Network Working Group                                         M. Vucinic
Internet-Draft                                                     Inria
Intended status: Informational                               G. Selander
Expires: 22 August 2020                                      J. Mattsson
                                                             Ericsson AB
                                                               D. Garcia
                                                     Odin Solutions S.L.
                                                        19 February 2020

             Requirements for a Lightweight AKE for OSCORE


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

Status of This Memo

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   This Internet-Draft will expire on 22 August 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem description . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  AKE for OSCORE  . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Credentials . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Mutual Authentication . . . . . . . . . . . . . . . . . .   6
     2.4.  Confidentiality . . . . . . . . . . . . . . . . . . . . .   7
     2.5.  Cryptographic Agility and Negotiation Integrity . . . . .   7
     2.6.  Identity Protection . . . . . . . . . . . . . . . . . . .   8
     2.7.  Auxiliary Data  . . . . . . . . . . . . . . . . . . . . .   9
     2.8.  Extensibility . . . . . . . . . . . . . . . . . . . . . .   9
     2.9.  Denial of Service . . . . . . . . . . . . . . . . . . . .  10
     2.10. Lightweight . . . . . . . . . . . . . . . . . . . . . . .  10
       2.10.1.  LoRaWAN  . . . . . . . . . . . . . . . . . . . . . .  12
       2.10.2.  6TiSCH . . . . . . . . . . . . . . . . . . . . . . .  13
       2.10.3.  NB-IoT . . . . . . . . . . . . . . . . . . . . . . .  14
       2.10.4.  Discussion . . . . . . . . . . . . . . . . . . . . .  16
       2.10.5.  AKE frequency  . . . . . . . . . . . . . . . . . . .  17
   3.  Requirements Summary  . . . . . . . . . . . . . . . . . . . .  17
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Informative References  . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   OSCORE [RFC8613] is a lightweight communication security protocol
   providing end-to-end security on application layer for constrained
   IoT settings (cf.  [RFC7228]).  OSCORE lacks a matching authenticated
   key exchange protocol (AKE).  The intention with LAKE is to create a
   simple yet secure AKE for implementation in embedded devices
   supporting OSCORE.

   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:

   *  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

   *  The IETF LPWAN WG charter (-01) identifies the need to improve the
      transport capabilities of LPWA networks such as NB-IoT and LoRa

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      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

   *  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:

   *  Open Connectivity Foundation (OCF) has been actively involved in
      the OSCORE development for the purpose of deploying OSCORE.

   *  Fairhair Alliance has defined an architecture [Fairhair] which
      adopts OSCORE for multicast, but it is not clear whether the
      architecture will support unicast OSCORE.  Fairhair Alliance
      merged with OCF in November 2019.

   This document compiles the requirements for the AKE for OSCORE.  It
   summarizes the security requirements that are expected from such an
   AKE, as well as the main characteristics of the environments where
   the solution is envisioned to be deployed.  The solution will
   presumably be useful in other scenarios as well since a low security
   overhead improves the overall performance.

2.  Problem description

2.1.  AKE for OSCORE

   The rationale for designing this protocol is that OSCORE is lacking a
   matching AKE.  OSCORE was designed for lightweight RESTful operations
   for example by minimizing the overhead, and applying the protection
   to the application layer, thereby limiting the data being encrypted
   and integrity protected for the other endpoint.  Moreover, OSCORE was
   tailored for use with lightweight primitives that are likely to be
   implemented in the device, specifically CoAP, CBOR and COSE.  The
   same properties must apply to the AKE.

   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]):

   *  A shared secret (OSCORE Master Secret) with Perfect Forward
      Secrecy (PFS, see Section 2.4) and a good amount of randomness.
      (The term "good amount of randomness" is borrowed from [HKDF] to
      signify not necessarily uniformly distributed randomness.)

   *  OSCORE Sender IDs of peer endpoints, arbitrarily short

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   *  COSE algorithms to use with OSCORE

   COSE provides the crypto primitives for OSCORE, and shall therefore
   be used also by the AKE, for several reasons including maintenance of
   crypto library.  COSE provides identification of credentials and
   algorithms for OSCORE and the AKE, and an extension point for new

   The AKE cannot rely on messages being exchanged in both directions
   after the AKE has completed, because CoAP/OSCORE requests may not
   have a response [RFC7967].  Furthermore, there is no assumption of
   dependence between CoAP client/server and AKE initiator/responder
   roles, and an OSCORE context may be used with CoAP client and server
   roles interchanged as is done, for example, in [LwM2M].

   Moreover, the AKE must support transport over CoAP.  Since the AKE
   messages most commonly will be encapsulated in CoAP, the AKE must not
   duplicate functionality provided by CoAP, or at least not duplicate
   functionality in such a way that it adds extra costs in terms of code
   size, code maintenance, etc.  It is therefore assumed that the AKE is
   being transported in a protocol that provides reliable transport,
   that can preserve packet ordering and handle message duplication,
   that can perform fragmentation and protect against denial of service
   attacks, such as provided by the CoAP Echo option

   The AKE may use other transport than CoAP.  In this case the
   underlying layers must correspondingly handle message loss,
   reordering, message duplication, fragmentation, and denial of service

2.2.  Credentials

   IoT deployments differ in terms of what credentials can be supported.
   Currently many systems use pre-shared keys (PSKs) provisioned out of
   band, for various reasons.  PSKs are often used in a first deployment
   because of their perceived simplicity.  The use of PSKs 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

   However, PSK-based provisioning has inherent weaknesses.  There has
   been reports of massive breaches of PSK provisioning systems, and as
   many systems use PSKs 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.

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   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
   the same scheme as PSKs, which allows for a more relaxed trust model
   since RPKs need not be secret.  The corresponding private keys are
   assumed to be provisioned to the party being authenticated beforehand
   (e.g. in factory or generated on-board).

   As a third option, by using a public key infrastructure and running
   an asymmetric key AKE with public key certificates instead of RPKs,
   key provisioning can be omitted, leading to a more automated ("zero-
   touch") bootstrapping procedure.  The root CA keys are assumed to be
   provisioned beforehand.

   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- (shared between two nodes), RPK- and certificate-based

   Multiple public key authentication credential types may need to be
   supported for RPK and certificate-based authentication.  In case of a
   Diffie-Hellman key exchange both the use of signature based public
   keys (for compatibility with existing ecosystem) and static DH public
   keys (for reduced message size) is expected.

   To further minimize the bandwidth consumption it is required to
   support transporting the certificates by reference rather than by
   value.  Considering the wide variety of deployments the AKE must
   support different schemes for transporting and identifying
   credentials, including those identified in Section 2 of

   The common lack of a user interface in constrained devices leads to
   various credential provisioning schemes.  The use of RPKs may be
   appropriate for the authentication of the AKE initiator but not for
   the AKE responder.  The AKE must support different credentials for
   authentication in different directions of the AKE run, e.g.
   certificate-based authentication for the initiating endpoint and RPK-
   based authentication for the responding endpoint.

   Assuming that both signature public keys and static DH public keys
   are in use, then also the case of mixed credentials need to be
   supported with one endpoint using a static DH public key and the

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   other using a signature public key.  The AKE shall support the
   initiator signaling which public key credential mix to be used in the
   protocol such that the responder knows and can verify that the
   intended variant was executed.

2.3.  Mutual Authentication

   The AKE must provide mutual authentication during the protocol run.
   At the end of the AKE protocol, each endpoint shall have
   authenticated the other's credential.  In particular, both endpoints
   must agree on a fresh session identifier, and the roles and
   credentials of both endpoints.

   Since the protocol may be initiated by different endpoints, it shall
   not be necessary to determine beforehand which endpoint takes the
   role of initiator of the AKE.

   The mutual authentication guarantees of the AKE shall at least
   guarantee the following properties:

   *  The AKE shall provide Key Compromise Impersonation (KCI)

   *  The AKE shall protect against identity misbinding attacks, when
      applicable.  Note that the identity may be directly related to a
      public key such as for example the public key itself, a hash of
      the public key, or data unrelated to a key.

   *  The AKE shall protect against reflection attacks, but need not
      protect against attacks when more than two parties legitimately
      share keys (cf. the Selfie attack on TLS 1.3) as that setting is
      out of scope.

   Moreover, it shall be possible for the receiving endpoint to detect a
   replayed AKE message.

   Furthermore, the endpoints shall be able to verify that the identity
   of the other endpoint is an acceptable identity that it is intended
   to authenticate to.  (This requirement extends beyond the AKE in that
   the application must enable access to information about acceptable
   identities without compromising the overall lightweightness of the

   As often is the case, it is expected that an AKE fulfilling these
   goals would have at least three flights of messages (with each flight
   potentially consisting of one or more messages, depending on the AKE
   design and the mapping to OSCORE).

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2.4.  Confidentiality

   The shared secret established by the AKE must be known only to the
   two authenticated endpoints.

   A passive network attacker should never learn any session keys, even
   if it knows both endpoints' long-term keys.

   An active attacker who has compromised the initiator or responder
   credential shall still not be able to compute past session keys
   (Perfect Forward Secrecy).  These properties can be achieved e.g.
   with an ephemeral Diffie-Hellman key exchange.

   Perfect Forward Secrecy may alternatively be achieved with a nonce
   exchange followed by appropriately derived new session keys provided
   that state can be kept in the form of a session counter.  Note that
   OSCORE specifies a method for session key update involving a nonce
   exchange (see Appendix B in [RFC8613]).

   The AKE shall provide a mechanism to use the output of one handshake
   to optimize future handshakes, e.g., by generating keying material
   which can be used to authenticate a future handshake, thus avoiding
   the need for public key authentication in that handshake.

   To mitigate against bad random number generators the AKE shall
   mandate randomness improvements such as

2.5.  Cryptographic Agility and Negotiation Integrity

   Motivated by long deployment lifetimes, the AKE is required to
   support cryptographic agility, including the modularity of COSE
   crypto algorithms and negotiation of preferred crypto algorithms for
   OSCORE and the AKE.

   *  The protocol shall support both pre-shared key and asymmetric key
      authentication.  PAKE and post-quantum key exchange is out of
      scope, but may be supported in a later version.

   *  The protocol shall allow multiple elliptic curves for Diffie-
      Hellman operations and signature-based authentication.

   *  The AKE shall support negotiation of all the COSE algorithms to be
      used in the AKE and in OSCORE.  A successful negotiation shall
      result in the most preferred algorithms of one of the parties
      which are supported by the other.

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   *  The AKE may choose different sets of symmetric crypto algorithms
      (AEAD, MAC, etc.) for AKE and for OSCORE.  In particular, the
      length of the MAC for the AKE may be required to be larger than
      for OSCORE.

   The AKE negotiation must provide strong integrity guarantees against
   active attackers.  At the end of the AKE protocol, both endpoints
   must agree on both the crypto algorithms that were proposed and those
   that were chosen.  In particular, the protocol must protect against
   downgrade attacks.

2.6.  Identity Protection

   In general, it is necessary to transport identities as part of the
   AKE run in order to provide authentication of an entity not
   identified beforehand.  In the case of constrained devices, the
   identity may contain sensitive information on the manufacturer of the
   device, the batch, default firmware version, etc.  Protecting
   identifying information from passive and active attacks is important
   from a privacy point of view, but needs to be balanced with the other
   requirements, including security and lightweightness.  For certain
   data we therefore need to make an exemption in order to obtain an
   efficient protocol.

   In the case of public key identities, the AKE is required to protect
   the identity of one of the peers against active attackers and the
   identity of the other peer against passive attackers.

   In case of a PSK identifier, this may be protected against passive
   attackers, for example with a key derived from a Diffie-Hellman
   shared secret at the earliest in flight 3.  As a consequence, in
   order to authenticate the responder within the AKE, at least four
   protocol flights are needed in case of symmetric key authentication
   with identity protection.  Considering the need to keep the number of
   round-trips at a minimum (see Section 2.10.4), unless there are other
   good reasons for having more than 3 flights, it is not required to
   protect the PSK identifier, and it may thus be sent in the first

   Other identifying information that needs to be transported in plain
   text is cipher suites and connection identifiers.  Encrypting crypto
   algorithms does not allow negotiation of cipher suite within 3
   flights.  Encryption of connection identifiers only works in
   asymmetric case and does not enable arbitrarily short identifiers
   (see Section 2.1).

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2.7.  Auxiliary Data

   In order to reduce round trips and the number of flights, and in some
   cases also streamline processing, certain security features may be
   integrated into the AKE by transporting auxiliary data together with
   the AKE messages.

   One example is the transport of third-party authorization information
   such as an access token or a voucher from initiator to responder or
   vice versa.  Such a scheme could enable the party receiving the
   authorization information to make a decision about whether the party
   being authenticated is also authorized before the protocol is
   completed, and if not then discontinue the protocol before it is
   complete, thereby saving time, message processing and data
   transmission.  This application can be further optimized by using an
   AKE with static DH keys [I-D.selander-ace-ake-authz].

   Another example is the embedding of a certificate enrolment request
   or a newly issued certificate.

   The AKE must support the transport of such auxiliary data together
   with the protocol messages.

   Auxiliary data may contain privacy sensitive information.  The
   auxiliary data must not violate the AKE security properties.  The AKE
   needs to provide clear guidance on the level of security provided to
   auxiliary data at different stages of the protocol.

   For example, for a SIGMA-I AKE it is expected that the 3 flights will
   provide the following protection of the auxiliary data:

   *  Auxiliary data in the first flight is unprotected

   *  Auxiliary data in the second flight is confidentiality protected
      against passive attackers and integrity protected against active

   *  Auxiliary data in the third flight is confidentiality and
      integrity protected against active attackers

2.8.  Extensibility

   It is desirable that the AKE supports some kind of extensibility, in
   particular, the ability to later include new AKE modes such as PAKE
   support.  Note that by supporting COSE, the AKE can already support
   new algorithms, new certificate formats, ways to identify
   credentials, etc.

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   The main objective with this work is to create a simple yet secure
   AKE.  The AKE should avoid having multiple ways to express the same
   thing.  When the underlying encodings offered by CBOR offer multiple
   possibility the AKE should be strongly opinioniated, and clearly
   specify which one will be used.

   While remaining extensible, the AKE should avoid optional mechanisms
   which introduce code paths that are less well tested.

   The AKE should avoid mechanisms where an initiator takes a guess at
   the policy, and when it receives a negative response, must guess,
   based upon what it has tried, what to do next.

2.9.  Denial of Service

   The AKE shall protect against denial of service attacks on responder
   and initiator to the extent that the protocol supports lightweight
   deployments (see Section 2.10) and without duplicating the DoS
   mitigation of the underlying transport (see Section 2.1).

   Jamming attacks, cutting cables etc. leading to long term loss of
   availability may not be possible to mitigate, but an attacker
   temporarily injecting messages or disturbing the communication shall
   not have a similar impact.

2.10.  Lightweight

   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:

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

   *  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 and
   technologies.  Some properties are difficult to evaluate for a given
   protocol, for example, because they depend on the radio conditions or
   other simultaneous network traffic.  Additionally, these properties
   are not independent.  Therefore the properties listed here should be
   taken as input for identifying plausible protocol metrics that can be
   more easily measured and compared between protocols.

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   Per 'bytes on the wire', it is desirable for the 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.  Note that the MTU size
   depends on radio technology and its characteristics, including data
   rates, number of hops, etc.  Example benchmarks are given further
   down in this section.

   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 2.10.4 for
   a discussion about the tradeoff between message size and number of

   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 as long as it
   fits into the MTU frame.  The power consumption thus increases with
   AKE message size and the largest impact is on average under poor
   network conditions.

   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

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   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

2.10.1.  LoRaWAN

   Reflecting deployment reality as of now, we focus on the European
   regulation as described in ETSI EN 300 220.  LoRaWAN employs
   unlicensed radio frequency bands in the 868 MHz ISM band.  For
   LoRaWAN the most relevant metric is the Time-on-Air, which determines
   the period before the next communication can occur and also which can
   be used as an indicator to calculate energy consumption.  LoRaWAN is
   legally required to use a duty cycle with values such as 0.1%, 1% and
   10% depending on the sub-band that is being used, leading to a
   payload split into fragments interleaved with unavailable times.  For
   Europe, the duty cycle is 1% (or smaller).  Although there are
   exceptions from the use of duty cycle, the use of an AKE for
   providing end-to-end security on application layer needs to comply
   with the duty cycle.  Bytes on the wire

   LoRaWAN has a variable MTU depending on the Spreading Factor (SF).
   The higher the spreading factor, the higher distances can be achieved
   and/or better reception.  If the coverage and distance allows it,
   with SF7 - corresponding to higher data rates - the maximum payload
   is 222 bytes.  For a SF12 - and low data rates - the maximum payload
   is 51 bytes.

   The benchmark used here is Data Rates 0-2 corresponding to a packet
   size of 51 bytes [LoRaWAN].  The use of larger frame size depend on
   good radio conditions which are not always present.  Some libraries/
   providers only support 51-bytes packet size.  Time

   The time it takes to send a message over the air in LoRaWAN can be
   calculated as a function of the different parameters of the
   communication.  These are the Spreading Factor (SF), the message
   size, the channel, bandwidth, coding rate, etc.  An important feature
   of LoRaWAN is the duty cycle limitation due to the use of the ISM
   band.  A duty cycle of 1% implies that the time to complete a
   fragmentation of the payload increases by at least 10,000%. This

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   limitation determines how long time the device will have to wait for
   next use, which encourages the reduction of the message size as much
   as possible.  Round trips and number of flights

   Considering the duty cycle of LoRaWAN and associated unavailable
   times, the round trips and number of LoRaWAN packets needs to be
   reduced as much as possible.  Power

   The calculation of the power consumption in LoRaWAN is dependent on
   several factors, such as the spreading factor used and the length of
   the messages sent, both having a clear dependency with the time it
   takes to transmit the messages.  The communication model (inherent to
   the different LoRaWAN classes of devices) also has an impact on the
   energy consumption, but overall the Time-on-Air is an important
   indication of the performance.

2.10.2.  6TiSCH

   6TiSCH operates in the 2.4 GHz unlicensed frequency band and uses
   hybrid Time Division/Frequency Division multiple access (TDMA/FDMA).
   Nodes in a 6TiSCH network form a mesh.  The basic unit of
   communication, a cell, is uniquely defined by its time and frequency
   offset in the communication schedule matrix.  Cells can be assigned
   for communication to a pair of nodes in the mesh and so be collision-
   free, or shared by multiple nodes, for example during network
   formation.  In case of shared cells, some collision-resolution scheme
   such as slotted-Aloha is employed.  Nodes exchange frames which are
   at most 127-bytes long, including the link-layer headers.  To
   preserve energy, the schedule is typically computed in such a way
   that nodes switch on their radio below 1% of the time ("radio duty
   cycle").  A 6TiSCH mesh can be several hops deep.  In typical use
   cases considered by the 6TiSCH working group, a network that is 2-4
   hops deep is commonplace; a network which is more than 8 hops deep is
   not common.  Bytes on the wire

   Increasing the number of bytes on the wire in a protocol message has
   an important effect on the 6TiSCH network in case the fragmentation
   is triggered.  More fragments contribute to congestion of shared
   cells (and concomitant error rates) in a non-linear way.

   The available size for key exchange messages depends on the topology
   of the network, whether the message is traveling uplink or downlink,

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   and other stack parameters.  A key performance indicator for a 6TiSCH
   network is "network formation", i.e. the time it takes from switching
   on all devices, until the last device has executed the AKE and
   securely joined.  As an example, given the size limit on the frames
   and taking into account the different headers (including link-layer
   security), if a 6TiSCH network is 5 hops deep, the maximum CoAP
   payload size to avoid fragmentation is 47/45 bytes (uplink/downlink)
   [AKE-for-6TiSCH].  Time

   Given the slotted nature of 6TiSCH, the number of bytes in a frame
   has insignificant impact on latency, but the number of frames has.
   The relevant metric for studying AKE is the network formation time,
   which implies parallel AKE runs among nodes that are attempting to
   join the network.  Network formation time directly affects the time
   installers need to spend on site at deployment time.  Round trips and number of flights

   Given the mesh nature of the 6TiSCH network, and given that each
   message may travel several hops before reaching its destination, it
   is highly desirable to minimize the number of round trips to reduce
   latency.  Power

   From the power consumption point of view, it is more favorable to
   send a small number of large frames than a larger number of short

2.10.3.  NB-IoT

   3GPP has specified Narrow-Band IoT (NB-IoT) for support of infrequent
   data transmission via user plane and via control plane.  NB-IoT is
   built on cellular licensed spectrum at low data rates for the purpose
   of supporting:

   *  operations in extreme coverage conditions,

   *  device battery life of 10 years or more,

   *  low device complexity and cost, and

   *  a high system capacity of millions of connected devices per square

   NB-IoT achieves these design objectives by:

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   *  Reduced baseband processing, memory and RF enabling low complexity
      device implementation.

   *  A lightweight setup minimizing control signaling overhead to
      optimize power consumption.

   *  In-band, guard-band, and stand-alone deployment enabling efficient
      use of spectrum and network infrastructure.  Bytes on the wire

   The number of bytes on the wire in a protocol message has a direct
   effect on the performance for NB-IoT.  In contrast to LoRaWAN and
   6TiSCH, the NB-IoT radio bearers are not characterized by a fixed
   sized PDU.  Concatenation, segmentation and reassembly are part of
   the service provided by the NB-IoT radio layer.  As a consequence,
   the byte count has a measurable impact on time and energy consumption
   for running the AKE.  Time

   Coverage significantly impacts the available bit rate and thereby the
   time for transmitting a message, and there is also a difference
   between downlink and uplink transmissions (see Section
   The transmission time for a message is essentially proportional to
   the number of bytes.

   Since NB-IoT is operating in licensed spectrum, in contrast to e.g.
   LoRaWAN, the packets on the radio interface can be transmitted back-
   to-back, so the time before sending OSCORE protected data is limited
   by the number of round trips/flights of the AKE and not by a duty
   cycle.  Round trips and number of flights

   As indicated in Section, the number of frames and round-
   trips is one limiting factor for protocol completion time.  Power

   Since NB-IoT is operating in licensed spectrum, the device is allowed
   to transmit at a relatively high power, which has a large impact on
   the energy consumption.

   The benchmark for NB-IoT energy consumption is based on the same
   computational model as was used by 3GPP in the design of this radio
   layer [NB-IoT-battery-life-evaluation].  The device power consumption
   is assumed to be 500mW for transmission and 80mW for reception.

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   Power consumption for "light sleep" (~ 3mW) and "deep sleep" (~
   0.015mW) are negligible in comparison.  The bitrates (uplink/
   downlink) are assumed to be 28/170 kbps for good coverage and
   0,37/2,5 kbps for bad coverage.

   The results [AKE-for-NB-IoT] show a high per-byte energy consumption
   for uplink transmissions, in particular in bad coverage.  Given that
   the application decides about the device being initiator or responder
   in the AKE, the protocol cannot be tailored for a particular message
   being uplink or downlink.  To perform well in both kind of
   applications the overall number of bytes of the protocol needs to be
   as low as possible.

2.10.4.  Discussion

   While "as small protocol messages as possible" does not lend itself
   to a sharp boundary threshold, "as few flights 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.  For example, with a frame size of 50 bytes, two 60-byte
   messages will fragment into 4 frames in total, whereas three 40-byte
   messages fragment into 3 frames in total.  While it may be possible
   to engineer such a solution for a particular radio technology and
   signature algorithm, the benefits in terms of fewer flights/round
   trips in general and for NB-IoT in particular (see Section 2.10.3)
   are considered more important than optimizing for a specific
   scenario.  Considering that an AKE protocol complying with these
   requirements is expected to have at least 3 messages, the optimal AKE
   has 3 messages and each message fits into as few frames as possible,
   ideally 1 frame per message.

   The difference between uplink and downlink performance should not be
   engineered into the protocol since it cannot be assumed that a
   particular protocol message will be sent uplink or downlink.

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2.10.5.  AKE frequency

   One question that has been asked in the context of lightweightness
   is: - How often is the AKE executed?  While it may be impossible to
   give a precise answer there are other perspectives to this question.

   1.  For some use cases, already one execution of the AKE is heavy,
       for example, because

       *  there are a number of parallel executions of the AKE which
          loads down the network, such as in a network formation
          setting, or

       *  the duty cycle makes the completion time long for even one run
          of the protocol.

   2.  If a device reboots it may not be able to recover the security
       context, e.g. due to lack of persistent storage, and is required
       to establish a new security context for which an AKE is
       preferred.  Reboot frequency may be difficult to predict in

   3.  To limit the impact of a key compromise, BSI, NIST and ANSSI and
       other organizations recommend in other contexts frequent renewal
       of keys by means of Diffie-Hellman key exchange.  This may be a
       symmetric key authenticated key exchange, where the symmetric key
       is obtained from a previous asymmetric key based run of the AKE.

   To summarize, even if it we are unable to give precise numbers for
   AKE frequency, a lightweight AKE:

   *  reduces the time for network formation and AKE runs in challenging
      radio technologies,

   *  allows devices to quickly re-establish security in case of
      reboots, and

   *  enables support for recommendations of frequent key renewal.

3.  Requirements Summary

   *  The AKE must support PSK, RPK and certificate based authentication
      with PFS and crypto agility for AKE as well as OSCORE, have 3
      flights and support transport over CoAP.  It is required to
      support different schemes for transporting and identifying

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

   *  The AKE must reuse CBOR, CoAP and COSE primitives and algorithms
      for low code complexity and to avoid duplicate maintenance of a
      combined OSCORE and AKE implementation.

   *  The messages should be as small as reasonably achievable.  The
      messages shall fit into as few LoRaWAN packets and 6TiSCH frames
      as possible.

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 mutual authentication, confidentiality,
   and negotiation integrity as is further detailed in the requirements.

5.  IANA Considerations



   The authors want to thank Richard Barnes, Karthik Bhargavan, Ivaylo
   Petrov, Eric Rescorla, Michael Richardson, and Claes Tidestav for
   providing valuable input.

Informative References

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

   [RFC7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <>.

   [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,

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              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Constrained Join Protocol (CoJP) for 6TiSCH", Work in
              Progress, Internet-Draft, draft-ietf-6tisch-minimal-
              security-15, 10 December 2019, <

              Minaburo, A., Toutain, L., and R. Andreasen, "LPWAN Static
              Context Header Compression (SCHC) for CoAP", Work in
              Progress, Internet-Draft, draft-ietf-lpwan-coap-static-
              context-hc-12, 10 December 2019, <

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Headers for carrying and referencing X.509 certificates",
              Work in Progress, Internet-Draft, draft-ietf-cose-x509-05,
              4 November 2019, <

              Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
              Request-Tag, and Token Processing", Work in Progress,
              Internet-Draft, draft-ietf-core-echo-request-tag-08, 4
              November 2019, <

              Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
              and C. Wood, "Randomness Improvements for Security
              Protocols", Work in Progress, Internet-Draft, draft-irtf-
              cfrg-randomness-improvements-09, 27 January 2020,

              Selander, G., Mattsson, J., Vucinic, M., and M.
              Richardson, "Lightweight Authorization for Authenticated
              Key Exchange.", Work in Progress, Internet-Draft, draft-
              selander-ace-ake-authz-00, 6 February 2020,

              "AKE for 6TiSCH", March 2019,

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              "AKE for NB-IoT", March 2019,

              "On mMTC, NB-IoT and eMTC battery life evaluation",
              January 2017,

   [HKDF]     Krawczyk, H., "Cryptographic Extraction and Key
              Derivation: The HKDF Scheme", May 2010,

   [LwM2M]    "OMA SpecWorks LwM2M", August 2018,

   [Fairhair] "Security Architecture for the Internet of Things (IoT) in
              Commercial Buildings, Fairhair Alliance white paper",
              March 2018, <https://www.fairhair-

   [LoRaWAN]  "LoRaWAN Regional Parameters v1.0.2rB", February 2017,

Authors' Addresses

   Malisa Vucinic


   Goeran Selander
   Ericsson AB


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   John Preuss Mattsson
   Ericsson AB


   Dan Garcia-Carrillo
   Odin Solutions S.L.


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