CoRE Working Group                                             M. Tiloca
Internet-Draft                                              RISE SICS AB
Intended status: Standards Track                             G. Selander
Expires: August 16, September 6, 2018                                  F. Palombini
                                                             Ericsson AB
                                                                 J. Park
                                             Universitaet Duisburg-Essen
                                                       February 12,
                                                          March 05, 2018

                  Secure group communication for CoAP
                  draft-ietf-core-oscore-groupcomm-00
                  draft-ietf-core-oscore-groupcomm-01

Abstract

   This document describes a method mode for protecting group communication
   over the Constrained Application Protocol (CoAP).  The proposed
   approach mode
   relies on Object Security for Constrained RESTful Environments
   (OSCORE) and the CBOR Object Signing and Encryption (COSE) format.  All
   In particular, it is defined how OSCORE should be used in a group
   communication setting, while fulfilling the same security
   requirements fulfilled by OSCORE are
   maintained for multicast OSCORE request messages and related OSCORE response messages.
   Source authentication of all messages exchanged within the group is
   ensured, by means of digital signatures produced through private keys
   of sender devices endpoints and embedded in the protected CoAP messages.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 16, September 6, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Assumptions and  OSCORE Security Objectives . . . . . . . . . . . . .   5
     2.1.  Assumptions . . Context . . . . . . . . . . . . . . . . . . .   5
     2.1.  Management of Group Keying Material . .   5
     2.2.  Security Objectives . . . . . . . . .   7
   3.  The COSE Object . . . . . . . . . .   7
   3.  OSCORE Security Context . . . . . . . . . . . . .   8
     3.1.  Example: Request  . . . . . .   7
     3.1.  Management of Group Keying Material . . . . . . . . . . .   9
   4.  The COSE Object . . .  10
     3.2.  Example: Response . . . . . . . . . . . . . . . . . . . .  10
   5.
   4.  Message Processing  . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  11
     4.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  12
     5.2.  11
     4.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  13
     5.3.  12
     4.3.  Protecting the Response . . . . . . . . . . . . . . . . .  13
     5.4.  12
     4.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  13
   6.  12
   5.  Synchronization of Sequence Numbers . . . . . . . . . . . . .  14  13
   6.  Responsibilities of the Group Manager . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14  15
     7.1.  Group-level Security  . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Assumptions and Security Objectives  . . . . . . . .  19
     A.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .  19
     A.2.  Security Objectives . . . . . . . . . . . . . . . . . . .  20
   Appendix B.  List of Use Cases  . . . . . . . . . . . . . . . . .  18  21
   Appendix B. C.  Example of Group Identifier Format . . . . . . . . .  20  23
   Appendix C. D.  Set-up of New Endpoints  . . . . . . . . . . . . . .  21
     C.1.  24
     D.1.  Join Process  . . . . . . . . . . . . . . . . . . . . . .  21
     C.2.  24
     D.2.  Provisioning and Retrieval of Public Keys . . . . . . . .  23
     C.3.  27
     D.3.  Group Joining Based on the ACE Framework  . . . . . . . .  24  29
   Appendix D. E.  Examples of Synchronization Approaches . . . . . . .  25
     D.1.  29
     E.1.  Best-Effort Synchronization . . . . . . . . . . . . . . .  25
     D.2.  30
     E.2.  Baseline Synchronization  . . . . . . . . . . . . . . . .  25
     D.3.  30
     E.3.  Challenge-Response Synchronization  . . . . . . . . . . .  26  30
   Appendix E. F.  No Verification of Signatures  . . . . . . . . . . .  27  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28  32

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a web
   transfer protocol specifically designed for constrained devices and
   networks [RFC7228].

   Group communication for CoAP [RFC7390] addresses use cases where
   deployed devices benefit from a group communication model, for
   example to reduce latencies and improve performance.  Use cases
   include lighting control, integrated building control, software and
   firmware updates, parameter and configuration updates, commissioning
   of constrained networks, and emergency multicast (see Appendix A). B).
   Furthermore, [RFC7390] recognizes the importance to introduce a
   secure mode for CoAP group communication.  This specification defines
   such a mode.

   Object Security for Constrained RESTful Environments
   (OSCORE)[I-D.ietf-core-object-security] describes a security protocol
   based on the exchange of protected CoAP messages.  OSCORE builds on
   CBOR Object Signing and Encryption (COSE) [RFC8152] and provides end-
   to-end encryption, integrity, and replay protection between a sending
   endpoint and a receiving endpoint across possibly involving intermediary nodes.
   endpoints.  To this end, a CoAP message is protected by including its
   payload (if any), certain options, and header fields in a COSE
   object, which finally replaces the authenticated and encrypted fields
   in the protected message.

   This document describes multicast group OSCORE, providing end-to-end security
   of CoAP messages exchanged between members of a multicast group.  In
   particular, the described approach defines how OSCORE should be used
   in a group communication context, while fulfilling setting, so that end-to-end security is
   assured by using the same security requirements. method.  That is, end-to-end
   security is assured for multicast CoAP requests sent by multicaster nodes
   endpoints to the group and for related CoAP responses sent as reply
   by multiple listener
   nodes.  Multicast endpoints.  Group OSCORE provides source
   authentication of all CoAP messages exchanged within the group, by
   means of digital signatures produced through private keys of sender
   devices and embedded in the protected CoAP messages.  As in OSCORE,
   it is still possible to simultaneously rely on DTLS to protect hop-by-hop hop-
   by-hop communication between a multicaster node endpoint and a proxy (and
   vice versa), and between a proxy and a listener node endpoint (and vice
   versa).

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are expected to be familiar with the terms and concepts
   described in CoAP [RFC7252]; [RFC7252] including "endpoint", "sender" and
   "recipient"; group communication for CoAP [RFC7390]; COSE and counter
   signatures [RFC8152].

   Readers are also expected to be familiar with the terms and concepts
   for protection and processing of CoAP messages through OSCORE, such
   as "Security Context", "Master Secret" and "Master Salt", defined in
   [I-D.ietf-core-object-security].

   Terminology for constrained environments, such as "constrained
   device", "constrained-node network", is defined in [RFC7228].

   This document refers also to the following terminology.

   o  Keying material: data that is necessary to establish and maintain
      secure communication among member of a multicast group. endpoints.  This includes, for
      instance, keys and IVs [RFC4949].

   o  Group Manager (GM): entity responsible for creating  Group: a multicast
      group, establishing and provisioning Security Contexts among
      authorized group members, as well as managing the joining set of new endpoints that share group members keying material and the leaving
      parameters (Common Context of current the group's Security Context, see
      Section 2).  That is, the term group members.  A GM can used in this specification
      refers to a "security group", not to be confused with network/
      multicast groups or application groups.

   o  Group Manager (GM): entity responsible for multiple multicast a set of OSCORE groups.  Besides,
      Each endpoint in a GM group securely communicates with the respective
      GM, which is not required to be an actual group member and to take
      part in the group communication.  The GM is also responsible for renewing/
      updating Security Contexts and related keying material in the
      multicast groups full list of
      responsibilities of its competence.  Each endpoint in a multicast
      group securely communicates with the respective GM. Group Manager is provided in Section 6.

   o  Multicaster: member of a multicast group that sends multicast CoAP request
      messages intended for all members of the group.  In a 1-to-N
      multicast group,
      communication model, only a single multicaster transmits data to
      the group; in an M-to-N multicast group communication model (where M and N do not
      necessarily have the same value), M group members are
      multicasters.  According to [RFC7390], any possible proxy entity
      is supposed to know about the multicasters in the group and to not
      perform aggregation of response messages.  Also, every multicaster
      expects and is able to handle multiple response messages
      associated to a given multicast request message that it has
      previously sent to the group.

   o  Listener: member of a multicast group that receives multicast CoAP request
      messages when listening to the multicast IP address associated to
      the multicast group.  A listener may reply back, by sending a response
      message to the multicaster which has sent the multicast request message.

   o  Pure listener: member of a multicast group that is configured as listener
      and never replies back to multicasters after receiving
      multicast request
      messages.

   o  Endpoint  Group ID: group identifier assigned by the Group Manager to an
      endpoint upon joining the group as a new member, unless configured
      exclusively as pure listener.  The group.  Group Manager generates and
      manages Endpoint IDs in order to ensure their uniqueness are
      unique within the set of groups of a same multicast group.  That is, within a single multicast group, Group Manager.

   o  Endpoint ID: Sender ID of the same endpoint, as defined in
      [I-D.ietf-core-object-security].  An Endpoint ID cannot be associated is provided to more endpoints at an
      endpoint upon joining a group, is valid only within that group,
      and is unique within the same time.  Endpoint IDs group.  Endpoints which are not necessarily related to any
      protocol-relevant identifiers, such
      configured only as IP addresses. pure listeners do not have an Endpoint ID.

   o  Group request: multicast CoAP request message sent by a
      multicaster in the group to all listeners in the group through
      multicast IP, unless otherwise specified.

   o  Source authentication: evidence that a received message in the
      group originated from a specifically identified group member.
      This also provides assurances that the message was not tampered
      with either by a different group member or by a non-group member.

2.  Assumptions and  OSCORE Security Objectives

   This section presents a set of assumptions and security objectives
   for the approach described in this document.

2.1.  Assumptions

   The following assumptions are assumed to be already addressed and are
   out of the scope of this document.

   o  Multicast communication topology: this document considers both
      1-to-N (one multicaster and multiple listeners) and M-to-N
      (multiple multicasters and multiple listeners) communication
      topologies.  The 1-to-N communication topology is the simplest Context

   To support group communication scenario that would serve the needs secured with OSCORE, each endpoint
   registered as member of a
      typical low-power and lossy network (LLN).  For instance, in a
      typical lighting control use case, a single switch is the only
      entity responsible for sending commands to a group maintains a Security Context as
   defined in Section 3 of lighting
      devices. [I-D.ietf-core-object-security].  In more advanced lighting control use cases, a M-to-N
      communication topology would be required, for instance
   particular, each endpoint in case
      multiple sensors (presence or day-light) are responsible to
      trigger events to a group of lighting devices.

   o  Multicast group size: security solutions for group communication
      should be able to adequately support different, possibly large,
      group sizes.  Group size is stores:

   1.  one Common Context, shared by all the combination of endpoints in the number of
      multicasters and listeners group.
       All the endpoints in a multicast group, with possible
      overlap (i.e. a multicaster may also be a listener at the group agree on the same
      time). COSE AEAD
       algorithm.  In the use cases mentioned addition to what is defined in this document, the number Section 3 of
      multicasters (normally
       [I-D.ietf-core-object-security], the controlling devices) is expected to be
      much smaller than Common Context includes the number of listeners (i.e.
       following information.

       *  Group Identifier (Gid).  Variable length byte string
          identifying the controlled
      devices). Security Context.  A security solution for group communication that
      supports 1 to 50 multicasters would Gid MUST have a random
          component and be able long enough, in order to properly cover the
      group sizes required for most use cases that are relevant for this
      document.  The total number achieve a negligible
          probability of group members collisions between Group Identifiers from
          different Group Managers.  A Group ID is expected to be in used i) alone or
          together with other parameters, such as the range of 2 to 100 devices.  Groups larger than that should be
      divided into smaller independent multicast groups, e.g. by
      grouping lights in a building on a per floor basis.

   o  Establishment and management IP
          address of Security Contexts: a the group, to retrieve the OSCORE Security Context must be established among
          of the associated group members by the Group
      Manager which manages the multicast group.  A secure mechanism
      must be used to generate, revoke and (re-)distribute keying
      material, multicast security policies (see Section 4); and security parameters in
      the multicast group. ii) as OSCORE
          Master Salt (see Section 3.1 of
          [I-D.ietf-core-object-security]).  The actual establishment and management choice of the Gid for a
          given group's Security Context is out application specific.  It is
          the role of the scope application to specify how to handle possible
          collisions.  An example of specific formatting of the Group
          Identifier that would follow this document, and it specification is anticipated that an activity given in IETF dedicated to
          Appendix C.

       *  Counter Signature Algorithm.  Value identifying the design algorithm
          used for source authenticating messages sent within the group,
          by means of a generic key management scheme will include this feature,
      preferably based on [RFC3740][RFC4046][RFC4535].

   o  Multicast data security ciphersuite: all group members MUST agree
      on a ciphersuite to provide authenticity, integrity and
      confidentiality counter signature (see Section 4.5 of messages
          [RFC8152]).  Its value is immutable once the Common Context is
          established.  All the endpoints in the multicast group. group agree on the same
          counter signature algorithm.  The
      ciphersuite list of supported signature
          algorithms is specified as part of the Security Context.

   o  Backward security: a new device joining the multicast group should
      not have access to any old Security Contexts communication policy and MUST
          include the EdDSA signature algorithm ed25519 [RFC8032].

   2.  one Sender Context, unless the endpoint is configured exclusively
       as pure listener.  The Sender Context is used before its
      joining.  This ensures that a new to secure outgoing
       group member messages and is not able initialized according to
      decrypt confidential data sent before it Section 3 of
       [I-D.ietf-core-object-security], once the endpoint has joined the
       group.
      The adopted key management scheme should ensure that  In practice, the Security
      Context is updated to ensure backward confidentiality.  The actual
      mechanism to update symmetric keying material in the Security Sender
       Context and renew of the sender endpoint is shared with all the recipient
       endpoints that have received group
      keying material upon a group member's joining has messages from that same sender
       endpoint.  Besides, in addition to be what is defined as
      part of in
       [I-D.ietf-core-object-security], the group key management scheme.

   o  Forward security: entities that leave Sender Context stores also
       the multicast endpoint's public-private key pair.

   3.  one Recipient Context for each distinct endpoint from which group should
      not have access
       messages are received, used to any future Security Contexts or process such incoming messages.
       The recipient endpoint creates a new Recipient Context upon
       receiving an incoming message
      exchanged within from another endpoint in the group after their leaving.  This ensures that
      a former group member is not able to decrypt confidential data
      sent within the group anymore.  Also, it ensures that a former
      member is not able to send encrypted and/or integrity protected
      messages to the group anymore.  The actual mechanism to update
       for the
      Security Context first time (see Section 4.2 and renew Section 4.4).  In
       practice, the group symmetric keying material upon in a group
      member's leaving has to be defined as part given Recipient
       Context of the group key
      management scheme.

2.2.  Security Objectives

   The approach described in this document aims at fulfilling the
   following security objectives:

   o  Data replay protection: replayed group request messages or
      response messages MUST be detected.

   o  Group-level data confidentiality: messages sent within the
      multicast group SHALL be encrypted if privacy sensitive data recipient endpoint is
      exchanged within shared with the group.  In fact, some control commands and/or associated responses could pose unforeseen security and privacy
      risks to the system users, when sent as plaintext.  This document
      considers group-level data confidentiality since
       sender endpoint from which group messages are
      encrypted at a group level, i.e. received.  Besides,
       in such a way that they can be
      decrypted by any member of the multicast group, but not by an
      external adversary or other external entities.

   o  Source authentication: messages sent within the multicast group
      SHALL be authenticated.  That is, it is essential addition to ensure that a
      message what is originated by a member of the group defined in
       [I-D.ietf-core-object-security], each Recipient Context stores
       also the first place
      (group authentication), and in particular by a specific member public key of the associated other endpoint from which
       group (source authentication).

   o  Message integrity: messages sent within are received.

   The table in Figure 1 overviews the multicast group SHALL
      be integrity protected.  That is, it is essential to ensure that a
      message has not been tampered with by an external adversary or
      other external entities which are not group members.

   o  Message ordering: it MUST be possible to determine the ordering of
      messages coming from a single sender endpoint.  In accordance with
      OSCORE [I-D.ietf-core-object-security], this results new information included in providing
      relative freshness of group requests and absolute freshness of
      responses.  It is not required to determine ordering of messages
      from different sender endpoints.

3. the
   OSCORE Security Context

   To support multicast communication secured Context, with OSCORE, each endpoint
   registered as member of a multicast group maintains a Security
   Context as respect to what defined in Section 3 of
   [I-D.ietf-core-object-security].
   In particular, each endpoint in a group stores:

   1.  one

         +---------------------------+-----------------------------+
         |      Context portion      |       New information       |
         +---------------------------+-----------------------------+
         |                           |                             |
         |      Common Context, received from the Context       | Group Manager upon joining
       the multicast group and shared by all Identifier (Gid)      |
         |                           |                             |
         |      Common Context       | Counter signature algorithm |
         |                           |                             |
         |      Sender Context       | Endpoint's private key      |
         |                           |                             |
         |      Sender Context       | Endpoint's public key       |
         |                           |                             |
         |  Each Recipient Context   | Public key of the endpoints in           |
         |                           | associated other endpoint   |
         |                           |                             |
         +---------------------------+-----------------------------+

            Figure 1: Additions to the group.
       All OSCORE Security Context

   Upon receiving a secure CoAP message, a recipient endpoint relies on
   the endpoints sender endpoint's public key, in order to verify the group agree on counter
   signature conveyed in the same COSE AEAD
       algorithm.  In addition to what is defined Object.

   If not already stored in Section 3 of
       [I-D.ietf-core-object-security], the Common Recipient Context includes associated to the
       following information.

       *  Group Identifier (Gid).  Variable length byte string
          identifying
   sender endpoint, the Security recipient endpoint retrieves the public key from
   a trusted key repository.  In such a case, the correct binding
   between the sender endpoint and the retrieved public key must be
   assured, for instance by means of public key certificates.  Further
   discussion about how public keys can be handled and retrieved in the
   group is provided in Appendix D.2.

   The Sender Key/IV stored in the Sender Context and used as Master Salt
          parameter the Recipient
   Keys/IVs stored in the derivation Recipient Contexts are derived according to
   the same scheme defined in Section 3.2 of keying material.
   [I-D.ietf-core-object-security].

2.1.  Management of Group Keying Material

   The Gid is
          used together with approach described in this specification should take into account
   the multicast IP address risk of compromise of the group to
          retrieve members.  In particular, the Security Context, upon receiving a adoption
   of key management schemes for secure
          multicast request message (see Section 5.2).  The Gid
          associated revocation and renewal of
   Security Contexts and group keying material should be considered.

   Consistently with the security assumptions in Appendix A.1, it is
   RECOMMENDED to adopt a multicast group is determined by key management scheme, and securely
   distribute a new value for the
          responsible Group Manager.  The choice Master Secret parameter of the Gid for a given group's
   Security Context Context, before a new joining endpoint is application specific.  However, added to the group
   or after a
          Gid MUST be random as well as long enough, currently present endpoint leaves the group.  This is
   necessary in order to achieve preserve backward security and forward security
   in the group.

   In particular, a negligible probability of collisions between new Group
          Identifiers from different Group Managers.  It is the role of Identifier (Gid) for that group and a new
   value for the application to specify how to handle possible collisions. Master Secret parameter must also be distributed.  An
   example of specific formatting of the Group Identifier that
          would follow format supporting this specification operation is given
   provided in Appendix B.

       *  Counter signature algorithm.  Value identifying the algorithm
          used for source authenticating messages sent within C.  Then, each group member re-derives the group,
          by means of a counter signature (see
   keying material stored in its own Sender Context and Recipient
   Contexts as described in Section 4.5 of
          [RFC8152]).  Its value 2, using the updated Group
   Identifier.

   Especially in dynamic, large-scale, groups where endpoints can join
   and leave at any time, it is immutable once important that the Security Context considered group key
   management scheme is established.  All efficient and highly scalable with the endpoints group
   size, in order to limit the group agree impact on performance due to the
          same counter signature algorithm. Security
   Context and keying material update.

3.  The Group Manager MUST
          define COSE Object

   When creating a list of supported signature algorithms as part of protected CoAP message, an endpoint in the group communication policy.  Such a list MUST include
   computes the
          EdDSA signature algorithm ed25519 [RFC8032].

   2.  one Sender Context, unless COSE object using the endpoint is configured exclusively untagged COSE_Encrypt0 structure
   [RFC8152] as pure listener.  The Sender Context is used to secure outgoing
       messages and is initialized according to defined in Section 3 5 of [I-D.ietf-core-object-security], once the endpoint has joined the
       multicast group.  In practice,
   with the sender endpoint shares following modifications.

   o  The value of the
       same symmetric keying material stored "kid" parameter in the Sender Context with
       all "unprotected" field of
      response messagess SHALL be set to the recipient endpoints receiving its outgoing OSCORE
       messages.  The Sender Endpoint ID in of the Sender Context coincides with endpoint
      transmitting the
       Endpoint ID received upon joining message, i.e. the group.  It is
       responsibility Sender ID.

   o  The "unprotected" field of the Group Manager to assign Endpoint IDs to new
       joining endpoints in such a way that uniquess is ensured within "Headers" field SHALL additionally
      include the multicast group.  Besides, in addition to what following parameter:

      *  CounterSignature0 : its value is defined in
       [I-D.ietf-core-object-security], set to the Sender Context stores also counter signature
         of the COSE object, computed by the endpoint's public-private key pair.

   3.  one Recipient Context for each distinct endpoint from which
       messages are received, used to process such incoming secure
       messages.  The endpoint creates a new Recipient Context upon
       receiving an incoming message from another endpoint by means of its
         own private key as described in Section 4.5 of [RFC8152].  The
         presence of this parameter is explicitly signaled, by using the group
       for
         reserved sixth least significant bit of the first time.  In practice, the recipient endpoint shares
       the symmetric keying material stored byte of flag
         bits in the Recipient Context
       with value of the associated other endpoint from which secure messages are
       received.  Besides, in addition Object-Security option (see
         Section 6.1 of [I-D.ietf-core-object-security]).

   o  The Additional Authenticated Data (AAD) considered to what compute the
      COSE object is extended, by adding the countersignature algorithm
      used to protect group messages.  In particular, the "external_aad"
      defined in
       [I-D.ietf-core-object-security], each Recipient Context stores
       also the public key Section 5.4 of [I-D.ietf-core-object-security] SHALL
      also include "alg_countersign", which contains the associated other endpoint Counter
      Signature Algorithm from which
       secure messages are received.

   Upon receiving a secure CoAP message, a recipient endpoint relies on the sender endpoint's public key, Common Context (see Section 2).

   external_aad = [
      oscore_version : uint,
      [alg_aead : int / tstr , alg_countersign : int / tstr],
      request_kid : bstr,
      request_piv : bstr,
      options : bstr
   ]

   o  The OSCORE compression defined in order to verify Section 6 of
      [I-D.ietf-core-object-security] is used, with the counter
   signature conveyed in following
      additions for the COSE Object.

   If not already stored in encoding of the Recipient Context associated Object-Security option.

      *  The fourth least significant bit of the first byte of flag bits
         SHALL be set to 1, to indicate the
   sender endpoint, presence of the recipient endpoint retrieves "kid"
         parameter for both group requests and responses.

      *  The fifth least significant bit of the public key from
   a trusted key repository.  In such a case, first byte of flag bits
         MUST be set to 1 for group requests, to indicate the correct binding
   between presence
         of the sender endpoint and kid context in the retrieved public key MUST OSCORE payload.  The kid context flag
         MAY be
   assured, set to 1 for instance by means responses.

      *  The sixth least significant bit of public key certificates.

   It is RECOMMENDED that the Group Manager acts first byte of flag bits
         is originally marked as trusted key
   repository, reserved in
         [I-D.ietf-core-object-security] and hence its usage is configured to store public keys of group
   members and provide them defined in
         this specification.  This bit is set to other members of 1 if the same group upon
   request.  Possible approaches
         "CounterSignature0" parameter is present, or to provision public keys upon joining
   the group and 0 otherwise.
         In order to retrieve public keys ensure source authentication of group members are discussed messages as
         described in Appendix C.2. this specification, this bit SHALL be set to 1.

      *  The Sender Key/IV stored in 'kid context' value encodes the Sender Context and the Recipient
   Keys/IVs stored in the Recipient Contexts are derived according to
   the same scheme defined in Section 3.2 of
   [I-D.ietf-core-object-security].

3.1.  Management of Group Keying Material Identifier value
         (Gid) of the group's Security Context.

      *  The approach described in this specification should take into account following q bytes (q given by the risk of compromise of group members.  Such a risk is reduced when
   multicast groups are deployed Counter Signature
         Algorithm specified in physically secured locations, like
   lighting inside office buildings.  Nevertheless, the adoption of key
   management schemes for secure revocation and renewal of Security
   Contexts and group keying material should be considered.

   Consistently with Context) encode the security assumptions in Section 2, it is
   RECOMMENDED to adopt a group key management scheme, and securely
   distribute a new value for the Master Secret parameter
         of the group's
   Security Context, before a new joining endpoint is added to the group
   or after a currently present endpoint leaves "CounterSignature0" parameter including the group.  This is
   necessary in order to preserve backward security and forward security
   in counter
         signature of the multicast group. COSE object.

      *  The Group Manager responsible for the group
   is entrusted with such a task.

   In particular, remaining bytes in the Group Manager MUST distribute also a new Group
   Identifier (Gid) for that group, together with a new Object-Security value for encode the
   Master Secret parameter.  An example
         value of how this can be done the "kid" parameter, which is
   provided always present both in Appendix B.  Then, each
         group member re-derives the
   keying material stored in its own Sender Context requests and Recipient
   Contexts as described in Section 3, using the updated Group
   Identifier.

   Especially in dynamic, large-scale, multicast groups where endpoints
   can join responses.

     0 1 2 3 4 5 6 7 <----------- n bytes -----------> <-- 1 byte -->
    +-+-+-+-+-+-+-+-+---------------------------------+--------------+
    |0 0|1|h|1|  n  |       Partial IV (if any)       |  s (if any)  |
    +-+-+-+-+-+-+-+-+---------------------------------+--------------+

    <------ s bytes ------> <--------- q bytes --------->
    -----------------------+-----------------------------+-----------+
       kid context = Gid   |      CounterSignature0      |    kid    |
    -----------------------+-----------------------------+-----------+

                      Figure 2: Object-Security Value

3.1.  Example: Request

   Request with kid = 0x25, Partial IV = 5 and leave at any time, it is important that kid context = 0x44616c,
   assuming the considered
   group key management scheme is efficient and highly scalable with label for the
   group size, new kid context defined in order to limit the impact on performance due to
   [I-D.ietf-core-object-security] has value 10.  COUNTERSIGN is the
   Security Context
   CounterSignature0 byte string as described in Section 3 and keying material update.

4. is 64
   bytes long in this example.  The COSE Object

   When creating a protected CoAP message, an endpoint ciphertext in this example is 14
   bytes long.

   Before compression (96 bytes):

   [
   h'',
   { 4:h'25', 6:h'05', 10:h'44616c', 9:COUNTERSIGN },
   h'aea0155667924dff8a24e4cb35b9'
   ]

   After compression (85 bytes):

   Flag byte: 0b00111001 = 0x39

   Option Value: 39 05 03 44 61 6c COUNTERSIGN 25 (7 bytes + size of
    COUNTERSIGN)

   Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)

3.2.  Example: Response

   Response with kid = 0x52.  COUNTERSIGN is the group
   computes the COSE object using the untagged COSE_Encrypt0 structure
   [RFC8152] CounterSignature0 byte
   string as defined described in Section 5 3 and is 64 bytes long in this
   example.  The ciphertext in this example is 14 bytes long.

   Before compression (88 bytes):

   [
   h'',
   { 4:h'52', 9:COUNTERSIGN },
   h'60b035059d9ef5667c5a0710823b'
   ]

   After compression (80 bytes):

   Flag byte: 0b00101000 = 0x28

   Option Value: 28 COUNTERSIGN 52 (2 bytes + size of COUNTERSIGN)

   Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b (14 bytes)

4.  Message Processing

   Each request message and response message is protected and processed
   as specified in [I-D.ietf-core-object-security], with the
   modifications described in the following modifications.

   o sections.  The value of the "kid" parameter following
   security objectives are fulfilled, as further discussed in
   Appendix A.2: data replay protection, group-level data
   confidentiality, source authentication, message integrity, and
   message ordering.

   Furthermore, endpoints in the "unprotected" field of
      responses SHALL be set to the Sender ID of the endpoint
      transmitting the group message.

   o  The "unprotected" field locally perform error handling
   and processing of the "Headers" field SHALL additionally
      include the following parameters:

      *  gid : its value is set invalid messages according to the Group Identifier (Gid) of the
         group's Security Context.  This parameter MAY be omitted if the
         message is same principles
   adopted in [I-D.ietf-core-object-security].  However, a CoAP response.

      *  countersign : its value receiver
   endpoint MUST stop processing and silently reject any message which
   is set malformed and does not follow the format specified in Section 3,
   without sending back any error message.  This prevents listener
   endpoints from sending multiple error messages to a multicaster
   endpoint, so avoiding the counter signature risk of flooding and possibly congesting
   the
         COSE object (Appendix C.3.3 of [RFC8152]), computed by group.

4.1.  Protecting the Request

   A multicaster endpoint by means of its own private key transmits a secure group request as described
   in Section 4.5 8.1 of [RFC8152].

   In particular, "gid" is included as COSE header parameter as defined
   in Figure 1.

    +------+-------+------------+----------------+-------------------+
    | name | label | value type | value registry | description       |
    +------+-------+------------+----------------+-------------------+
    | gid  | TBD   | bstr       |                | Identifies the    |
    |      |       |            |                | OSCORE group      |
    |      |       |            |                | Security Context  |
    +------+-------+------------+----------------+-------------------+

     Figure 1: Additional common header parameter for [I-D.ietf-core-object-security], with the COSE object

   o following
   modifications.

   1.  The Additional Authenticated Data (AAD) considered to compute the
      COSE object is extended, in order to include also multicaster endpoint stores the association Token - Group
      Identifier (Gid) of
       Identifier.  That is, it SHALL be able to find the correct
       Security Context used to protect the group request message.  In particular, and verify the "external_aad"
       response(s) by using the CoAP Token used in Section 5.3
      of [I-D.ietf-core-object-security] SHALL include also gid as
      follows:

   external_aad = [
      version : uint,
      alg : int,
      request_kid : bstr,
      request_piv : bstr,
      gid : bstr,
      options : bstr
   ]

   o the message exchange.

   2.  The OSCORE compression multicaster computes the COSE object as defined in Section 8 3
       of
      [I-D.ietf-core-object-security] is used, with the following
      additions for this specification.

4.2.  Verifying the encoding Request

   Upon receiving a secure group request, a listener endpoint proceeds
   as described in Section 8.2 of [I-D.ietf-core-object-security], with
   the object-security option.

      * following modifications.

   1.  The fourth least significant bit of the first byte of the
         object-security option value SHALL be set to 1, to indicate listener endpoint retrieves the
         presence of Group Identifier from the "kid"
       'kid context' parameter for both multicast requests and
         responses.

      *  The fifth least significant bit of the first byte MUST be set
         to 1 for multicast requests, to indicate the presence of received COSE object.  Then, it
       uses the
         Context Hint in Group Identifier together with the OSCORE payload.  The Context Hint flag MAY
         be set to 1 for responses.

      *  The sixth least significant bit destination IP
       address of the first byte is set to 1
         if the "countersign" parameter is present, or to 0 otherwise.
         In order to ensure source authentication of group messages as
         described in this specification, this bit SHALL be set request to 1.

      *  The Context Hint value encodes the Group Identifier value (Gid)
         of identify the correct group's
       Security Context.

      *

   2.  The following q bytes (q given by the counter signature
         algorithm specified in the Security Context) encode listener endpoint retrieves the value
         of Sender ID from the "countersign" "kid"
       parameter including the counter signature of the received COSE object.

      *  The remaining bytes in the Object-Security value encode the
         value of  Then, the "kid" parameter, which Sender ID is always present both in
         multicast requests and in responses.

     0 1 2 3 4 5 6 7 <----------- n bytes -----------> <-- 1 byte -->
    +-+-+-+-+-+-+-+-+---------------------------------+--------------+
    |0 0|1|h|1|  n  |        Partial IV (if any)      |  s (if any)  |
    +-+-+-+-+-+-+-+-+---------------------------------+--------------+

    <------ s bytes ------> <--------- q bytes --------->
    -----------------------+-----------------------------+-----------+
          Gid (if any)     |         countersign         |    kid    |
    -----------------------+-----------------------------+-----------+

                      Figure 2: Object-Security Value

5.  Message Processing

   Each multicast request message and response message is protected and
   processed as specified in [I-D.ietf-core-object-security], with the
   modifications described in
       used to retrieve the following sections.

   Furthermore, endpoints in correct Recipient Context associated to the multicast group locally perform error
   handling
       multicaster endpoint and processing of invalid messages according used to process the same
   principles adopted in [I-D.ietf-core-object-security].  However, group request.  When
       receiving a
   receiver endpoint MUST stop processing and silently reject any secure group request message which is malformed and does not follow from that multicaster
       endpoint for the first time, the format specified
   in Section 4, without sending back any error message.  This prevents listener endpoints from sending multiple error messages to endpoint creates a new
       Recipient Context, initializes it according to Section 3 of
       [I-D.ietf-core-object-security], and includes the multicaster endpoint, so avoiding
       endpoint's public key.

   3.  The listener endpoint retrieves the risk corresponding public key of flooding
       the multicast
   group.

5.1. multicaster endpoint from the associated Recipient Context.
       Then, it verifies the counter signature and decrypts the group
       request.

4.3.  Protecting the Request Response

   A multicaster listener endpoint transmits that has received a secure multicast group request message may
   reply with a secure response, which is protected as described in
   Section 7.1 8.3 of [I-D.ietf-core-object-security], with the following
   modifications.

   1.  The multicaster listener endpoint stores the association Token - Group
       Identifier.  That is, it SHALL be able to find the correct
       Security Context used to protect the multicast request and verify
       the response(s) by using the CoAP Token used in the message
       exchange.

   2.  The multicaster computes computes the COSE object as defined in
       Section 4
       of this specification.

5.2.  Verifying the Request

   Upon receiving a secure multicast request message, a listener
   endpoint proceeds as described in Section 7.2 of
   [I-D.ietf-core-object-security], with the following modifications.

   1.  The listener endpoint retrieves the Group Identifier from the
       "gid" parameter of the received COSE object.  Then, it uses the
       Group Identifier together with the destination IP address of the
       multicast request message to identify the correct group's
       Security Context.

   2.  The listener endpoint retrieves the Sender ID from the "kid"
       parameter of the received COSE object.  Then, the Sender ID is
       used to retrieve the correct Recipient Context associated to the
       multicaster endpoint and used to process the request message.
       When receiving a secure multicast CoAP request message from that
       multicaster endpoint for the first time, the listener endpoint
       creates a new Recipient Context, initializes it according to
       Section 3 of [I-D.ietf-core-object-security], and includes the
       multicaster endpoint's public key.

   3.  The listener endpoint retrieves the corresponding public key of
       the multicaster endpoint from the associated Recipient Context.
       Then, it verifies the counter signature and decrypts the request
       message.

5.3.  Protecting the Response

   A listener endpoint that has received a multicast request message may
   reply with a secure response message, which is protected as described
   in Section 7.3 of [I-D.ietf-core-object-security], with the following
   modifications.

   1.  The listener endpoint computes the COSE object as defined in
       Section 4 of this specification.

5.4.

4.4.  Verifying the Response

   Upon receiving a secure response message, a multicaster endpoint
   proceeds as described in Section 7.4 8.4 of
   [I-D.ietf-core-object-security], with the following modifications.

   1.  The multicaster endpoint retrieves the Security Context by using
       the Token of the received response message.

   2.  The multicaster endpoint retrieves the Sender ID from the "kid"
       parameter of the received COSE object.  Then, the Sender ID is
       used to retrieve the correct Recipient Context associated to the
       listener endpoint and used to process the response message.  When
       receiving a secure CoAP response message from that listener endpoint
       for the first time, the multicaster endpoint creates a new
       Recipient Context, initializes it according to Section 3 of
       [I-D.ietf-core-object-security], and includes the listener
       endpoint's public key.

   3.  The multicaster endpoint retrieves the corresponding public key
       of the listener endpoint from the associated Recipient Context.
       Then, it verifies the counter signature and decrypts the response
       message.

   The mapping between response messages from listener endpoints and the
   associated multicast group request message from a multicaster endpoint relies on the 3-tuple (Group ID, Sender
   pair (Sender ID, Partial IV) associated to the secure multicast request message. group request.
   This is used by listener endpoints as part of the Additional
   Authenticated Data when protecting their own response message, as
   described in Section 4.

6. 3.

5.  Synchronization of Sequence Numbers

   Upon joining the multicast group, new listeners are not aware of the sequence
   number values currently used by different multicasters to transmit multicast request messages.
   group requests.  This means that, when such listeners receive a
   secure multicast group request from a given multicaster for the first time,
   they are not able to verify if that request is fresh and has not been
   replayed.  The same applies holds when a listener endpoint loses
   synchronization with sequence numbers of multicasters, for instance
   after a device reboot.

   The exact way to address this issue depends on the specific use case
   and its synchronization requirements.  The Group Manager should
   define also how list of methods to handle
   synchronization of sequence numbers, as numbers is part of the policies enforced in the multicast group.  In particular,
   the Group Manager can suggest to single specific group
   communication policy, and different listener endpoints
   how they can exceptionally behave in order to synchronize with
   sequence numbers of multicasters. use
   different methods.  Appendix D E describes three possible approaches
   that can be considered.

7.  Security Considerations

   The same security considerations from OSCORE (Section 11 of
   [I-D.ietf-core-object-security]) apply to this specification.
   Additional security aspects to be taken into account are discussed
   below.

7.1.  Group-level Security

   The approach described in this document relies on commonly shared
   group keying material to protect communication within a multicast
   group.  This means that messages are encrypted at a group level
   (group-level data confidentiality), i.e. they can be decrypted by any
   member

6.  Responsibilities of the multicast group, but not by an external adversary or
   other external entities.

   In addition, it Group Manager

   The Group Manager is required that all group members are trusted, i.e.
   they do not forward responsible for performing the content following tasks:

   o  Creating and managing OSCORE groups.  This includes the assignment
      of group messages a Group ID to unauthorized
   entities.  However, in many use cases, every newly created group, as well as ensuring
      uniqueness of Group IDs within the devices in set of its OSCORE groups.

   o  Defining policies for authorizing the multicast
   group belong to a common authority and are configured joining of its OSCORE
      groups.  Such policies can be enforced by a
   commissioner.  For instance, third party, which is
      in a professional lighting scenario, trust relation with the
   roles of multicaster Group Manager and listener are configured by enforces join
      policies on behalf of the lighting
   commissioner, Group Manager.

   o  Driving the join process to add new endpoints as group members.

   o  Establishing Security Common Contexts and devices strictly follow those roles.

8.  IANA Considerations

   TBD.  Header parameter 'gid'.

9.  Acknowledgments

   The authors sincerely thank Stefan Beck, Rolf Blom, Carsten Bormann,
   Klaus Hartke, Richard Kelsey, John Mattsson, Jim Schaad providing them to
      authorized group members during the join process, together with a
      corresponding Security Sender Context.

   o  Generating and Ludwig
   Seitz for their feedback managing Endpoint IDs within its OSCORE groups, as
      well as assigning and comments.

10.  References

10.1.  Normative References

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., providing them to new endpoints during the
      join process.  This includes ensuring uniqueness of Endpoints IDs
      within each of its OSCORE groups.

   o  Defining a set of supported signature algorithms as part of the
      communication policy of each of its OSCORE groups, and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-08 (work in
              progress), January 2018.

   [RFC2119]  Bradner, S., "Key words for signalling
      it to new endpoints during the join process.

   o  Defining the methods to handle loss of synchronization with
      sequence numbers as part of the communication policy of each of
      its OSCORE groups, and signaling the one(s) to use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7252]  Shelby, Z., Hartke, K., new
      endpoints during the join process.

   o  Renewing the Security Context of an OSCORE group upon membership
      change, by revoking and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC8032]  Josefsson, S. renewing common security parameters and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase
      keying material (rekeying).

   o  Providing the management keying material that a new endpoint
      requires to participate in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.  Informative References

   [I-D.amsuess-core-repeat-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "Repeat And
              Request-Tag", draft-amsuess-core-repeat-request-tag-00
              (work the rekeying process, consistently with
      the key management scheme used in progress), July 2017.

   [I-D.aragon-ace-ipsec-profile]
              Aragon, S., Tiloca, M., and S. Raza, "IPsec profile the group joined by the new
      endpoint.

   o  Updating the Group ID of
              ACE", draft-aragon-ace-ipsec-profile-01 (work in
              progress), October 2017.

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer its OSCORE groups, upon renewing the
      respective Security (DTLS)
              Profiles for Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-dtls-
              authorize-02 (work in progress), October 2017.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization Context.

   The Group Manager may additionally be responsible for
              Constrained Environments (ACE)", draft-ietf-ace-oauth-
              authz-09 (work the following
   tasks:

   o  Acting as trusted key repository, in progress), November 2017.

   [I-D.ietf-ace-oscore-profile]
              Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE
              profile order to store the public
      keys of the Authentication and Authorization for
              Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-00 (work in progress), December 2017.

   [I-D.somaraju-ace-multicast]
              Somaraju, A., Kumar, S., Tschofenig, H., and W. Werner,
              "Security for Low-Latency Group Communication", draft-
              somaraju-ace-multicast-02 (work in progress), October
              2016.

   [I-D.tiloca-ace-oscoap-joining]
              Tiloca, M. and J. Park, "Joining members of its OSCORE multicast
              groups in ACE", draft-tiloca-ace-oscoap-joining-02 (work
              in progress), October 2017.

   [RFC2093]  Harney, H. groups, and C. Muckenhirn, "Group Key Management
              Protocol (GKMP) Specification", RFC 2093,
              DOI 10.17487/RFC2093, July provide such public
      keys to other members of the same group upon request.  This
      specification recommends that the Group Manager is entrusted to
      perform this task.

   o  Acting as network router device where endpoints register to
      correctly receive group messages sent to the multicast IP address
      of that group.

   o  Autonomously and locally enforcing access policies to authorize
      new endpoints to join its OSCORE groups.

7.  Security Considerations

   The same security considerations from OSCORE (Section 11 of
   [I-D.ietf-core-object-security]) apply to this specification.
   Additional security aspects to be taken into account are discussed
   below.

7.1.  Group-level Security

   The approach described in this document relies on commonly shared
   group keying material to protect communication within a group.  This
   means that messages are encrypted at a group level (group-level data
   confidentiality), i.e. they can be decrypted by any member of the
   group, but not by an external adversary or other external entities.

   In addition, it is required that all group members are trusted, i.e.
   they do not forward the content of group messages to unauthorized
   entities.  However, in many use cases, the devices in the group
   belong to a common authority and are configured by a commissioner
   (see Appendix B).

8.  IANA Considerations

   This document has no actions for IANA.

9.  Acknowledgments

   The authors sincerely thank Stefan Beck, Rolf Blom, Carsten Bormann,
   Esko Dijk, Klaus Hartke, Richard Kelsey, John Mattsson, Jim Schaad,
   Ludwig Seitz and Peter van der Stok for their feedback and comments.

   The work on this document has been partly supported by the EIT-
   Digital High Impact Initiative ACTIVE.

10.  References

10.1.  Normative References

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-08 (work in
              progress), January 2018.

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

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

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.  Informative References

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profiles for Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-dtls-
              authorize-02 (work in progress), October 2017.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-oauth-
              authz-10 (work in progress), February 2018.

   [I-D.ietf-ace-oscore-profile]
              Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE
              profile of the Authentication and Authorization for
              Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-00 (work in progress), December 2017.

   [I-D.ietf-core-echo-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "Echo and
              Request-Tag", draft-ietf-core-echo-request-tag-00 (work in
              progress), October 2017.

   [I-D.palombini-ace-key-groupcomm]
              Palombini, F. and M. Tiloca, "Key Provisioning for Group
              Communication using ACE", draft-palombini-ace-key-
              groupcomm-00 (work in progress), March 2018.

   [I-D.somaraju-ace-multicast]
              Somaraju, A., Kumar, S., Tschofenig, H., and W. Werner,
              "Security for Low-Latency Group Communication", draft-
              somaraju-ace-multicast-02 (work in progress), October
              2016.

   [I-D.tiloca-ace-oscoap-joining]
              Tiloca, M. and J. Park, "Joining of OSCORE multicast
              groups in ACE", draft-tiloca-ace-oscoap-joining-02 (work
              in progress), October 2017.

   [RFC2093]  Harney, H. and C. Muckenhirn, "Group Key Management
              Protocol (GKMP) Specification", RFC 2093,
              DOI 10.17487/RFC2093, July 1997,
              <https://www.rfc-editor.org/info/rfc2093>.

   [RFC2094]  Harney, H. and C. Muckenhirn, "Group Key Management
              Protocol (GKMP) Architecture", RFC 2094,
              DOI 10.17487/RFC2094, July 1997,
              <https://www.rfc-editor.org/info/rfc2094>.

   [RFC2627]  Wallner, D., Harder, E., C. Muckenhirn, "Group Key Management
              Protocol (GKMP) Architecture", RFC 2094,
              DOI 10.17487/RFC2094, July 1997,
              <https://www.rfc-editor.org/info/rfc2094>.

   [RFC2627]  Wallner, D., Harder, E., and R. Agee, "Key Management for
              Multicast: Issues and Architectures", RFC 2627,
              DOI 10.17487/RFC2627, June 1999,
              <https://www.rfc-editor.org/info/rfc2627>.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
              <https://www.rfc-editor.org/info/rfc3376>.

   [RFC3740]  Hardjono, T. and B. Weis, "The Multicast Group Security
              Architecture", RFC 3740, DOI 10.17487/RFC3740, March 2004,
              <https://www.rfc-editor.org/info/rfc3740>.

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.

   [RFC4046]  Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
              "Multicast Security (MSEC) Group Key Management
              Architecture", RFC 4046, DOI 10.17487/RFC4046, April 2005,
              <https://www.rfc-editor.org/info/rfc4046>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP: Group Secure Association Key Management
              Protocol", RFC 4535, DOI 10.17487/RFC4535, June 2006,
              <https://www.rfc-editor.org/info/rfc4535>.

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

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <https://www.rfc-editor.org/info/rfc6282>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [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>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/info/rfc7390>.

Appendix A.  Assumptions and Security Objectives

   This section presents a set of assumptions and security objectives
   for the approach described in this document.

A.1.  Assumptions

   The following assumptions are assumed to be already addressed and are
   out of the scope of this document.

   o  Multicast communication topology: this document considers both
      1-to-N (one multicaster and multiple listeners) and M-to-N
      (multiple multicasters and multiple listeners) communication
      topologies.  The 1-to-N communication topology is the simplest
      group communication scenario that would serve the needs of a
      typical low-power and lossy network (LLN).  Examples of use cases
      that benefit from secure group communication are provided in
      Appendix B.

   o  Group size: security solutions for group communication should be
      able to adequately support different and possibly large groups.
      The group size is the current number of members in a group.  In
      the use cases mentioned in this document, the number of
      multicasters (normally the controlling devices) is expected to be
      much smaller than the number of listeners (i.e. the controlled
      devices).  A security solution for group communication that
      supports 1 to 50 multicasters would be able to properly cover the
      group sizes required for most use cases that are relevant for this
      document.  The maximum group size is expected to be in the range
      of 2 to 100 devices.  Groups larger than that should be divided
      into smaller independent groups, e.g. by grouping lights in a
      building on a per floor basis.

   o  Communication with the Group Manager: an endpoint must use a
      secure dedicated channel when communicating with the Group
      Manager, even when not registered as group member.  In particular,
      communications with the Group Manager occuring during the join
      process to become a group member must also be secured.

   o  Establishment and management of Security Contexts: an OSCORE
      Security Context must be established among the group members.  In
      particular, a Common Context must be provided to a new joining
      endpoint together with a corresponding Sender Context.  On the
      other hand, Recipient Contexts are locally and R. Agee, "Key Management for
              Multicast: Issues individually
      derived by each group member.  A secure mechanism must be used to
      generate, revoke and Architectures", RFC 2627,
              DOI 10.17487/RFC2627, June 1999,
              <https://www.rfc-editor.org/info/rfc2627>.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., (re-)distribute keying material, multicast
      security policies and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
              <https://www.rfc-editor.org/info/rfc3376>.

   [RFC3740]  Hardjono, T. security parameters in the group.  The
      actual establishment and B. Weis, "The Multicast Group management of the Security
              Architecture", RFC 3740, DOI 10.17487/RFC3740, March 2004,
              <https://www.rfc-editor.org/info/rfc3740>.

   [RFC3810]  Vida, R., Ed. Context is out
      of the scope of this document, and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.

   [RFC4046]  Baugher, M., Canetti, R., Dondeti, L., it is anticipated that an
      activity in IETF dedicated to the design of a generic key
      management scheme will include this feature, preferably based on
      [RFC3740][RFC4046][RFC4535].

   o  Multicast data security ciphersuite: all group members must agree
      on a ciphersuite to provide authenticity, integrity and F. Lindholm,
              "Multicast
      confidentiality of messages in the group.  The ciphersuite is
      specified as part of the Security (MSEC) Group Key Management
              Architecture", RFC 4046, DOI 10.17487/RFC4046, April 2005,
              <https://www.rfc-editor.org/info/rfc4046>.

   [RFC4301]  Kent, S. Context.

   o  Backward security: a new device joining the group should not have
      access to any old Security Contexts used before its joining.  This
      ensures that a new group member is not able to decrypt
      confidential data sent before it has joined the group.  The
      adopted key management scheme should ensure that the Security
      Context is updated to ensure backward confidentiality.  The actual
      mechanism to update the Security Context and K. Seo, "Security Architecture for renew the group
      keying material upon a group member's joining has to be defined as
      part of the group key management scheme.

   o  Forward security: entities that leave the group should not have
      access to any future Security Contexts or message exchanged within
      the group after their leaving.  This ensures that a former group
      member is not able to decrypt confidential data sent within the
      group anymore.  Also, it ensures that a former member is not able
      to send encrypted and/or integrity protected messages to the group
      anymore.  The actual mechanism to update the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP: Group Secure Association Key Management
              Protocol", RFC 4535, DOI 10.17487/RFC4535, June 2006,
              <https://www.rfc-editor.org/info/rfc4535>.

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

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <https://www.rfc-editor.org/info/rfc6282>.

   [RFC6347]  Rescorla, E. Context and N. Modadugu, "Datagram Transport Layer
      renew the group keying material upon a group member's leaving has
      to be defined as part of the group key management scheme.

A.2.  Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [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>.

   [RFC7390]  Rahman, A., Ed. Objectives

   The approach described in this document aims at fulfilling the
   following security objectives:

   o  Data replay protection: replayed group request messages or
      response messages must be detected.

   o  Group-level data confidentiality: messages sent within the group
      shall be encrypted if privacy sensitive data is exchanged within
      the group.  This document considers group-level data
      confidentiality since messages are encrypted at a group level,
      i.e. in such a way that they can be decrypted by any member of the
      group, but not by an external adversary or other external
      entities.

   o  Source authentication: messages sent within the group shall be
      authenticated.  That is, it is essential to ensure that a message
      is originated by a member of the group in the first place, and E. Dijk, Ed., "Group Communication for in
      particular by a specific member of the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/info/rfc7390>. group.

   o  Message integrity: messages sent within the group shall be
      integrity protected.  That is, it is essential to ensure that a
      message has not been tampered with by an external adversary or
      other external entities which are not group members.

   o  Message ordering: it must be possible to determine the ordering of
      messages coming from a single sender endpoint.  In accordance with
      OSCORE [I-D.ietf-core-object-security], this results in providing
      relative freshness of group requests and absolute freshness of
      responses.  It is not required to determine ordering of messages
      from different sender endpoints.

Appendix A. B.  List of Use Cases

   Group Communication for CoAP [RFC7390] provides the necessary
   background for multicast-based CoAP communication, with particular
   reference to low-power and lossy networks (LLNs) and resource
   constrained environments.  The interested reader is encouraged to
   first read [RFC7390] to understand the non-security related details.
   This section discusses a number of use cases that benefit from secure
   group communication.  Specific security requirements for these use
   cases are discussed in Section 2. Appendix A.

   o  Lighting control: consider a building equipped with IP-connected
      lighting devices, switches, and border routers.  The devices are
      organized into groups according to their physical location in the
      building.  For instance, lighting devices and switches in a room
      or corridor can be configured as members of a single multicast group.
      Switches are then used to control the lighting devices by sending
      on/off/dimming commands to all lighting devices in a group, while
      border routers connected to an IP network backbone (which is also
      multicast-enabled) can be used to interconnect routers in the
      building.  Consequently, this would also enable logical multicast groups to
      be formed even if devices in the lighting group may be physically
      in different subnets (e.g. on wired and wireless networks).
      Connectivity between ligthing lighting devices may be realized, for
      instance, by means of IPv6 and (border) routers supporting 6LoWPAN
      [RFC4944][RFC6282].  Group communication enables synchronous
      operation of a group of connected lights, ensuring that the light
      preset (e.g. dimming level or color) of a large group of
      luminaires are changed at the same perceived time.  This is
      especially useful for providing a visual synchronicity of light
      effects to the user.  As a practical guideline, events within a
      200 ms interval are perceived as simultaneous by humans, which is
      necessary to ensure in many setups.  Devices may reply back to the
      switches that issue on/off/dimming commands, in
      order order to report
      about the execution of the requested operation (e.g.  OK, failure,
      error) and their current operational status.  In a typical
      lighting control scenario, a single switch is the only entity
      responsible for sending commands to a group of lighting devices.
      In more advanced lighting control use cases, a M-to-N
      communication topology would be required, for instance in case
      multiple sensors (presence or day-light) are responsible to report about
      trigger events to a group of lighting devices.  Especially in
      professional lighting scenarios, the execution roles of multicaster and
      listener are configured by the requested operation
      (e.g.  OK, failure, error) lighting commissioner, and their current operational status. devices
      strictly follow those roles.

   o  Integrated building control: enabling Building Automation and
      Control Systems (BACSs) to control multiple heating, ventilation
      and air-conditioning units to pre-defined presets.  Controlled
      units can be organized into multicast groups in order to reflect their
      physical position in the building, e.g. devices in the same room
      can be configured as members of a single multicast group.
      Furthermore, controlled  As a practical
      guideline, events within intervals of seconds are typically
      acceptable.  Controlled units are expected to possibly reply back
      to the BACS issuing control commands, in order to report about the
      execution of the requested operation (e.g.  OK, failure, error)
      and their current operational status.

   o  Software and firmware updates: software and firmware updates often
      comprise quite a large amount of data.  This can overload a LLN
      that is otherwise typically used to deal with only small amounts
      of data, on an infrequent base.  Rather than sending software and
      firmware updates as unicast messages to each individual device,
      multicasting such updated data to a larger group of devices at
      once displays a number of benefits.  For instance, it can
      significantly reduce the network load and decrease the overall
      time latency for propagating this data to all devices.  Even if
      the complete whole update process itself is secured, securing the
      individual messages is important, in case updates consist of
      relatively large amounts of data.  In fact, checking individual
      received data piecemeal for tampering avoids that devices store
      large amounts of partially corrupted data and that they detect
      tampering hereof only after all data has been received.  Devices
      receiving software and firmware updates are expected to possibly
      reply back, in order to provide a feedback about the execution of
      the update operation (e.g.  OK, failure, error) and their current
      operational status.

   o  Parameter and configuration update: by means of multicast
      communication, it is possible to update the settings of a group of
      similar devices, both simultaneously and efficiently.  Possible
      parameters are related, for instance, to network load management
      or network access controls.  Devices receiving parameter and
      configuration updates are expected to possibly reply back, to
      provide a feedback about the execution of the update operation
      (e.g.  OK, failure, error) and their current operational status.

   o  Commissioning of LLNs systems: a commissioning device is
      responsible for querying all devices in the local network or a
      selected subset of them, in order to discover their presence, and
      be aware of their capabilities, default configuration, and
      operating conditions.  Queried devices displaying similarities in
      their capabilities and features, or sharing a common physical
      location can be configured as members of a single multicast group.  Queried
      devices are expected to reply back to the commissioning device, in
      order to notify their presence, and provide the requested
      information and their current operational status.

   o  Emergency multicast: a particular emergency related information
      (e.g. natural disaster) is generated and multicast by an emergency
      notifier, and relayed to multiple devices.  The latters may reply
      back to the emergency notifier, in order to provide their feedback
      and local information related to the ongoing emergency.  This kind
      of setups should additionally rely on a fault tolerance multicast
      algorithm, such as MPL.

Appendix B. C.  Example of Group Identifier Format

   This section provides an example of how the Group Identifier (Gid)
   can be specifically formatted.  That is, the Gid can be composed of
   two parts, namely a Group Prefix and a Group Epoch.

   The Group Prefix is uniquely defined in the set of all the multicast groups
   associated to the same Group Manager.  The choice of the Group Prefix
   for a given group's Security Context is application specific.  A
   Group Prefixes are random as well as Prefix is random, constant over time, and long enough, in order enough to
   achieve a negligible probability of collisions between Group
   Identifiers from different Group Managers.  The size of the Group
   Prefix directly impact on the maximum number of distinct groups under
   the same Group Manager.

   The Group Epoch is set to 0 upon the group's initialization, and is
   incremented by 1 upon completing each renewal of the Security Context
   and keying material in the group (see Section 3.1). 2.1).  In particular,
   once a new Master Secret has been distributed to the group, all the
   group members increment by 1 the Group Epoch in the Group Identifier
   of that group (see Section 3). group.

   As an example, a 3-byte Group Identifier can be composed of: i) a
   1-byte Group Prefix '0xb1' interpreted as a raw byte string; and ii)
   a 2-byte Group Epoch interpreted as an unsigned integer ranging from
   0 to 65535.  Then, after having established the Security Common
   Context 61532 times in the group, its Group Identifier will assume
   value '0xb1f05c'.

Appendix C. D.  Set-up of New Endpoints

   An endpoint joins a multicast group by explicitly interacting with the
   responsible Group Manager.  All communications  Communications between a joining endpoint
   and the Group Manager rely on the CoAP protocol and MUST must be secured.
   Specific details on how to secure communications between joining
   endpoints and a Group Manager are out of the scope of this
   specification. scope.

   In order to receive multicast messages sent to the group, a joining
   endpoint has to register with a network router device
   [RFC3376][RFC3810], signaling its intent to receive packets sent to
   the multicast IP address of that group.  As a particular case, the
   Group Manager can also act as such a network router device.  Upon
   joining the group, endpoints are not required to know how many and
   what endpoints are active in the same group.

   Furthermore, in order to participate in the secure group
   communication, an endpoint needs to maintain be properly initialized upon
   joining the group.  In particular, the Group Manager provides keying
   material and parameters to a number of information
   elements stored in joining endpoint, which can then
   initialize its own Security Context (see Section 3). 2).

   The following Appendix C.1 describes which of this D.1 provides an example describing how such
   information is can be provided to an endpoint upon joining a multicast group
   through the responsible Group Manager.

C.1.  Then, Appendix D.2 discusses
   how public keys of group members can be handled and made available to
   group members.  Finally, Appendix D.3 overviews how the ACE framework
   for Authentication and Authorization in constrained environments
   [I-D.ietf-ace-oauth-authz] can be possibly used to support such a
   join process.

D.1.  Join Process

   An endpoint requests to join a multicast group by sending a confirmable CoAP
   POST request to the Group Manager responsible for that group.  The  This
   join request is can reflect the format of the Key Distribution Request
   message defined in Section 4.1 of [I-D.palombini-ace-key-groupcomm].
   Besides, it can be addressed to a CoAP resource associated to that
   group and carries the following information.

   o  Group identifier: the Group Identifier (Gid) of the group, as
      known to the joining endpoint at this point in time.  This may not
      fully coincide with the Gid currently associated to the group,
      e.g. if it includes a dynamic component.  This information can be
      mapped to the first element of the "scope" parameter of the Key
      Distribution Request message defined in Section 4.1 of
      [I-D.palombini-ace-key-groupcomm].

   o  Role: the exact role of the joining endpoint in the multicast group.
      Possible values are: "multicaster", "listener", "pure listener",
      "multicaster and listener", or "multicaster and pure listener".
      This information can be mapped to the second element of the
      "scope" parameter of the Key Distribution Request message defined
      in Section 4.1 of [I-D.palombini-ace-key-groupcomm].

   o  Retrieval flag: indication of interest to receive the public keys
      of the endpoints currently in the group, as included in the
      following join response.  This flag must not be present if the
      Group Manager is not configured to store the public keys of group
      members, or if the joining endpoint is configured exclusively as
      pure listener for the group to join.  This information can be
      mapped to the "get_pub_keys" parameter of the Key Distribution
      Request message defined in Section 4.1 of
      [I-D.palombini-ace-key-groupcomm].

   o  Identity credentials: information elements to enforce source
      authentication of group messages from the joining endpoint, such
      as its public key.  The exact content depends on whether the Group
      Manager is configured to store the public keys of group members.
      If this is the case, this information is omitted if it has been
      provided to the same Group Manager upon previously joining the
      same or a different multicast group under its control.  This
      information is also omitted if the joining endpoint is configured
      exclusively as pure listener for the joined group.  Appendix C.2
      discusses additional details on provisioning of public keys and
      other information to enforce source authentication of joining
      node's messages.

   o  Retrieval flag: indication of interest to receive the public keys
      of the endpoints currently in the multicast group, as included in
      the following join response.  This flag MUST be set to false if
      the Group Manager is not configured to store the public keys of
      group members, or its control.  This information is
      also omitted if the joining endpoint is configured exclusively as
      pure listener for the joined group.  Appendix D.2 discusses
      additional details on provisioning of public keys and other
      information to enforce source authentication of joining
      endpoints's messages.  This information can be mapped to the
      "client_cred" parameter of the Key Distribution Request message
      defined in Section 4.1 of [I-D.palombini-ace-key-groupcomm].

   The Group Manager MUST must be able to verify that the joining enpoint endpoint is
   authorized to become a member of the multicast group.  To this end, the Group
   Manager can directly authorize the joining endpoint, or expect it to
   provide authorization evidence previously obtained from a trusted
   entity.  Appendix C.3 D.3 describes how this can be achieved by
   leveraging the ACE framework for Authentication and Authorization in
   constrained environments [I-D.ietf-ace-oauth-authz].

   In case of successful authorization check, the Group Manager
   generates an Endpoint ID assigned to the joining node, endpoint, before
   proceeding with the rest of the join process.  Instead, in case the
   authorization check fails, the Group Manager MUST abort aborts the join process.
   Further details about the authorization of joining endpoint are out
   of the scope of this specification. scope.

   As discussed in Section 3.1, 2.1, it is then RECOMMENDED recommended that the Security
   Context is renewed before the joining endpoint receives the group
   keying material and becomes a new active member of the multicast group.  This
   is achieved by securely distributing a new Master Secret and a new
   Group Identifier Identifier to the endpoints currently present in the same
   group.

   Once renewed the Security Context in the group, the Group Manager
   replies to the joining endpoint with a CoAP response carrying the
   following information.  This join response can reflect the format of
   the Key Distribution Response message defined in Section 4.2 of
   [I-D.palombini-ace-key-groupcomm].

   o  Security Common Context: the OSCORE Security Common Context
      associated to the joined group (see Section 2).  This information
      can be mapped to the "key" parameter of the Key Distribution
      Response message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].

   o  Endpoint ID: the Endpoint ID associated to the joining endpoint.
      This information is not included in case "Role" in the join
      request is equal to "pure listener".  This information can be
      mapped to the "clientID" parameter within the "key" parameter of
      the Key Distribution Response message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].

   o  Member public keys: the public keys of the endpoints currently
      present in the group.  This includes: the public keys of the non-
      pure listeners currently in the group, if the joining endpoint is
      configured (also) as multicaster; and the public keys of the
      multicasters currently in the group, if the joining endpoint is
      configured (also) as listener or pure listener.  This information
      is omitted in case the Group Manager is not configured to store
      the
   endpoints currently public keys of group members or if the "Retrieval flag" was
      not present in the same group.

   Once renewed join request.  Appendix D.2 discusses
      additional details on provisioning public keys upon joining the Security Context in
      group and on retrieving public keys of group members.  This
      information can be mapped to the multicast group, "pub_keys" parameter of the Key
      Distribution Response message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].

   o  Group
   Manager replies to the joining endpoint with policies: a CoAP response carrying list of key words indicating the following information.

   o  Security Common Context: particular
      policies enforced in the OSCORE Security Common Context
      associated to group.  This includes, for instance, the joined multicast group (see Section 3).

   o  Endpoint ID:
      list of supported signature algorithms and the Endpoint ID associated method to the joining node. achieve
      synchronization of sequence numbers among group members (see
      Appendix E).  This information is not included in case "Role" in the join request is
      equal can be mapped to "pure listener". the
      "group_policies" parameter of the Key Distribution Response
      message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].

   o  Management keying material: the set of administrative keying
      material used to participate in the group rekeying process run by
      the Group Manager (see Section 3.1). 2.1).  The specific elements of
      this management keying material depend on the group rekeying
      protocol used in the group.  For instance, this can simply consist
      in a group key encryption key and a pairwise symmetric key shared
      between the joining node endpoint and the Group Manager, in case GKMP
      [RFC2093][RFC2094] is used.  Instead, if key-tree based rekeying
      protocols like LKH [RFC2627] are used, it can consist in the set
      of symmetric keys associated to the key-tree leaf representing the
      group member up to the key-tree root representing the group key
      encryption key.

   o  Member public keys: the public keys of the endpoints currently
      present in the multicast group.  This includes: the public keys of
      the non-pure listeners currently in the group, if the joining
      endpoint is configured (also) as multicaster; and the public keys
      of the multicasters currently in the group, if the joining
      endpoint is configured (also) as listener or pure listener.  This
      information is omitted in case the Group Manager is not configured symmetric keys associated to store the public keys of group members or if key-tree leaf representing the "Retrieval
      flag" was set
      group member up to false in the join request.  Appendix C.2
      discusses additional details on provisioning public keys upon
      joining key-tree root representing the group and on retrieving public keys key
      encryption key.  This information can be mapped to the
      "mgt_key_material" parameter of group members.

C.2. the Key Distribution Response
      message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].

D.2.  Provisioning and Retrieval of Public Keys

   As mentioned in Section 3, 6, it is RECOMMENDED recommended that the Group Manager
   acts as trusted key repository, stores so storing public keys of group
   members and provide providing them to other members of the same group upon
   request.  In such a case, a joining endpoint provides its own public
   key to the Group Manager, as "Identity credentials" of the join
   request, when joining the multicast group (see Appendix C.1). D.1).

   After that, the Group Manager MUST should verify that the joining endpoint
   actually owns the associated private key, for instance by performing
   a proof-of-possession challenge-response. challenge-response, whose details are out of
   scope.  In case of failure, the Group Manager performs up to a pre-
   defined maximum number of retries, after which it aborts the join
   process.

   In case of success, successful challenge-response, the Group Manager stores
   the received public key as associated to the joining endpoint and its
   Endpoint ID, before sending the join
   response and continuing with the rest of the join process. ID.  From then on, that public key will be available for
   secure and trusted delivery to other endpoints in the multicast group.
   Finally, the Group Manager sends the join response to the joining
   endpoint, as described in Appendix D.1.

   The joining node endpoint does not have to provide its own public key if
   that already occurred upon previously joining the same or a different
   multicast
   group under the same Group Manager.  However, separately for each multicast
   group under its control, the Group Manager maintains an updated list
   of active Endpoint IDs associated to a same the respective endpoint's public
   key.

   Instead, in case the Group Manager does not act as trusted key
   repository, the following information is exchanged exchange with the Group Manager can occur
   during the join process.

   1.  The joining endpoint signs its own certificate by using its own
       private key.  The certificate includes also the identifier of the
       issuer Certification Authority (CA).  There is no restriction on
       the Certificate Subject included in the joining node's endpoint's
       certificate.

   2.  The joining endpoint includes specifies the following information signed certificate as
       "Identity credentials" in the join request (Appendix C.1): the
       signed certificate; and the identifier of the Certification
       Authority that issued the certificate. D.1).  The
       joining endpoint can optionally specify also a list of public key
       repositories storing its own certificate.  In such a case, this
       information can be mapped to the "pub_keys_repos" parameter of
       the Key Distribution Request message defined in Section 4.1 of
       [I-D.palombini-ace-key-groupcomm].

   3.  When processing the join request, the Group Manager first
       validates the certificate by verifying the signature of the
       issuer CA, and then verifies the signature of the joining node.
       endpoint.

   4.  The Group Manager stores the association between the Certificate
       Subject of the joining node's endpoint's certificate and the pair {Group
       ID, Endpoint ID of the joining node}. endpoint}. If received from the
       joining endpoint, the Group Manager also stores the list of
       public key repositories storing the certificate of the joining
       endpoint.

   When a group member X wants to retrieve the public key of another
   group member Y in the same multicast group, the endpoint X proceeds as follows.

   1.  The endpoint X contacts the Group Manager, specifying the pair
       {Group ID, Endpoint ID of the endpoint Y}.

   2.  The Group Manager provides the endpoint X with the Certificate
       Subject CS from the certificate of endpoint Y.  If available, the
       Group Manager provides the endpoint X also with the list of
       public key repositories storing the certificate of the endpoint
       Y.

   3.  The endpoint X retrieves the certificate of the endpoint X from a
       key repository storing it, by using the Certificate Subject CS.

C.3.

D.3.  Group Joining Based on the ACE Framework

   The join process to register an endpoint as a new member of a
   multicast group
   can be based on the ACE framework for Authentication and
   Authorization in constrained environments [I-D.ietf-ace-oauth-authz],
   built on re-use of OAuth 2.0 [RFC6749].

   In particular, the approach described in
   [I-D.tiloca-ace-oscoap-joining] uses the ACE framework to delegate
   the authentication and authorization of joining endpoints to an
   Authorization Server in a trust relation with the Group Manager.  At
   the same time, it allows a joining endpoint to establish a secure
   channel with the Group Manager, by leveraging protocol-specific
   profiles of ACE ACE, such as [I-D.ietf-ace-oscore-profile]
   [I-D.ietf-ace-dtls-authorize] [I-D.aragon-ace-ipsec-profile] and
   [I-D.ietf-ace-dtls-authorize], to achieve communication security,
   proof-of-possession and server authentication.

   More specifically and with reference to the terminology defined in
   OAuth 2.0:

   o  The joining endpoint acts as Client;

   o  The Group Manager acts as Resource Server, with different CoAP
      resources for different multicast groups it is responsible for;

   o  An Authorization Server enables and enforces authorized access of
      the joining endpoint to the Group Manager and its CoAP resources
      paired with multicast groups to join.

   Messages exchanged among the participants follow the formats defined
   in [I-D.palombini-ace-key-groupcomm].  Both the joining endpoint and
   the Group Manager MUST have to adopt secure communication also for any
   message exchange with the Authorization Server.  To this end,
   different alternatives are possible, such as OSCORE, DTLS [RFC6347]
   or IPsec [RFC4301].

Appendix D. E.  Examples of Synchronization Approaches

   This section describes three possible approaches that can be
   considered by listener endpoints to synchronize with sequence numbers
   of multicasters.

D.1.

E.1.  Best-Effort Synchronization

   Upon receiving a multicast request from a multicaster, a listener
   endpoint does not take any action to synchonize with the sequence
   number of that multicaster.  This provides no assurance at all as to
   message freshness, which can be acceptable in non-critical use cases.

D.2.

E.2.  Baseline Synchronization

   Upon receiving a multicast request from a given multicaster for the
   first time, a listener endpoint initializes its last-seen sequence
   number in its Recipient Context associated to that multicaster.
   However, the listener drops the multicast request without delivering
   it to the application layer.  This provides a reference point to
   identify if future multicast group requests from the same multicaster are
   fresher than the last one received.

   A replay time interval exists, between when a possibly replayed
   message is originally transmitted by a given multicaster and the
   first authentic fresh message from that same multicaster is received.
   This can be acceptable for use cases where listener endpoints admit
   such a trade-off between performance and assurance of message
   freshness.

D.3.

E.3.  Challenge-Response Synchronization

   A listener endpoint performs a challenge-response exchange with a
   multicaster, by using the Repeat Option for CoAP described in
   Section 2 of [I-D.amsuess-core-repeat-request-tag]. [I-D.ietf-core-echo-request-tag].

   That is, upon receiving a multicast group request from a particular multicaster
   for the first time, the listener processes the message as described
   in Section 5.2 4.2 of this specification, but, even if valid, does not
   deliver it to the application.  Instead, the listener replies to the
   multicaster with a 4.03 Forbidden response message including a Repeat
   Option, and stores the option value included therein.

   Upon receiving a 4.03 Forbidden response that includes a Repeat
   Option and originates from a verified group member, a multicaster
   MUST send
   sends a group request as a unicast message addressed to the same listener,
   echoing the Repeat Option value.  In particular, the multicaster does
   not necessarily resend the same group request, but can instead send a
   more recent one, if the application permits it.  This makes it
   possible for the multicaster to not retain previously sent group
   requests for full retransmission, unless the application explicitly
   requires otherwise.  In either case, the multicaster uses the
   sequence number value currently stored in its own Sender Context.  If
   the multicaster stores group requests for possible retransmission
   with the Repeat Option, it should not store a given request for
   longer than a pre-configured time interval.  Note that the unicast
   request echoing the Repeat Option is correctly treated and processed
   as a group message, since the "gid" 'kid context' field including the Group
   Identifier of the OSCORE group is still present in the Object-
   Security Option as part of the COSE object (see Section 4). 3).

   Upon receiving the unicast group request including the Repeat Option, the
   listener verifies that the option value equals the stored and
   previously sent value; otherwise, the request is silently discarded.
   Then, the listener verifies that the unicast group request has been
   received within a pre-configured time interval, as described in
   [I-D.amsuess-core-repeat-request-tag].
   [I-D.ietf-core-echo-request-tag].  In such a case, the request is
   further processed and verified; otherwise, it is silently discarded.
   Finally, the listener updates the Recipient Context associated to
   that multicaster, by setting the Replay Window according to the
   Sequence Number from the unicast group request conveying the Repeat Option.
   The listener either delivers the request to the application if it is
   an actual retransmission of the original one, or discard discards it
   otherwise.  Mechanisms to signal whether the resent request is a full
   retransmission of the original one are out of the scope of this
   specification.

   In case it does not receive a valid group unicast request including the
   Repeat Option within the configured time interval, the listener node
   SHOULD
   endpoint should perform the same challenge-response upon receiving
   the next multicast request from that same multicaster.

   A listener SHOULD NOT should not deliver group request messages requests from a given multicaster
   to the application until one valid group request from that same multicaster
   has been verified as fresh, as conveying an echoed Repeat Option [I-D.amsuess-core-repeat-request-tag].
   [I-D.ietf-core-echo-request-tag].  Also, a listener MAY may perform the
   challenge-response described above at any time, if synchronization
   with sequence numbers of multicasters is (believed to be) lost, for
   instance after a device reboot.  It is the role of the application to
   define under what circumstances sequence numbers lose
   synchronization.  This can include a minimum gap between the sequence
   number of the latest accepted group request from a multicaster and
   the sequence number of a group request just received from the same
   multicaster.  A multicaster MUST always has to be always ready to perform the
   challenge-response based on the Repeat Option in case a listener
   starts it.

   Note that endpoints configured as pure listeners are not able to
   perform the challenge-response described above, as they do not store
   a Sender Context to secure the 4.03 Forbidden response to the
   multicaster.  Therefore, pure listeners should adopt alternative
   approaches to achieve and maintain synchronization with sequence
   numbers of multicasters.

   This approach provides an assurance of absolute message freshness.
   However, it can result in an impact on performance which is
   undesirable or unbearable, especially in large multicast groups where many nodes
   endpoints at the same time might join as new members or lose
   synchronization.

Appendix E. F.  No Verification of Signatures

   There are some application scenarios using group communications communication that
   have particularly strict requirements.  One example of this is the
   requirement of low message latency in non-emergency lighting
   applications [I-D.somaraju-ace-multicast].  For those applications
   which have tight performance constraints and relaxed security
   requirements, it can be inconvenient for some endpoints to verify
   digital signatures in order to assert source authenticity of received
   group messages.  In other cases, the signature verification can be
   deferred or only checked for specific actions.  For instance, a
   command to turn a bulb on where the bulb is already on does not need
   the signature to be checked.  In such situations, the counter
   signature needs to be included anyway as part of the group message,
   so that an endpoint that needs to validate the signature for any
   reason has the ability to do so.

   In this specification, it is NOT RECOMMENDED that endpoints do not
   verify the counter signature of received group messages.  However, it
   is recognized that there may be situations where it is not always
   required.  The consequence of not doing the signature validation is
   that security in the group is based only on the group-authenticity of
   the shared keying material used for encryption.  That is, endpoints
   in the multicast group have evidence that a received message has been
   originated by a group member, although not specifically identifiable
   in a secure way.  This can violate a number of security requirements,
   as the compromise of any element in the group means that the attacker
   has the ability to control the entire group.  Even worse, the group
   may not be limited in scope, and hence the same keying material might
   be used not only for light bulbs but for locks as well.  Therefore,
   extreme care must be taken in situations where the security
   requirements are relaxed, so that deployment of the system will
   always be done safely.

Authors' Addresses
   Marco Tiloca
   RISE SICS AB
   Isafjordsgatan 22
   Kista  SE-16440 Stockholm
   Sweden

   Email: marco.tiloca@ri.se

   Goeran Selander
   Ericsson AB
   Torshamnsgatan 23
   Kista  SE-16440 Stockholm
   Sweden

   Email: goran.selander@ericsson.com

   Francesca Palombini
   Ericsson AB
   Torshamnsgatan 23
   Kista  SE-16440 Stockholm
   Sweden

   Email: francesca.palombini@ericsson.com

   Jiye Park
   Universitaet Duisburg-Essen
   Schuetzenbahn 70
   Essen  45127
   Germany

   Email: ji-ye.park@uni-due.de