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Versions: (draft-tiloca-core-multicast-oscoap) 00 01 02 03 04

CoRE Working Group                                             M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                             G. Selander
Expires: September 9, 2019                                  F. Palombini
                                                             Ericsson AB
                                                                 J. Park
                                             Universitaet Duisburg-Essen
                                                          March 08, 2019


           Group OSCORE - Secure Group Communication for CoAP
                draft-ietf-core-oscore-groupcomm-04

Abstract

   This document describes a mode for protecting group communication
   over the Constrained Application Protocol (CoAP).  The proposed mode
   relies on Object Security for Constrained RESTful Environments
   (OSCORE) and the CBOR Object Signing and Encryption (COSE) format.
   In particular, it defines how OSCORE is used in a group communication
   setting, while fulfilling the same security requirements for group
   requests and responses.  Source authentication of all messages
   exchanged within the group is provided by means of digital signatures
   produced by the sender and embedded in the protected CoAP messages.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 9, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  OSCORE Security Context . . . . . . . . . . . . . . . . . . .   5
     2.1.  Management of Group Keying Material . . . . . . . . . . .   8
     2.2.  Wrap-Around of Partial IVs  . . . . . . . . . . . . . . .   9
   3.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Updated external_aad  . . . . . . . . . . . . . . . . . .   9
     3.2.  Use of the 'kid' Parameter  . . . . . . . . . . . . . . .  10
     3.3.  Updated 'unprotected' Field . . . . . . . . . . . . . . .  10
   4.  OSCORE Header Compression . . . . . . . . . . . . . . . . . .  10
     4.1.  Encoding of the OSCORE Option Value . . . . . . . . . . .  11
     4.2.  Encoding of the OSCORE Payload  . . . . . . . . . . . . .  12
     4.3.  Examples of Compressed COSE Objects . . . . . . . . . . .  12
   5.  Message Binding, Sequence Numbers, Freshness and Replay
       Protection  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Synchronization of Sender Sequence Numbers  . . . . . . .  13
   6.  Message Processing  . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  14
     6.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  15
     6.3.  Protecting the Response . . . . . . . . . . . . . . . . .  15
     6.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  16
   7.  Responsibilities of the Group Manager . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
     8.1.  Group-level Security  . . . . . . . . . . . . . . . . . .  18
     8.2.  Uniqueness of (key, nonce)  . . . . . . . . . . . . . . .  18
     8.3.  Management of Group Keying Material . . . . . . . . . . .  19
     8.4.  Update of Security Context and Key Rotation . . . . . . .  19
     8.5.  Collision of Group Identifiers  . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Counter Signature Parameters Registry . . . . . . . . . .  20
     9.2.  Expert Review Instructions  . . . . . . . . . . . . . . .  22
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     10.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Assumptions and Security Objectives  . . . . . . . .  25
     A.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .  25



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     A.2.  Security Objectives . . . . . . . . . . . . . . . . . . .  26
   Appendix B.  List of Use Cases  . . . . . . . . . . . . . . . . .  27
   Appendix C.  Example of Group Identifier Format . . . . . . . . .  29
   Appendix D.  Set-up of New Endpoints  . . . . . . . . . . . . . .  30
   Appendix E.  Examples of Synchronization Approaches . . . . . . .  31
     E.1.  Best-Effort Synchronization . . . . . . . . . . . . . . .  31
     E.2.  Baseline Synchronization  . . . . . . . . . . . . . . . .  31
     E.3.  Challenge-Response Synchronization  . . . . . . . . . . .  32
   Appendix F.  No Verification of Signatures  . . . . . . . . . . .  33
   Appendix G.  Document Updates . . . . . . . . . . . . . . . . . .  34
     G.1.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  34
     G.2.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  35
     G.3.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  36
     G.4.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  36
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

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, improve performance and reduce bandwidth
   utilisation.  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 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, replay protection and binding of
   response to request between a sender and a receipient, also in the
   presence of intermediaries.  To this end, a CoAP message is protected
   by including its payload (if any), certain options, and header fields
   in a COSE object, which replaces the authenticated and encrypted
   fields in the protected message.

   This document defines Group OSCORE, providing end-to-end security of
   CoAP messages exchanged between members of a group, and preserving
   independence of transport layer.  In particular, the described
   approach defines how OSCORE should be used in a group communication



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   setting, so that end-to-end security is assured in the same way as
   OSCORE for unicast communication.  That is, end-to-end security is
   provided for CoAP multicast requests sent by a client to the group,
   and for related CoAP responses sent by multiple servers.  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
   [RFC6347] to protect hop-by-hop communication between a sender and a
   proxy (and vice versa), and between a proxy and a recipient (and vice
   versa).  Note that DTLS cannot be used to secure messages sent over
   multicast.

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] including "endpoint", "client", "server",
   "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" and "Master Secret", 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 endpoints.  This includes, for
      instance, keys and IVs [RFC4949].

   o  Group: a set of endpoints that share group keying material and
      security parameters (Common Context, see Section 2).  The term
      group used in this specification refers thus to a "security
      group", not to be confused with network/multicast group or
      application group.




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   o  Group Manager: entity responsible for a group.  Each endpoint in a
      group communicates securely with the respective Group Manager,
      which is neither required to be an actual group member nor to take
      part in the group communication.  The full list of
      responsibilities of the Group Manager is provided in Section 7.

   o  Silent server: member of a group that never responds to requests.
      Note that a silent server can act as a client, the two roles are
      independent.

   o  Group Identifier (Gid): identifier assigned to the group.  Group
      Identifiers should be unique within the set of groups of a given
      Group Manager, in order to avoid collisions.  In case they are
      not, the considerations in Section 8.5 apply.

   o  Group request: CoAP request message sent by a client in the group
      to all servers in that group.

   o  Source authentication: evidence that a received message in the
      group originated from a specific identified group member.  This
      also provides assurance that the message was not tampered with by
      anyone, be it a different legitimate group member or an endpoint
      which is not a group member.

2.  OSCORE Security Context

   To support group communication secured with OSCORE, each endpoint
   registered as member of a group maintains a Security Context as
   defined in Section 3 of [I-D.ietf-core-object-security], extended as
   defined below.  Each endpoint in a group makes use of:

   1.  one Common Context, shared by all the endpoints in a given group.
       In particular:

       *  The ID Context parameter contains the Gid of the group, which
          is used to retrieve the Security Context for processing
          messages intended to the endpoints of the group (see
          Section 6).  The choice of the Gid is application specific.
          An example of specific formatting of the Gid is given in
          Appendix C.  The application needs to specify how to handle
          possible collisions between Gids, see Section 8.5.

       *  A new parameter Counter Signature Algorithm is included.  Its
          value identifies the digital signature algorithm used to
          compute a counter signature on the COSE object (see
          Section 4.5 of [RFC8152]) which provides source authentication
          within the group.  Its value is immutable once the Common
          Context is established.  The used Counter Signature Algorithm



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          MUST be selected among the signing ones defined in the COSE
          Algorithms Registry (see section 16.4 of [RFC8152]).  The
          EdDSA signature algorithm ed25519 [RFC8032] is mandatory to
          implement.  If Elliptic Curve Digital Signature Algorithm
          (ECDSA) is used, it is RECOMMENDED that implementations
          implement "deterministic ECDSA" as specified in [RFC6979].

       *  A new parameter Counter Signature Parameters is included.
          This parameter identifies the parameters associated to the
          digital signature algorithm specified in the Counter Signature
          Algorithm.  This parameter MAY be empty and is immutable once
          the Common Context is established.  The exact structure of
          this parameter depends on the value of Counter Signature
          Algorithm, and is defined in the Counter Signature Parameters
          Registry (see Section 9.1), where each entry indicates a
          specified structure of the Counter Signature Parameters.

   2.  one Sender Context, unless the endpoint is configured exclusively
       as silent server.  The Sender Context is used to secure outgoing
       messages and is initialized according to Section 3 of
       [I-D.ietf-core-object-security], once the endpoint has joined the
       group.  The Sender Context of a given endpoint matches the
       corresponding Recipient Context in all the endpoints receiving a
       protected message from that endpoint.  Besides, in addition to
       what is defined in [I-D.ietf-core-object-security], the Sender
       Context stores also the endpoint's private key.

   3.  one Recipient Context for each distinct endpoint from which
       messages are received, used to process incoming messages.  The
       recipient may generate the Recipient Context upon receiving an
       incoming message from another endpoint in the group for the first
       time (see Section 6.2 and Section 6.4).  Each Recipient Context
       matches the Sender Context of the endpoint from which protected
       messages are received.  Besides, in addition to what is defined
       in [I-D.ietf-core-object-security], each Recipient Context stores
       also the public key of the associated other endpoint from which
       messages are received.

   The table in Figure 1 overviews the new information included in the
   OSCORE Security Context, with respect to what defined in Section 3 of
   [I-D.ietf-core-object-security].










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         +---------------------------+------------------------------+
         |      Context portion      |       New information        |
         +---------------------------+------------------------------+
         |                           |                              |
         |      Common Context       | Counter signature algorithm  |
         |                           |                              |
         |      Common Context       | Counter signature parameters |
         |                           |                              |
         |      Sender Context       | Endpoint's own private key   |
         |                           |                              |
         |  Each Recipient Context   | Public key of the            |
         |                           | associated other endpoint    |
         |                           |                              |
         +---------------------------+------------------------------+

            Figure 1: Additions to the OSCORE Security Context

   Upon receiving a secure CoAP message, a recipient uses the sender's
   public key, in order to verify the counter signature of the COSE
   Object (see Section 3).

   If not already stored in the Recipient Context associated to the
   sender, the recipient retrieves the sender's public key from the
   Group Manager, which collects public keys upon endpoints' joining the
   group, acts as trusted key repository and ensures the correct
   association between the public key and the identifier of the sender,
   for instance by means of public key certificates.

   Note that a group member can retrieve public keys from the Group
   Manager and generate the Recipient Context associated to another
   group member at any point in time, as long as this is done before
   verifying a received secure CoAP message.  The exact configuration is
   application dependent.  For example, an application can configure a
   group member to retrieve all the required information and to create
   the Recipient Context exactly upon receiving a message from another
   group member for the first time.  As an alternative, the application
   can configure a group member to asynchronously retrieve the required
   information and update its list of Recipient Contexts well before
   receiving any message, e.g. by Observing [RFC7641] the Group Manager
   to get updates on the group membership.

   It is RECOMMENDED that the Group Manager collects public keys and
   provides them to group members upon request as described in
   [I-D.ietf-ace-key-groupcomm-oscore], where the join process is based
   on the ACE framework for Authentication and Authorization in
   constrained environments [I-D.ietf-ace-oauth-authz].  Further details
   about how public keys can be handled and retrieved in the group is
   out of the scope of this document.



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   An endpoint receives its own Sender ID from the Group Manager upon
   joining the group.  That Sender ID is valid only within that group,
   and is unique within the group.  An endpoint uses its own Sender ID
   (together with other data) to generate unique AEAD nonces for
   outgoing messages, as in [I-D.ietf-core-object-security].  Endpoints
   which are configured only as silent servers do not have a Sender ID.

   The Sender/Recipient Keys and the Common IV are derived according to
   the same scheme defined in Section 3.2 of
   [I-D.ietf-core-object-security].  The mandatory-to-implement HKDF and
   AEAD algorithms for Group OSCORE are the same as in
   [I-D.ietf-core-object-security].

2.1.  Management of Group Keying Material

   In order to establish a new Security Context in a group, a new Group
   Identifier (Gid) for that group and a new value for the Master Secret
   parameter MUST be distributed.  An example of Gid format supporting
   this operation is provided in Appendix C.  Then, each group member
   re-derives the keying material stored in its own Sender Context and
   Recipient Contexts as described in Section 2, using the updated Gid.

   After a new Gid has been distributed, a same Recipient ID ('kid')
   should not be considered as a persistent and reliable indicator of
   the same group member.  Such an indication can be actually achieved
   only by verifying countersignatures of received messages.

   As a consequence, group members may end up retaining stale Recipient
   Contexts, that are no longer useful to verify incoming secure
   messages.  Applications may define policies to delete (long-)unused
   Recipient Contexts and reduce the impact on storage space.

   If the application requires so (see Appendix A.1), it is RECOMMENDED
   to adopt a group key management scheme, and securely distribute a new
   value for the Gid and 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 the group.  This is
   necessary to preserve backward security and forward security in the
   group, if the application requires it.

   The specific approach used to distribute the new Gid and Master
   Secret parameter to the group is out of the scope of this document.
   However, it is RECOMMENDED that the Group Manager supports the
   distribution of the new Gid and Master Secret parameter to the group
   according to the Group Rekeying Process described in
   [I-D.ietf-ace-key-groupcomm-oscore].





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2.2.  Wrap-Around of Partial IVs

   A client can eventually experience a wrap-around of its own Sender
   Sequence Number, which is used as Partial IV in outgoing requests and
   incremented after each request.

   When this happens, the endpoint MUST NOT transmit further group
   requests until it has derived a new Sender Context, in order to avoid
   reusing nonces with the same keys.

   Furthermore, the endpoint SHOULD inform the Group Manager, that can
   take one of the following actions:

   o  The Group Manager renews the OSCORE Security Context in the group
      (see Section 2.1).

   o  The Group Manager provides a new Sender ID value to the endpoint
      that has experienced the wrap-around.  Then, the endpoint derives
      a new Sender Context using the new Sender ID, as described in
      Section 3.2 of [I-D.ietf-core-object-security].

   Either case, same considerations from Section 2.1 hold about possible
   retaining of stale Recipient Contexts.

3.  The COSE Object

   Building on Section 5 of [I-D.ietf-core-object-security], this
   section defines how to use COSE [RFC8152] to wrap and protect data in
   the original message.  OSCORE uses the untagged COSE_Encrypt0
   structure with an Authenticated Encryption with Additional Data
   (AEAD) algorithm.  For Group OSCORE, the following modifications
   apply.

3.1.  Updated external_aad

   The external_aad in the Additional Authenticated Data (AAD) is
   extended with the counter signature algorithm and related parameters
   used to sign messages.  In particular, compared with Section 5.4 of
   [I-D.ietf-core-object-security], the 'algorithms' array in the
   aad_array MUST also include:

   o  'alg_countersign', which contains the Counter Signature Algorithm
      from the Common Context (see Section 2).  This parameter has the
      value specified in the "Value" field of the Counter Signature
      Parameters Registry (see Section 9.1) for this counter signature
      algorithm.

   The 'algorithms' array in the aad_array MAY also include:



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   o  'par_countersign', which contains the Counter Signature Parameters
      from the Common Context (see Section 2).  This parameter contains
      the counter signature parameters encoded as specified in the
      "Parameters" field of the Counter Signature Parameters Registry
      (see Section 9.1), for the used counter signature algorithm.  Note
      that if the Counter Signature Parameters in the Common Context is
      empty, 'par_countersign' is not present.

   This external_aad structure is used both for the encryption process
   producing the ciphertext (see Section 5.3 of [RFC8152]) and for the
   signing process producing the counter signature, as defined below.

   external_aad = bstr .cbor aad_array

   aad_array = [
      oscore_version : uint,
      algorithms : [alg_aead : int / tstr ,
                    alg_countersign : int / tstr ,
                    ? par_countersign : any],
      request_kid : bstr,
      request_piv : bstr,
      options : bstr
   ]

3.2.  Use of the 'kid' Parameter

   The value of the 'kid' parameter in the 'unprotected' field of
   response messages MUST be set to the Sender ID of the endpoint
   transmitting the message.  That is, unlike in
   [I-D.ietf-core-object-security], the 'kid' parameter is always
   present in all messages, i.e. both requests and responses.

3.3.  Updated 'unprotected' Field

   The 'unprotected' field MUST additionally include the following
   parameter:

   o  CounterSignature0 : its value is set to the counter signature of
      the COSE object, computed by the sender using its own private key
      as described in Appendix A.2 of [RFC8152].  In particular, the
      Sig_structure contains the external_aad as defined above and the
      ciphertext of the COSE_Encrypt0 object as payload.

4.  OSCORE Header Compression

   The OSCORE compression defined in Section 6 of
   [I-D.ietf-core-object-security] is used, with the following additions
   for the encoding of the OSCORE Option and the OSCORE Payload.



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4.1.  Encoding of the OSCORE Option Value

   Analogously to [I-D.ietf-core-object-security], the value of the
   OSCORE option SHALL contain the OSCORE flag bits, the Partial IV
   parameter, the kid context parameter (length and value), and the kid
   parameter, with the following modifications:

   o  The first byte, containing the OSCORE flag bits, has the following
      encoding modifications:

      *  The fourth least significant bit MUST be set to 1 in every
         message, to indicate the presence of the 'kid' parameter for
         all group requests and responses.  That is, unlike in
         [I-D.ietf-core-object-security], the 'kid' parameter is always
         present in all messages.

      *  The fifth least significant bit MUST be set to 1 for group
         requests, to indicate the presence of the 'kid context'
         parameter in the compressed COSE object.  The 'kid context' MAY
         be present in responses if the application requires it.  In
         such a case, the kid context flag MUST be set to 1.

   The flag bits are registered in the OSCORE Flag Bits registry
   specified in Section 13.7 of [I-D.ietf-core-object-security].

   o  The 'kid context' value encodes the Group Identifier value (Gid)
      of the group's Security Context.

   o  The remaining bytes in the OSCORE Option value encode the value of
      the 'kid' parameter, which is always present both in group
      requests and in responses.

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

            <-- 1 byte ---> <------ s bytes ------>
           +---------------+-----------------------+-----------+
           |  s (if any)   |   kid context = Gid   |    kid    |
           +---------------+-----------------------+-----------+

                       Figure 2: OSCORE Option Value








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4.2.  Encoding of the OSCORE Payload

   The payload of the OSCORE message SHALL encode the ciphertext of the
   COSE object concatenated with the value of the CounterSignature0 of
   the COSE object, computed as in Appendix A.2 of [RFC8152] according
   to the Counter Signature Algorithm and Counter Signature Parameters
   in the Security Context.

4.3.  Examples of Compressed COSE Objects

   This section covers a list of OSCORE Header Compression examples for
   group requests and responses.  The examples assume that the
   COSE_Encrypt0 object is set (which means the CoAP message and
   cryptographic material is known).  Note that the examples do not
   include the full CoAP unprotected message or the full security
   context, but only the input necessary to the compression mechanism,
   i.e. the COSE_Encrypt0 object.  The output is the compressed COSE
   object as defined in Section 4 and divided into two parts, since the
   object is transported in two CoAP fields: OSCORE option and payload.

   The examples assume that the label for the new kid context defined in
   [I-D.ietf-core-object-security] has value 10.  COUNTERSIGN is the
   CounterSignature0 byte string as described in Section 3 and is 64
   bytes long.

   1.  Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
       0x25, Partial IV = 5 and kid context = 0x44616c

   Before compression (96 bytes):

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

   After compression (85 bytes):

   Flag byte: 0b00011001 = 0x19

   Option Value: 19 05 03 44 61 6c 25 (7 bytes)

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

   1.  Response with ciphertext = 60b035059d9ef5667c5a0710823b, kid =
       0x52 and no Partial IV.




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   Before compression (88 bytes):

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

   After compression (80 bytes):

   Flag byte: 0b00001000 = 0x08

   Option Value: 08 52 (2 bytes)

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

5.  Message Binding, Sequence Numbers, Freshness and Replay Protection

   The requirements and properties described in Section 7 of
   [I-D.ietf-core-object-security] also apply to OSCORE used in group
   communication.  In particular, group OSCORE provides message binding
   of responses to requests, which provides relative freshness of
   responses, and replay protection of requests.

   Besides, group OSCORE provides additional assurances on the client
   side, upon receiving responses bound to a same request.  That is, as
   long as the client retains the CoAP Token used in a request (see
   Section 2.5 of [RFC7390]), group OSCORE ensures that: any possible
   response sent to that request is not a replay; and at most one
   response to that request from a given server is accepted, if required
   by the application.

   More details about error processing for replay detection in group
   OSCORE are specified in Section 6 of this specification.  The
   mechanisms describing replay protection and freshness of Observe
   notifications do not apply to group OSCORE, as Observe is not defined
   for group settings.

5.1.  Synchronization of Sender Sequence Numbers

   Upon joining the group, new servers are not aware of the Sender
   Sequence Number values currently used by different clients to
   transmit group requests.  This means that, when such servers receive
   a secure group request from a given client for the first time, they
   are not able to verify if that request is fresh and has not been
   replayed or (purposely) delayed.  The same holds when a server loses




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   synchronization with Sender Sequence Numbers of clients, for instance
   after a device reboot.

   The exact way to address this issue is application specific, and
   depends on the particular use case and its synchronization
   requirements.  The list of methods to handle synchronization of
   Sender Sequence Numbers is part of the group communication policy,
   and different servers can use different methods.

   Appendix E describes three possible approaches that can be considered
   for synchronization of sequence numbers.

6.  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 sections.  The following
   security objectives are fulfilled, as further discussed in
   Appendix A.2: data replay protection, group-level data
   confidentiality, source authentication, message integrity.

   As per [RFC7252][RFC7390], group requests sent over multicast MUST be
   Non-Confirmable.  Thus, senders should store their outgoing messages
   for an amount of time defined by the application and sufficient to
   correctly handle possible retransmissions.  However, this does not
   prevent the acknowledgment of Confirmable group requests in non-
   multicast environments.  Besides, according to Section 5.2.3 of
   [RFC7252], responses to Non-Confirmable group requests SHOULD be also
   Non-Confirmable.  However, endpoints MUST be prepared to receive
   Confirmable responses in reply to a Non-Confirmable group request.

   Furthermore, endpoints in the group locally perform error handling
   and processing of invalid messages according to the same principles
   adopted in [I-D.ietf-core-object-security].  However, a recipient
   MUST stop processing and silently reject any message which is
   malformed and does not follow the format specified in Section 3, or
   which is not cryptographically validated in a successful way.  Either
   case, it is RECOMMENDED that the recipient does not send back any
   error message.  This prevents servers from replying with multiple
   error messages to a client sending a group request, so avoiding the
   risk of flooding and possibly congesting the group.

6.1.  Protecting the Request

   A client transmits a secure group request as described in Section 8.1
   of [I-D.ietf-core-object-security], with the following modifications.





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   o  In step 2, the 'algorithms' array in the Additional Authenticated
      Data is modified as described in Section 3.

   o  In step 4, the encryption of the COSE object is modified as
      described in Section 3.  The encoding of the compressed COSE
      object is modified as described in Section 4.

   o  In step 5, the counter signature is computed and the format of the
      OSCORE mesage is modified as described in Section 4.2.  In
      particular, the payload of the OSCORE message includes also the
      counter signature.

6.2.  Verifying the Request

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

   o  In step 2, the decoding of the compressed COSE object follows
      Section 4.  If the received Recipient ID ('kid') does not match
      with any Recipient Context for the retrieved Gid ('kid context'),
      then the server creates a new Recipient Context, initializes it
      according to Section 3 of [I-D.ietf-core-object-security], also
      retrieving the client's public key.

   o  In step 4, the 'algorithms' array in the Additional Authenticated
      Data is modified as described in Section 3.

   o  In step 6, the server also verifies the counter signature using
      the public key of the client from the associated Recipient
      Context.

   o  Additionally, if the used Recipient Context was created upon
      receiving this group request and the message is not verified
      successfully, the server MAY delete that Recipient Context.  Such
      a configuration, which is specified by the application, would
      prevent attackers from overloading the server's storage and
      creating processing overhead on the server.

6.3.  Protecting the Response

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

   o  In step 2, the 'algorithms' array in the Additional Authenticated
      Data is modified as described in Section 3.




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   o  In step 4, the encryption of the COSE object is modified as
      described in Section 3.  The encoding of the compressed COSE
      object is modified as described in Section 4.

   o  In step 5, the counter signature is computed and the format of the
      OSCORE mesage is modified as described in Section 4.2.  In
      particular, the payload of the OSCORE message includes also the
      counter signature.

6.4.  Verifying the Response

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

   o  In step 2, the decoding of the compressed COSE object is modified
      as described in Section 3.  The client also checks whether it
      previously received a secure response to this request, such that
      it was successfully verified and included the same Recipient ID
      ('kid') of the just received response.  In case of positive match
      the client SHALL stop processing the response.  If the received
      Recipient ID ('kid') does not match with any Recipient Context for
      the retrieved Gid ('kid context'), then the client creates a new
      Recipient Context, initializes it according to Section 3 of
      [I-D.ietf-core-object-security], also retrieving the server's
      public key.

   o  In step 3, the 'algorithms' array in the Additional Authenticated
      Data is modified as described in Section 3.

   o  In step 5, the client also verifies the counter signature using
      the public key of the server from the associated Recipient
      Context.  In case of success, the client also records the received
      Recipient ID ('kid') as included in a successfully verified
      response to the request.

   o  Additionally, if the used Recipient Context was created upon
      receiving this response and the message is not verified
      successfully, the client MAY delete that Recipient Context.  Such
      a configuration, which is specified by the application, would
      prevent attackers from overloading the client's storage and
      creating processing overhead on the client.

   Upon freeing up the Token value of a secure group request for
   possible reuse [RFC7390], the client MUST delete the list of recorded
   Recipient IDs associated to that request (see step 5 above).





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7.  Responsibilities of the Group Manager

   The Group Manager is responsible for performing the following tasks:

   1.   Creating and managing OSCORE groups.  This includes the
        assignment of a Gid to every newly created group, as well as
        ensuring uniqueness of Gids within the set of its OSCORE groups.

   2.   Defining policies for authorizing the joining of its OSCORE
        groups.  Such policies can be enforced locally by the Group
        Manager, or by a third party in a trust relation with the Group
        Manager and entrusted to enforce join policies on behalf of the
        Group Manager.

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

   4.   Establishing Security Common Contexts and providing them to
        authorized group members during the join process, together with
        a corresponding Security Sender Context.

   5.   Generating and managing Sender IDs within its OSCORE groups, as
        well as assigning and providing them to new endpoints during the
        join process.  This includes ensuring uniqueness of Sender IDs
        within each of its OSCORE groups.

   6.   Defining a communication policy for each of its OSCORE groups,
        and signalling it to new endpoints during the join process.

   7.   Renewing the Security Context of an OSCORE group upon membership
        change, by revoking and renewing common security parameters and
        keying material (rekeying).

   8.   Providing the management keying material that a new endpoint
        requires to participate in the rekeying process, consistent with
        the key management scheme used in the group joined by the new
        endpoint.

   9.   Updating the Gid of its OSCORE groups, upon renewing the
        respective Security Context.

   10.  Acting as key repository, in order to handle the public keys of
        the members of its OSCORE groups, and providing such public keys
        to other members of the same group upon request.  The actual
        storage of public keys may be entrusted to a separate secure
        storage device.






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8.  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.

8.1.  Group-level Security

   The approach described in this document relies on commonly shared
   group keying material to protect communication within a group.  This
   has the following implications.

   o  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.

   o  The AEAD algorithm provides only group authentication, i.e. it
      ensures that a message sent to a group has been sent by a member
      of that group, but not by the alleged sender.  This is why source
      authentication of messages sent to a group is ensured through a
      counter signature, which is computed by the sender using its own
      private key and then appended to the message payload.

   Note that, even if an endpoint is authorized to be a group member and
   to take part in group communications, there is a risk that it behaves
   inappropriately.  For instance, it can forward the content of
   messages in the group 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), which results in a
   practically limited risk and enables a prompt detection/reaction in
   case of misbehaving.

8.2.  Uniqueness of (key, nonce)

   The proof for uniqueness of (key, nonce) pairs in Appendix D.3 of
   [I-D.ietf-core-object-security] is also valid in group communication
   scenarios.  That is, given an OSCORE group:

   o  Uniqueness of Sender IDs within the group is enforced by the Group
      Manager.

   o  The case A in Appendix D.3 of [I-D.ietf-core-object-security] for
      messages including a Partial IV concerns only group requests, and
      same considerations from [I-D.ietf-core-object-security] apply
      here as well.




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   o  The case B in Appendix D.3 of [I-D.ietf-core-object-security] for
      messages not including a Partial IV concerns all group responses,
      and same considerations from [I-D.ietf-core-object-security] apply
      here as well.

   As a consequence, each message encrypted/decrypted with the same
   Sender Key is processed by using a different (ID_PIV, PIV) pair.
   This means that nonces used by any fixed encrypting endpoint are
   unique.  Thus, each message is processed with a different (key,
   nonce) pair.

8.3.  Management of Group Keying Material

   The approach described in this specification should take into account
   the risk of compromise of group members.  In particular, this
   document specifies that a key management scheme for secure revocation
   and renewal of Security Contexts and group keying material should be
   adopted.

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

8.4.  Update of Security Context and Key Rotation

   A group member can receive a message shortly after the group has been
   rekeyed, and new security parameters and keying material have been
   distributed by the Group Manager.  In the following two cases, this
   may result in misaligned Security Contexts between the sender and the
   recipient.

   In the first case, the sender protects a message using the old
   Security Context, i.e. before having installed the new Security
   Context.  However, the recipient receives the message after having
   installed the new Security Context, hence not being able to correctly
   process it.  A possible way to ameliorate this issue is to preserve
   the old, recent, Security Context for a maximum amount of time
   defined by the application.  By doing so, the recipient can still try
   to process the received message using the old retained Security
   Context as second attempt.  Note that a former (compromised) group
   member can take advantage of this by sending messages protected with
   the old retained Security Context.  Therefore, a conservative
   application policy should not admit the storage of old Security
   Contexts.





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   In the second case, the sender protects a message using the new
   Security Context, but the recipient receives that request before
   having installed the new Security Context.  Therefore, the recipient
   would not be able to correctly process the request and hence discards
   it.  If the recipient receives the new Security Context shortly after
   that and the sender endpoint uses CoAP retransmissions, the former
   will still be able to receive and correctly process the message.  In
   any case, the recipient should actively ask the Group Manager for an
   updated Security Context according to an application-defined policy,
   for instance after a given number of unsuccessfully decrypted
   incoming messages.

8.5.  Collision of Group Identifiers

   In case endpoints are deployed in multiple groups managed by
   different non-synchronized Group Managers, it is possible for Group
   Identifiers of different groups to coincide.  That can also happen if
   the application can not guarantee unique Group Identifiers within a
   given Group Manager.  However, this does not impair the security of
   the AEAD algorithm.

   In fact, as long as the Master Secret is different for different
   groups and this condition holds over time, and as long as the Sender
   IDs within a group are unique, AEAD keys are different among
   different groups.

9.  IANA Considerations

   Note to RFC Editor: Please replace all occurrences of "[This
   Document]" with the RFC number of this specification and delete this
   paragraph.

   This document has the following actions for IANA.

9.1.  Counter Signature Parameters Registry

   This specification establishes the IANA "Counter Signature
   Parameters" Registry.  The Registry has been created to use the
   "Expert Review Required" registration procedure [RFC8126].  Expert
   review guidelines are provided in Section 9.2.

   The columns of this table are:

   o  Name: A value that can be used to identify an algorithm in
      documents for easier comprehension.  Its value is taken from the
      'Name' column of the "COSE Algorithms" Registry.





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   o  Value: The value to be used to identify this algorithm.  Its
      content is taken from the 'Value' column of the "COSE Algorithms"
      Registry.  The value MUST be the same one used in the "COSE
      Algorithms" Registry for the entry with the same 'Name' field.

   o  Parameters: This indicates the CBOR encoding of the parameters (if
      any) for the counter signature algorithm indicated by the 'Value'
      field.

   o  Description: A short description of the parameters encoded in the
      'Parameters' field (if any).

   o  Reference: This contains a pointer to the public specification for
      the field, if one exists.

   Initial entries in the registry are as follows.

   +-------------+-------+-------------+-----------------+-----------+
   |    Name     | Value | Parameters  |   Description   | Reference |
   +-------------+-------+-------------+-----------------+-----------+
   |             |       |             |                 |           |
   |    EdDSA    |  -8   |  crv : int  | crv value taken | [This     |
   |             |       |             | from the COSE   | Document] |
   |             |       |             | Elliptic Curve  |           |
   |             |       |             | Registry        |           |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+
   |             |       |             |                 |           |
   |    ES256    |  -7   |  crv : int  | crv value taken | [This     |
   |             |       |             | from the COSE   | Document] |
   |             |       |             | Elliptic Curve  |           |
   |             |       |             | Registry        |           |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+
   |             |       |             |                 |           |
   |    ES384    |  -35  |  crv : int  | crv value taken | [This     |
   |             |       |             | from the COSE   | Document] |
   |             |       |             | Elliptic Curve  |           |
   |             |       |             | Registry        |           |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+
   |             |       |             |                 |           |
   |    ES512    |  -36  |  crv : int  | crv value taken | [This     |
   |             |       |             | from the COSE   | Document] |
   |             |       |             | Elliptic Curve  |           |
   |             |       |             | Registry        |           |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+



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   |             |       |             |                 |           |
   |    PS256    |  -37  |             | Parameters not  | [This     |
   |             |       |             | present         | Document] |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+
   |             |       |             |                 |           |
   |    PS384    |  -38  |             | Parameters not  | [This     |
   |             |       |             | present         | Document] |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+
   |             |       |             |                 |           |
   |    PS512    |  -39  |             | Parameters not  | [This     |
   |             |       |             | present         | Document] |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+
   |             |       |             |                 |           |
   | RSAES-OAEP  |  -40  |             | Parameters not  | [This     |
   | w/ RFC 8017 |       |             | present         | Document] |
   | default     |       |             |                 |           |
   | parameters  |       |             |                 |           |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+
   |             |       |             |                 |           |
   | RSAES-OAEP  |  -41  |             | Parameters not  | [This     |
   | w/ SHA-256  |       |             | present         | Document] |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+
   |             |       |             |                 |           |
   | RSAES-OAEP  |  -42  |             | Parameters not  | [This     |
   | w/ SHA-512  |       |             | present         | Document] |
   |             |       |             |                 |           |
   +-------------+-------+-------------+-----------------+-----------+

9.2.  Expert Review Instructions

   The IANA Registry established in this document is defined as expert
   review.  This section gives some general guidelines for what the
   experts should be looking for, but they are being designated as
   experts for a reason so they should be given substantial latitude.

   Expert reviewers should take into consideration the following points:

   TBD








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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-16 (work in
              progress), March 2019.

   [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>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/info/rfc6979>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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







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   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm-
              oscore-00 (work in progress), December 2018.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-22
              (work in progress), March 2019.

   [I-D.ietf-core-echo-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "Echo and
              Request-Tag", draft-ietf-core-echo-request-tag-03 (work in
              progress), October 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.

   [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>.

   [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>.






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   [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>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

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 sender and multiple recipients) and M-to-N (multiple
      senders and multiple recipients) 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.

      In a 1-to-N communication model, only a single client transmits
      data to the group, in the form of request messages; in an M-to-N
      communication model (where M and N do not necessarily have the
      same value), M group members are clients.  According to [RFC7390],
      any possible proxy entity is supposed to know about the clients in
      the group and to not perform aggregation of response messages from
      multiple servers.  Also, every client expects and is able to
      handle multiple response messages associated to a same request
      sent to the group.

   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 clients
      (normally the controlling devices) is expected to be much smaller
      than the number of servers (i.e. the controlled devices).  A
      security solution for group communication that supports 1 to 50
      clients 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



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      devices.  Groups larger than that should be divided into smaller
      independent groups.

   o  Communication with the Group Manager: an endpoint must use a
      secure dedicated channel when communicating with the Group
      Manager, also when not registered as group member.

   o  Provisioning and management of Security Contexts: an OSCORE
      Security Context must be established among the group members.  A
      secure mechanism must be used to generate, revoke and
      (re-)distribute keying material, multicast security policies and
      security parameters in the group.  The actual provisioning and
      management of the Security Context is out of the scope of this
      document.

   o  Multicast data security ciphersuite: all group members must agree
      on a ciphersuite to provide authenticity, integrity and
      confidentiality of messages in the group.  The ciphersuite is
      specified as part of the Security 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 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 Security Context and
      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 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.




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   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 in
      particular by a specific member of the 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.  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 senders.

Appendix 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 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 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 groups to
      be formed even if devices in the lighting group may be physically
      in different subnets (e.g. on wired and wireless networks).



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      Connectivity between 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 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
      trigger events to a group of lighting devices.  Especially in
      professional lighting scenarios, the roles of client and server
      are configured by the lighting commissioner, and 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 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 group.  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



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      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 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 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 constant over time and is uniquely defined in the
   set of all the groups associated to the same Group Manager.  The
   choice of the Group Prefix for a given group's Security Context is
   application specific.  The size of the Group Prefix directly impact
   on the maximum number of distinct groups under the same Group
   Manager.




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   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 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.

   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'.

   Using an immutable Group Prefix for a group assumes that enough time
   elapses between two consecutive usages of the same Group Epoch value
   in that group.  This ensures that the Gid value is temporally unique
   during the lifetime of a given message.  Thus, the expected highest
   rate for addition/removal of group members and consequent group
   rekeying should be taken into account for a proper dimensioning of
   the Group Epoch size.

   As discussed in Section 8.5, if endpoints are deployed in multiple
   groups managed by different non-synchronized Group Managers, it is
   possible that Group Identifiers of different groups coincide at some
   point in time.  In this case, a recipient has to handle coinciding
   Group Identifiers, and has to try using different OSCORE Security
   Contexts to process an incoming message, until the right one is found
   and the message is correctly verified.  Therefore, it is favourable
   that Group Identifiers from different Group Managers have a size that
   result in a small probability of collision.  How small this
   probability should be is up to system designers.

Appendix D.  Set-up of New Endpoints

   An endpoint joins a group by explicitly interacting with the
   responsible Group Manager.  When becoming members of a group,
   endpoints are not required to know how many and what endpoints are in
   the same group.

   Communications between a joining endpoint and the Group Manager rely
   on the CoAP protocol and must be secured.  Specific details on how to
   secure communications between joining endpoints and a Group Manager
   are out of the scope of this document.

   The Group Manager must verify that the joining endpoint is authorized
   to join the group.  To this end, the Group Manager can directly
   authorize the joining endpoint, or expect it to provide authorization



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   evidence previously obtained from a trusted entity.  Further details
   about the authorization of joining endpoints are out of scope.

   In case of successful authorization check, the Group Manager
   generates a Sender ID assigned to the joining endpoint, before
   proceeding with the rest of the join process.  That is, the Group
   Manager provides the joining endpoint with the keying material and
   parameters to initialize the OSCORE Security Context (see Section 2).
   The actual provisioning of keying material and parameters to the
   joining endpoint is out of the scope of this document.

   It is RECOMMENDED that the join process adopts the approach described
   in [I-D.ietf-ace-key-groupcomm-oscore] and based on the ACE framework
   for Authentication and Authorization in constrained environments
   [I-D.ietf-ace-oauth-authz].

Appendix E.  Examples of Synchronization Approaches

   This section describes three possible approaches that can be
   considered by server endpoints to synchronize with sender sequence
   numbers of client endpoints sending group requests.

E.1.  Best-Effort Synchronization

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

E.2.  Baseline Synchronization

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

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








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E.3.  Challenge-Response Synchronization

   A server performs a challenge-response exchange with a client, by
   using the Echo Option for CoAP described in Section 2 of
   [I-D.ietf-core-echo-request-tag] and according to Section 7.5.2 of
   [I-D.ietf-core-object-security].

   That is, upon receiving a group request from a particular client for
   the first time, the server processes the message as described in
   Section 6.2 of this specification, but, even if valid, does not
   deliver it to the application.  Instead, the server replies to the
   client with a 4.03 Forbidden response message including an Echo
   Option, and stores the option value included therein.

   Upon receiving a 4.03 Forbidden response that includes an Echo Option
   and originates from a verified group member, a client sends a request
   as a unicast message addressed to the same server, echoing the Echo
   Option value.  In particular, the client 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 client to
   not retain previously sent group requests for full retransmission,
   unless the application explicitly requires otherwise.  In either
   case, the client uses the sender sequence number value currently
   stored in its own Sender Context.  If the client stores group
   requests for possible retransmission with the Echo Option, it should
   not store a given request for longer than a pre-configured time
   interval.  Note that the unicast request echoing the Echo Option is
   correctly treated and processed as a message, since the 'kid context'
   field including the Group Identifier of the OSCORE group is still
   present in the OSCORE Option as part of the COSE object (see
   Section 3).

   Upon receiving the unicast request including the Echo Option, the
   server verifies that the option value equals the stored and
   previously sent value; otherwise, the request is silently discarded.
   Then, the server verifies that the unicast request has been received
   within a pre-configured time interval, as described in
   [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 server updates the Recipient Context associated to that
   client, by setting the Replay Window according to the Sequence Number
   from the unicast request conveying the Echo Option.  The server
   either delivers the request to the application if it is an actual
   retransmission of the original one, or 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.




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   In case it does not receive a valid unicast request including the
   Echo Option within the configured time interval, the server endpoint
   should perform the same challenge-response upon receiving the next
   group request from that same client.

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

   Note that endpoints configured as silent servers 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 client.
   Therefore, silent servers should adopt alternative approaches to
   achieve and maintain synchronization with sender sequence numbers of
   clients.

   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 groups where many
   endpoints at the same time might join as new members or lose
   synchronization.

Appendix F.  No Verification of Signatures

   There are some application scenarios using group 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
   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 message, so that an




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   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 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 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.

Appendix G.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

G.1.  Version -03 to -04

   o  Added the new "Counter Signature Parameters" in the Security
      Common Context (see Section 2).

   o  Added recommendation on using "deterministic ECDSA" if ECDSA is
      used as counter signature algorithm (see Section 2).

   o  Clarified possible asynchronous retrieval of key material from the
      Group Manager, in order to process incoming messages (see
      Section 2).

   o  Structured Section 3 into subsections.

   o  Added the new 'par_countersign' to the aad_array of the
      external_aad (see Section 3.1).

   o  Clarified non reliability of 'kid' as identity indicator for a
      group member (see Section 2.1).

   o  Described possible provisioning of new Sender ID in case of
      Partial IV wrap-around (see Section 2.2).





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   o  The former signature bit in the Flag Byte of the OSCORE option
      value is reverted to reserved (see Section 4.1).

   o  Updated examples of compressed COSE object, now with the sixth
      less significant bit in the Flag Byte of the OSCORE option value
      set to 0 (see Section 4.3).

   o  Relaxed statements on sending error messages (see Section 6).

   o  Added explicit step on computing the counter signature for
      outgoing messages (see Setions 6.1 and 6.3).

   o  Handling of just created Recipient Contexts in case of
      unsuccessful message verification (see Sections 6.2 and 6.4).

   o  Handling of replied/repeated responses on the client (see
      Section 6.4).

   o  New IANA Registry "Counter Signature Parameters" (see
      Section 9.1).

G.2.  Version -02 to -03

   o  Revised structure and phrasing for improved readability and better
      alignment with draft-ietf-core-object-security.

   o  Added discussion on wrap-Around of Partial IVs (see Section 2.2).

   o  Separate sections for the COSE Object (Section 3) and the OSCORE
      Header Compression (Section 4).

   o  The countersignature is now appended to the encrypted payload of
      the OSCORE message, rather than included in the OSCORE Option (see
      Section 4).

   o  Extended scope of Section 5, now titled " Message Binding,
      Sequence Numbers, Freshness and Replay Protection".

   o  Clarifications about Non-Confirmable messages in Section 5.1
      "Synchronization of Sender Sequence Numbers".

   o  Clarifications about error handling in Section 6 "Message
      Processing".

   o  Compacted list of responsibilities of the Group Manager in
      Section 7.

   o  Revised and extended security considerations in Section 8.



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   o  Added IANA considerations for the OSCORE Flag Bits Registry in
      Section 9.

   o  Revised Appendix D, now giving a short high-level description of a
      new endpoint set-up.

G.3.  Version -01 to -02

   o  Terminology has been made more aligned with RFC7252 and draft-
      ietf-core-object-security: i) "client" and "server" replace the
      old "multicaster" and "listener", respectively; ii) "silent
      server" replaces the old "pure listener".

   o  Section 2 has been updated to have the Group Identifier stored in
      the 'ID Context' parameter defined in draft-ietf-core-object-
      security.

   o  Section 3 has been updated with the new format of the Additional
      Authenticated Data.

   o  Major rewriting of Section 4 to better highlight the differences
      with the message processing in draft-ietf-core-object-security.

   o  Added Sections 7.2 and 7.3 discussing security considerations
      about uniqueness of (key, nonce) and collision of group
      identifiers, respectively.

   o  Minor updates to Appendix A.1 about assumptions on multicast
      communication topology and group size.

   o  Updated Appendix C on format of group identifiers, with practical
      implications of possible collisions of group identifiers.

   o  Updated Appendix D.2, adding a pointer to draft-palombini-ace-key-
      groupcomm about retrieval of nodes' public keys through the Group
      Manager.

   o  Minor updates to Appendix E.3 about Challenge-Response
      synchronization of sequence numbers based on the Echo option from
      draft-ietf-core-echo-request-tag.

G.4.  Version -00 to -01

   o  Section 1.1 has been updated with the definition of group as
      "security group".

   o  Section 2 has been updated with:




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      *  Clarifications on etablishment/derivation of security contexts.

      *  A table summarizing the the additional context elements
         compared to OSCORE.

   o  Section 3 has been updated with:

      *  Examples of request and response messages.

      *  Use of CounterSignature0 rather than CounterSignature.

      *  Additional Authenticated Data including also the signature
         algorithm, while not including the Group Identifier any longer.

   o  Added Section 6, listing the responsibilities of the Group
      Manager.

   o  Added Appendix A (former section), including assumptions and
      security objectives.

   o  Appendix B has been updated with more details on the use cases.

   o  Added Appendix C, providing an example of Group Identifier format.

   o  Appendix D has been updated to be aligned with draft-palombini-
      ace-key-groupcomm.

Acknowledgments

   The authors sincerely thank Stefan Beck, Rolf Blom, Carsten Bormann,
   Esko Dijk, Klaus Hartke, Rikard Hoeglund, 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 VINNOVA and
   the Celtic-Next project CRITISEC; and by the EIT-Digital High Impact
   Initiative ACTIVE.

Authors' Addresses

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   Kista  SE-16440 Stockholm
   Sweden

   Email: marco.tiloca@ri.se




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   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


























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