Subject: [Ace] draft-ietf-ace-key-groupcomm-02

From: Daniel Migault <[email-masked]>
Date: Tue, 9 Jul 2019 13:20:31 -0400

Hi,

Please find some comments while reading the document. I also have some high
level questions that I might have missed in the draft or that are defined
somewhere else.

The group seems to be defined by its group members and each member are
registered with read or write access. What is unclear to me is how these
properties (read, write) affect the group communication. Are there any
specific cryptographic material or different access policies implemented by
the RS for each role ?

I am not entirely clear how the group communication is protected. In
particular, I am wondering if the source is authenticated or if the key is
shared between the different members of the group. I am also wondering how
the public key associated to each group member is used by another member.

Please find some comments on the documents.

Yours,
Daniel

ACE Working Group                                           F. Palombini
Internet-Draft                                               Ericsson AB
Intended status: Standards Track                               M. Tiloca
Expires: January 6, 2020                                         RISE AB
                                                           July 05, 2019

           Key Provisioning for Group Communication using ACE
                  draft-ietf-ace-key-groupcomm-02

Abstract

   This document defines message formats and procedures for requesting
   and distributing group keying material using the ACE framework, to
   protect communications between group members.

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 January 6, 2020.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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.

Palombini & Tiloca       Expires January 6, 2020                [Page 1]

Internet-Draft  Key Provisioning for Group Communication       July 2019

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Authorization to Join a Group . . . . . . . . . . . . . . . .   6
     3.1.  Authorization Request . . . . . . . . . . . . . . . . . .   7
     3.2.  Authorization Response  . . . . . . . . . . . . . . . . .   7
     3.3.  Token Post  . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Key Distribution  . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Key Distribution Request  . . . . . . . . . . . . . . . .  12
     4.2.  Key Distribution Response . . . . . . . . . . . . . . . .  13
   5.  Removal of a Node from the Group  . . . . . . . . . . . . . .  16
     5.1.  Expired Authorization . . . . . . . . . . . . . . . . . .  16
     5.2.  Request to Leave the Group  . . . . . . . . . . . . . . .  16
   6.  Retrieval of New or Updated Keying Material . . . . . . . . .  17
     6.1.  Key Re-Distribution Request . . . . . . . . . . . . . . .  18
     6.2.  Key Re-Distribution Response  . . . . . . . . . . . . . .  19
   7.  Retrieval of Public Keys for Group Members  . . . . . . . . .  19
     7.1.  Public Key Request  . . . . . . . . . . . . . . . . . . .  20
     7.2.  Public Key Response . . . . . . . . . . . . . . . . . . .  20
   8.  ACE Groupcomm Parameters  . . . . . . . . . . . . . . . . . .  21
   9.  ACE Groupcomm Request Type  . . . . . . . . . . . . . . . . .  22
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  23
     10.1.  Update of Keying Material  . . . . . . . . . . . . . . .  24
     10.2.  Block-Wise Considerations  . . . . . . . . . . . . . . .  24
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
     11.1.  ACE Authorization Server Request Creation Hints Registry  25
     11.2.  ACE Public Key Encoding Registry . . . . . . . . . . . .  25
     11.3.  ACE Groupcomm Parameters Registry  . . . . . . . . . . .  26
     11.4.  Ace Groupcomm Request Type Registry  . . . . . . . . . .  26
     11.5.  ACE Groupcomm Key Registry . . . . . . . . . . . . . . .  27
     11.6.  ACE Groupcomm Profile Registry . . . . . . . . . . . . .  28
     11.7.  ACE Groupcomm Policy Registry  . . . . . . . . . . . . .  28
     11.8.  Sequence Number Synchronization Method Registry  . . . .  29
     11.9.  Expert Review Instructions . . . . . . . . . . . . . . .  29
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     12.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Requirements on Application Profiles . . . . . . . .  33
   Appendix B.  Document Updates . . . . . . . . . . . . . . . . . .  33
     B.1.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  34
     B.2.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  34
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

Palombini & Tiloca       Expires January 6, 2020                [Page 2]

Internet-Draft  Key Provisioning for Group Communication       July 2019

1.  Introduction

   This document expands the ACE framework [I-D.ietf-ace-oauth-authz] to
   define the format of messages used to request, distribute and renew
   the keying material in a group communication scenario, e.g. based on
   multicast [RFC7390][I-D.dijk-core-groupcomm-bis] or on publishing-
   subscribing [I-D.ietf-core-coap-pubsub].
<mglt>

This is a nit which may be due to my english. My my reading of "e.g.
based on " seems to be misleading here - at least to me. I am
interpreting the text as 7390 and the two other drafts as examples that
could take advantage of the current document. As these document do not
seem to consider multicast communication in general - 7390 seem slimited
to one-to-many -, this gives, in my opinion a false impression that the
scheme describe in this document is much more general.  While we do not
want to prevent other protocol to re-use the scheme defined here, I
believe it would be safer to restrict the current document to these
latest document as they provide the group communication framework we are
using here. In these were only examples, I would expect the document to
provide a description how a more generic framework.

</mglt>
   The ACE framework is based
   on CBOR [RFC7049], so CBOR is the format used in this specification.
   However, using JSON [RFC8259] instead of CBOR is possible, using the
   conversion method specified in Sections 4.1 and 4.2 of [RFC7049].

<mglt>
This is a comment and there is probably a common understanding of the
text above I am missing. My reading is that CBOR and JSON are supported,
meaning that I could send a JSON packet or a CBOR packet. Unless this is
natively supported, it seems to indicate that two format are supported
and I woudl be inclined to say we shoudl maybe pick one of them for
inter-operability. In this case CBOR might be appropriated.

I am reading that the support for JSON is easily adapted from the
current documentation, but not so much recommended.
</mglt>

   Profiles that use group communication can build on this document to
   specify the selection of the message parameters defined in this
   document to use and their values.  Known applications that can
   benefit from this document would be, for example, those addressing
   group communication based on multicast
   [RFC7390][I-D.dijk-core-groupcomm-bis] or publishing/subscribing
   [I-D.ietf-core-coap-pubsub] in ACE.

<mglt>
I would suggest to limit the document to these documents, especially if
we do not envision any other cases while writing this document.

I believe it woudl be also important to remind some characteristics of
these groups. Typically it seems these groups envisioned one-to-many
communication and are not highly dynamic. I believe it woudl be helpful
to have the limitations or characteristics of these groups, so future
application could easily see if the key management propose fits their
requirements.
</mglt>

   If the application requires backward and forward security, updated
   keying material is generated and distributed to the group members
   (rekeying), when membership changes.  A key management scheme
   performs the actual distribution of the updated keying material to
   the group.  In particular, the key management scheme rekeys the
   current group members when a new node joins the group, and the
   remaining group members when a node leaves the group.  This document
   provides a message format for group rekeying that allows to fulfill
   these requirements.  Rekeying mechanisms can be based on [RFC2093],
   [RFC2094] and [RFC2627].

<mglt>
The latest paragraph provides a high level view of how the protocol
works. This is very helpful, but maybe additional text - or a dedicated
use case section may be needed to provide the full life cycle of the
keys. Typically, I am wondering one a group keys are managed at the
initialization of the group. My interpretation of the text is that if I
have 1000 battery power motes in my home and each of them join the group
one by one, I will have 1000 key updates. I believe it would be good to
clarify this.

</mglt>

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  These
   words may also appear in this document in lowercase, absent their
   normative meanings.

   Readers are expected to be familiar with the terms and concepts
   described in [I-D.ietf-ace-oauth-authz] and [RFC8152], such as
   Authorization Server (AS) and Resource Server (RS).

   This document additionally uses the following terminology:

   o  Transport profile, to indicate a profile of ACE as per
      Section 5.6.4.3 of [I-D.ietf-ace-oauth-authz].  That is, a
      transport profile specifies the communication protocol and
      communication security protocol between an ACE Client and Resource

Palombini & Tiloca       Expires January 6, 2020                [Page 3]

Internet-Draft  Key Provisioning for Group Communication       July 2019

      Server, as well as proof-of-possession methods, if it supports
      proof-of-possession access tokens.  Tranport profiles of ACE
      include, for instance, [I-D.ietf-ace-oscore-profile],
      [I-D.ietf-ace-dtls-authorize] and [I-D.ietf-ace-mqtt-tls-profile].

   o  Application profile, to indicate a profile of ACE that defines how
      applications enforce and use supporting security services they
      require.  These services include, for instance, provisioning,
      revocation and (re-)distribution of keying material.  An
      application profile may define specific procedures and message
      formats.

2.  Overview

       +------------+                  +-----------+
       |     AS     |                  |    KDC    |
       |            |        .-------->|           |
       +------------+       /          +-----------+
             ^             /
             |            /
             v           /                           +-----------+
       +------------+   /      +------------+        |+-----------+
       |   Client   |<-'       | Dispatcher |        ||+-----------+
       |            |<-------->|    (RS)    |<------->||   Group   |
       +------------+          +------------+         +|  members  |
                                                       +-----------+

                  Figure 1: Key Distribution Participants

   The following participants (see Figure 1) take part in the
   authorization and key distribution.

   o  Client (C): node that wants to join the group communication.  It
      can request write and/or read rights.
<mglt>
I am wondering if one can have write access without having read access.
It seems to me - but I might be wrong - that write access provides read
access. If that is correct, I am wondering if we could have read or
write.

The figure seems to give two roles of the Client: 1) the ability to join
the group with some interactions with KDC as well as 2) the ability to
send message with some interactions with the Dispatcher. I am wondering
if there are any reasons the latest is not mentioned.
</mglt>

   o  Authorization Server (AS): same as AS in the ACE Framework; it
      enforces access policies, and knows if a node is allowed to join
      the group with write and/or read rights.

   o  Key Distribution Center (KDC): maintains the keying material to
      protect group communications, and provides it to Clients
      authorized to join the group.  During the first part of the
      exchange (Section 3), it takes the role of the RS in the ACE
      Framework.  During the second part (Section 4), which is not based
      on the ACE Framework, it distributes the keying material.  In
      addition, it provides the latest keying material to group members
      when requested.  If required by the application, the KDC renews

Palombini & Tiloca       Expires January 6, 2020                [Page 4]

Internet-Draft  Key Provisioning for Group Communication       July 2019

      and re-distributes the keying material in the group when
      membership changes.

   o  Dispatcher: entity through which the Clients communicate with the
      group and which distributes messages to the group members.
      Examples of dispatchers are: the Broker node in a pub-sub setting;
      a relayer node for group communication that delivers group
      messages as multiple unicast messages to all group members; an
      implicit entity as in a multicast communication setting, where
      messages are transmitted to a multicast IP address and delivered
      on the transport channel.

   This document specifies the message flows and formats for:

   o  Authorizing a new node to join the group (Section 3), and
      providing it with the group keying material to communicate with
      the other group members (Section 4).

   o  Removing of a current member from the group (Section 5).

   o  Retrieving keying material as a current group member (Section 6
      and Section 7).

   o  Renewing and re-distributing the group keying material (rekeying)
      upon a membership change in the group (Section 4.2 and Section 5).

   Figure 2 provides a high level overview of the message flow for a
   node joining a group communication setting.

  C                              AS     KDC   Dispatcher          Group
  |                              |       |        |               Member
  |                              |       |        | \               |
  |     Authorization Request    |       |        | | Defined       |
  |----------------------------->|       |        | | in the ACE    |
  |                              |       |        | | framework     |
  |     Authorization Response   |       |        | |               |
  |<-----------------------------|       |        | |               |
  |                              |       |        | |               |
  |--------- Token Post ---------------->|        | /               |
  |                                      |        |                 |
  |---- Key Distribution Request ------->|        |                 |
  |                                      |        |                 |
  |<--- Key Distribution Response ------ | --- Group Rekeying ----->|
  |                                               |                 |
  |<================== Protected communication ===|================>|
  |                                               |                 |

              Figure 2: Message Flow Upon New Node's Joining
<mglt>
"Defined in the ACE framework" : it is unclear to me if it designates
the DIspatcher of the Authorization and Token Post exchanges. I think
that is the latest.

I understand the Dispatcher is not involved in the distribution of the
keying material, but only to the distribution of the messages. If that
is correct, it does not appear clearly on the figure, maybe placing
Dispatcher and Group Members below the Group Keying arrow would clarify
this.

I think Protected communication is not in the scope of this draft, so it
might specified as well as it is being done for the ACE framework - if
that is correct.

Protected communication is usually interpreted as end-to-end security
and this may not be achieved with group communication. Maybe secure
group communication might be more appropriated.
</mglt>

Palombini & Tiloca       Expires January 6, 2020                [Page 5]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   The exchange of Authorization Request and Authorization Response
   between Client and AS MUST be secured, as specified by the transport
   profile of ACE used between Client and KDC.

   The exchange of Key Distribution Request and Key Distribution
   Response between Client and KDC MUST be secured, as a result of the
   transport profile of ACE used between Client and KDC.

   All further communications between the Client and the KDC MUST be
   secured, for instance with the same security mechanism used for the
   Key Distribution exchange.

   All communications between a Client and the other group members MUST
   be secured using the keying material provided in Section 4.

3.  Authorization to Join a Group

   This section describes in detail the format of messages exchanged by
   the participants when a node requests access to a group.  The first
   part of the exchange is based on ACE [I-D.ietf-ace-oauth-authz].

   As defined in [I-D.ietf-ace-oauth-authz], the Client requests from
   the AS an authorization to join the group through the KDC (see
   Section 3.1).  If the request is approved and authorization is
   granted, the AS provides the Client with a proof-of-possession access
   token and parameters to securely communicate with the KDC (see
   Section 3.2).  Communications between the Client and the AS MUST be
   secured, according to the transport profile of ACE used.  The
   Content-Format used in the messages is the one specified by the used
   transport profile of ACE (e.g. application/ace+cbor for the first two
   messages and application/cwt for the third message, depending on the
   format of the access token).

   Figure 3 gives an overview of the exchange described above.

         Client                                            AS  KDC
            |                                               |   |
            |---- Authorization Request: POST /token ------>|   |
            |                                               |   |
            |<--- Authorization Response: 2.01 (Created) ---|   |
            |                                               |   |
            |----- POST Token: POST /authz-info --------------->|
            |                                                   |

               Figure 3: Message Flow of Join Authorization

Palombini & Tiloca       Expires January 6, 2020                [Page 6]

Internet-Draft  Key Provisioning for Group Communication       July 2019

3.1.  Authorization Request

   The Authorization Request sent from the Client to the AS is as
   defined in Section 5.6.1 of [I-D.ietf-ace-oauth-authz] and MUST
   contain the following parameters:

   o  'grant_type', with value "client_credentials".

   Additionally, the Authorization Request MAY contain the following
   parameters, which, if included, MUST have the corresponding values:

<mglt>
I believe the value is understood as the value in a dictionary (key,
value) - rather then the value of the value itself.

I suggest to provide explicitly how the AS behaves if the value format
is not recognized or if the provided value is unknown to the AS. More
specifically, is the parameter ignored or the exchange aborted.
</mglt>

   o  'scope', containing the identifier of the specific group (or topic
      in the case of pub-sub) that the Client wishes to access, and
      optionally the role(s) that the Client wishes to take.  This value
      is a CBOR array encoded as a byte string, which contains:

      *  As first element, the identifier of the specific group or
         topic.

      *  Optionally, as second element, the role (or CBOR array of
         roles) the Client wishes to take in the group.

      The encoding of the group or topic identifier and of the role
      identifiers is application specific.

   o  'audience', with an identifier of a KDC.

   o  'req_cnf', as defined in Section 3.1 of
      [I-D.ietf-ace-oauth-params], optionally containing the public key
      or a reference to the public key of the Client, if it wishes to
      communicate that to the AS.

   o  Other additional parameters as defined in
      [I-D.ietf-ace-oauth-authz], if necessary.

3.2.  Authorization Response

   The Authorization Response sent from the AS to the Client is as
   defined in Section 5.6.2 of [I-D.ietf-ace-oauth-authz] and MUST
   contain the following parameters:

   o  'access_token', containing the proof-of-possession access token.

   o  'cnf' if symmetric keys are used, not present if asymmetric keys
      are used.  This parameter is defined in Section 3.2 of
      [I-D.ietf-ace-oauth-params] and contains the symmetric proof-of-
      possession key that the Client is supposed to use with the KDC.

Palombini & Tiloca       Expires January 6, 2020                [Page 7]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   o  'rs_cnf' if asymmetric keys are used, not present if symmetric
      keys are used.  This parameter is as defined in Section 3.2 of
      [I-D.ietf-ace-oauth-params] and contains information about the
      public key of the KDC.

   o  'exp', contains the lifetime in seconds of the access token.  This
      parameter MAY be omitted if the application defines how the
      expiration time is communicated to the Client via other means, or
      if it establishes a default value.

   Additionally, the Authorization Response MAY contain the following
   parameters, which, if included, MUST have the corresponding values:

<mglt>
Same comment as above regarding the MUST statement.
</mglt>

   o  'scope', which mirrors the 'scope' parameter in the Authorization
      Request (see Section 3.1).  Its value is a CBOR array encoded as a
      byte string, containing:

      *  As first element, the identifier of the specific group or topic
         the Client is authorized to access.

      *  Optionally, as second element, the role (or CBOR array of
         roles) the Client is authorized to take in the group.

      The encoding of the group or topic identifier and of the role
      identifiers is application specific.

   o  Other additional parameters as defined in
      [I-D.ietf-ace-oauth-authz], if necessary.

   The access token MUST contain all the parameters defined above
   (including the same 'scope' as in this message, if present, or the
   'scope' of the Authorization Request otherwise), and additionally
   other optional parameters that the transport profile of ACE requires.

   When receiving an Authorization Request from a Client that was
   previously authorized, and which still owns a valid non expired
   access token, the AS replies with an Authorization Response with a
   new access token.

3.3.  Token Post

   The Client sends a CoAP POST request including the access token to
   the KDC, as specified in Section 5.8.1 of [I-D.ietf-ace-oauth-authz].
   If the specific transport profile of ACE defines it, the Client MAY
   use a different endpoint than /authz-info at the KDC to post the
   access token to.

Palombini & Tiloca       Expires January 6, 2020                [Page 8]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   Optionally, the Client might need to request necessary information
   concerning the public keys in the group, as well as concerning the
   algorithm and related parameters for computing signatures in the
   group.  In such a case, the joining node MAY ask for that information
   to the KDC in this same request.  To this end, it sends the CoAP POST
   request to the /authz-info endpoint using the Content-Format
   "application/ace+cbor" defined in Section 8.14 of
   [I-D.ietf-ace-oauth-authz], and includes also the following
   parameters:

<mglt>
That the Client MAY ask for this information suggests that this is not
necessary. It would maybe worth indicating when these information are not
requested. I see two cases 1) when public key is not involved, and 2)
when this is pre-configured or provided by other means.

I am not sure these cases make senses, but in the latest cases, it would
be good to specify why we do not end up in a dead lock.

Public key is associated with authentication. I understand that the
authentication is necessary to the RS rather than the group members. If
that is correct, I believe that specifying the entity associated to the
public key - here the RS - would be clearer.

</mglt>

   o  'sign_info' defined in Section 3.3.1, encoding the CBOR simple
      value Null, to require information and parameters on the signature
      algorithm and on the public keys used in the group.

   o  'pub_key_enc' defined in Section 3.3.2, encoding the CBOR simple
      value Null, to require information on the exact encoding of public
      keys used in the group.

   The CDDL notation of the 'sign_info' and 'pub_key_enc' parameters
   formatted as in the request is given below.

      sign_info_req = nil

      pub_key_enc_req = nil

   Alternatively, the joining node may retrieve this information by
   other means.

   After successful verification, the Client is authorized to receive
   the group keying material from the KDC and join the group.  In
   particular, the KDC replies to the Client with a 2.01 (Created)
   response, using Content-Format "application/ace+cbor" defined in
   Section 8.14 of [I-D.ietf-ace-oauth-authz].

   The payload of the 2.01 response is a CBOR map, which MUST include a
   nonce N generated by the KDC.  The Client may use this nonce for
   proving the possession of its own private key (see the
   'client_cred_verify' parameter in Section 4).

   Optionally, if they were included in the request, the AS MAY include
   the 'sign_info' parameter as well as the 'pub_key_enc' parameter
   defined in Section 3.3.1 and Section 3.3.2 of this specification,
   respectively.

   The 'sign_info' parameter MUST be present if the POST request
   included the 'sign_info' parameter with value Null.  If present, the
   'sign_info' parameter of the 2.01 (Created) response is a CBOR array
   formatted as follows.

Palombini & Tiloca       Expires January 6, 2020                [Page 9]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   o  The first element 'sign_alg' is an integer or a text string,
      indicating the signature algorithm used in the group.  It is
      required of the application profiles to define specific values for
      this parameter.

   o  The second element 'sign_parameters' indicates the parameters of
      the signature algorithm.  Its structure depends on the value of
      'sign_alg'.  It is required of the application profiles to define
      specific values for this parameter.  If no parameters of the
      signature algorithm are specified, 'sign_parameters' MUST be
      encoding the CBOR simple value Null.

   o  The third element 'sign_key_parameters' indicates the parameters
      of the key used with the signature algorithm.  Its structure
      depends on the value of 'sign_alg'.  It is required of the
      application profiles to define specific values for this parameter.
      If no parameters of the key used with the signature algorithm are
      specified, 'sign_key_parameters' MUST be encoding the CBOR simple
      value Null.

   The 'pub_key_enc' parameter MUST be present if the POST request
   included the 'pub_key_enc' parameter with value Null.  If present,
   the 'pub_key_enc' parameter of the 2.01 (Created) response is a CBOR
   integer, indicating the encoding of public keys used in the group.
   The values of this field are registered in the "ACE Public Key
   Encoding" Registry, defined in Section 11.2.  It is required of the
   application profiles to define specific values to use for this
   parameter.

   The CDDL notation of the 'sign_info' and 'pub_key_enc' parameters
   formatted as in the response is given below.

      sign_info_res = [
        sign_alg : int / tstr,
        sign_parameters : any / nil,
        sign_key_parameters : any / nil
      ]

      pub_key_enc_res = int

   Note that the CBOR map specified as payload of the 2.01 (Created)
   response may include further parameters, e.g. according to the
   signalled transport profile of ACE.

   Note that this step could be merged with the following message from
   the Client to the KDC, namely Key Distribution Request.

Palombini & Tiloca       Expires January 6, 2020               [Page 10]

Internet-Draft  Key Provisioning for Group Communication       July 2019

3.3.1.  'sign_info' Parameter

   The 'sign_info' parameter is an OPTIONAL parameter of the AS Request
   Creation Hints message defined in Section 5.1.2. of
   [I-D.ietf-ace-oauth-authz].  This parameter contains information and
   parameters about the signature algorithm and the public keys to be
   used between the Client and the RS.  Its exact content is application
   specific.

3.3.2.  'pub_key_enc' Parameter

   The 'pub_key_enc' parameter is an OPTIONAL parameter of the AS
   Request Creation Hints message defined in Section 5.1.2. of
   [I-D.ietf-ace-oauth-authz].  This parameter contains information
   about the exact encoding of public keys to be used between the Client
   and the RS.  Its exact content is application specific.

4.  Key Distribution

   This section defines how the keying material used for group
   communication is distributed from the KDC to the Client, when joining
   the group as a new member.

   If not previously established, the Client and the KDC MUST first
   establish a pairwise secure communication channel using ACE.

<mglt>
I have the impression that such agreement is performed during the
registration request / resopnse with the AS. Is that correct or are we
considering another exchange ?
</mglt>

The
   exchange of Key Distribution Request-Response MUST occur over that
   secure channel.  The Client and the KDC MAY use that same secure
   channel to protect further pairwise communications, that MUST be
   secured.

   During this exchange, the Client sends a request to the AS,
   specifying the group it wishes to join (see Section 4.1).  Then, the
   KDC verifies the access token and that the Client is authorized to
   join that group; if so, it provides the Client with the keying
   material to securely communicate with the member of the group (see
   Section 4.2).  The Content-Format used in the messages is set to
   application/cbor.

<mglt>
I believe the paragraph above describes how the secure channel is
established between the Client and the KDC. If that is correct that may
be mention explicitly. I would suggest to replace "During this exchange"
by "During the Authorization request/respons e exchange".

</mglt>

   Figure 4 gives an overview of the exchange described above.

         Client                                               KDC
            |                                                  |
            |---- Key Distribution Request: POST /group-id --->|
            |                                                  |
            |<--- Key Distribution Response: 2.01 (Created) ---|
            |                                                  |

     Figure 4: Message Flow of Key Distribution to a New Group Member

Palombini & Tiloca       Expires January 6, 2020               [Page 11]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   The same set of message can also be used for the following cases,
   when the Client is already a group member:

   o  The Client wishes to (re-)get the current keying material, for
      cases such as expiration, loss or suspected mismatch, due to e.g.
      reboot or missed group rekeying.  This is further discussed in
      Section 6.

   o  The Client wishes to (re-)get the public keys of other group
      members, e.g. if it is aware of new nodes joining the group after
      itself.  This is further discussed in Section 7.

   Additionally, the format of the payload of the Key Distribution
   Response (Section 4.2) can be reused for messages sent by the KDC to
   distribute updated group keying material, in case of a new node
   joining the group or of a current member leaving the group.  The key
   management scheme used to send such messages could rely on, e.g.,
   multicast in case of a new node joining or unicast in case of a node
   leaving the group.

   Note that proof-of-possession to bind the access token to the Client
   is performed by using the proof-of-possession key bound to the access
   token for establishing secure communication between the Client and
   the KDC.

   If the application requires backward security, the KDC SHALL generate
   new group keying material and securely distribute it to all the
   current group members, using the message format defined in this
   section.  Application profiles may define alternative message
   formats.

<mglt>
I am wondering why SHALL is introduced here as MUST has been used
throughout the document. I do not see any difference between SHALL and
MUST, so if that is correct, I would recommend to use MUST throughout
the document.
</mglt>

4.1.  Key Distribution Request

   The Client sends a Key Distribution Request to the KDC.  This
   corresponds to a CoAP POST request to the endpoint in the KDC
   associated to the group to join.  The endpoint in the KDC is
   associated to the 'scope' value of the Authorization Request/
   Response.  The payload of this request is a CBOR map which MUST
   contain the following fields:

   o  'type', encoded as a CBOR int, with value 1 ("key distribution").

   Additionally, the CBOR map in the payload MAY contain the following
   fields, which, if included, MUST have the corresponding values:

   o  'scope', with value the specific resource that the Client is
      authorized to access (i.e. group or topic identifier) and role(s),
      encoded as in Section 3.1.

Palombini & Tiloca       Expires January 6, 2020               [Page 12]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   o  'get_pub_keys', if the Client wishes to receive the public keys of
      the other nodes in the group from the KDC.  The value is an empty
      CBOR array.  This parameter may be present if the KDC stores the
      public keys of the nodes in the group and distributes them to the
      Client; it is useless to have here if the set of public keys of
      the members of the group is known in another way, e.g. it was
      provided by the AS.

<mglt>
For large groups, there might be a need to limit the size of the
response and so to enable that the batch of keys be retrieved in
multiple exchanges or via another redirection mechanism.
</mglt>

   o  'client_cred', with value the public key or certificate of the
      Client, encoded as a CBOR byte string.  If the KDC is managing
      (collecting from/distributing to the Client) the public keys of
      the group members, this field contains the public key of the
      Client.  The default encoding for public keys is COSE Keys.
      Alternative specific encodings of this parameter MAY be defined in
      applications of this specification.

   o  'client_cred_verify', encoded as a CBOR byte string.  This
      parameter contains a signature computed by the Client over the
      nonce N received from the KDC in the 2.01 (Created) response to
      the token POST request (see Section 3.3).  The Client computes the
      signature by using its own private key, whose corresponding public
      key is either directly specified in the 'client_cred' parameter or
      included in the certificate specified in the 'client_cred'
      parameter.  This parameter MUST be present if the 'client_cred'
      parameter is present.

   o  'pub_keys_repos', can be present if a certificate is present in
      the 'client_cred' field, with value a list of public key
      repositories storing the certificate of the Client.  This
      parameter is encoded as a CBOR array of CBOR text strings, each of
      which specifies the URI of a key repository.

4.2.  Key Distribution Response

   The KDC verifies that the 'scope' received in the Key Distribution
   Request, if present, is a subset of the 'scope' stored in the access
   token associated to this client.  If verification fails, the KDC MUST
   respond with a 4.01 (Unauthorized) error message.

   If the Key Distribution Request is not formatted correctly (e.g. no
   'scope' field present while expected, or unknown fields present), the
   KDC MUST respond with 4.00 (Bad Request) error message.

   If verification succeeds, the KDC sends a Key Distribution success
   Response to the Client.  The Key Distribution success Response
   corresponds to a 2.01 Created message.  The payload of this response
   is a CBOR map, which MUST contain:

Palombini & Tiloca       Expires January 6, 2020               [Page 13]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   o  'kty', identifying the key type of the 'key' parameter.  The set
      of values can be found in the "Key Type" column of the "ACE
      Groupcomm Key" Registry.  Implementations MUST verify that the key
      type matches the application profile being used, if present, as
      registered in the "ACE Groupcomm Key" registry.

   o  'key', containing the keying material for the group communication,
      or information required to derive it.

   The exact format of the 'key' value MUST be defined in applications
   of this specification.  Additionally, documents specifying the key
   format MUST register it in the "ACE Groupcomm Key" registry,
   including its name, type and application profile to be used with, as
   defined in the "ACE Groupcomm Key" registry, defined in Section 11.5.

     +----------+----------------+---------+-------------------------+
     | Name     | Key Type Value | Profile | Description             |
     +----------+----------------+---------+-------------------------+
     | Reserved | 0              |         | This value is reserved  |
     +----------+----------------+---------+-------------------------+

                         Figure 5: Key Type Values

   Optionally, the Key Distribution Response MAY contain the following
   parameters, which, if included, MUST have the corresponding values:

   o  'profile', with value a CBOR integer that MUST be used to uniquely
      identify the application profile for group communication.  The
      value MUST be registered in the "ACE Groupcomm Profile" Registry.

   o  'exp', with value the expiration time of the keying material for
      the group communication, encoded as a CBOR unsigned integer or
      floating-point number.  This field contains a numeric value
      representing the number of seconds from 1970-01-01T00:00:00Z UTC
      until the specified UTC date/time, ignoring leap seconds,
      analogous to what specified in Section 2 of [RFC7519].

   o  'pub_keys', may only be present if 'get_pub_keys' was present in
      the Key Distribution Request.  This parameter is a CBOR byte
      string, which encodes the public keys of all the group members
      paired with the respective member identifiers.  The default
      encoding for public keys is COSE Keys, so the default encoding for
      'pub_keys' is a CBOR byte string wrapping a COSE_KeySet (see
      [RFC8152]), which contains the public keys of all the members of
      the group.  In particular, each COSE Key in the COSE_KeySet
      includes the identifier of the corresponding group member as value
      of its 'kid' key parameter.  Alternative specific encodings of

Palombini & Tiloca       Expires January 6, 2020               [Page 14]

Internet-Draft  Key Provisioning for Group Communication       July 2019

      this parameter MAY be defined in applications of this
      specification.

   o  'group_policies', with value a CBOR map, whose entries specify how
      the group handles specific management aspects.  These include, for
      instance, approaches to achieve synchronization of sequence
      numbers among group members.  The elements of this field are
      registered in the "ACE Groupcomm Policy" Registry.  This
      specification defines the two elements "Sequence Number
      Synchronization Method" and "Key Update Check Interval", which are
      summarized in Figure 6.  Application profiles that build on this
      document MUST specify the exact content format of included map
      entries.

+-----------------+-------+----------|--------------------|------------+
|      Name       | CBOR  |   CBOR   |    Description     | Reference  |
|                 | label |   type   |                    |            |
|-----------------+-------+----------|--------------------|------------|
| Sequence Number | TBD1  | tstr/int | Method for a re-   | [[this     |
| Synchronization |       |          | cipient node to    | document]] |
| Method          |       |          | synchronize with   |            |
|                 |       |          | sequence numbers   |            |
|                 |       |          | of a sender node.  |            |
|                 |       |          | Its value is taken |            |
|                 |       |          | from the 'Value'   |            |
|                 |       |          | column of the      |            |
|                 |       |          | Sequence Number    |            |
|                 |       |          | Synchronization    |            |
|                 |       |          | Method registry    |            |
|                 |       |          |                    |            |
| Key Update      | TBD2  |   int    | Polling interval   | [[this     |
| Check Interval  |       |          | in seconds, to     | document]] |
|                 |       |          | check for new      |            |
|                 |       |          | keying material at |            |
|                 |       |          | the KDC            |            |
+-----------------+-------+----------|--------------------|------------+

                     Figure 6: ACE Groupcomm Policies

   o  'mgt_key_material', encoded as a CBOR byte string and containing
      the administrative keying material to participate in the group
      rekeying performed by the KDC.  The exact format and content
      depend on the specific rekeying scheme used in the group, which
      may be specified in the application profile.

   Specific application profiles that build on this document need to
   specify how exactly the keying material is used to protect the group
   communication.

Palombini & Tiloca       Expires January 6, 2020               [Page 15]

Internet-Draft  Key Provisioning for Group Communication       July 2019

5.  Removal of a Node from the Group

   This section describes at a high level how a node can be removed from
   the group.

   If the application requires forward security, the KDC SHALL generate
   new group keying material and securely distribute it to all the
   current group members but the leaving node, using the message format
   defined in Section 4.2.  Application profiles may define alternative
   message formats.

5.1.  Expired Authorization

   If the AS provides Token introspection (see Section 5.7 of
   [I-D.ietf-ace-oauth-authz]), the KDC can optionally use and check
   whether:

   o  the node is not authorized anymore;

   o  the access token is still valid, upon its expiration.

   Either case, once aware that a node is not authorized anymore, the
   KDC has to remove the unauthorized node from the list of group
   members, if the KDC keeps track of that.

5.2.  Request to Leave the Group

   A node can actively request to leave the group.  In this case, the
   Client can send a request formatted as follows to the KDC, to abandon
   the group.  The client MUST use the protected channel established
   with ACE, mentioned in Section 4.

   To request to leave a group, the client MUST send a CoAP POST request
   to the endpoint in the KDC associated to the group to leave (same
   endpoint used in Section 4.1 for Key Distribution requests).  The
   payload of this Leave Request is a CBOR map which MUST contain:

   o  'type', encoded as a CBOR int, with value 2 ("leave").

   o  'scope', with value the specific resource that the Client is
      authorized to access (i.e. group or topic identifier) and wants to
      leave, encoded as in Section 3.1.  The 'role' field is omitted.

   Note that the 'role' field is omitted since such a request should
   only be used to leave a group altogether.  If the leaving node wants
   to be part of a group with fewer roles, it does not need to
   communicate that to the KDC, and can simply stop acting according to
   such roles.

Palombini & Tiloca       Expires January 6, 2020               [Page 16]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   If the Leave Request is such that the KDC cannot extract all the
   necessary information to understand and process it correctly (e.g. no
   'scope' field present), the KDC MUST respond with a 4.00 (Bad
   Request) error message.  Otherwise, the KDC MUST remove the leaving
   node from the list of group members, if the KDC keeps track of that.

   Note that, after having left the group, a node may wish to join it
   again.  Then, as long as the node is still authorized to join the
   group, i.e. it has a still valid access token, it can re-request to
   join the group directly to the KDC without needing to retrieve a new
   access token from the AS.  This means that the KDC needs to keep
   track of nodes with valid access tokens, before deleting all
   information about the leaving node.

6.  Retrieval of New or Updated Keying Material

   A node stops using the group keying material upon its expiration,
   according to the 'exp' parameter specified in the retained COSE Key.
   Then, if it wants to continue participating in the group
   communication, the node has to request new updated keying material to
   the KDC.  In this case, and depending on what part of the keying
   material is expired, the client may need to communicate to the KDC
   its need for that part to be renewed: for example, if the Client uses
   an individual key to protect outgoing traffic and has to renew it,
   the node may request a new one, or new input material to derive it,
   without renewing the whole group keying material.

   The Client may perform the same request to the KDC also upon
   receiving messages from other group members without being able to
   retrieve the material to correctly decrypt them.  This may be due to
   a previous update of the group keying material (rekeying) triggered
   by the KDC, that the Client was not able to receive or decrypt.

   Note that policies can be set up so that the Client sends a request
   to the KDC only after a given number of unsuccessfully decrypted
   incoming messages.  It is application dependent and pertaining to the
   particular message exchange (e.g.  [I-D.ietf-core-oscore-groupcomm])
   to set up policies that instruct clients to retain unsuccessfully
   decrypted messages and for how long, so that they can be decrypted
   after getting updated keying material, rather than just considered
   non valid messages to discard right away.

   The same request could also be sent by the client without being
   triggered by a failed decryption of a message, if the client wants to
   confirm that it has the latest group keying material.  If that is the
   case, the client will receive from the KDC the same group keying
   material it has in memory.

Palombini & Tiloca       Expires January 6, 2020               [Page 17]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   Note that the difference between the keying material renewal request
   and the keying material update request is that the first one triggers
   the KDC to produce new keying material for that node, while the
   second one only triggers distribution (the renewal might have
   happened independently, because of expiration).  Once a node receives
   new individual keying material, other group members may need to use
   the update keying material request to retrieve it.

   Alternatively, the re-distribution of keying material can be
   initiated by the KDC, which e.g.:

   o  Can maintain an Observable resource to send notifications to
      Clients when the keying material is updated.  Such a notification
      would have the same payload as the Key Re-Distribution Response
      defined in Section 6.2.

   o  Can send the payload of the Key Re-Distribution Response as one or
      multiple multicast requests to the members of the group, using
      secure rekeying schemes such as [RFC2093][RFC2094][RFC2627].

   o  Can send unicast requests to each Client over a secure channel,
      with the Key Re-Distribution Response as payload.

   o  Can act as a publisher in a pub-sub scenario, and update the
      keying material by publishing on a specific topic on a broker,
      which all the members of the group are subscribed to.

   Note that these methods of KDC-initiated key re-distribution have
   different security properties and require different security
   associations.

6.1.  Key Re-Distribution Request

   To request a re-distribution of keying material, the Client sends a
   shortened Key Distribution Request to the KDC (Section 4.1),
   formatted as follows.  The payload MUST contain the following fields:

   o  'type', encoded as a CBOR int, with value 3 ("update key") if the
      request is intended to retrieve updated group keying material, and
      4 ("new") if the request is intended for the KDC to produce and
      provide new individual keying material for the Client.

   o  'scope', which contains only the identifier of the specific group
      or topic, encoded as in Section 3.1.  That is, the role field is
      not present.

Palombini & Tiloca       Expires January 6, 2020               [Page 18]

Internet-Draft  Key Provisioning for Group Communication       July 2019

6.2.  Key Re-Distribution Response

   The KDC receiving a Key Re-Distribution Request MUST check that it is
   storing a valid access token from that client for that scope.

   If that is not the case, i.e. it does not store the token or the
   token is not valid for that client for the scope requested, the KDC
   MUST respond with a 4.01 (Unauthorized) error message.  Analogously
   to Section 4.2, if the Key Re-Distribution Request is not formatted
   correctly (e.g. no 'scope' field present, or unknown fields present),
   the KDC MUST respond with a 4.00 (Bad Request) error message.

   Otherwise, the KDC replies to the Client with a Key Distribution
   Response, which MUST include the 'kty', 'key' and 'exp' parameters
   specified in Section 4.2.  The Key Distribution Response MAY also
   include the 'profile', 'group_policies' and 'mgt_key_material'
   parameters specified in Section 4.2.

   Note that this response might simply re-provide the same keying
   material currently owned by the Client, if it has not been renewed.

7.  Retrieval of Public Keys for Group Members

   In case the KDC maintains the public keys of group members, a node in
   the group can contact the KDC to request public keys of either all
   group members or a specified subset, using the messages defined
   below.

   Figure 7 gives an overview of the exchange described above.

            Client                                         KDC
               |                                            |
               |---- Public Key Request: POST /group-id --->|
               |                                            |
               |<--- Public Key Response: 2.01 (Created) ---|
               |                                            |

           Figure 7: Message Flow of Public Key Request-Response

   Note that these messages can be combined with the Key Re-Distribution
   messages in Section 6, to request at the same time the keying
   material and the public keys.  In this case, either a new endpoint at
   the KDC may be used, or additional information needs to be sent in
   the request payload, to distinguish these combined messages from the
   Public Key messages described below, since they would be identical
   otherwise.

<mglt>
I am wondering why public key is needed in a context of group
communication. I do not expect group members but only by the RS.

It is unclear to me how the
</mglt>

Palombini & Tiloca       Expires January 6, 2020               [Page 19]

Internet-Draft  Key Provisioning for Group Communication       July 2019

7.1.  Public Key Request

   To request public keys, the Client sends a shortened Key Distribution
   Request to the KDC (Section 4.1), formatted as follows.  The payload
   of this request MUST contain the following fields:

   o  'type', encoded as a CBOR int, with value 5 ("pub keys").

   o  'get_pub_keys', which has as value a CBOR array including either:

      *  no elements, i.e. an empty array, in order to request the
         public key of all current group members; or

      *  N elements, each of which is the identifier of a group member
         encoded as a CBOR byte string, in order to request the public
         key of the specified nodes.

   o  'scope', which contains only the identifier of the specific group
      or topic, encoded as in Section 3.1.  That is, the role field is
      not present.

7.2.  Public Key Response

   The KDC replies to the Client with a Key Distribution Response
   containing only the 'pub_keys' parameter, as specified in
   Section 4.2.  The payload of this response contains the following
   field:

   o  'pub_keys', which contains either:

      *  the public keys of all the members of the group, if the
         'get_pub_keys' parameter of the Public Key request was an empty
         array; or

      *  the public keys of the group members with the identifiers
         specified in the 'get_pub_keys' parameter of the Public Key
         request.

   The KDC may enforce one of the following policies, in order to handle
   possible identifiers that are included in the 'get_pub_keys'
   parameter of the Public Key request but are not associated to any
   current group member.

   o  The KDC silently ignores those identifiers.

   o  The KDC retains public keys of group members for a given amount of
      time after their leaving, before discarding them.  As long as such

Palombini & Tiloca       Expires January 6, 2020               [Page 20]

Internet-Draft  Key Provisioning for Group Communication       July 2019

      public keys are retained, the KDC provides them to a requesting
      Client.

   Either case, a node that has left the group should not expect any of
   its outgoing messages to be successfully processed, if received after
   its leaving, due to a possible group rekeying occurred before the
   message reception.

8.  ACE Groupcomm Parameters

   This specification defines a number of fields used during the message
   exchange.  The table below summarizes them, and specifies the CBOR
   key to use instead of the full descriptive name.

Palombini & Tiloca       Expires January 6, 2020               [Page 21]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   +--------------+----------+---------------+
   | Name         | CBOR Key | CBOR Type     |
   +--------------+----------+---------------+
   | scope        |   TBD    | array         |
   +--------------+----------+---------------+
   | get_pub_keys |   TBD    | array         |
   +--------------+----------+---------------+
   | client_cred  |   TBD    | byte string   |
   +--------------+----------+---------------+
   | client_cred_ |   TBD    | byte string   |
   | verify       |          |               |
   +--------------+----------+---------------+
   | pub_keys_    |   TBD    | array         |
   | repos        |          |               |
   +--------------+----------+---------------+
   | kty          |   TBD    | int / byte    |
   |              |          | string        |
   +--------------+----------+---------------+
   | key          |   TBD    | see "ACE      |
   |              |          | Groupcomm     |
   |              |          | Key" Registry |
   +--------------+----------+---------------+
   | profile      |   TBD    | int           |
   +--------------+----------+---------------+
   | exp          |   TBD    | int / float   |
   +--------------+----------+---------------+
   | pub_keys     |   TBD    | byte string   |
   +--------------+----------+---------------+
   | group_       |   TBD    | map           |
   | policies     |          |               |
   +--------------+----------+---------------+
   | mgt_key_     |   TBD    | byte string   |
   | material     |          |               |
   +--------------+----------+---------------+
   | type         |   TBD    | int           |
   +--------------+----------+---------------+

9.  ACE Groupcomm Request Type

   This specification defines a number of types of requests.  The table
   below summarizes them.

Palombini & Tiloca       Expires January 6, 2020               [Page 22]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   +------------------+----------+
   |     Name         |  Value   |
   +------------------+----------+
   | key distribution |    1     |
   +------------------+----------+
   | leave            |    2     |
   +------------------+----------+
   | update key       |    3     |
   +------------------+----------+
   | new              |    4     |
   +------------------+----------+
   | pub keys         |    5     |
   +------------------+----------+

10.  Security Considerations

   When a Client receives a message from a sender for the first time, it
   needs to have a mechanism in place to avoid replay, e.g.
   Appendix B.2 of [I-D.ietf-core-object-security].

   The KDC must renew the group keying material upon its expiration.

   The KDC should renew the keying material upon group membership
   change, and should provide it to the current group members through
   the rekeying scheme used in the group.

   The KDC may enforce a rekeying policy that takes into account the
   overall time required to rekey the group, as well as the expected
   rate of changes in the group membership.

   That is, the KDC may not rekey the group at every membership change,
   for instance if members' joining and leaving occur frequently and
   performing a group rekeying takes too long.  Instead, the KDC may
   rekey the group after a minum number of group members have joined or
   left within a given time interval, or during predictable network
   inactivity periods.

   However, this would result in the KDC not constantly preserving
   backward and forward security.  In fact, newly joining group members
   could be able to access the keying material used before their
   joining, and thus could access past group communications.  Also,
   until the KDC performs a group rekeying, the newly leaving nodes
   would still be able to access upcoming group communications that are
   protected with the keying material that has not yet been updated.

Palombini & Tiloca       Expires January 6, 2020               [Page 23]

Internet-Draft  Key Provisioning for Group Communication       July 2019

10.1.  Update of Keying Material

   A group member can receive a message shortly after the group has been
   rekeyed, and new keying material has been distributed by the KDC.  In
   the following two cases, this may result in misaligned keying
   material between the group members.

   In the first case, the sender protects a message using the old keying
   material.  However, the recipient receives the message after having
   received the new keying material, hence not being able to correctly
   process it.  A possible way to ameliorate this issue is to preserve
   the old, recent, keying material 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 keying material
   as second attempt.  Note that a former (compromised) group member can
   take advantage of this by sending messages protected with the old
   retained keying material.  Therefore, a conservative application
   policy should not admit the storage of old keying material.

   In the second case, the sender protects a message using the new
   keying material, but the recipient receives that request before
   having received the new keying material.  Therefore, the recipient
   would not be able to correctly process the request and hence discards
   it.  If the recipient receives the new keying material 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 KDC for an updated
   keying material according to an application-defined policy, for
   instance after a given number of unsuccessfully decrypted incoming
   messages.

10.2.  Block-Wise Considerations

   If the block-wise options [RFC7959] are used, and the keying material
   is updated in the middle of a block-wise transfer, the sender of the
   blocks just changes the keying material to the updated one and
   continues the transfer.  As long as both sides get the new keying
   material, updating the keying material in the middle of a transfer
   will not cause any issue.  Otherwise, the sender will have to
   transmit the message again, when receiving an error message from the
   recipient.

   Compared to a scenario where the transfer does not use block-wise,
   depending on how fast the keying material is changed, the nodes might
   consume a larger amount of the network resending the blocks again and
   again, which might be problematic.

Palombini & Tiloca       Expires January 6, 2020               [Page 24]

Internet-Draft  Key Provisioning for Group Communication       July 2019

11.  IANA Considerations

   This document has the following actions for IANA.

11.1.  ACE Authorization Server Request Creation Hints Registry

   IANA is asked to register the following entries in the "ACE
   Authorization Server Request Creation Hints" Registry defined in
   Section 8.1 of [I-D.ietf-ace-oauth-authz].

   o  Name: sign_info

   o  CBOR Key: TBD (range -256 to 255)

   o  Value Type: any

   o  Reference: [[This specification]]

   o  Name: pub_key_enc

   o  CBOR Key: TBD (range -256 to 255)

   o  Value Type: integer

   o  Reference: [[This specification]]

11.2.  ACE Public Key Encoding Registry

   This specification establishes the "ACE Public Key Encoding" IANA
   Registry.  The Registry has been created to use the "Expert Review
   Required" registration procedure [RFC8126].  Expert review guidelines
   are provided in Section 11.9.  It should be noted that, in addition
   to the expert review, some portions of the Registry require a
   specification, potentially a Standards Track RFC, be supplied as
   well.

   The columns of this Registry are:

   o  Name: This is a descriptive name that enables easier reference to
      the item.  The name MUST be unique.  It is not used in the
      encoding.

   o  Value: The value to be used to identify this public key encoding.
      This value MUST be unique.  The value can be a positive or a
      negative integer.  Integer values between 0 and 255 are designated
      as Standards Track Document required.  Integer values from 256 to
      65535 are designated as Specification Required.  Integer values of

Palombini & Tiloca       Expires January 6, 2020               [Page 25]

Internet-Draft  Key Provisioning for Group Communication       July 2019

      greater than 65535 are designated as expert review.  Integer
      values less than -65536 are marked as private use.

   o  Description: This field contains a brief description for this
      public key encoding.

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

   The value 0 is to be marked as "Reserved".

11.3.  ACE Groupcomm Parameters Registry

   This specification establishes the "ACE Groupcomm Parameters" IANA
   Registry.  The Registry has been created to use the "Expert Review
   Required" registration procedure [RFC8126].  Expert review guidelines
   are provided in Section 11.9.

   The columns of this Registry are:

   o  Name: This is a descriptive name that enables easier reference to
      the item.  The name MUST be unique.  It is not used in the
      encoding.

   o  CBOR Key: This is the value used as CBOR key of the item.  These
      values MUST be unique.  The value can be a positive integer, a
      negative integer, or a string.

   o  CBOR Type: This contains the CBOR type of the item, or a pointer
      to the registry that defines its type, when that depends on
      another item.

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

   This Registry has been initially populated by the values in
   Section 8.  The specification column for all of these entries will be
   this document.

11.4.  Ace Groupcomm Request Type Registry

   This specification establishes the "ACE Groupcomm Request Type" IANA
   Registry.  The Registry has been created to use the "Expert Review
   Required" registration procedure [RFC8126].  Expert review guidelines
   are provided in Section 11.9.

   The columns of this Registry are:

Palombini & Tiloca       Expires January 6, 2020               [Page 26]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   o  Name: This is a descriptive name that enables easier reference to
      the item.  The name MUST be unique.  It is not used in the
      encoding.

   o  Value: This is the value used to identify the request.  These
      values MUST be unique.  The value must be a positive integer.

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

   This Registry has been initially populated by the values in
   Section 9.  The reference column for all of these entries will be
   this document.  The value 0 is to be marked as "Reserved".

11.5.  ACE Groupcomm Key Registry

   This specification establishes the "ACE Groupcomm Key" IANA Registry.
   The Registry has been created to use the "Expert Review Required"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 11.9.

   The columns of this Registry are:

   o  Name: This is a descriptive name that enables easier reference to
      the item.  The name MUST be unique.  It is not used in the
      encoding.

   o  Key Type Value: This is the value used to identify the keying
      material.  These values MUST be unique.  The value can be a
      positive integer, a negative integer, or a string.

   o  Profile: This field may contain one or more descriptive strings of
      application profiles to be used with this item.  The values should
      be taken from the Name column of the "ACE Groupcomm Profile"
      Registry.

   o  Description: This field contains a brief description of the keying
      material.

   o  References: This contains a pointer to the public specification
      for the format of the keying material, if one exists.

   This Registry has been initially populated by the values in Figure 5.
   The specification column for all of these entries will be this
   document.

Palombini & Tiloca       Expires January 6, 2020               [Page 27]

Internet-Draft  Key Provisioning for Group Communication       July 2019

11.6.  ACE Groupcomm Profile Registry

   This specification establishes the "ACE Groupcomm Profile" IANA
   Registry.  The Registry has been created to use the "Expert Review
   Required" registration procedure [RFC8126].  Expert review guidelines
   are provided in Section 11.9.  It should be noted that, in addition
   to the expert review, some portions of the Registry require a
   specification, potentially a Standards Track RFC, be supplied as
   well.

   The columns of this Registry are:

   o  Name: The name of the application profile, to be used as value of
      the profile attribute.

   o  Description: Text giving an overview of the application profile
      and the context it is developed for.

   o  CBOR Value: CBOR abbreviation for the name of this application
      profile.  Different ranges of values use different registration
      policies [RFC8126].  Integer values from -256 to 255 are
      designated as Standards Action.  Integer values from -65536 to
      -257 and from 256 to 65535 are designated as Specification
      Required.  Integer values greater than 65535 are designated as
      Expert Review.  Integer values less than -65536 are marked as
      Private Use.

   o  Reference: This contains a pointer to the public specification of
      the abbreviation for this application profile, if one exists.

11.7.  ACE Groupcomm Policy Registry

   This specification establishes the "ACE Groupcomm Policy" IANA
   Registry.  The Registry has been created to use the "Expert Review
   Required" registration procedure [RFC8126].  Expert review guidelines
   are provided in Section 11.9.  It should be noted that, in addition
   to the expert review, some portions of the Registry require a
   specification, potentially a Standards Track RFC, be supplied as
   well.

   The columns of this Registry are:

   o  Name: The name of the group communication policy.

   o  CBOR label: The value to be used to identify this group
      communication policy.  Key map labels MUST be unique.  The label
      can be a positive integer, a negative integer or a string.
      Integer values between 0 and 255 and strings of length 1 are

Palombini & Tiloca       Expires January 6, 2020               [Page 28]

Internet-Draft  Key Provisioning for Group Communication       July 2019

      designated as Standards Track Document required.  Integer values
      from 256 to 65535 and strings of length 2 are designated as
      Specification Required.  Integer values of greater than 65535 and
      strings of length greater than 2 are designated as expert review.
      Integer values less than -65536 are marked as private use.

   o  CBOR type: the CBOR type used to encode the value of this group
      communication policy.

   o  Description: This field contains a brief description for this
      group communication policy.

   o  Reference: This field contains a pointer to the public
      specification providing the format of the group communication
      policy, if one exists.

   This registry will be initially populated by the values in Figure 6.

11.8.  Sequence Number Synchronization Method Registry

   This specification establishes the "Sequence Number Synchronization
   Method" IANA Registry.  The Registry has been created to use the
   "Expert Review Required" registration procedure [RFC8126].  Expert
   review guidelines are provided in Section 11.9.  It should be noted
   that, in addition to the expert review, some portions of the Registry
   require a specification, potentially a Standards Track RFC, be
   supplied as well.

   The columns of this Registry are:

   o  Name: The name of the sequence number synchronization method.

   o  Value: The value to be used to identify this sequence number
      synchronization method.

   o  Description: This field contains a brief description for this
      sequence number synchronization method.

   o  Reference: This field contains a pointer to the public
      specification describing the sequence number synchronization
      method.

11.9.  Expert Review Instructions

   The IANA Registries established in this document are 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.

Palombini & Tiloca       Expires January 6, 2020               [Page 29]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   Expert reviewers should take into consideration the following points:

   o  Point squatting should be discouraged.  Reviewers are encouraged
      to get sufficient information for registration requests to ensure
      that the usage is not going to duplicate one that is already
      registered and that the point is likely to be used in deployments.
      The zones tagged as private use are intended for testing purposes
      and closed environments, code points in other ranges should not be
      assigned for testing.

   o  Specifications are required for the standards track range of point
      assignment.  Specifications should exist for specification
      required ranges, but early assignment before a specification is
      available is considered to be permissible.  Specifications are
      needed for the first-come, first-serve range if they are expected
      to be used outside of closed environments in an interoperable way.
      When specifications are not provided, the description provided
      needs to have sufficient information to identify what the point is
      being used for.

   o  Experts should take into account the expected usage of fields when
      approving point assignment.  The fact that there is a range for
      standards track documents does not mean that a standards track
      document cannot have points assigned outside of that range.  The
      length of the encoded value should be weighed against how many
      code points of that length are left, the size of device it will be
      used on, and the number of code points left that encode to that
      size.

12.  References

12.1.  Normative References

   [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-24
              (work in progress), March 2019.

   [I-D.ietf-ace-oauth-params]
              Seitz, L., "Additional OAuth Parameters for Authorization
              in Constrained Environments (ACE)", draft-ietf-ace-oauth-
              params-05 (work in progress), March 2019.

Palombini & Tiloca       Expires January 6, 2020               [Page 30]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Group OSCORE - Secure Group Communication for CoAP",
              draft-ietf-core-oscore-groupcomm-05 (work in progress),
              July 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>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [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>.

12.2.  Informative References

   [I-D.dijk-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              dijk-core-groupcomm-bis-00 (work in progress), March 2019.

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-dtls-
              authorize-08 (work in progress), April 2019.

   [I-D.ietf-ace-mqtt-tls-profile]
              Sengul, C., Kirby, A., and P. Fremantle, "MQTT-TLS profile
              of ACE", draft-ietf-ace-mqtt-tls-profile-00 (work in
              progress), May 2019.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE profile of the Authentication and Authorization
              for Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-07 (work in progress), February 2019.

Palombini & Tiloca       Expires January 6, 2020               [Page 31]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", draft-ietf-core-coap-pubsub-08 (work in
              progress), March 2019.

   [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.

   [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., 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>.

   [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>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

Palombini & Tiloca       Expires January 6, 2020               [Page 32]

Internet-Draft  Key Provisioning for Group Communication       July 2019

Appendix A.  Requirements on Application Profiles

   This section lists the requirements on application profiles of this
   specification,for the convenience of application profile designers.

   o  Specify the communication protocol the members of the group must
      use (e.g., multicast CoAP).

   o  Specify the security protocol the group members must use to
      protect their communication (e.g., group OSCORE).  This must
      provide encryption, integrity and replay protection.

   o  Specify the encoding and value of the identifier of group or topic
      and role of 'scope' (see Section 3.1).

   o  Specify and register the application profile identifier (see
      Section 4.1).

   o  Specify the acceptable values of 'kty' (see Section 4.2).

   o  Specify the format and content of 'group_policies' entries (see
      Section 4.2).

   o  Optionally, specify the format and content of 'mgt_key_material'
      (see Section 4.2).

   o  Optionally, specify tranport profile of ACE
      [I-D.ietf-ace-oauth-authz] to use between Client and KDC.

   o  Optionally, specify the encoding of public keys, of 'client_cred',
      and of 'pub_keys' if COSE_Keys are not used (see Section 4.2).

   o  Optionally, specify the acceptable values for parameters related
      to signature algorithm and signature keys: 'sign_alg',
      'sign_parameters', 'sign_key_parameters', 'pub_key_enc' (see
      Section 3.3).

   o  Optionally, specify the negotiation of parameter values for
      signature algorithm and signature keys, if 'sign_info' and
      'pub_key_enc' are not used (see Section 3.3).

Appendix B.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

Palombini & Tiloca       Expires January 6, 2020               [Page 33]

Internet-Draft  Key Provisioning for Group Communication       July 2019

B.1.  Version -01 to -02

   o  Editorial fixes.

   o  Distinction between transport profile and application profile
      (Section 1.1).

   o  New parameters 'sign_info' and 'pub_key_enc' to negotiate
      parameter values for signature algorithm and signature keys
      (Section 3.3).

   o  New parameter 'type' to distinguish different Key Distribution
      Request messages (Section 4.1).

   o  New parameter 'client_cred_verify' in the Key Distribution Request
      to convey a Client signature (Section 4.1).

   o  Encoding of 'pub_keys_repos' (Section 4.1).

   o  Encoding of 'mgt_key_material' (Section 4.1).

   o  Improved description on retrieval of new or updated keying
      material (Section 6).

   o  Encoding of 'get_pub_keys' in Public Key Request (Section 7.1).

   o  Extended security considerations (Sections 10.1 and 10.2).

   o  New "ACE Public Key Encoding" IANA Registry (Section 11.2).

   o  New "ACE Groupcomm Parameters" IANA Registry (Section 11.3),
      populated with the entries in Section 8.

   o  New "Ace Groupcomm Request Type" IANA Registry (Section 11.4),
      populated with the values in Section 9.

   o  New "ACE Groupcomm Policy" IANA Registry (Section 11.7) populated
      with two entries "Sequence Number Synchronization Method" and "Key
      Update Check Interval" (Section 4.2).

   o  Improved list of requirements for application profiles
      (Appendix A).

B.2.  Version -00 to -01

   o  Changed name of 'req_aud' to 'audience' in the Authorization
      Request (Section 3.1).

Palombini & Tiloca       Expires January 6, 2020               [Page 34]

Internet-Draft  Key Provisioning for Group Communication       July 2019

   o  Defined error handling on the KDC (Sections 4.2 and 6.2).

   o  Updated format of the Key Distribution Response as a whole
      (Section 4.2).

   o  Generalized format of 'pub_keys' in the Key Distribution Response
      (Section 4.2).

   o  Defined format for the message to request leaving the group
      (Section 5.2).

   o  Renewal of individual keying material and methods for group
      rekeying initiated by the KDC (Section 6).

   o  CBOR type for node identifiers in 'get_pub_keys' (Section 7.1).

   o  Added section on parameter identifiers and their CBOR keys
      (Section 8).

   o  Added request types for requests to a Join Response (Section 9).

   o  Extended security considerations (Section 10).

   o  New IANA registries "ACE Groupcomm Key Registry", "ACE Groupcomm
      Profile Registry", "ACE Groupcomm Policy Registry" and "Sequence
      Number Synchronization Method Registry" (Section 11).

   o  Added appendix about requirements for application profiles of ACE
      on group communication (Appendix A).

Acknowledgments

   The following individuals were helpful in shaping this document: Ben
   Kaduk, John Mattsson, Jim Schaad, Ludwig Seitz, Goeran Selander and
   Peter van der Stok.

   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

Palombini & Tiloca       Expires January 6, 2020               [Page 35]

Internet-Draft  Key Provisioning for Group Communication       July 2019

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

   Email: [email-masked]

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

   Email: [email-masked]

Palombini & Tiloca       Expires January 6, 2020               [Page 36]