, which is a list of
            KeyIdLookupDescriptor entries. A KeyIdLookupDescriptor is
            composed by a set of parameters (see Tab. 65 of the IEEE
            802.15.4 standard for more details [IEEE802154]), i.e.,
            KeyIdMode, KeySource, KeyIndex, DeviceAddMode, DevicePANId,
            and DeviceAddress, that are used to identify the key within
            the macKeyTable.

            - The DeviceDescriptorHandleList, which contains pointers to
            DeviceDescriptor elements stored within the macDeviceTable.
            It is used to identify which devices may use the key.

            - The KeyUsageList, which is a list of KeyUsageDescriptor
            elements. A KeyUsageDescriptor is composed by the FrameType
            and the CommandFrameIdentifies fields that indicate the
            frame type with which the considered key may be used (see
            Tab. 62 of the IEEE 802.15.4 standard for more details
            [IEEE802154]).

            - The Key.

      - macDeviceTable: it is composed by a set of DeviceDescriptor
      elements, providing some information about remote devices which
      the node can establish a secure communication with. A dedicated
      DeviceDescriptor element is associated to each remote device. It
      is composed by a number of fields, i.e., PANId, ShortAddress,
      ExtAddress, FrameCounter, and Extemp, which collect information
      related to a specific remote device (see Tab. 64 of the IEEE
      802.15.4 standard for more details [IEEE802154]).



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      - macSecurityLevelTable: it is made by a set of
      SecurityLevelDescriptor elements, which store details about the
      security level required for each MAC frame type and subtype.
      Fields belonging to the SecurityLevelDescriptor data structure
      are: FrameType, ComamndFrameIdentifier, SecurityMinimum,
      DeviceOverrideSecurityMinimum, and AllowedSecurityLevels (see Tab.
      63 of the IEEE 802.15.4 standard for more details [IEEE802154]).

      - macFrameCounter: it is an integer value storing the outgoing
      frame counter for the considered device.

      - macAutoRequestSecurityLevel: it is an integer value providing
      the security level used for automatic data requests.

      - macAutoRequestKeyIdMode: it is an integer value indicating the
      key identifier mode used for automatic data requests. It is not
      valid if the macAutoRequestSecurityLevel attribute is set to 0x00.

      - macAutoRequestKeySource: it represents a short or extended IEEE
      802.15.4 MAC address, indicating the originator of the key used
      for automatic data requests. This attribute is not valid if the
      macAutoRequestKeyIdMode element is not valid or set to 0x00.

      - macAutoRequestKeyIndex: it is an integer value storing the index
      of the key used for automatic data requests. It is not valid if
      the macAutoRequestKeyIdMode attribute is not valid or set to
      0x00.

      - macDefaultKeySource: it is the extended IEEE 802.15.4 MAC
      address of the originator of the default key used for key
      identifier mode 0x01.



During the outgoing security procedure, the high layer uses the
KeyIdMode parameter to select a specific key in the macKeyTable to be
used for protecting the MAC frame.

The KeyIdMode is set to 00, 01, 10, and 11 in the case the key can be
derived implicitly by both sender and the receiver and its is not
specified in the message, the key is determined explicitly by the
KeyIndex parameter stored into the MAC header and the
macDefaultKeySource, the key can be derived by considering KeyIndex and
KeySource fields stored into the MAC header (with KeySource representing
the short address of the device that has generated the key), and the key
can be derived by considering KeyIndex and KeySource fields stored into
the MAC header (with KeySource representing the IEEE extended address of
the device that has generated the key), respectively.



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The IEEE 802.15.4 standard does not provide any guideline to create (and
or negotiate) keys, as well as to configure the aforementioned security
MAC attributes.







4  Definition of the secured domain

In this document, the "secured domain" concept refers to the portion of
a LLN network where procedures and techniques described in this proposal
must be performed in order to configure and maintain secured
communications.

Two types of network nodes, i.e., the Full Function Device (FFD) and the
Reduced Function Device (RFD), can be found in an IEEE 802.15.4 network.
They can be arranged in both peer-to-peer and star topologies. A FFD
device has the highest computational capabilities and it works as the
coordinator of the network (also called PAN coordinator), thus being a
reference node for a group of other RFD devices. Instead, a RFD device
has lower resource and communication capabilities and it is able to
communicate only with its reference FFD. Whereas RFD nodes must be
connected only to coordinator, FFD devices can connect among them
forming more complex meshed network architectures.

The concept of secured domain adopted in this Internet Draft coincides
with the broadcast domain of an IEEE 802.15.4 network, which is composed
by a number of RFD connected to a coordinator and the coordinator
itself.

The IEEE 802.15.4e amendment does not introduces any variations to the
network architecture, thus the previous definition is still valid in
this context.

The same consideration can be done for network architectures and models
designed within the 6TiSCH Working Group that, as explained before, are
based on top of the IEEE 802.15.4e amendment.

Definitively, to the sake of clarity, from this moment on, we will focus
on the terminology related to the IEEE 802.14.5 standard, implying that
the entire framework can be adopted also for IEEE 802.15.4e networks and
6TiSCH's proposals.


5  Security Configurations



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The security framework described in this document supports five network
configurations, which differ among them according to the offered
security features. They are:


      - Fully Secured network: all the IEEE 802.15.4 devices forming the
      network are configured to fully support security services. It
      represents the most secured configuration: all packets,
      independently from the message they carry, are encrypted and
      authenticated. Nodes that do not support security capabilities (or
      that are not in posses of all the information to joining the
      network, such as key materials and encryption and decryption
      algorithms) are not allowed to join the network.

      - Unsecured network: security services are not supported. Even if
      in possession of security capabilities, any pair of nodes is not
      allowed to establish a secured communication. Differently for the
      Fully Secured scheme, this is offers the lowest security level.
      Since the data encryption, the message integrity, and the peer
      authentication are not implemented, all the MAC frames are
      exchanged in clear. Hence, the setup and the maintaining of the
      network are described by the standard and no further upgrades are
      required.

      - Partial Secured network: only the integrity of message is
      supported.

      - Hybrid Secured network: the network can be composed by
      heterogeneous nodes that could or could not support security
      features. As default, the network is created in an unsecured
      manner. All the non-unicast control messages sent by the
      coordinator should be transmitted in clear. In this way, in fact,
      it is ensured that all the devices are able to read the content of
      packets. A RFD node with security capabilities, that intends to
      exchange encrypted and/or authenticated packets with the
      coordinator, could negotiate a set of link key with its reference
      FFD.

      - Flexible Secured network: as default, the network is setup with
      the Fully Secured configuration and all packets are encrypted and
      authenticated. If there is at least one node that have not
      security capabilities, the coordinator could decides to switch to
      the Hybrid Secured configuration.



5.1  Minimum security requirements




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The IEEE 802.15.4 standard imposes to specify, for each kind of MAC
packet, minimum security levels that should be guaranteed. These
restrictions must be detailed for each remote device.

To this end, SecurityMinimum, DeviceOverrideSecurityMinimum, and
AllowedSecurityLevels parameters are stored into the DeviceDescriptor
element (see Sec. 3) to define the minimum security level (i.e., one of
those reported in Fig.1), the possibility to override the minimum
security level (i.e., DeviceOverrideSecurityMinimum is just a boolean
flag), and the list of allowed security levels in the case the minimum
one could be overridden, respectively.

With reference to secured network configurations presented in Sec. 3.1,
these parameters must be set as reported in Fig. 2.


+------------------------------------------------------------------+
| Attribute      | Secured Network Configurations                  |
|                |Unsecured| Fully   | Partial | Hybrid  |Flexible |
+----------------+---------+---------+---------+---------+---------+
| SecurityMinimum|0        | from 5  | from 1  | 0       | from 1  |
|                |         | to 7    | to 4    |         | to 7    |
+----------------+---------+---------+---------+---------+---------+
| DeviceOverride-| FALSE   | FALSE   | FALSE   | FALSE   | TRUE    |
| SecurityMinimum|         |         |         |         |         |
+----------------+---------+---------+---------+---------+---------+
| AllowedSecuri- | 0       | from 5  | from 1  | from 0  | from 0  |
| tyLevelsvels   |         | to 7    | to 4    | to 7    | to 7    |
+----------------+---------+---------+---------+---------+---------+
Figure 2. Setting of security attributes of the DeviceDescriptor element
in each proposed secure network configuration.


The Unsecured network configuration does not support any security
features. Hence, both minimum and allowable security levels are set to 0
for all the MAC frames and the possibility to override such constraints
is disabled for all devices.

If the Fully Secured configuration is enabled, the minimum security
level must be chosen in the range [5,7], thus allowing the possibility
to support the encryption and the authentication of messages. The
manufacturer must set the default value to 7; it can be updated by the
network administrator. The minimum security level must not be overridden
by any devices and, as a consequence, the field AllowedSecurityLevels
should contain only one value, equal to the minimum security level.

If the Partial Secured configuration is enabled, the minimum security
level must be chosen in the range [1,4], thus allowing the possibility



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to support the authentication of messages. The manufacturer must set the
default value to 4; it can be updated by the network administrator. The
minimum security level must not be overridden by any devices and, as a
consequence, the field AllowedSecurityLevels should contain only one
value, equal to the minimum security level.

If the Hybrid Secured configuration is enabled, the minimum security
level must be set to 0, thus supporting the joining of devices having
different security capabilities. All the security levels could be
allowed and the network administrator could decide to enable only a
subset of them according to the network design.

If the Flexible Secured configuration is enabled, the minimum security
level must be set to 1. The joining of nodes without (or with limited)
security capabilities is permitted by setting the
DeviceOverrideSecurityMinimum variable to TRUE and by including lower
security levels in the list of AllowedSecurityLevels.


6  Initialization of a secured IEEE 802.15.4 domain

A secured 802.15.4 domain is created by means of three different phases:
setting-up, bootstrapping, and key negotiation.


6.1  Setting-up phase

The setting-up phase is used to properly configure the device that will
operate in an IEEE 802.15.4 network.

It consists in storing, within the device, parameters and initial
secrets (i.e., the masterKey), which will be used by secure algorithms
and procedure to setup the secure domain. They include. (i) the
MasterKey, (ii) the PrimeNumbersTable, and (iii) the
GlobalSecurityLevelsTable.

This operation may be performed by the manufacturer or by the network
administrator. The MasterKey can be used to generate two different
keys:
      - the DefaultKey, exploited to protect broadcast messages (i.e.,
      the beacon frame);

      - the LinkKey, used to encrypt and authenticate unicast packets
      (i.e., those exchanged between only two specific nodes).

The GlobalSecurityLevelsTable, that has been reported in Fig. 3, is used
to store the minimum security level and the list of allowed security
levels that must be adopted for each kind of MAC frame and for each



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security configuration defined in Sec. 5. The table reported in Fig. 3
does not consider ACK messages because they must not be protected (as
imposed by the IEEE 802.15.4 standard [IEEE802154]).

Both the minimum security level and the list of allowed security levels
must be chosen by the manufacturer or by the network administrator,
according to restrictions reported in Fig. 2.

+-------------+------------+---------------------------------------+
| Attribute   | Frame Type |     Secured Network Configurations    |
|             |            | Fully   | Partial | Hybrid  |Flexible |
+-------------+------------+---------+---------+---------+---------+
| Security    | Beacon     |         |         |         |         |
| Minimum     |            |         |         |         |         |
+-------------+------------+---------+---------+---------+---------+
| Security    | Data       |         |         |         |         |
| Minimum     |            |         |         |         |         |
+-------------+------------+---------+---------+---------+---------+
| Security    | Command MAC|         |         |         |         |
| Minimum     |            |         |         |         |         |
+-------------+------------+---------+---------+---------+---------+
| AllowedSe-  | Beacon     |         |         |         |         |
| curityLevels|            |         |         |         |         |
+-------------+------------+---------+---------+---------+---------+
| AllowedSe-  | Data       |         |         |         |         |
| curityLevels|            |         |         |         |         |
+-------------+------------+---------+---------+---------+---------+
| AllowedSe-  | Command MAC|         |         |         |         |
| curityLevels|            |         |         |         |         |
+-------------+------------+---------+---------+---------+---------+
Figure 3. Structure of the GlobalSecurityLevelsTable.



The PrimeNumbersTable stores a set of prime numbers and their respective
primitive roots, which are used during the key negotiation procedure.
Its implementation is reported in Fig. 4.

+----------------+--------------+----------------+
| PrimeNumberId  | Prime Number | Primitive Root |
+----------------+--------------+----------------+
| 1              | p1           | g1             |
+----------------+--------------+----------------+
| 2              | p2           | g2             |
+----------------+--------------+----------------+
:                                                :
+----------------+--------------+----------------+
| N              | pN           | gN             |



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+----------------+--------------+----------------+
Figure 4. Structure of the PimeNumbersTable.




6.2  Bootstrap phase for a FFD device

The PAN is initialized by a FFD node. The MAC entity of the FFD node
starts scanning the channel (for discovering the presence of other
active coordinators and identifying the portion of the spectrum which it
could operate in) after the reception of the MLME-START.request
primitive, generated by the so called Next Higher Layer. Then, it will
answer with a MLME-START.confirm primitive reporting a SUCCESS status.
From this moment on, the device behaves as the PAN coordinator and it is
able to select the identification number for the PAN, i.e., the PAN_ID,
and the short MAC address of the IEEE 802.15.4 network [IEEE802154]. In
this phase, the FFD generates the DefaultKey, D_k, and updates MAC
security attributes accordingly. The DefaultKey, D_k, will be used to
encrypt/decrypt all the broadcast messages. To this end, the following
tasks will be executed.

      a) The DefaultKey, D_k, is obtained from the MasterKey, M_k, by
      using a 128-bit hash function, H_128(.)

                       D_k= H_128(PAN_ID | M_k).

      b) The FFD creates the KeyDescriptor associated to the DefaultKey,
      D_k, by following these steps:

            b.1) A new KeyIdLookupList data structure is created and
            stored within the KeyDescriptor element. A
            KeyIdLookupDescriptor is generated and stored into the
            KeyIdLookupList data structure. The KeyIdMode, the
            KeySource, and the KeyIndex variables of this
            KeyIdLookupDescriptor are set to 0x03, the MAC address of
            the device, and 1, respectively. Instead, DeviceAddrMode,
            DevicePANId, and DeviceAddress are not set due to the
            selected KeyIdMode (see Tab. 65 of the IEEE 802.15.4
            standard for more details [IEEE802154]).

            b.2) A KeyUsageList data structure is created and stored
            within the KeyDescriptor element. One KeyUsageDescriptor for
            each broadcast message is create and stored into the
            KeyUsageList data structure.

            b.3) The DeviceDescriptorHandleList is left blank because
            the FFD does not yet know the list of devices that may use



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            this key.

            b.4) The DefaultKey, D_k, is stored within the Key field.

      c) The KeyDescriptor created in the previous step is added to the
      macKeyTable.

      d) The macDefaultKeySource is set to the MAC address of the
      device.


6.3  Bootstrap phase for a RFD device in a Beacon-enabled network

      To join the network, the RFD device should associate with the
      coordinator. The Next Higher Layer sends to the MAC entity the
      MLME-ASSOCIATE.request primitive, starting the association
      procedure.

      In this phase, the FFD generates the DefaultKey, D_k, and updates
      MAC security attributes accordingly. The DefaultKey, D_k, will be
      used to encrypt/decrypt all the broadcast messages. To this end,
      the following tasks will be executed:

      a) The RFD node receives a Beacon messages sent by the coordinator
      and extracts from it the PAN_ID, the MAC address of the
      coordinator and the FrameCounter of the received frame.

      b) A new DeviceDescriptor element, associated to the coordinator
      (i.e., the FFD node that sent the Beacon message) is created and
      stored into the macDeviceTable. It is built considering these
      specifications (see Tab. 64 of the IEEE 802.15.4 standard
      [IEEE802154] for more details):

            b.1) The PANId variable is associated to the PAN_ID value
            extracted from the Beacon message.

            b.2) The ShortAddress is set to the MAC address of the
            coordinator whenever the short addressing mode is used. This
            parameter is set to 0xfffe if only the extended addressing
            mode is used. If its value is unknown, the ShortAddress
            parameter is set to 0xfff.

            b.3) The ExtAddress is set to the IEEE MAC address of the
            coordinator.

            b.4) The FrameCounter parameter is set to the FrameCounter
            value extracted from the Beacon message.




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            b.5) The Extempt boolean flag is set to the allowed value of
            the DeviceOverriddeSecurityMinimum variable described in
            Fig. 2.

      c) The DefaultKey, D_k, is obtained from the MasterKey, M_k, by
      using a 128-bit hash function, H_128(.):

                       D_k= H_128(PAN_ID | M_k).

      d) The RFD creates the keyDescriptor associated to the DefaultKey,
      D_k, by following these steps:

            d.1) a new KeyIdLookupList data structure is created and
            stored within the KeyDescriptor element. A
            KeyIdLookupDescriptor is generated and stored into the
            KeyIdLookupList data structure. The KeyIdMode, the
            KeySource, and the KeyIndex variables of this
            KeyIdLookupDescriptor are set to 0x03, the MAC address of
            the coordinator that sent the Beacon frame, and 1,
            respectively. Instead, DeviceAddrMode, DevicePANId, and
            DeviceAddress are not set due to the selected KeyIdMode (see
            Tab. 65 of the IEEE 802.15.4 standard for more details
            [IEEE802154]).

            d.2) A KeyUsageList data structure is created and stored
            within the KeyDescriptor element. One KeyUsageDescriptor for
            each broadcast message is create and stored into the
            KeyUsageList data structure.

            d.3) The DeviceDescriptorHandleList is created and populated
            with the pointer to the DeviceDescriptor created at the
            point 2.

            d.4) The DefaultKey, D_k, is stored within the Key field.

      e) The KeyDescriptor created in the previous step is added to the
      macKeyTable.

      f) The macDefaultKeySource is set to the MAC address of the
      coordinator.


6.4  Bootstrap phase for a RFD device in a not-Beacon-enabled network

In the case the not-beacon-enabled scheme is enabled, the RFD device
must explicitly request its generation to the coordinator. The payload
of the Beacon Request packet must be protected using an ephemeral key,
phi_k, obtained from the MasterKey, M_k, and the source address of the



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RFD node, srcMACaddress, as

                  phi_k = H_128(srcMACaddress | M_K).

The KeyIdMode of the Beacon Request packet is set to 00, thus enabling
the coordinator to implicitly obtain the ephemeral key.

Once received the Beacon frame, the RFD node will execute all the steps
described in Sec. 6.3.


6.5  Key negotiation phase Since resource-constrained devices are unable
to perform complex algorithms and protocols, a simple key agreement
protocol is adopted during the execution of the key negotiation phase.

A new command message, which is identified with a CommandFrameIdentifier
set to 0xAA, is used for this purpose. It is composed by four different
fields: KeyGenControlField, Rand, KeyMaterial, and AuthenticationField.

The structure of the new command MAC frame has been reported in Fig. 5.
The structure of the KeyGenControlField, instead, are shown in Fig. 6.
The introduction of these new fields respects the constraints imposed by
the standard about the maximum packet size.

+--------------+--------------+--------------+----------------+
| Octects: 2   | 0/2          | 0/S          | 0/16           |
+--------------+--------------+--------------+----------------+
| KeyGen       | Rand         | Key          | Authentication |
| ControlFiled |              | Material     | Filed          |
+--------------+--------------+--------------+----------------+
Figure 5. A new command MAC frame adopted during the key negotiation
phase.

+--------+--------+--------+--------+--------+
| Bits: 2| 2      | 1      | 1      | 10     |
+--------+--------+--------+--------+--------+
| Message| KeyGen | Key    | Auth   | Key    |
| Type   | Mode   | Flag   | Flag   | Size   |
+--------+--------+--------+--------+--------+
Figure 6. KeyGenControlField of the new command MAC frame adopted during
the key negotiation phase.


The KeyGenControlField (2 bytes long) stores details about the content
of the message. It is composed by the following fields:

      - the MessageType (2 bits long), which identifies the type of
      message exchanged during the procedure. It may assume these



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      values:

            - MessageType=00 identifies a message storing key materials
            (i.e., DH parameters).

            - MessageType=01 identifies a message storing key materials
            belonging to a different approach selected before by the
            remote node. This message can be generate only by the FFD in
            the case the key negotiation algorithm chosen by the RFD is
            not supported.

            - MessageType=10 identifies final messages belonging to the
            key negotiation phase that are used to verify the mutual
            authentication of nodes.

            - MessageType=11 is reserved for future upgrades.

      - the KeyGenMode (2 bits long), which describes the algorithm
      adopted for key generation. In this Internet draft we describe a
      key negotiation procedure based on the DH algorithm, which is
      identified by the code 00. Other values, i.e., 01, 10 and 11, are
      reserved and can be used for future upgrades.

      - the boolean KeyFlag (1 bit long), which is set to TRUE in the
      case the message delivers key materials or to FALSE otherwise.

      - the boolean AuthFlag (1 bit long) ), which is set to TRUE in the
      case the message delivers an authentication field or to FALSE
      otherwise.

      - the KeySize (10 bits long), which indicates the size of the
      transported key material. Its value is set to 0 in the case the
      message does not contain any key materials.

The Rand field (0/2 bytes long) contains a random value used for
generating the PreLinkKey, P_k, and for verifying the authenticity of
the remote device. It is present only if MessageType is equal to 00 or
01.

The KeyMaterial field (0/S bytes long, where S is the size of the prime
number) contains key materials, such as DH parameters. It is present
only if MessageType is equal to 00 or 01.

AuthenticationField field (0/16 bytes long) is used to verify the
authenticity of the remote device. It is present only if MessageType is
equal to 10.





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6.5.1  Key exchange mechanism based on DH

The key exchange mechanism based on the DH algorithm is reported in Fig.
7.

It is initialized by the RFD device that wants to establish a secure
link with the remote FFD. The procedure assumes that both RFD and FFD
devices store into the PrimeNumbersTable the same set of N prime numbers
and their primitive roots, each one having size equal to S (see Sec. 6.1
for more details).

The number of bits needed to identify each prime number of the
PrimeNumbersTable, i.e., P_bits, is equal to

                           P_bits = log2 (N).

The key exchange mechanism is provides the execution of these
operations:

      a) The RFD identifies in the PrimeNumbersTable a prime number, P,
      and the corresponding primitive root, g, by extracting the latest
      P_bits bits from the output of the following hash function

                          H_128(PAN_ID | D_k).

      b) The RFD computes the private key and the public key, according
      to the DH algorithm. Hence, the private key, PVK_RFD, is extracted
      as a random number. Then, the public key, PBK_RFD, is created as:

                      PBK_RFD = g^PVK_RFD * mod P

      c) The RFD extract another random number, RAND_1, that will be
      used for the mutual authentication.

      d) The RDF sends its public key, PBK_RFD, to the remote FFD
      through a specific MAC command frame, which is composed by the
      following fields (see Fig. 7): MessageType=00, KeyGenMode=01,
      KeyFalg=TRUE, AuthFlag=FALSE, KeySize=S, Rand=RAND_1, and
      KeyMaterial=PBK_RFD. This message is encrypted with the
      DefaultKey, D_k.

      e) Once received the aforementioned MAC command frame, the FFD
      node generates, in turn, its private and public key through the DH
      algorithm. Hence, it identifies in the PrimeNumbersTable a prime
      number, P, and the corresponding primitive root, g, by extracting
      the latest P_bits bits from the output of the following hash
      function




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                         H_128 (PAN_ID | D_k).

      Then, it generates a random variable, PVK_FFD, which is the
      private key and computes the public key, PBK_FFD, as in the
      following:

                      PBK_FFD = g^PVK_FFD * mod P.

      f) The FFD extract another random number, RAND_2, that will be
      used for the mutual authentication;

      f) The RDF sends its public key, PBK_FFD, to the remote RFD
      through a specific MAC command frame, which is composed by the
      following fields (see Fig. 7): MessageType=00, KeyGenMode=01,
      KeyFalg=TRUE, AuthFlag=FALSE, KeySize=S, Rand=RAND_2, and
      KeyMaterial=PBK_FFD. This message is encrypted with the
      DefaultKey, D_k.

      h) The RFD computes the PreLinkKey, P_k

                     P_k = PBK_FFD^PVK_RDF * mod P.

      i) The FFD computes the PreLinkKey, P_k,

                     P_k = PBK_RFD^PVK_FFD * mod P.

      j) Both RFD and FFD devices computes the LinkKey by using the
      procedure described in Sec. 6.5.2 and update MAC security
      attributes according to procedures described in Secs. 6.5.3 and
      6.5.4.

      k) The RFD node computes the authentication parameters, AUTH_RFD,
      through the 128-bit hash function, H_128(.),

                AUTH_RFD=H_128(P_k || RAND_2 || RAND_1).

      Then, it sends to the coordinator a new MAC command message to
      demonstrate its authenticity. This message is composed by the
      following fields (see Fig. 7): MessageType=10, KeyGenMode=01,
      KeyFalg=FALSE, AuthFlag=TRUE, KeySize=0, Rand=RAND_2, and
      AuthenticationField=AUTH_RFD. This message is protected by using
      the LinkKey computed before.

      l) The FFD node computes the authentication parameters, AUTH_FFD,
      through the 128-bit hash function, H_128(.)

                AUTH_FFD=H_128 (P_k || RAND_1|| RAND_2).




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      Then, it sends to the RFD node a new MAC command message to
      demonstrate its authenticity. This message is composed by the
      following fields(see Fig. 7): MessageType=10, KeyGenMode=01,
      KeyFalg=FALSE, AuthFlag=TRUE, KeySize=0, Rand=RAND_2, and
      AuthenticationField=AUTH_FFD. This message is protected through
      the LinkKey computed before.

      m) Both RFD and FFD verify the authenticity of the remote.




              RFD Device                                FFD Device
                   |                                           |
                   |                                           |
              compute DH                                       |
              parameters                                       |
                   |                                           |
     +---------    |                                           |
     |             | [MessageType=00, KeyGenMode=00, keyFlag=1 |
     |             | AuthFlag=0, keySize=S,                    |
 messages          | KeyMaterial=PBK_RFD, Rand=RAND_1 ]        |
 encrypted with    |-----------------------------------------> |
 the DefaultKey    |                                           |
     |             |                                   compute DH
     |             |                                    parameters
     |             |                                           |
     |             | [MessageType=00, KeyGenMode=00, keyFlag=1 |
     |             |  AuthFlag=0, KeySize=S,                   |
     |             |  KeyMaterial=PBK_FFD, Rand=RAND_2 ]       |
     |             | <---------------------------------------- |
     |             |                                           |
     +--------     |                                           |
             compute P_k                              compute P_k
             and LinkKey                               and LinkKey
                   |                                           |
                   |                                           |
     +---------    |                                           |
     |             | [MessageType=11, KeyGenMode=00, keyFlag=0 |
     |             | AuthFlag=1, keySize=0,                    |
 messages          | AuthenticationField=AUTH_RFD ]            |
 encrypted with    |-----------------------------------------> |
 the LinkKey       |                                           |
     |             |                                           |
     |             | [MessageType=11, KeyGenMode=00, keyFlag=0 |
     |             |  AuthFlag=1, KeySize=0,                   |
     |             |  AuthenticationField=AUTH_FFD ]           |
     |             | <---------------------------------------- |



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     |             |                                           |
     +--------     |                                           |
                   V                                           V

             Figure 7. Key exchange mechanism based on DH.






6.5.2  Generation of the LinkKeys

The standard imposes to use the CCM* algorithm and a 128-bit key to
protect MAC frames. Independently from the size the PreLinkKey, both RFD
and FFD must create a set of link keys, each one 128 bits long. Firstly,
each node computes a NewPreLinkKey, NP_k128, through the 128-bit hash
function, H_128(.):

                     NP_k128 = H_128(PAN_ID | P_k).

The NP_k128 key will be used to compute LinkKeys. The CCM* algorithm
assumes that each key must be used for a specific number of block
ciphers. For each i-th group of block ciphers, the LinkKey, L_k, is
computed as in the following:

                    L_k = H_128 (i | PAN_ID | P_k).

Finally, both FFD and RFD devices update their MAC security attributes
by using the procedures described in Secs. 6.5.3 and 6.5.4,
respectively.




6.5.3  Update of MAC security attribute for the FFD node after the
generation of the LinkKey

After the calculation of the i-th LinkKey, the FFD updates its MAC
security attributes as described in what follows.

      a) If i=1, a new DeviceDescriptor element, associated to the RFD
      node with which it has negotiated a link key, is created and
      stored into the macDeviceTable. It is composed by:

            a.1) the PANId, which is set to the PAN_ID value.

            a.2) The ShortAddress, which is set to the MAC address of



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            the RFD node whenever the short addressing mode is used.
            This parameter is set to 0xfffe if only the extended
            addressing mode is used. In the case its value is unknown,
            this parameter is set to 0xfff.

            a.3) The ExtAddress, which is set to the IEEE MAC address of
            the RFD node.

            a.4) The FrameCounter, which is set to the FrameCounter
            value extracted from the latest packet received by the RFD
            node.

            a.5) The Extempt boolean flag, which is set to the allowed
            value of the DeviceOverriddeSecurityMinimum variable
            described in Fig. 2.

      b) The FFD creates the keyDescriptor associated to the i-th
      LinkKey, L_k, by following these steps:

            b.1) A new KeyIdLookupList data structure is created and
            stored within the KeyDescriptor element. A
            KeyIdLookupDescriptor is generated and stored into the
            KeyIdLookupList data structure. The KeyIdMode, the
            KeySource, and the KeyIndex variables of this
            KeyIdLookupDescriptor are set to 0x03, the MAC address of
            the RFD node that initialized the key negotiation phase, and
            1, respectively. DeviceAddrMode, DevicePANId, and
            DeviceAddress are not set because of the selected KeyIdMode
            (see Tab. 65 of the IEEE 802.15.4 standard for more details
            [IEEE802154]).

            b.2) A KeyUsageList data structure is created and stored
            within the KeyDescriptor element. One KeyUsageDescriptor
            associated to data MAC frames is created and stored into the
            KeyUsageList data structure.

            b.3) The DeviceDescriptorHandleList is created and populated
            with the pointer to the DeviceDescriptor created before.
            b.4) The i-th portion of the LinkKey, L_k, is stored within
            the Key field.

      c) The KeyDescriptor created in the previous step is added to the
      macKeyTable.








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6.5.4  Update of MAC security attribute for the RFD node after the
generation of the LinkKey

After the calculation of the i-th LinkKey, the RFD updates its MAC
security attributes as described in what follows:


      a) A keyDescriptor associated to the i-th LinkKey, L_k, is created
      by following these steps:

            a.1) a new KeyIdLookupList data structure is created and
            stored within the KeyDescriptor element. A
            KeyIdLookupDescriptor is generated and stored into the
            KeyIdLookupList data structure. The KeyIdMode, the
            KeySource, and the KeyIndex variables of this
            KeyIdLookupDescriptor are set to 0x03, the MAC address of
            the RFD node, and 1, respectively. DeviceAddrMode,
            DevicePANId, and DeviceAddress are not set because of the
            selected KeyIdMode (see Tab. 65 of the IEEE 802.15.4
            standard for more details [IEEE802154]).

            a.2) A KeyUsageList data structure is created and stored
            within the KeyDescriptor element. One KeyUsageDescriptor
            associated to data MAC frames is created and stored into the
            KeyUsageList data structure.

            a.3) The DeviceDescriptorHandleList is created and populated
            with the pointer to the DeviceDescriptor associate dto the
            coordinator.

            a.4) The i-th portion of the LinkKey, L_k, is stored within
            the Key field.

      b) The KeyDescriptor created in the previous step is added to the
      macKeyTable.





7  Additional features

There is the possibility to switch from the Flexible Secured to the
Hybrid Secure configuration.

To this aim, during the association phase, a device without security
capabilities sends to the coordinator a Beacon Request message with the
SecurityEnabled flag set to FALSE.



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The FFD device then switches to the Hybrid Secure configuration and
update all the MAC security attributes accordingly.

From this moment on, the coordinator sends control messages in clear.















































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8  Security Considerations

There are no security considerations for this document.

9  IANA Considerations

There is no IANA action required for this document.


10  References

10.1  Normative References

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC1776]  Crocker, S., "The Address is the Message", RFC 1776, April
              1 1995.

   [TRUTHS]   Callon, R., "The Twelve Networking Truths", RFC 1925,
              April 1 1996.


10.2  Informative References

   [EVILBIT]  Bellovin, S., "The Security Flag in the IPv4 Header",
              RFC 3514, April 1 2003.

   [RFC5513]  Farrel, A., "IANA Considerations for Three Letter
              Acronyms", RFC 5513, April 1 2009.

   [RFC5514]  Vyncke, E., "IPv6 over Social Networks", RFC 5514, April 1
              2009.



Authors' Addresses


   G. Piro
   DEI, Dep. of Electrical and Information Engineering
   Politecnico di Bari
   Via Orabona 4, 70125, Bari, ITALY
   Phone: +39 0805963301

   Email: g.piro@poliba.it





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   G. Boggia
   DEI, Dep. of Electrical and Information Engineering
   Politecnico di Bari
   Via Orabona 4, 70125, Bari, ITALY
   Phone: +39 0805963913

   Email: g.boggia@poliba.it



   L.A. Grieco
   DEI, Dep. of Electrical and Information Engineering
   Politecnico di Bari
   Via Orabona 4, 70125, Bari, ITALY
   Phone: +39 0805963911

   Email: a.grieco@poliba.it


































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