Requirements for a Lightweight AKE for OSCORE.
draft-selander-lake-reqs-02

Network Working Group                                         M. Vucinic
Internet-Draft                                                     INRIA
Intended status: Informational                               G. Selander
Expires: April 18, 2020                                      J. Mattsson
                                                             Ericsson AB
                                                        October 16, 2019

             Requirements for a Lightweight AKE for OSCORE.
                      draft-selander-lake-reqs-02

Abstract

   This document compiles the requirements for a lightweight
   authenticated key exchange protocol for OSCORE.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem description . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Credentials . . . . . . . . . . . . . . . . . . . . . . .   2
     2.2.  Crypto Agility  . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  AKE for OSCORE  . . . . . . . . . . . . . . . . . . . . .   3
     2.4.  Lightweight . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Requirements Summary  . . . . . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   OSCORE [RFC8613] is a lightweight communication security protocol
   providing end-to-end security on application layer for constrained
   IoT settings (cf.  [RFC7228]).  It is expected to be deployed with
   standards and frameworks using CoAP such as 6TiSCH, LPWAN, OMA
   Specworks LwM2M, Fairhair Alliance and Open Connectivity Foundation.
   OSCORE lacks a matching authenticated key exchange protocol (AKE).
   This document compiles the requirements for such an AKE.

2.  Problem description

2.1.  Credentials

   IoT deployments differ in terms of what credentials can be supported.
   Currently many systems use pre-shared keys (PSK) provisioned out of
   band, for various reasons.  PSK are often used in a first deployment
   because of its percieved simplicity.  The use of PSK allows for
   protection of communication without major additional security
   processing, and also enables the use of symmetric crypto algorithms
   only, reducing the implementation and computational effort in the
   endpoints.

   However, PSK based provisioning has inherent weaknesses.  There has
   been reports of massive breaches of PSK provisioning systems, and as
   many systems use PSK without perfect forward secrecy (PFS) they are
   vulnerable to passive pervasive monitoring.  The security of these
   systems can be improved by adding PFS through an AKE authenticated by
   the provisioned PSK.

   Shared keys can alternatively be established in the endpoints using
   an AKE protocol authenticated with asymmetric public keys instead of
   symmetric secret keys.  Raw public keys (RPK) can be provisioned with

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   the same scheme as PSKs, and allows a more relaxed trust model since
   RPKs need not be secret.

   By running the same asymmetric key AKE with public key certificates
   instead of RPK, key provisioning can be omitted, leading to a more
   automated bootstrapping procedure.

   These steps provide an example of a migration path in limited scoped
   steps from simple to more robust security bootstrapping and
   provisioning schemes where each step improves the overall security
   and/or simplicity of deployment of the IoT system, although not all
   steps are necessarily feasible for the most constrained settings.

   In order to allow for these different schemes, the AKE must support
   PSK, RPK and certificate based authentication.

   Considering the wide variety of deployments it is desirable to
   support different schemes for transporting and identifying
   credentials, see Section 2 of [I-D.ietf-cose-x509].

2.2.  Crypto Agility

   Motivated by long deployment lifetimes, the AKE is required to
   support crypto agility, including modularity of COSE crypto
   algorithms and negotiation of preferred crypto algorithms for OSCORE
   and the AKE.  The AKE negotiation must be protected against downgrade
   attacks.

2.3.  AKE for OSCORE

   In order to be suitable for OSCORE, at the end of the AKE protocol
   run the two parties must agree on (see Section 3.2 of [RFC8613]):

   o  a shared secret (OSCORE Master Secret) with PFS and a good amount
      of randomness.  (The term "good amount of randomness" is borrowed
      from [HKDF] to signify not necessarily uniformly distributed
      randomness.)

   o  identifiers providing a hint to the receiver of what security
      context to use when decrypting the message (OSCORE Sender IDs of
      peer endpoints), arbitrarily short

   o  COSE algorithms to use with OSCORE

   Moreover, the AKE must support the same transport as OSCORE, in
   particular any protocol where CoAP can be transported.

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   To ensure that the AKE is efficient for the expected applications of
   OSCORE, we list the relevant public specifications of technologies
   where OSCORE is included:

   o  The IETF 6TiSCH WG charter (-02) identifies the need to "secur[e]
      the join process and mak[e] that fit within the constraints of
      high latency, low throughput and small frame sizes that
      characterize IEEE802.15.4 TSCH".  OSCORE protects the join
      protocol as described in 6TiSCH Minimal Security
      [I-D.ietf-6tisch-minimal-security].

   o  The IETF LPWAN WG charter (-01) identifies the need to improve the
      transport capabilities of LPWA networks such as NB-IoT and LoRa
      whose "common traits include ... frame sizes ... [on] the order of
      tens of bytes transmitted a few times per day at ultra-low
      speeds".  The application of OSCORE is described in
      [I-D.ietf-lpwan-coap-static-context-hc].

   o  OMA Specworks LwM2M version 1.1 [LwM2M] defines bindings to two
      challenging radio technologies where OSCORE will be deployed:
      LoRaWAN and NB-IoT.

   Other industry fora which plan to use OSCORE:

   o  Fairhair Alliance has defined an architecture [Fairhair] which
      adopts OSCORE for multicast, but it is not clear whether the
      architecture will support unicast OSCORE.

   o  Open Connectivity Foundation (OCF) has been actively involved in
      the OSCORE development for the purpose of deploying OSCORE, but no
      public reference is available since OCF only references RFCs.  We
      believe that these OSCORE consumers reflect similar levels of
      constraints on the devices and networks in question.

   The solution will presumably be useful in other scenarios as well
   since a low security overhead improves the overall performance, but
   we do not require the solution to necessarily be applicable anywhere
   else.

2.4.  Lightweight

   As motivated in Section 2.3 we target an AKE which is efficiently
   deployable in 6TiSCH multi-hop networks, LoRaWAN networks and NB-IoT
   networks.  The desire is to optimize the AKE to be 'as lightweight as
   reasonably achievable' in these environments, where 'lightweight'
   refers to:

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   o  resource consumption, measured by bytes on the wire, wall-clock
      time and number of round trips to complete, or power consumption

   o  the amount of new code required on end systems which already have
      an OSCORE stack

   These properties need to be considered in the context of the use of
   an existing CoAP/OSCORE stack in the targeted networks.  However,
   some properties may be difficult to evaluate for a given protocol,
   for example, because they depend on the radio conditions or other
   simultaneous network traffic.  Therefore these properties should be
   taken as input for identifying plausible protocol metrics that can be
   more easily measured and compared between protocols.

   Per 'bytes on the wire', it is desirable for these AKE messages to
   fit into the MTU size of these protocols; and if not possible, within
   as few frames as possible, since using multiple MTUs can have
   significant costs in terms of time and power.

   Per 'time', it is desirable for the AKE message exchange(s) to
   complete in a reasonable amount of time, both for a single
   uncongested exchange and when multiple exchanges are running in an
   interleaved fashion, like e.g. in a "network formation" setting when
   multiple devices connect for the first time.  This latency may not be
   a linear function depending on congestion and the specific radio
   technology used.  As these are relatively low data rate networks, the
   latency contribution due to computation is in general not expected to
   be dominant.

   Per 'round-trips', it is desirable that the number of completed
   request/response message exchanges required before the initiating
   endpoint can start sending protected traffic data is as small as
   possible, since this reduces completion time.  See Section 2.4.4 for
   a discussion about the tradeoff between message size and number of
   messages.

   Per 'power', it is desirable for the transmission of AKE messages and
   crypto to draw as little power as possible.  The best mechanism for
   doing so differs across radio technologies.  For example, NB-IoT uses
   licensed spectrum and thus can transmit at higher power to improve
   coverage, making the transmitted byte count relatively more important
   than for other radio technologies.  In other cases, the radio
   transmitter will be active for a full MTU frame regardless of how
   much of the frame is occupied by message content, which makes the
   byte count less sensitive for the power consumption.  Increased power
   consumption is unavoidable in poor network conditions, such as most
   wide-area settings including LoRaWAN.

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   Per 'new code', it is desirable to introduce as little new code as
   possible onto OSCORE-enabled devices to support this new AKE.  These
   devices have on the order of 10s of kB of memory and 100 kB of
   storage on which an embedded OS; a COAP stack; CORE and AKE
   libraries; and target applications would run.  It is expected that
   the majority of this space is available for actual application logic,
   as opposed to the support libraries.  In a typical OSCORE
   implementation COSE encrypt and signature structures will be
   available, as will support for COSE algorithms relevant for IoT
   enabling the same algorithms as is used for OSCORE (e.g.  COSE
   algorithm no. 10 = CCM* used by 6TiSCH).  The use of those, or CBOR
   or CoAP, would not add to the footprint.

   While the large variety of settings and capabilities of the devices
   and networks makes it challenging to produce exact values of some
   these dimensions, there are some key benchmarks that are tractable
   for security protocol engineering and which have a significant
   impact.

2.4.1.  LoRaWAN

   LoRaWAN employs unlicensed radio frequency bands in the 868MHz ISM
   band, in Europe regulated by ETSI EN 300 220.  For LoRaWAN the most
   relevant metric is the Time-on-Air, which determines the back-off
   times and can be used an indicator to calculate energy consumption.
   LoRaWAN is legally required to use a 1% (or smaller) duty cycle, a
   payload split into two fragments instead of one increases the time to
   complete the sending of this payload by at least 10,000%. The use of
   an AKE for providing end-to-end security on application layer need to
   comply with the duty cycle.  One relevant benchmark is performance in
   low coverage with Data Rates 0-2 corresponding to a packet size of 51
   bytes [LoRaWAN].  While larger frame sizes are also defined, their
   use depend on good radio conditions.  Some libraries/providers only
   support 51 bytes packet size.

2.4.2.  6TiSCH

   For 6TiSCH specifically, as a time-sliced network, bytes of the wire
   (or rather, the quantization into frame count) is particularly
   noteworthy, since more frames contribute to congestion for spectrum
   (and concomitant error rates) in a non-linear way, especially in
   scenarios when large numbers of independent nodes are attempting to
   execute an AKE to join a network.

   The available size for key exchange messages depends the topology of
   the network and other parameters.  One benchmark which is relevant
   for studying AKE is the network formation setting.  For a 6TiSCH
   production network 5 hops deep in a network formation setting, the

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   available CoAP overhead to avoid fragmentation is 47/45 bytes
   (uplink/downlink) [AKE-for-6TiSCH].

2.4.3.  NB-IoT

   For NB-IoT, in contrast to the other two technologies below, the
   radio bearers are not characterized by a fixed sized PDU.
   Concatenation, segmentation and reassembly are part of the service
   provided by the radio layer.  Furthermore, since NB-IoT is operating
   in licensed spectrum, the packets on the radio interface can be
   transmitted back-to-back, so the time before sending OSCORE protected
   data is dependent on the number of round trips/messages of the AKE.
   An AKE providing challenge-response based mutual authentication
   requires at least three messages/one round trip before it is possible
   to encrypt traffic data between peers meeting for the first time.
   NB-IoT has a high per byte energy consumption component for uplink
   transfers, implying that those messages should be as small as
   possible.

2.4.4.  Discussion

   While "as small protocol messages as possible" does not lend itself
   to a sharp boundary threshold, "as few protocol messages as possible"
   does and is relevant in all settings above.

   The penalty is high for not fitting into the frame sizes of 6TiSCH
   and LoRaWAN networks.  Fragmentation is not defined within these
   technologies so requires fragmentation scheme on a higher layer in
   the stack.  With fragmentation increases the number of frames per
   message, each with its associated overhead in terms of power
   consumption and latency.  Additionally the probability for errors
   increases, which leads to retransmissions of frames or entire
   messages that in turn increases the power consumption and latency.

   There are trade-offs between "few messages" and "few frames"; if
   overhead is spread out over more messages such that each message fits
   into a particular frame this may reduce the overall power
   consumption.  While it may be possible to engineer such a solution
   for a particular radio technology and signature algorithm, the
   benefits in terms of fewer messages/round trips in general and for
   NB-IoT in particular (see Section 2.4.3) are considered more
   important than optimizing for a specific scenario.  Hence an optimal
   AKE protocol has 3 messages and each message fits into as few frames
   as possible, ideally 1 frame per message.

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3.  Requirements Summary

   o  The AKE must support PSK, RPK and certificate based authentication
      and crypto agility, be 3-pass and support the same transport as
      OSCORE.  It is desirable to support different schemes for
      transporting and identifying credentials.

   o  After the AKE run, the peers must agree on a shared secret with
      PFS and good amount of randomness, peer identifiers (potentially
      short), and COSE algorithms to use.

   o  The AKE must reuse CBOR, CoAP and COSE primitives and algorithms
      for low code complexity of a combined OSCORE and AKE
      implementation.

   o  The messages must be as small as reasonably achievable and fit
      into as few LoRaWAN packets and 6TiSCH frames as possible,
      optimally 1 for each message.

4.  Security Considerations

   This document compiles the requirements for an AKE and provides some
   related security considerations.

   The AKE must provide the security properties expected of IETF
   protocols, e.g., providing confidentiality protection, integrity
   protection, and authentication with strong work factor.

5.  IANA Considerations

   None.

6.  Informative References

   [AKE-for-6TiSCH]
              "AKE for 6TiSCH", March 2019,
              <https://docs.google.com/document/
              d/1wLoIexMLG3U9iYO5hzGzKjkvi-VDndQBbYRNsMUlh-k>.

   [Fairhair]
              "Security Architecture for the Internet of Things (IoT) in
              Commercial Buildings, Fairhair Alliance white paper",
              March 2018, <https://www.fairhair-
              alliance.org/data/downloadables/1/9/
              fairhair_security_wp_march-2018.pdf>.

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   [HKDF]     Krawczyk, H., "Cryptographic Extraction and Key
              Derivation: The HKDF Scheme", May 2010,
              <https://eprint.iacr.org/2010/264.pdf>.

   [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-12 (work in progress), July 2019.

   [I-D.ietf-cose-x509]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Headers for carrying and referencing X.509 certificates",
              draft-ietf-cose-x509-04 (work in progress), September
              2019.

   [I-D.ietf-lpwan-coap-static-context-hc]
              Minaburo, A., Toutain, L., and R. Andreasen, "LPWAN Static
              Context Header Compression (SCHC) for CoAP", draft-ietf-
              lpwan-coap-static-context-hc-11 (work in progress),
              October 2019.

   [LoRaWAN]  "LoRaWAN Regional Parameters v1.0.2rB", February 2017,
              <https://lora-alliance.org/resource-hub/lorawantm-
              regional-parameters-v102rb>.

   [LwM2M]    "OMA SpecWorks LwM2M", August 2018,
              <http://www.openmobilealliance.org/release/LightweightM2M/
              V1_1-20180710-A/OMA-TS-LightweightM2M_Transport-
              V1_1-20180710-A.pdf>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

Authors' Addresses

   Malisa Vucinic
   INRIA

   Email: malisa.vucinic@inria.fr

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   Goeran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com

   John Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com

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