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Constrained Bootstrapping Remote Secure Key Infrastructure (BRSKI)
draft-ietf-anima-constrained-voucher-19

Document Type Active Internet-Draft (anima WG)
Authors Michael Richardson , Peter Van der Stok , Panos Kampanakis , Esko Dijk
Last updated 2023-01-02
Replaces draft-richardson-anima-ace-constrained-voucher
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Intended RFC status Proposed Standard
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draft-ietf-anima-constrained-voucher-19
anima Working Group                                        M. Richardson
Internet-Draft                                  Sandelman Software Works
Updates: 8366, 8995 (if approved)                        P. van der Stok
Intended status: Standards Track                  vanderstok consultancy
Expires: 6 July 2023                                       P. Kampanakis
                                                           Cisco Systems
                                                                 E. Dijk
                                                       IoTconsultancy.nl
                                                          2 January 2023

   Constrained Bootstrapping Remote Secure Key Infrastructure (BRSKI)
                draft-ietf-anima-constrained-voucher-19

Abstract

   This document defines the Constrained Bootstrapping Remote Secure Key
   Infrastructure (Constrained BRSKI) protocol, which provides a
   solution for secure zero-touch bootstrapping of resource-constrained
   (IoT) devices into the network of a domain owner.  This protocol is
   designed for constrained networks, which may have limited data
   throughput or may experience frequent packet loss.  Constrained BRSKI
   is a variant of the BRSKI protocol, which uses an artifact signed by
   the device manufacturer called the "voucher" which enables a new
   device and the owner's network to mutually authenticate.  While the
   BRSKI voucher is typically encoded in JSON, Constrained BRSKI defines
   a compact CBOR-encoded voucher.  The BRSKI voucher is extended with
   new data types that allow for smaller voucher sizes.  The Enrollment
   over Secure Transport (EST) protocol, used in BRSKI, is replaced with
   EST-over-CoAPS; and HTTPS used in BRSKI is replaced with CoAPS.  This
   document Updates RFC 8366 and RFC 8995.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-anima-constrained-
   voucher/.

   Discussion of this document takes place on the anima Working Group
   mailing list (mailto:anima@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/anima/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/anima/.

   Source for this draft and an issue tracker can be found at
   https://github.com/anima-wg/constrained-voucher.

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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
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   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 6 July 2023.

Copyright Notice

   Copyright (c) 2023 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   4.  Overview of Protocol  . . . . . . . . . . . . . . . . . . . .   7
   5.  Updates to RFC8366 and RFC8995  . . . . . . . . . . . . . . .   8
   6.  BRSKI-EST Protocol  . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  DTLS Connection . . . . . . . . . . . . . . . . . . . . .   9
       6.1.1.  DTLS Version  . . . . . . . . . . . . . . . . . . . .   9
       6.1.2.  TLS Client Certificates: IDevID authentication  . . .  10
       6.1.3.  DTLS Handshake Fragmentation Considerations . . . . .  10
       6.1.4.  Registrar and the Server Name Indicator (SNI) . . . .  10
     6.2.  Resource Discovery, URIs and Content Formats  . . . . . .  11
       6.2.1.  RFC8995 Telemetry Returns . . . . . . . . . . . . . .  14
     6.3.  Join Proxy options  . . . . . . . . . . . . . . . . . . .  15
     6.4.  Extensions to BRSKI . . . . . . . . . . . . . . . . . . .  15

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       6.4.1.  CoAP EST Resource Discovery and BRSKI . . . . . . . .  15
       6.4.2.  CoAP responses  . . . . . . . . . . . . . . . . . . .  16
     6.5.  Extensions to EST-coaps . . . . . . . . . . . . . . . . .  16
       6.5.1.  Pledge Extensions . . . . . . . . . . . . . . . . . .  17
       6.5.2.  EST-client Extensions . . . . . . . . . . . . . . . .  18
       6.5.3.  Registrar Extensions  . . . . . . . . . . . . . . . .  21
   7.  BRSKI-MASA Protocol . . . . . . . . . . . . . . . . . . . . .  22
     7.1.  Protocol and Formats  . . . . . . . . . . . . . . . . . .  22
     7.2.  Registrar Voucher Request . . . . . . . . . . . . . . . .  23
     7.3.  MASA and the Server Name Indicator (SNI)  . . . . . . . .  23
       7.3.1.  Registrar Certificate Requirement . . . . . . . . . .  23
   8.  Pinning in Voucher Artifacts  . . . . . . . . . . . . . . . .  24
     8.1.  Registrar Identity Selection and Encoding . . . . . . . .  24
     8.2.  MASA Pinning Policy . . . . . . . . . . . . . . . . . . .  25
     8.3.  Pinning of Raw Public Keys  . . . . . . . . . . . . . . .  26
     8.4.  Considerations for use of IDevID-Issuer . . . . . . . . .  27
   9.  Artifacts . . . . . . . . . . . . . . . . . . . . . . . . . .  28
     9.1.  Voucher Request artifact  . . . . . . . . . . . . . . . .  28
       9.1.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . .  28
       9.1.2.  SID values  . . . . . . . . . . . . . . . . . . . . .  29
       9.1.3.  YANG Module . . . . . . . . . . . . . . . . . . . . .  30
       9.1.4.  Example Pledge voucher request (PVR) artifact . . . .  34
       9.1.5.  Example Registrar voucher request (RVR) artifact  . .  34
     9.2.  Voucher artifact  . . . . . . . . . . . . . . . . . . . .  35
       9.2.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . .  35
       9.2.2.  SID values  . . . . . . . . . . . . . . . . . . . . .  35
       9.2.3.  YANG Module . . . . . . . . . . . . . . . . . . . . .  36
       9.2.4.  Example voucher artifacts . . . . . . . . . . . . . .  39
     9.3.  Signing voucher and voucher-request artifacts with
           COSE  . . . . . . . . . . . . . . . . . . . . . . . . . .  40
       9.3.1.  Signing of Registrar Voucher Request (RVR)  . . . . .  41
       9.3.2.  Signing of Pledge Voucher Request (PVR) . . . . . . .  42
       9.3.3.  Signing of voucher by MASA  . . . . . . . . . . . . .  43
   10. Extensions to Discovery . . . . . . . . . . . . . . . . . . .  44
     10.1.  Discovery operations by Pledge . . . . . . . . . . . . .  45
       10.1.1.  GRASP discovery  . . . . . . . . . . . . . . . . . .  45
       10.1.2.  CoAP Discovery . . . . . . . . . . . . . . . . . . .  46
     10.2.  Discovery operations by Join Proxy . . . . . . . . . . .  47
       10.2.1.  GRASP Discovery  . . . . . . . . . . . . . . . . . .  47
       10.2.2.  CoAP discovery . . . . . . . . . . . . . . . . . . .  48
   11. Deployment-specific Discovery Considerations  . . . . . . . .  48
     11.1.  6TSCH Deployments  . . . . . . . . . . . . . . . . . . .  48
     11.2.  Generic networks using GRASP . . . . . . . . . . . . . .  48
     11.3.  Generic networks using mDNS  . . . . . . . . . . . . . .  48
     11.4.  Thread networks using Mesh Link Establishment (MLE)  . .  49
   12. Design Considerations . . . . . . . . . . . . . . . . . . . .  49
   13. Raw Public Key Use Considerations . . . . . . . . . . . . . .  49
     13.1.  The Registrar Trust Anchor . . . . . . . . . . . . . . .  50

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     13.2.  The Pledge Voucher Request . . . . . . . . . . . . . . .  50
     13.3.  The Voucher Response . . . . . . . . . . . . . . . . . .  50
   14. Use of constrained vouchers with HTTPS  . . . . . . . . . . .  51
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  52
     15.1.  Duplicate serial-numbers . . . . . . . . . . . . . . . .  52
     15.2.  IDevID security in Pledge  . . . . . . . . . . . . . . .  52
     15.3.  Security of CoAP and UDP protocols . . . . . . . . . . .  54
     15.4.  Registrar Certificate may be self-signed . . . . . . . .  54
     15.5.  Use of RPK alternatives to proximity-registrar-cert  . .  54
     15.6.  MASA support of CoAPS  . . . . . . . . . . . . . . . . .  55
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  55
     16.1.  GRASP Discovery Registry . . . . . . . . . . . . . . . .  55
     16.2.  Resource Type Registry . . . . . . . . . . . . . . . . .  55
     16.3.  The IETF XML Registry  . . . . . . . . . . . . . . . . .  56
     16.4.  The YANG Module Names Registry . . . . . . . . . . . . .  56
     16.5.  The RFC SID range assignment sub-registry  . . . . . . .  57
     16.6.  Media Types Registry . . . . . . . . . . . . . . . . . .  57
       16.6.1.  application/voucher-cose+cbor  . . . . . . . . . . .  57
     16.7.  CoAP Content-Format Registry . . . . . . . . . . . . . .  58
     16.8.  Update to BRSKI Parameters Registry  . . . . . . . . . .  58
   17. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  59
   18. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  60
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .  60
     19.1.  Normative References . . . . . . . . . . . . . . . . . .  60
     19.2.  Informative References . . . . . . . . . . . . . . . . .  63
   Appendix A.  Library Support for BRSKI  . . . . . . . . . . . . .  66
     A.1.  OpensSSL  . . . . . . . . . . . . . . . . . . . . . . . .  66
     A.2.  mbedTLS . . . . . . . . . . . . . . . . . . . . . . . . .  67
   Appendix B.  Constrained BRSKI-EST Message Examples . . . . . . .  68
     B.1.  enrollstatus  . . . . . . . . . . . . . . . . . . . . . .  68
     B.2.  voucher_status  . . . . . . . . . . . . . . . . . . . . .  69
   Appendix C.  COSE-signed Voucher (Request) Examples . . . . . . .  70
     C.1.  Pledge, Registrar and MASA Keys . . . . . . . . . . . . .  70
       C.1.1.  Pledge IDevID private key . . . . . . . . . . . . . .  70
       C.1.2.  Registrar private key . . . . . . . . . . . . . . . .  71
       C.1.3.  MASA private key  . . . . . . . . . . . . . . . . . .  71
     C.2.  Pledge, Registrar, Domain CA and MASA Certificates  . . .  72
       C.2.1.  Pledge IDevID Certificate . . . . . . . . . . . . . .  72
       C.2.2.  Registrar Certificate . . . . . . . . . . . . . . . .  74
       C.2.3.  Domain CA Certificate . . . . . . . . . . . . . . . .  76
       C.2.4.  MASA Certificate  . . . . . . . . . . . . . . . . . .  78
     C.3.  COSE-signed Pledge Voucher Request (PVR)  . . . . . . . .  80
     C.4.  COSE-signed Registrar Voucher Request (RVR) . . . . . . .  81
     C.5.  COSE-signed Voucher from MASA . . . . . . . . . . . . . .  84
   Appendix D.  Generating Certificates with OpenSSL . . . . . . . .  87
   Appendix E.  Pledge Device Class Profiles . . . . . . . . . . . .  91
     E.1.  Minimal Pledge  . . . . . . . . . . . . . . . . . . . . .  91
     E.2.  Typical Pledge  . . . . . . . . . . . . . . . . . . . . .  92

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     E.3.  Full-featured Pledge  . . . . . . . . . . . . . . . . . .  92
     E.4.  Comparison Chart of Pledge Classes  . . . . . . . . . . .  92
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  93

1.  Introduction

   Secure enrollment of new nodes into constrained networks with
   constrained nodes presents unique challenges.  As explained in
   [RFC7228], the networks are challenged and the nodes are constrained
   by energy, memory space, and code size.

   The Bootstrapping Remote Secure Key Infrastructure (BRSKI) protocol
   described in [RFC8995] provides a solution for secure zero-touch
   (automated) bootstrap of new (unconfigured) devices.  In it, new
   devices, such as IoT devices, are called "pledges", and equipped with
   a factory-installed Initial Device Identifier (IDevID) (see
   [ieee802-1AR]), are enrolled into a network.

   The BRSKI solution described in [RFC8995] was designed to be modular,
   and this document describes a version scaled to the constraints of
   IoT deployments.

   Therefore, this document defines a constrained version of the voucher
   artifact (described in [RFC8366]), along with a constrained version
   of BRSKI.  This constrained-BRSKI protocol makes use of the
   constrained CoAP-based version of EST (EST-coaps from [RFC9148])
   rather than the EST over HTTPS [RFC7030].  Constrained-BRSKI is
   itself scalable to multiple resource levels through the definition of
   optional functions.  Appendix E illustrates this.

   In BRSKI, the [RFC8366] voucher is by default serialized to JSON with
   a signature in CMS [RFC5652].  This document defines a new voucher
   serialization to CBOR [RFC8949] with a signature in COSE [RFC9052].

   This COSE-signed CBOR-encoded voucher is transported using both
   secured CoAP and HTTPS.  The CoAP connection (between Pledge and
   Registrar) is to be protected by either OSCORE+EDHOC
   [I-D.ietf-lake-edhoc] or DTLS (CoAPS).  The HTTP connection (between
   Registrar and MASA) is to be protected using TLS (HTTPS).

   This document specifies a constrained voucher-request artifact based
   on Section 3 of [RFC8995], and voucher(-request) transport over CoAP
   based on Section 3 of [RFC8995] and on [RFC9148].

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   The CBOR definitions for the constrained voucher format are defined
   using the mechanism described in [RFC9254] using the SID mechanism
   explained in [I-D.ietf-core-sid].  As the tooling to convert YANG
   documents into a list of SID keys is still in its infancy, the table
   of SID values presented here MUST be considered normative rather than
   the output of the tool specified in [I-D.ietf-core-sid].

2.  Terminology

   The following terms are defined in [RFC8366], and are used
   identically as in that document: artifact, domain, imprint, Join
   Registrar/Coordinator (JRC), Manufacturer Authorized Signing
   Authority (MASA), Pledge, Registrar, Trust of First Use (TOFU), and
   Voucher.

   The following terms from [RFC8995] are used identically as in that
   document: Domain CA, enrollment, IDevID, Join Proxy, LDevID,
   manufacturer, nonced, nonceless, PKIX.

   The term Pledge Voucher Request, or acronym PVR, is introduced to
   refer to the voucher request between the Pledge and the Registrar.

   The term Registrar Voucher Request, or acronym RVR, is introduced to
   refer to the voucher request between the Registrar and the MASA.

   This document uses the term "PKIX Certificate" to refer to the
   X.509v3 profile described in [RFC5280].

   In code examples, the string "<CODE BEGINS>" denotes the start of a
   code example and "<CODE ENDS>" the end of the code example.  Four
   dots ("....") in a CBOR diagnostic notation byte string denotes a
   further sequence of bytes that is not shown for brevity.

3.  Requirements Language

   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.

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4.  Overview of Protocol

   [RFC8366] defines a voucher that can assert proximity, authenticates
   the Registrar, and can offer varying levels of anti-replay
   protection.  The proximity proof provided by a voucher is an
   assertion that the Pledge and the Registrar are believed to be close
   together, from a network topology point of view.  Similar to BRSKI
   [RFC8995], proximity is proven by making a DTLS connection between a
   Pledge and a Registrar.  The Pledge initiates this connection using a
   link-local source address.

   The secure DTLS connection is then used by the Pledge to make a
   Pledge Voucher Request (PVR).  The Registrar then includes the PVR
   into its own Registrar Voucher Request (RVR), sent to an agent (MASA)
   of the Pledge's manufacturer.  The MASA verifies the PVR and RVR and
   issues a signed voucher.  The voucher provides an authorization
   statement from the manufacturer indicating that the Registrar is the
   intended owner of the Pledge.  The voucher refers to the Registrar
   through pinning of the Registrar's identity.

   After verification of the voucher, the Pledge enrolls into the
   Registrar's domain by obtaining a certificate using the EST-coaps
   [RFC9148] protocol, suitable for constrained devices.  Once the
   Pledge has obtained its domain identity (LDevID) in this manner, it
   can use this identity to obtain network access credentials,
   to join the local IP network.  The method to obtain such credentials
   depends on the particular network technology used and is outside the
   scope of this document.

   This document does not make any extensions to the semantic meaning of
   vouchers, only the encoding has been changed to optimize for
   constrained devices and networks.

   The two main parts of the BRSKI protocol are named separately in this
   document: BRSKI-EST for the protocol between Pledge and Registrar,
   and BRSKI-MASA for the protocol between the Registrar and the MASA.

   Time-based vouchers are supported in this definition, but given that
   constrained devices are extremely unlikely to have accurate time,
   their use will be uncommon.  Most Pledges using constrained vouchers
   will be online during enrollment and will use live nonces to provide
   anti-replay protection rather than expiry times.

   [RFC8366] defines the voucher artifact, while the Voucher Request
   artifact was defined in [RFC8995].  This document defines both a
   constrained voucher and a constrained voucher-request.  They are
   presented in the order "voucher-request", followed by a "voucher"
   response as this is the order that they occur in the protocol.

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   The constrained voucher request MUST be signed by the Pledge.  It
   signs using the private key associated with its IDevID certificate.
   This also holds for the most constrained types of Pledges that are
   unable to perform certain PKIX operations (such as certificate chain
   validation).  These types of Pledge still contain an IDevID identity
   that is used for authentication.  See Section 13 for additional
   details on PKIX-less operations.

   The constrained voucher MUST be signed by the MASA.

   For the constrained voucher request (PVR) this document defines two
   distinct methods for the Pledge to identify the Registrar: using
   either the Registrar's full PKIX certificate, or using a Raw Public
   Key (RPK).  The method depends on which type of Registrar identity is
   obtained by the Pledge during the DTLS handshake process.  Normally,
   the Pledge obtains the PKIX certificate.  But when operating PKIX-
   less as described in Section 13, the Registrar's RPK is obtained.

   For the constrained voucher also both methods are supported to
   indicate (pin) a trusted domain identity: using either a pinned
   domain PKIX certificate, or a pinned RPK.

   The BRSKI architectures mandates that the MASA be aware of the
   capabilities of the Pledge.  This is not a drawback as a Pledges is
   constructed by a manufacturer which also arranges for the MASA to be
   aware of the inventory of devices.  The MASA therefore knows if the
   Pledge supports PKIX operations, or if it is limited to Raw Public
   Key (RPK) operations only.  Based upon this, the MASA can select
   which attributes to use in the voucher for certain operations, like
   the pinning of the Registrar identity.

5.  Updates to RFC8366 and RFC8995

   This section details the ways in which this document updates other
   RFCs.  The terminology for Updates is taken from
   [I-D.kuehlewind-update-tag].

   This document Updates [RFC8366].  It Extends [RFC8366] by creating a
   new serialization format, and creates a mechanism to pin a Raw Public
   Key (RPK).

   This document Updates [RFC8995].  It Amends [RFC8995]

   *  by clarifying how pinning is done,

   *  adopts clearer explanation of the TLS Server Name Indicator (SNI),
      see Section 6.1.4 and Section 7.3,

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   *  clarifies when new trust anchors should be retrieved
      (Section 6.5.1),

   *  clarifies what kinds of Extended Key Usage attributes are
      appropriate for each certificate (Section 7.3.1).

   It Extends [RFC8995] as follows:

   *  defines the CoAP version of the BRSKI protocol,

   *  makes some messages optional if the results can be inferred from
      other validations (Section 6.5),

   *  provides the option to return trust anchors in a simpler format
      (Section 6.5.3),

   *  extends the BRSKI-MASA protocol to carry the new voucher-cose+cbor
      format.

6.  BRSKI-EST Protocol

   This section describes the constrained BRSKI extensions to EST-coaps
   [RFC9148] to transport the voucher between Registrar and Pledge
   (optionally via a Join Proxy) over CoAP.  The extensions are
   targeting low-resource networks with small packets.

   The constrained BRSKI-EST protocol described in this section is
   between the Pledge and the Registrar only.

6.1.  DTLS Connection

   A DTLS connection is established between the Pledge and the
   Registrar, similar to the TLS connection described in Section 5.1 of
   [RFC8995].  This may occur via a Join Proxy as described in
   Section 6.3.  Regardless of the Join Proxy presence or particular
   mechanism used, the DTLS connection should operate identically.  The
   Constrained BRSKI and EST-coaps requests and responses for
   bootstrapping are carried over this DTLS connection.

6.1.1.  DTLS Version

   DTLS version 1.3 [RFC9147] SHOULD be used in any implementation of
   this specification.  An exception case where DTLS 1.2 [RFC6347] MAY
   be used is in a Pledge that uses a software platform where DTLS 1.3
   is not available (yet).  This may occur for example if a legacy
   device gets software-upgraded to support Constrained BRSKI.  For this
   reason, a Registrar MUST by default support both DTLS 1.3 and DTLS
   1.2 client connections.  However, for security reasons the Registrar

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   MAY be administratively configured to support only a particular DTLS
   version or higher.

   An EST-coaps server [RFC9148] that implements this specification also
   MUST support both DTLS 1.3 and DTLS 1.2 client connections by
   default.  However, for security reasons the EST-coaps server MAY be
   administratively configured to support only a particular DTLS version
   or higher.

6.1.2.  TLS Client Certificates: IDevID authentication

   As described in Section 5.1 of [RFC8995], the Pledge makes a
   connection to the Registrar using a TLS Client Certificate for
   authentication.  This is the Pledge's IDevID certificate.

   Subsequently the Pledge will send a Pledge Voucher Request (PVR).
   Further elements of Pledge authentication may be present in the PVR,
   as detailed in Section 9.3.

6.1.3.  DTLS Handshake Fragmentation Considerations

   DTLS includes a mechanism to fragment handshake messages.  This is
   described in Section 4.4 of [RFC9147].  Constrained BRSKI will often
   be used with a Join Proxy, described in
   [I-D.ietf-anima-constrained-join-proxy], which relays each DTLS
   message to the Registrar.  A stateless Join Proxy will need some
   additional space to wrap each DTLS message inside a CoAP request,
   while the wrapped result needs to fit in the maximum packet sized
   guaranteed on 6LoWPAN networks, which is 1280 bytes.

   For this reason it is RECOMMENDED that a PMTU of 1024 bytes be
   assumed for the DTLS handshake and appropriate DTLS fragmentation is
   used.  It is unlikely that any Packet Too Big indications [RFC4443]
   will be relayed by the Join Proxy back to the Pledge.

   During the operation of the constrained BRSKI-EST protocol, the CoAP
   Blockwise transfer mechanism will be used when message sizes exceed
   the PMTU.  A Pledge/EST-client on a constrained network MUST use the
   (D)TLS maximum fragment length extension ("max_fragment_length")
   defined in Section 4 of [RFC6066] with the maximum fragment length
   set to a value of either 2^9 or 2^10.

6.1.4.  Registrar and the Server Name Indicator (SNI)

   The SNI issue described below affects [RFC8995] as well, and is
   reported in errata: https://www.rfc-editor.org/errata/eid6648

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   As the Registrar is discovered by IP address, and typically connected
   via a Join Proxy, the name of the Registrar is not known to the
   Pledge.  The Pledge will not know what the hostname for the Registrar
   is, so it cannot do DNS-ID validation ([I-D.ietf-uta-rfc6125bis]) on
   the Registrar's certificate.  Instead, it must do validation using
   the voucher.

   As the Pledge does not know the name of the Registrar, the Pledge
   cannot put any reasonable value into the [RFC6066] Server Name
   Indicator (SNI).  Threfore the Pledge SHOULD omit the SNI extension
   as per Section 9.2 of [RFC8446].

   In some cases, particularly while testing BRSKI, a Pledge may be
   given the hostname of a particular Registrar to connect to directly.
   Such a bypass of the discovery process may result in the Pledge
   taking a different code branch to establish a DTLS connection, and
   may result in the SNI being inserted by a library.  The Registrar
   MUST ignore any SNI seen.

   A primary motivation for making the SNI ubiquitous in the public web
   is because it allows for multi-tenant hosting of HTTPS sites on a
   single (scarce) IPv4 address.  This consideration does not apply to
   the server function in the Registrar because:

   *  it uses DTLS and CoAP, not HTTPS

   *  it typically uses IPv6, often [RFC4193] Unique Local Address,
      which are plentiful

   *  the server port number is typically discovered, so multiple
      tenants can be accomodated via unique port numbers.

   As per Section 3.6.1 of [RFC7030], the Registrar certificate MUST
   have the Extended Key Usage (EKU) id-kp-cmcRA.  This certificate is
   also used as a TLS Server Certificate, so it MUST also have the EKU
   id-kp-serverAuth.  See Appendix C.2.2 for an example of a Registrar
   certificate with these EKUs set.

6.2.  Resource Discovery, URIs and Content Formats

   To keep the protocol messages small the EST-coaps and constrained-
   BRSKI URIs are shorter than the respective EST and BRSKI URIs.

   The EST-coaps server URIs differ from the EST URIs by replacing the
   scheme https by coaps and by specifying shorter resource path names.
   Below are some examples; the first two using a discovered short path
   name and the last one using the well-known URI of EST which requires
   no resource discovery by the EST client.

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     coaps://estserver.example.com/est/<short-name>
     coaps://estserver.example.com/e/<short-name>
     coaps://estserver.example.com/.well-known/est/<short-name>

   Similarly the constrained BRSKI Registrar URIs differ from the RFC
   8995 BRSKI URIs by replacing the scheme https by coaps and by
   specifying shorter resource path names.  Below are some examples; the
   first two are using a discovered short path name and the last one is
   using the well-known URI prefix which requires no resource discovery
   by the Pledge.  This is the same "/.well-known/brski" prefix as
   defined in Section 5 of [RFC8995].

     coaps://registrar.example.com/brski/<short-name>
     coaps://registrar.example.com/b/<short-name>
     coaps://registrar.example.com/.well-known/brski/<short-name>

   Figure 5 in Section 3.2.2 of [RFC7030] enumerates the operations
   supported by EST, for which Table 1 in Section 5.1 of [RFC9148]
   enumerates the corresponding EST-coaps short path names.  Similarly,
   Table 1 below provides the mapping from the supported BRSKI extension
   URI paths to the constrained-BRSKI URI paths.

             +=================+============================+
             | BRSKI resource  | constrained-BRSKI resource |
             +=================+============================+
             | /requestvoucher | /rv                        |
             +-----------------+----------------------------+
             | /voucher_status | /vs                        |
             +-----------------+----------------------------+
             | /enrollstatus   | /es                        |
             +-----------------+----------------------------+

                   Table 1: BRSKI URI paths mapping to
                       Constrained BRSKI URI paths

   Note that /requestvoucher occurs between the Pledge and Registrar (in
   scope of the BRSKI-EST protocol), but it also occurs between
   Registrar and MASA.  However, as described in Section 6, this section
   and above table addresses only the BRSKI-EST protocol.

   Pledges that wish to discover the available BRSKI bootstrap options/
   formats, or reduce the size of the CoAP headers by eliminating the
   "/.well-known/brski" path, can do a discovery operation using
   Section 4 of [RFC6690] by sending a discovery query to the Registrar
   over the secured DTLS connection.

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   For example, if the Registrar supports a short BRSKI URL (/b) and
   supports the voucher format "application/voucher-cose+cbor" (836),
   and status reporting in both CBOR and JSON formats, a CoAP resource
   discovery request and response may look as follows:

     REQ: GET /.well-known/core?rt=brski*

     RES: 2.05 Content
     Content-Format: 40
     Payload:
     </b>;rt=brski,
     </b/rv>;rt=brski.rv;ct=836,
     </b/vs>;rt=brski.vs;ct="50 60",
     </b/es>;rt=brski.es;ct="50 60"

   The Registrar is under no obligation to provide shorter URLs, and MAY
   respond to this query with only the "/.well-known/brski/<short-name>"
   resources for the short names as defined in Table 1.

   When responding to a discovery request for BRSKI resources, the
   Registrar MAY in addition return the full resource paths and the
   content types which are supported by these resources as shown in
   above example.  This is useful when multiple content types are
   specified for a particular resource on the Registrar.

   Registrars that have implemented shorter URLs MUST also respond in
   equivalent ways to the corresponding "/.well-known/brski/<short-
   name>" URLs, and MUST NOT distinguish between them.  In particular, a
   Pledge MAY use the longer (e.g. well-known) and shorter URLs in any
   combination.

   In case the client queries for only rt=brski type resources, the
   Registrar responds with only the root path for the BRSKI resources
   (rt=brski, resource /b in above example) and no others.  (So, a query
   for rt=brski, without the wildcard character.)  This is shown in the
   below example.  The Pledge requests only the BRSKI root resource of
   type rt=brski to check if short names are supported or not.  In this
   case, the Pledge is not interested to check what voucher request
   formats, or status telemetry formats -- other than the mandatory
   default formats -- are supported.  The compact response then shows
   that the Registrar indeed supports a short-name BRSKI resource at /b:

     REQ: GET /.well-known/core?rt=brski

     RES: 2.05 Content
     Content-Format: 40
     Payload:
     </b>;rt=brski

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   In above example, the well-known resource present under /.well-known/
   brski is not returned because this is assumed to be well-known to the
   Pledge and would not require discovery anyway.  Effectively, the
   client is guided to preferably use the short names under resource /b
   instead.

   Without discovery, a Pledge can only use the longer well-known URI
   for its voucher request, such as:

     REQ: GET /.well-known/brski/rv

   while with discovery of shorter URLs, a request such as:

     REQ: GET /b/rv

   is possible.

   The return of multiple content-types in the "ct" attribute allows the
   Pledge to choose the most appropriate one for a particular operation,
   and allows extension with new voucher (request) formats.  Note that
   only Content-Format 836 ("application/voucher-cose+cbor") is defined
   in this document for the voucher request resource (/rv).

   Content-Format 836 MUST be supported by the Registrar for the /rv
   resource.  If the "ct" attribute is not indicated for the /rv
   resource in the link format description, this implies that at least
   format 836 is supported.

   Note that this specification allows for voucher-cose+cbor format
   requests and vouchers to be transmitted over HTTPS, as well as for
   voucher-cms+json and other formats yet to be defined over CoAP.  The
   burden for this flexibility is placed upon the Registrar.  A Pledge
   on constrained hardware is expected to support a single format only.

   The Pledge and MASA need to support one or more formats (at least
   format 836) for the voucher and for the voucher request.  The MASA
   needs to support all formats that the Pledge supports.

   Section 11 details how the Pledge discovers the Registrar and Join
   Proxy in different deployment scenarios.

6.2.1.  RFC8995 Telemetry Returns

   [RFC8995] defines two telemetry returns from the Pledge which are
   sent to the Registrar.  These are the BRSKI Status Telemetry
   [RFC8995], Section 5.7 and the Enrollment Status Telemetry [RFC8995],
   Section 5.9.4.  These are two POST operations made the by Pledge at
   two key steps in the process.

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   [RFC8995] defines the content of these POST operations in CDDL, which
   are serialized as JSON.  This document extends the list of acceptable
   formats to CBOR as well as JSON, using the rules from [RFC8610].

   The existing JSON format is described as CoAP Content-Format 50
   ("application/json"), and it MAY be supported.  The new CBOR format
   described as CoAP Content-Format 60 ("application/cbor"), MUST be
   supported by the Registrar for both the /vs and /es resources.

6.3.  Join Proxy options

   [I-D.ietf-anima-constrained-join-proxy] specifies the details for a
   stateful and stateless constrained Join Proxy which is equivalent to
   [RFC8995], Section 4.

6.4.  Extensions to BRSKI

   The following section explains extension within the BRSKI/CoAP
   connection itself.  Section 10 explains ways in which a pledge may
   discover the capability to use constrained vouchers, and to use the
   CoAPS transport.

6.4.1.  CoAP EST Resource Discovery and BRSKI

   Once the Pledge discovers an IP address and port number that connects
   to the Registrar (probably via a Join Proxy), and it establishes a
   DTLS connection.

   No further discovery of hosts or port numbers is required, but a
   pledge that can do more than one kind of enrollment (future work
   offers protocols other than [RFC9148]), then a pledge may need to use
   CoAP Discovery to determine what other protocols are available.

   A Pledge that only supports the EST-coaps enrollment method SHOULD
   NOT use CoAP discovery for BRSKI/EST resources.  It is more efficient
   to just try the supported enrollment method via the well-known BRSKI/
   EST-coaps resources.  This also avoids the Pledge having to do any
   CoRE Link Format parsing, which is specified in [RFC9148],
   Section 4.1.

   The Registrar MUST support all of the EST resources at their default
   ".well-known" locations (on the specified port) as well as any
   server-specific shorter form that might also be supported.

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   However, if discovery is done by the Pledge, it is possible for the
   Registrar to return references to resources which are on different
   port numbers.  The Registrar SHOULD NOT use different ports numbers
   by default, because a Pledge that is connected via a Join Proxy can
   only access a single UDP port.

   A Pledge that receives different port numbers or names SHOULD ignore
   those port numbers and continue to use the DTLS connection that it
   has already created.  Or it MAY fail the entire transaction and look
   for another Join Proxy/Registrar to do onboarding with.  (If the
   resources without the port numbers do not work, then the Pledge will
   fail anyway)

   A Registrar configured to never use Join Proxies MAY be configured to
   use multiple port numbers.  Therefore a Registrar MUST host all
   discoverable BRSKI resources on the same (UDP) server port that the
   Pledge's DTLS connection is using.  However, using the same UDP
   server port for all resources allows the Pledge to continue via the
   same DTLS connection which is more efficient.

6.4.2.  CoAP responses

   [RFC8995], Section 5 defines a number of HTTP response codes that the
   Registrar is to return when certain conditions occur.

   The 401, 403, 404, 406 and 415 response codes map directly to CoAP
   codes 4.01, 4.03, 4.04, 4.06 and 4.15.

   The 202 Retry process which occurs in the voucher request, is to be
   handled in the same way as Section 5.7 of [RFC9148] process for
   Delayed Responses.

6.5.  Extensions to EST-coaps

   This document extends [RFC9148], and it inherits the functions
   described in that document: specifically, the mandatory Simple
   (Re-)Enrollment (/sen and /sren) and Certification Authority
   certificates request (/crts).  Support for CSR Attributes Request
   (/att) and server-side key generation (/skg, /skc) remains optional
   for the EST server.

   Collecting the resource definitions from both [RFC8995], [RFC7030],
   and [RFC9148] results in the following shorter forms of URI paths for
   the commonly used resources:

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       +=================+=========================+===============+
       | BRSKI + EST     | Constrained-BRSKI + EST | Well-known    |
       |                 |                         | URI namespace |
       +=================+=========================+===============+
       | /requestvoucher | /rv                     | brski         |
       +-----------------+-------------------------+---------------+
       | /voucher_status | /vs                     | brski         |
       +-----------------+-------------------------+---------------+
       | /csrattrs       | /att                    | est           |
       +-----------------+-------------------------+---------------+
       | /simpleenroll   | /sen                    | est           |
       +-----------------+-------------------------+---------------+
       | /cacerts        | /crts                   | est           |
       +-----------------+-------------------------+---------------+
       | /enrollstatus   | /es                     | brski         |
       +-----------------+-------------------------+---------------+
       | /simplereenroll | /sren                   | est           |
       +-----------------+-------------------------+---------------+

            Table 2: BRSKI/EST URI paths mapping to Constrained
                         BRSKI/EST short URI paths

6.5.1.  Pledge Extensions

   This section defines extensions to the BRSKI Pledge, which are
   applicable during the BRSKI bootstrap procedure.  A Pledge which only
   supports the EST-coaps enrollment method, SHOULD NOT use discovery
   for EST-coaps resources, because it is more efficient to enroll (e.g.
   /sen) via the well-known EST resource on the current DTLS connection.
   This avoids an additional round-trip of packets and avoids the Pledge
   having to unnecessarily implement CoRE Link Format parsing.

   A constrained Pledge SHOULD NOT perform the optional EST "CSR
   attributes request" (/att) to minimize network traffic.  The Pledge
   selects which attributes to include in the CSR.

   One or more Subject Distinguished Name fields MUST be included.  If
   the Pledge has no specific information on what attributes/fields are
   desired in the CSR, it MUST use the Subject Distinguished Name fields
   from its IDevID unmodified.  The Pledge can receive such information
   via the voucher (encoded in a vendor-specific way) or via some other,
   out-of-band means.

   A constrained Pledge MAY use the following optimized EST-coaps
   procedure to minimize network traffic.

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   1.  if the voucher, that validates the current Registrar, contains a
       single pinned domain CA certificate, the Pledge provisionally
       considers this certificate as the EST trust anchor, as if it were
       the result of "CA certificates request" (/crts) to the Registrar.

   2.  Using this CA certificate as trust anchor it proceeds with EST
       simple enrollment (/sen) to obtain its provisionally trusted
       LDevID certificate.

   3.  If the Pledge validates that the trust anchor CA was used to sign
       its LDevID certificate, the Pledge accepts the pinned domain CA
       certificate as the legitimate trust anchor CA for the Registrar's
       domain and accepts the associated LDevID certificate.

   4.  If the trust anchor CA was NOT used to sign its LDevID
       certificate, the Pledge MUST perform an actual "CA certificates
       request" (/crts) to the EST server to obtain the EST CA trust
       anchor(s) since these can differ from the (temporary) pinned
       domain CA.

   5.  When doing this /crts request, the Pledge MAY use a CoAP Accept
       Option with value 287 ("application/pkix-cert") to limit the
       number of returned EST CA trust anchors to only one.  A
       constrained Pledge MAY support only this format in a /crts
       response, per Section 5.3 of [RFC9148].

   6.  If the Pledge cannot obtain the single CA certificate or the
       finally validated CA certificate cannot be chained to the LDevID
       certificate, then the Pledge MUST abort the enrollment process
       and report the error using the enrollment status telemetry (/es).

   Note that even though the Pledge may avoid performing any /crts
   request using the above EST-coaps procedure during bootstrap, it
   SHOULD support retrieval of the trust anchor CA periodically as
   detailed in the next section.

6.5.2.  EST-client Extensions

   This section defines extensions to EST-coaps clients, used after the
   BRSKI bootstrap procedure is completed.  (Note that such client is
   not called "Pledge" in this section, since it is already enrolled
   into the domain.)  A constrained EST-coaps client MAY support only
   the Content-Format 287 ("application/pkix-cert") in a /crts response,
   per Section 5.3 of [RFC9148].  In this case, it can only store one
   trust anchor of the domain.

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   An EST-coaps client that has an idea of the current time (internally,
   or via NTP) SHOULD consider the validity time of the trust anchor CA,
   and MAY begin requesting a new trust anchor CA using the /crts
   request when the CA has 50% of it's validity time (notAfter -
   notBefore) left.  A client without access to the current time cannot
   decide if the trust anchor CA has expired, and SHOULD poll
   periodically for a new trust anchor using the /crts request at an
   interval of approximately 1 month.  An EST-coaps server SHOULD
   include the CoAP ETag Option in every response to a /crts request, to
   enable clients to perform low-overhead validation whether their trust
   anchor CA is still valid.  The EST-coaps client SHOULD store the ETag
   resulting from a /crts response in memory and SHOULD use this value
   in an ETag Option in its next GET /crts request.

   The above-mentioned limitation that an EST-coaps client may support
   only one trust anchor CA is not an issue in case the domain trust
   anchor remains stable.  However, special consideration is needed for
   cases where the domain trust anchor can change over time.  Such a
   change may happen due to relocation of the client device to a new
   domain, or due to key update of the trust anchor as described in
   [RFC4210], Section 4.4.

   From the client's viewpoint, a trust anchor change typically happens
   during EST re-enrollment: a change of domain CA requires all devices
   operating under the old domain CA to acquire a new LDevID issued by
   the new domain CA.  A client's re-enrollment may be triggered by
   various events, such as an instruction to re-enroll sent by a domain
   entity, or an imminent expiry of its LDevID certificate.  How the re-
   enrollment is explicitly triggered on the client by a domain entity,
   such as a commissioner or a Registrar, is out of scope of this
   specification.

   The mechanism described in [RFC4210], Section 4.4 for Root CA key
   update requires four certificates: OldWithOld, OldWithNew,
   NewWithOld, and NewWithNew.  The OldWithOld certificate is already
   stored in the EST client's trust store.  The NewWithNew certificate
   will be distributed as the single certificate in a /crts response,
   during EST re-enrollment.  Since the EST client can only accept a
   single certificate in a /crts response it implies that the EST client
   cannot obtain the certificates OldWithNew and NewWithOld in this way,
   to perform the complete verification of the new domain CA.  Instead,
   the client only verifies the EST-coaps server using its old domain CA
   certificate in its trust store as detailed below, and based on this
   trust in the active and valid DTLS connection it automatically trusts
   the new (NewWithNew) domain CA certificate that the EST-coaps server
   provides in the /crts response.

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   In this manner, even during rollover of trust anchors, it is possible
   to have only a single trust anchor provided in a /crts response.

   During the period of the certificate renewal, it is not possible to
   create new communication channels between devices with NewCA
   certificates devices with OldCA certificates.  One option is that
   devices should avoid restarting existing DTLS or OSCORE connections
   during this interval that new certificates are being deployed.  The
   recommended period for certificate renewal is 24 hours.  For re-
   enrollment, the constrained EST-coaps client MUST support the
   following EST-coaps procedure, where optional re-enrollment to a new
   domain is under control of the EST-coaps server:

   1.  The client connects with DTLS to the EST-coaps server, and
       authenticates with its present domain certificate (LDevID
       certificate) as usual.  The EST-coaps server authenticates itself
       with its domain certificate that is trusted by the client, i.e.
       it chains to the single trust anchor that the client has stored.
       This is the "old" trust anchor, the one that will be eventually
       replaced in case the server decides to re-enroll the client into
       a new domain.  The client also checks that the server is a
       Registration Authority (RA) of the domain as required by
       Section 3.6.1 of [RFC7030].

   2.  The client performs the simple re-enrollment request (/sren) and
       upon success it obtains a new LDevID.

   3.  The client verifies the new LDevID against its (single) existing
       domain trust anchor.  If it chains successfully, this means the
       trust anchor did not change and the client MAY skip retrieving
       the current CA certificate using the "CA certificates request"
       (/crts).  If it does not chain successfully, this means the trust
       anchor was changed/updated and the client then MUST retrieve the
       new domain trust anchor using the "CA certificates request"
       (/crts).

   4.  If the client retrieved a new trust anchor in step 3, then it
       MUST verify that the new trust anchor chains with the new LDevID
       certificate it obtained in step 2.  If it chains successfully,
       the client stores both, accepts the new LDevID certificate and
       stops using it prior LDevID certificate.  If it does not chain
       successfully, the client MUST NOT update its LDevID certificate,
       it MUST NOT update its (single) domain trust anchor, and the
       client MUST abort the enrollment process and MUST attempt to
       report the error to the EST-coaps server using enrollment status
       telemetry (/es).

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   Note that even though the EST-coaps client may skip the /crts request
   in step 3, it SHOULD support retrieval of the trust anchor CA
   periodically as detailed earlier in this section.

   Note that an EST-coaps server that is also a Registrar will already
   support the enrollment status telemetry resource (/es) in step 4,
   while an EST-coaps server that purely implements [RFC9148], and not
   the present specification, will not support this resource.

6.5.3.  Registrar Extensions

   A Registrar SHOULD host any discoverable EST-coaps resources on the
   same (UDP) server port that the Pledge's DTLS initial connection is
   using.  This avoids the overhead of the Pledge reconnecting using
   DTLS, when it performs EST enrollment after the BRSKI voucher
   request.

   The Content-Format 50 (application/json) MUST be supported and 60
   (application/cbor) MUST be supported by the Registrar for the /vs and
   /es resources.

   Content-Format 836 MUST be supported by the Registrar for the /rv
   resource.

   When a Registrar receives a "CA certificates request" (/crts) request
   with a CoAP Accept Option with value 287 ("application/pkix-cert") it
   SHOULD return only the single CA certificate that is the envisioned
   or actual authority for the current, authenticated Pledge making the
   request.

   If the Pledge included in its request an Accept Option for only the
   287 ("application/pkix-cert") Content Format, but the domain has been
   configured to operate with multiple CA trust anchors only, then the
   Registrar returns a 4.06 Not Acceptable error to signal that the
   Pledge needs to use the Content Format 281 ("application/pkcs7-mime;
   smime-type=certs-only") to retrieve all the certificates.

   If the current authenticated client is an EST-coaps client that was
   already enrolled in the domain, and the Registrar is configured to
   assign this client to a new domain CA trust anchor during the next
   EST re-enrollment procedure, then the Registrar MUST respond with the
   new domain CA certificate in case the client performs the "CA
   Certificates request" (/crts) with an Accept Option for 287 only.
   This signals the client that a new domain is assigned to it.  The
   client follows the procedure as defined in Section 6.5.2.

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7.  BRSKI-MASA Protocol

   This section describes extensions to and clarifications of the BRSKI-
   MASA protocol between Registrar and MASA.

7.1.  Protocol and Formats

   Section 5.4 of [RFC8995] describes a connection between the Registrar
   and the MASA as being a normal TLS connection using HTTPS.  This
   document does not change that.  The Registrar MUST use the format
   "application/voucher-cose+cbor" in its voucher request to MASA, when
   the Pledge uses this format in its request to the Registrar
   [RFC8995].

   The MASA only needs to support formats for which it has constructed
   Pledges that use that format.

   The Registrar MUST use the same format for the RVR as the Pledge used
   for its PVR.

   The Registrar indicates the voucher format it wants to receive from
   MASA using the HTTP Accept header.  This format MUST be the same as
   the format of the PVR, so that the Pledge can parse it.

   At the moment of writing the creation of coaps based MASAs is deemed
   unrealistic.  The use of CoAP for the BRSKI-MASA connection can be
   the subject of another document.  Some consideration was made to
   specify CoAP support for consistency, but:

   *  the Registrar is not expected to be so constrained that it cannot
      support HTTPS client connections.

   *  the technology and experience to build Internet-scale HTTPS
      responders (which the MASA is) is common, while the experience
      doing the same for CoAP is much less common.

   *  a Registrar is likely to provide onboarding services to both
      constrained and non-constrained devices.  Such a Registrar would
      need to speak HTTPS anyway.

   *  a manufacturer is likely to offer both constrained and non-
      constrained devices, so there may in practice be no situation in
      which the MASA could be CoAP-only.  Additionally, as the MASA is
      intended to be a function that can easily be oursourced to a
      third-party service provider, reducing the complexity would also
      seem to reduce the cost of that function.

   *  security-related considerations: see Section 15.6.

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7.2.  Registrar Voucher Request

   If the PVR contains a proximity assertion, the Registrar MUST
   propagate this assertion into the RVR by including the "assertion"
   field with the value "proximity".  This conforms to the example in
   Section 3.3 of [RFC8995] of carrying the assertion forward.

7.3.  MASA and the Server Name Indicator (SNI)

   A TLS/HTTPS connection is established between the Registrar and MASA.

   Section 5.4 of [RFC8995] explains this process, and there are no
   externally visible changes.  A MASA that supports the unconstrained
   voucher formats should be able to support constrained voucher formats
   equally well.

   There is no requirement that a single MASA be used for both
   constrained and unconstrained voucher requests: the choice of MASA is
   determined by the id-mod-MASAURLExtn2016 extension contained in the
   IDevID.

   The Registrar MUST do DNS-ID checks ([I-D.ietf-uta-rfc6125bis]) on
   the contents of the certificate provided by the MASA.

   In constrast to the Pledge/Registrar situation, the Registrar always
   knows the name of the MASA, and MUST always include an [RFC6066]
   Server Name Indicator.  The SNI is optional in TLS1.2, but common.
   The SNI it considered mandatory with TLS1.3.

   The presence of the SNI is needed by the MASA, in order for the
   MASA's server to host multiple tenants (for different customers).

   The Registrar SHOULD use a TLS Client Certificate to authenticate to
   the MASA per Section 5.4.1 of [RFC8995].  If the certificate that the
   Registrar uses is marked as a id-kp-cmcRA certificate, via Extended
   Key Usage, then it MUST also have the id-kp-clientAuth EKU attribute
   set.

7.3.1.  Registrar Certificate Requirement

   In summary for typical Registrar use, where a single Registrar
   certificate is used, then the certificate MUST have EKU of: id-kp-
   cmcRA, id-kp-serverAuth, id-kp-clientAuth.

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8.  Pinning in Voucher Artifacts

   The voucher is a statement by the MASA for use by the Pledge that
   provides the identity of the Pledge's owner.  This section describes
   how the owner's identity is determined and how it is specified within
   the voucher.

8.1.  Registrar Identity Selection and Encoding

   Section 5.5 of [RFC8995] describes BRSKI policies for selection of
   the owner identity.  It indicates some of the flexibility that is
   possible for the Registrar, and recommends the Registrar to include
   only certificates in the voucher request (CMS) signing structure that
   participate in the certificate chain that is to be pinned.

   The MASA is expected to evaluate the certificates included by the
   Registrar in its voucher request, forming them into a chain with the
   Registrar's (signing) identity on one end.  Then, it pins a
   certificate selected from the chain.  For instance, for a domain with
   a two-level certification authority (see Figure 1), where the
   voucher-request has been signed by "Registrar", its signing structure
   includes two additional CA certificates.  The arrows in the figure
   indicate the issuing of a certificate, i.e. author of (1) issued (2)
   and author of (2) issued (3).

    .------------------.
    |  domain CA (1)   |
    |  trust anchor    |
    '------------------'
              |
              v
       .------------.
       | domain (2) |
       | Sub-CA     |
       '------------'
              |
              |
              v
     .----------------.
     |   domain       |
     | Registrar (3)  |
     | EE certificate |
     '----------------'

                         Figure 1: Two-Level CA PKI

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   When the Registrar is using a COSE-signed constrained voucher request
   towards MASA, instead of a regular CMS-signed voucher request, the
   COSE_Sign1 object contains a protected and an unprotected header.
   The Registrar MUST place all the certificates needed to validate the
   signature chain from the Registrar on the RVR in an "x5bag" attribute
   in the unprotected header [I-D.ietf-cose-x509].

   The "x5bag" attribute is very important as it provides the required
   signals from the Registrar to control what identity is pinned in the
   resulting voucher.  This is explained in the next section.

8.2.  MASA Pinning Policy

   The MASA, having assembled and verified the chain in the signing
   structure of the voucher request needs to select a certificate to
   pin.  (For the case that only the Registrar's End-Entity certificate
   is included, only this certificate can be selected and this section
   does not apply.)  The BRSKI policy for pinning by the MASA as
   described in Section 5.5.2 of [RFC8995] leaves much flexibility to
   the manufacturer.

   The present document adds the following rules to the MASA pinning
   policy to reduce the network load:

   1.  for a voucher containing a nonce, it SHOULD select the most
       specific (lowest-level) CA certificate in the chain.

   2.  for a nonceless voucher, it SHOULD select the least-specific
       (highest-level) CA certificate in the chain that is allowed under
       the MASA's policy for this specific domain.

   The rationale for 1. is that in case of a voucher with nonce, the
   voucher is valid only in scope of the present DTLS connection between
   Pledge and Registrar anyway, so there is no benefit to pin a higher-
   level CA.  By pinning the most specific CA the constrained Pledge can
   validate its DTLS connection using less crypto operations.  The
   rationale for pinning a CA instead of the Registrar's End-Entity
   certificate directly is based on the following benefit on constrained
   networks: the pinned certificate in the voucher can in common cases
   be re-used as a Domain CA trust anchor during the EST enrollment and
   during the operational phase that follows after EST enrollment, as
   explained in Section 6.5.1.

   The rationale for 2. follows from the flexible BRSKI trust model for,
   and purpose of, nonceless vouchers (Sections 5.5.* and 7.4.1 of
   [RFC8995]).

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   Refering to Figure 1 of a domain with a two-level certification
   authority, the most specific CA ("Sub-CA") is the identity that is
   pinned by MASA in a nonced voucher.  A Registrar that wished to have
   only the Registrar's End-Entity certificate pinned would omit the
   "domain CA" and "Sub-CA" certificates from the voucher-request.

   In case of a nonceless voucher, depending on the trust level, the
   MASA pins the "Registrar" certificate (low trust in customer), or the
   "Sub-CA" certificate (in case of medium trust, implying that any
   Registrar of that sub-domain is acceptable), or even the "domain CA"
   certificate (in case of high trust in the customer, and possibly a
   pre-agreed need of the customer to obtain flexible long-lived
   vouchers).

8.3.  Pinning of Raw Public Keys

   Specifically for constrained use cases, the pinning of the raw public
   key (RPK) of the Registrar is also supported in the constrained
   voucher, instead of a PKIX certificate.  If an RPK is pinned it MUST
   be the RPK of the Registrar.

   When the Pledge is known by MASA to support RPK but not PKIX
   certificate operations, the voucher produced by the MASA pins the RPK
   of the Registrar in either the "pinned-domain-pubk" or "pinned-
   domain-pubk-sha256" field of a voucher.  This is described in more
   detail in Section 9.2.3 and Section 13.  A Pledge that does not
   support PKIX certificates cannot use EST to enroll; it has to use
   another method for enrollment without certificates and the Registrar
   has to support this method also.  It is possible that the Pledge will
   not enroll, but instead only a network join operation will occur (See
   [RFC9031]).  How the Pledge discovers this method and details of the
   enrollment method are out of scope of this document.

   When the Pledge is known by MASA to support PKIX format certificates,
   the "pinned-domain-cert" field present in a voucher typically pins a
   domain certificate.  That can be either the End-Entity certificate of
   the Registrar, or the certificate of a domain CA of the Registrar's
   domain as specified in Section 8.2.  However, if the Pledge is known
   to also support RPK pinning and the MASA intends to identify the
   Registrar in the voucher (not the CA), then MASA MUST pin the RPK
   (RPK3 in Figure 2) of the Registrar instead of the Registrar's End-
   Entity certificate to save space in the voucher.

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    .------------.
    | pub-CA (1) |
    '------------'
           |
           v
    .------------.
    | sub-CA (2) |
    '------------'
           |
           v
   .--------------.
   | Registrar(3) |
   |    RPK3      |
   '--------------'

                   Figure 2: Raw Public Key (RPK) pinning

8.4.  Considerations for use of IDevID-Issuer

   [RFC8366] and [RFC8995] defines the idevid-issuer attribute for
   voucher and voucher-request (respectively), but they summarily
   explain when to use it.

   The use of idevid-issuer is provided so that the serial-number to
   which the issued voucher pertains can be relative to the entity that
   issued the devices' IDevID.  In most cases there is a one to one
   relationship between the trust anchor that signs vouchers (and is
   trusted by the pledge), and the Certification Authority that signs
   the IDevID.  In that case, the serial-number in the voucher must
   refer to the same device as the serial-number that is in IDevID
   certificate.

   However, there situations where the one to one relationship may be
   broken.  This occurs whenever a manufacturer has a common MASA, but
   different products (on different assembly lines) are produced with
   identical serial numbers.  A system of serial numbers which is just a
   simple counter is a good example of this.  A system of serial numbers
   where there is some prefix relating the product type does not fit
   into this, even if the lower digits are a counter.

   It is not possible for the Pledge or the Registrar to know which
   situation applies.  The question to be answered is whether or not to
   include the idevid-issuer in the PVR and the RVR.  A second question
   arisews as to what the format of the idevid-issuer contents are.

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   Analysis of the situation shows that the pledge never needs to
   include the idevid-issuer in it's PVR, because the pledge's IDevID
   certificate is available to the Registrar, and the Authority Key
   Identifier is contained within that.  The pledge therefore has no
   need to repeat this.

   For the RVR, the Registrar has to examine the pledge's IDevID
   certificate to discover the serial number for the Registrar's Voucher
   Request (RVR).  This is clear in Section 5.5 of [RFC8995].  That
   section also clarifies that the idevid-issuer is to be included in
   the RVR.

   Concerning the Authority Key Identifier, [RFC8366] specifies that the
   entire object i.e. the extnValue OCTET STRING is to be included:
   comprising the AuthorityKeyIdentifier, SEQUENCE, Choice as well as
   the OCTET STRING that is the keyIdentifier.

9.  Artifacts

   This section describes for both the voucher request and the voucher
   first the abstract (tree) definition as explained in [RFC8340].  This
   provides a high-level view of the contents of each artifact.

   Then the assigned SID values are presented.  These have been assigned
   using the rules in [I-D.ietf-core-sid].

9.1.  Voucher Request artifact

9.1.1.  Tree Diagram

   The following diagram is largely a duplicate of the contents of
   [RFC8366], with the addition of the fields proximity-registrar-pubk,
   proximity-registrar-pubk-sha256, proximity-registrar-cert, and prior-
   signed-voucher-request.

   prior-signed-voucher-request is only used between the Registrar and
   the MASA. proximity-registrar-pubk or proximity-registrar-pubk-sha256
   optionally replaces proximity-registrar-cert for the most constrained
   cases where RPK is used by the Pledge.

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   module: ietf-voucher-request-constrained

     grouping voucher-request-constrained-grouping:
       +-- voucher
          +-- created-on?                        yang:date-and-time
          +-- expires-on?                        yang:date-and-time
          +-- assertion                          enumeration
          +-- serial-number                      string
          +-- idevid-issuer?                     binary
          +-- pinned-domain-cert?                binary
          +-- domain-cert-revocation-checks?     boolean
          +-- nonce?                             binary
          +-- last-renewal-date?                 yang:date-and-time
          +-- proximity-registrar-pubk?          binary
          +-- proximity-registrar-pubk-sha256?   binary
          +-- proximity-registrar-cert?          binary
          +-- prior-signed-voucher-request?      binary

9.1.2.  SID values

         SID Assigned to
   --------- --------------------------------------------------
        2501 data /ietf-voucher-request-constrained:voucher
        2502 data .../assertion
        2503 data .../created-on
        2504 data .../domain-cert-revocation-checks
        2505 data .../expires-on
        2506 data .../idevid-issuer
        2507 data .../last-renewal-date
        2508 data /ietf-voucher-request-constrained:voucher/nonce
        2509 data .../pinned-domain-cert
        2510 data .../prior-signed-voucher-request
        2511 data .../proximity-registrar-cert
        2513 data .../proximity-registrar-pubk
        2512 data .../proximity-registrar-pubk-sha256
        2514 data .../serial-number

   The "assertion" attribute is an enumerated type [RFC8366], and the
   current PYANG tooling does not document the valid values for this
   attribute.  In the JSON serialization, the literal strings from the
   enumerated types are used so there is no ambiguity.  In the CBOR
   serialization, a small integer is used.  This following values are
   documented here, but the YANG module should be considered
   authoritative.  No IANA registry is provided or necessary because the
   YANG module provides for extensions.

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                       +=========+================+
                       | Integer | Assertion Type |
                       +=========+================+
                       | 0       | verified       |
                       +---------+----------------+
                       | 1       | logged         |
                       +---------+----------------+
                       | 2       | proximity      |
                       +---------+----------------+

                          Table 3: CBOR integers
                           for the "assertion"
                              attribute enum

9.1.3.  YANG Module

   In the voucher-request-constrained YANG module, the voucher is
   "augmented" within the "used" grouping statement such that one
   continuous set of SID values is generated for the voucher-request-
   constrained module name, all voucher attributes, and the voucher-
   request-constrained attributes.  Two attributes of the voucher are
   "refined" to be optional.

   <CODE BEGINS> file "ietf-voucher-request-constrained@2021-04-15.yang"
   module ietf-voucher-request-constrained {
     yang-version 1.1;

     namespace
       "urn:ietf:params:xml:ns:yang:ietf-voucher-request-constrained";
     prefix "constrained";

     import ietf-restconf {
       prefix rc;
       description
         "This import statement is only present to access
          the yang-data extension defined in RFC 8040.";
       reference "RFC 8040: RESTCONF Protocol";
     }

     import ietf-voucher {
       prefix "v";
     }

     organization
      "IETF ANIMA Working Group";

     contact
      "WG Web:   <http://tools.ietf.org/wg/anima/>

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       WG List:  <mailto:anima@ietf.org>
       Author:   Michael Richardson
                 <mailto:mcr+ietf@sandelman.ca>
       Author:   Peter van der Stok
                 <mailto: consultancy@vanderstok.org>
       Author:   Esko Dijk
                 <mailto: esko.dijk@iotconsultancy.nl>
       Author:   Panos Kampanakis
                 <mailto: pkampana@cisco.com>";

     description
      "This module defines the format for a voucher request,
       which is produced by a pledge to request a voucher.
       The voucher-request is sent to the potential owner's
       Registrar, which in turn sends the voucher request to
       the manufacturer or its delegate (MASA).

       A voucher is then returned to the pledge, binding the
       pledge to the owner.  This is a constrained version of the
       voucher-request present in
       {{I-D.ietf-anima-bootstrap-keyinfra}}

       This version provides a very restricted subset appropriate
       for very constrained devices.
       In particular, it assumes that nonce-ful operation is
       always required, that expiration dates are rather weak, as no
       clocks can be assumed, and that the Registrar is identified
       by either a pinned Raw Public Key of the Registrar, or by a
       pinned X.509 certificate of the Registrar or domain CA.

       The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
       'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY',
       and 'OPTIONAL' in the module text are to be interpreted as
       described in RFC 2119.";

     revision "2021-04-15" {
       description
        "Initial version";
       reference
        "RFC XXXX: Voucher Profile for Constrained Devices";
     }

     rc:yang-data voucher-request-constrained-artifact {
       // YANG data template for a voucher.
       uses voucher-request-constrained-grouping;
     }

     // Grouping defined for future usage

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     grouping voucher-request-constrained-grouping {
       description
         "Grouping to allow reuse/extensions in future work.";

       uses v:voucher-artifact-grouping {

         refine voucher/created-on {
             mandatory  false;
         }

         refine voucher/pinned-domain-cert {
             mandatory  false;
         }

         augment "voucher" {
           description "Base the constrained voucher-request upon the
             regular one";

           leaf proximity-registrar-pubk {
             type binary;
             description
               "The proximity-registrar-pubk replaces
                the proximity-registrar-cert in constrained uses of
                the voucher-request.
                The proximity-registrar-pubk is the
                Raw Public Key of the Registrar. This field is encoded
                as specified in RFC7250, section 3.
                The ECDSA algorithm MUST be supported.
                The EdDSA algorithm as specified in
                draft-ietf-tls-rfc4492bis-17 SHOULD be supported.
                Support for the DSA algorithm is not recommended.
                Support for the RSA algorithm is a MAY, but due to
                size is discouraged.";
           }

           leaf proximity-registrar-pubk-sha256 {
             type binary;
             description
               "The proximity-registrar-pubk-sha256
                is an alternative to both
                proximity-registrar-pubk and pinned-domain-cert.
                In many cases the public key of the domain has already
                been transmitted during the key agreement protocol,
                and it is wasteful to transmit the public key another
                two times.
                The use of a hash of public key info, at 32-bytes for
                sha256 is a significant savings compared to an RSA

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                public key, but is only a minor savings compared to
                a 256-bit ECDSA public-key.
                Algorithm agility is provided by extensions to this
                specification which may define a new leaf for another
                hash type.";
           }

           leaf proximity-registrar-cert {
             type binary;
             description
               "An X.509 v3 certificate structure as specified by
                RFC 5280,
                Section 4 encoded using the ASN.1 distinguished encoding
                rules (DER), as specified in ITU-T X.690.

                The first certificate in the Registrar TLS server
                certificate_list sequence  (see [RFC5246]) presented by
                the Registrar to the Pledge. This field or one of its
                alternatives MUST be populated in a
                Pledge's voucher request if the proximity assertion is
                populated.";
           }

           leaf prior-signed-voucher-request {
             type binary;
             description
               "If it is necessary to change a voucher, or re-sign and
                forward a voucher that was previously provided along a
                protocol path, then the previously signed voucher
                SHOULD be included in this field.

                For example, a pledge might sign a proximity voucher,
                which an intermediate registrar then re-signs to
                make its own proximity assertion.  This is a simple
                mechanism for a chain of trusted parties to change a
                voucher, while maintaining the prior signature
                information.

                The pledge MUST ignore all prior voucher information
                when accepting a voucher for imprinting. Other
                parties MAY examine the prior signed voucher
                information for the purposes of policy decisions.
                For example, this information could be useful to a
                MASA to determine that both pledge and registrar
                agree on proximity assertions. The MASA SHOULD
                remove all prior-signed-voucher-request information when
                signing a voucher for imprinting so as to minimize the
                final voucher size.";

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           }
         }
       }
     }
   }
   <CODE ENDS>

9.1.4.  Example Pledge voucher request (PVR) artifact

   Below, an example constrained voucher request (PVR) from a Pledge to
   a Registrar is shown in CBOR diagnostic notation.  Long CBOR byte
   strings have been shortened (with "....") for readability.  The enum
   value of the assertion field is calculated to be 2 for the
   "proximity" assertion by following the algorithm described in section
   9.6.4.2 of [RFC7950].  For reference, Section 9.1.2 shows a table
   with all currently defined assertion values.

   {
    2501: {      / SID=2501, ietf-voucher-request-constrained:voucher /
       1: 2,                     / SID=2502, assertion 2 = "proximity"/
       7: h'831D5198A6CA2C7F',   / SID=2508, nonce                    /
      12: h'30593013....D29A54', / SID=2513, proximity-registrar-pubk /
      13: "JADA123456789"        / SID=2514, serial-number            /
    }
   }

   The Pledge has included the item proximity-registrar-pubk which
   carries the public key of the Registrar, instead of including the
   full Registrar certificate in a proximity-registrar-cert item.  This
   is done to reduce the size of the PVR.  Also note that the Pledge did
   not include the created-on field since it lacks an internal real-time
   clock and has no knowledge of the current time at the moment of
   performing the bootstrapping.

9.1.5.  Example Registrar voucher request (RVR) artifact

   Next, an example constrained voucher request (RVR) from a Registrar
   to a MASA is shown in CBOR diagnostic notation.  The Registrar has
   created this request triggered by the reception of the Pledge voucher
   request (PVR) of the previous example.  Again, long CBOR byte strings
   have been shortened for readability.

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   {
    "ietf-request-voucher:voucher": {
       "assertion":     2,
       "created-on":    "2022-12-05T19:19:19.536Z",
       "nonce":         h'831D5198A6CA2C7F',
       "idevid-issuer": h'04183016....1736C3E0',
       "serial-number": "JADA123456789",
       "prior-signed-voucher-request": h'A11909....373839'
    }
   }

   Note that the Registrar uses here the string data type for all key
   names, instead of the more compact SID integer keys.  This is fine
   for any use cases where the network between Registrar and MASA is an
   unconstrained network where data size is not critical.  The
   constrained voucher request format supports both the string and SID
   key types.

9.2.  Voucher artifact

   The voucher's primary purpose is to securely assign a Pledge to an
   owner.  The voucher informs the Pledge which entity it should
   consider to be its owner.

9.2.1.  Tree Diagram

   The following diagram is largely a duplicate of the contents of
   [RFC8366], with only the addition of the fields pinned-domain-pubk
   and pinned-domain-pubk-sha256.

   module: ietf-voucher-constrained

     grouping voucher-constrained-grouping:
       +-- voucher
          +-- created-on?                      yang:date-and-time
          +-- expires-on?                      yang:date-and-time
          +-- assertion                        enumeration
          +-- serial-number                    string
          +-- idevid-issuer?                   binary
          +-- pinned-domain-cert?              binary
          +-- domain-cert-revocation-checks?   boolean
          +-- nonce?                           binary
          +-- last-renewal-date?               yang:date-and-time
          +-- pinned-domain-pubk?              binary
          +-- pinned-domain-pubk-sha256?       binary

9.2.2.  SID values

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         SID Assigned to
   --------- --------------------------------------------------
        2451 data /ietf-voucher-constrained:voucher
        2452 data /ietf-voucher-constrained:voucher/assertion
        2453 data /ietf-voucher-constrained:voucher/created-on
        2454 data .../domain-cert-revocation-checks
        2455 data /ietf-voucher-constrained:voucher/expires-on
        2456 data /ietf-voucher-constrained:voucher/idevid-issuer
        2457 data .../last-renewal-date
        2458 data /ietf-voucher-constrained:voucher/nonce
        2459 data .../pinned-domain-cert
        2460 data .../pinned-domain-pubk
        2461 data .../pinned-domain-pubk-sha256
        2462 data /ietf-voucher-constrained:voucher/serial-number

   The "assertion" enumerated attribute is numbered as per
   Section 9.1.2.

9.2.3.  YANG Module

   In the voucher-constrained YANG module, the voucher is "augmented"
   within the "used" grouping statement such that one continuous set of
   SID values is generated for the voucher-constrained module name, all
   voucher attributes, and the voucher-constrained attributes.  Two
   attributes of the voucher are "refined" to be optional.

   <CODE BEGINS> file "ietf-voucher-constrained@2021-04-15.yang"
   module ietf-voucher-constrained {
     yang-version 1.1;

     namespace
       "urn:ietf:params:xml:ns:yang:ietf-voucher-constrained";
     prefix "constrained";

     import ietf-restconf {
       prefix rc;
       description
         "This import statement is only present to access
          the yang-data extension defined in RFC 8040.";
       reference "RFC 8040: RESTCONF Protocol";
     }

     import ietf-voucher {
       prefix "v";
     }

     organization
      "IETF ANIMA Working Group";

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     contact
      "WG Web:   <http://tools.ietf.org/wg/anima/>
       WG List:  <mailto:anima@ietf.org>
       Author:   Michael Richardson
                 <mailto:mcr+ietf@sandelman.ca>
       Author:   Peter van der Stok
                 <mailto: consultancy@vanderstok.org>
       Author:   Esko Dijk
                 <mailto: esko.dijk@iotconsultancy.nl>
       Author:   Panos Kampanakis
                 <mailto: pkampana@cisco.com>";

   description
     "This module defines the format for a voucher, which
      is produced by a pledge's manufacturer or its delegate
      (MASA) to securely assign one or more pledges to an 'owner',
      so that a pledge may establish a secure connection to the
      owner's network infrastructure.

      This version provides a very restricted subset appropriate
      for very constrained devices.
      In particular, it assumes that nonce-ful operation is
      always required, that expiration dates are rather weak, as no
      clocks can be assumed, and that the Registrar is identified
      by either a pinned Raw Public Key of the Registrar, or by a
      pinned X.509 certificate of the Registrar or domain CA.

      The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
      'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY',
      and 'OPTIONAL' in the module text are to be interpreted as
      described in RFC 2119.";

     revision "2021-04-15" {
       description
        "Initial version";
       reference
        "RFC XXXX: Voucher Profile for Constrained Devices";
     }

     rc:yang-data voucher-constrained-artifact {
       // YANG data template for a voucher.
       uses voucher-constrained-grouping;
     }

     // Grouping defined for future usage
     grouping voucher-constrained-grouping {
       description
         "Grouping to allow reuse/extensions in future work.";

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       uses v:voucher-artifact-grouping {

         refine voucher/created-on {
             mandatory  false;
         }

         refine voucher/pinned-domain-cert {
             mandatory  false;
         }

         augment "voucher" {
           description "Base the constrained voucher
                                      upon the regular one";
           leaf pinned-domain-pubk {
             type binary;
             description
               "The pinned-domain-pubk may replace the
                pinned-domain-cert in constrained uses of
                the voucher. The pinned-domain-pubk
                is the Raw Public Key of the Registrar.
                This field is encoded as a Subject Public Key Info block
                as specified in RFC7250, in section 3.
                The ECDSA algorithm MUST be supported.
                The EdDSA algorithm as specified in
                draft-ietf-tls-rfc4492bis-17 SHOULD be supported.
                Support for the DSA algorithm is not recommended.
                Support for the RSA algorithm is a MAY.";
           }

           leaf pinned-domain-pubk-sha256 {
             type binary;
             description
               "The pinned-domain-pubk-sha256 is a second
                alternative to pinned-domain-cert.  In many cases the
                public key of the domain has already been transmitted
                during the key agreement process, and it is wasteful
                to transmit the public key another two times.
                The use of a hash of public key info, at 32-bytes for
                sha256 is a significant savings compared to an RSA
                public key, but is only a minor savings compared to
                a 256-bit ECDSA public-key.
                Algorithm agility is provided by extensions to this
                specification which can define a new leaf for another
                hash type.";
           }
         }
       }
     }

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   }
   <CODE ENDS>

9.2.4.  Example voucher artifacts

   Below, an example constrained voucher is shown in CBOR diagnostic
   notation.  It was created by a MASA in response to
   receiving the Registrar Voucher Request (RVR) shown in Section 9.1.5.
   The enum value of the assertion field is set to 2, to acknowledge to
   both the Pledge and the Registrar that the proximity of the Pledge to
   the Registrar is considered proven.

   {
    2451: {            / SID = 2451, ietf-voucher-constrained:voucher /
       1: 2,                      / SID = 2452, assertion "proximity" /
       2: "2022-12-05T19:19:23Z", / SID = 2453, created-on            /
       3: false,       / SID = 2454, domain-cert-revocation-checks    /
       7: h'831D5198A6CA2C7F',    / SID = 2508, nonce                 /
       8: h'308201F8....8CFF',    / SID = 2459, pinned-domain-cert    /
      11: "JADA123456789"         / SID = 2462, serial-number         /
    }
   }

   While the above example voucher includes the nonce from the PVR, the
   next example is a nonce-less voucher.  Instead of a nonce, it
   includes an expires-on field with the date and time on which the
   voucher expires.  Because the MASA did not verify the proximity of
   the Pledge and Registrar in this case, the assertion field contains a
   weaker assertion of "verified" (0).  This indicates that the MASA
   verified the domain's ownership of the Pledge via some other means.
   The enum value of the assertion field for "verified" is calculated to
   be 0 by following the algorithm described in section 9.6.4.2 of
   [RFC7950].

   {
    2451: {            / SID = 2451, ietf-voucher-constrained:voucher /
       1: 0,                      / SID = 2452, assertion "verified"  /
       2: "2022-12-06T10:15:32Z", / SID = 2453, created-on            /
       3: false,       / SID = 2454, domain-cert-revocation-checks    /
       4: "2022-12-13T10:15:32Z", / SID = 2455, expires-on            /
       8: h'308201F8....8CFF',    / SID = 2459, pinned-domain-cert    /
      11: "JADA123456789"         / SID = 2462, serial-number         /
    }
   }

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   The voucher is valid for one week.  To verify the voucher's validity,
   the Pledge would either need an internal real-time clock or some
   external means of obtaining the current time, such as Network Time
   Protocol (NTP) or a radio time signal receiver.

9.3.  Signing voucher and voucher-request artifacts with COSE

   The COSE_Sign1 structure is discussed in Section 4.2 of [RFC9052].
   The CBOR object that carries the body, the signature, and the
   information about the body and signature is called the COSE_Sign1
   structure.  It is used when only one signature is used on the body.

   Support for ECDSA with SHA2-256 using curve secp256r1 (aka
   prime256k1) is RECOMMENDED.  Most current low power hardware has
   support for acceleration of this algorithm.  Future hardware designs
   could omit this in favour of a newer algorithms.  This is the ES256
   keytype from Table 1 of [RFC9053].  Support for curve secp256k1 is
   OPTIONAL.

   Support for EdDSA using Curve 25519 is RECOMMENDED in new designs if
   hardware support is available.  This is keytype EDDSA (-8) from
   Table 2 of [RFC9053].  A "crv" parameter is necessary to specify the
   Curve, which from Table 18.  The 'kty' field MUST be present, and it
   MUST be 'OKP'.  (Table 17)

   A transition towards EdDSA is occurring in the industry.  Some
   hardware can accelerate only some algorithms with specific curves,
   other hardware can accelerate any curve, and still other kinds of
   hardware provide a tool kit for acceleration of any eliptic curve
   algorithm.

   In general, the Pledge is expected to support only a single
   algorithm, while the Registrar, usually not constrained, is expected
   to support a wide variety of algorithms: both legacy ones and up-and-
   coming ones via regular software updates.

   An example of the supported COSE_Sign1 object structure containing a
   Pledge Voucher Request (PVR) is shown in Figure 3.

   18(                  / tag for COSE_Sign1                       /
     [
       h'A10126',       / protected COSE header encoding: {1: -7}  /
                        /            which means { "alg": ES256 }  /
       {},              / unprotected COSE header parameters       /
       h'A119....3839', / voucher-request binary content (in CBOR) /
       h'4567....1234'  / voucher-request binary Sign1 signature   /
     ]
   )

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        Figure 3: COSE_Sign1 PVR example in CBOR diagnostic notation

   The [COSE-registry] specifies the integers/encoding for the "alg"
   field in Figure 3.  The "alg" field restricts the key usage for
   verification of this COSE object to a particular cryptographic
   algorithm.

9.3.1.  Signing of Registrar Voucher Request (RVR)

   A Registrar MUST include a COSE "x5bag" structure in the RVR as
   explained in Section 8.1.  Figure 4 shows an example Registrar
   Voucher Request (RVR) that includes the x5bag as an unprotected
   header parameter (32).  The bag contains two certificates in this
   case.

   18(                  / tag for COSE_Sign1                       /
     [
       h'A10126',       / protected COSE header encoding: {1: -7}  /
                        /            which means { "alg": ES256 }  /
       {
         32: [h'308202....9420AE', h'308201....E08CFF']  / x5bag   /
       },
       h'A178....7FED', / voucher-request binary content (in CBOR) /
       h'E1B7....2925'  / voucher-request binary Sign1 signature   /
     ]
   )

        Figure 4: COSE_Sign1 RVR example in CBOR diagnostic notation

   A "kid" (key ID) field is optionally present in the unprotected COSE
   header parameters map of a COSE object.  If present, it identifies
   the public key of the key pair that was used to sign the COSE
   message.  The value of the key identifier "kid" parameter may be in
   any format agreed between signer and verifier.  Usually a hash of the
   public key is used to identify the public key; but the choice of key
   identifier method is vendor-specific.  If "kid" is not present, then
   a verifying party needs to use other context information to retrieve
   the right public key to verify the COSE_Sign1 object against.

   By default, a Registrar does not include a "kid" parameter in the RVR
   since the signing key is already identified by the signing
   certificates included in the COSE "x5bag" structure.  A Registrar
   nevertheless MAY use a "kid" parameter in its RVR to identify its
   signing key/identity.

   The method of generating such "kid" value is vendor-specific and
   SHOULD be configurable in the Registrar to support commonly used
   methods.  In order to support future business cases and supply chain

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   integrations, a Registrar using the "kid" field MUST be configurable,
   on a per-manufacturer basis, to select a particular method for
   generating the "kid" value such that it is compatible with the method
   that the manufacturer.  Both binary and string values MUST be
   supported per [RFC9052], respectively encoded in the "kid" field
   using a CBOR byte string (bstr) or text string (tstr).

9.3.2.  Signing of Pledge Voucher Request (PVR)

   Like in the RVR, a "kid" (key ID) field is also optionally present in
   the PVR.  It can be used to identify the signing key/identity to the
   MASA.

   A Pledge by default SHOULD NOT use a "kid" parameter in its PVR,
   because its signing key is already identified by the Pledge's unique
   serial number that is included in the PVR and (by the Registrar) in
   the RVR.  This achieves the smallest possible PVR data size while
   still enabling the MASA to verify the PVR.  Still, when required the
   Pledge MAY use a "kid" parameter in its PVR to help the MASA identify
   the right public key to verify against.  This can occur for example
   if a Pledge has multiple IDevID identities.  The "kid" parameter in
   this case may be a simple integer identifying one out of N identities
   present, or it may be a hash of the public key, or anything else the
   Pledge vendor decides.  A Registrar normally SHOULD ignore a "kid"
   parameter used in a received PVR, as this information is intended for
   the MASA.  In other words, there is no need for the Registrar to
   verify the contents of this field, but it may include it in an audit
   log.

   The example in Figure 5 shows a PVR with the "kid" parameter present.

   18(                  / tag for COSE_Sign1                       /
     [
       h'A10126',       / protected COSE header encoding: {1: -7}  /
                        /            which means { "alg": ES256 }  /
       {
          4: h'59AB3E'  / COSE "kid" header parameter              /
       },
       h'A119....3839', / voucher-request binary content (in CBOR) /
       h'5678....7890'  / voucher-request binary Sign1 signature   /
     ]
   )

         Figure 5: COSE_Sign1 PVR example with "kid" field present

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   The Pledge SHOULD NOT use the "x5bag" structure in the PVR.  A
   Registrar that processes a PVR with an "x5bag" structure MUST ignore
   it, and MUST use only the TLS Client Certificate extension for
   authentication of the Pledge.

   A situation where the Pledge MAY use the x5bag structure is for
   communication of certificate chains to the MASA.  This would arise in
   some vendor- specific situations involving outsourcing of MASA
   functionality, or rekeying of the IDevID certification authority.

   In Appendix C further examples of signed PVRs are shown.

9.3.3.  Signing of voucher by MASA

   The MASA SHOULD NOT use a "kid" parameter in the voucher response,
   because the MASA's signing key is already known to the Pledge.
   Still, where needed the MASA MAY use a "kid" parameter in the voucher
   response to help the Pledge identify the right MASA public key to
   verify against.  This can occur for example if a Pledge has multiple
   IDevID identities.

   The MASA SHOULD NOT include an x5bag attribute in the voucher
   response - the exception is if the MASA knows that the Pledge doesn't
   pre-store the signing public key and certificate, and thus the MASA
   needs to provide a cert or cert chain that will enable linking the
   signing identity to the pre-stored Trust Anchor (CA) in the Pledge.
   This approach is not recommended, because including certificates in
   the x5bag attribute will significantly increase the size of the
   voucher which impacts operations on constrained networks.

   If the MASA signing key is based upon a PKI (see
   [I-D.richardson-anima-masa-considerations] Section 2.3), and the
   Pledge only pre-stores a manufacturer (root) CA identity in its Trust
   Store which is not the identity that signs the voucher, then a
   certificate chain needs to be included with the voucher in order for
   the Pledge to validate the MASA signing CA's signature by validating
   the chain up to the CA in its Trust Store.

   In BRSKI CMS signed vouchers [RFC8995], the CMS structure has a place
   for such certificates.  In the COSE-signed constrained vouchers
   described in this document, the x5bag attribute [I-D.ietf-cose-x509]
   is used to contain the needed certificates to form the chain.  A
   Registrar MUST NOT remove the x5bag attribute from the unprotected
   COSE header parameters when sending the voucher back to the Pledge.

   In Figure 6 an example is shown of a COSE-signed voucher.  This
   example shows the common case where the "x5bag" attribute is not
   used.

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   18(                  / tag for COSE_Sign1                       /
     [
       h'A10126',       / protected COSE header encoding: {1: -7}  /
                        /            which means { "alg": ES256 }  /
       {},              / unprotected COSE header parameters       /
       h'A119....3839', / voucher binary content (in CBOR)         /
       h'2A2C....7FBF'  / voucher binary Sign1 signature by MASA   /
     ]
   )

      Figure 6: COSE_Sign1 signed voucher in CBOR diagnostic notation

10.  Extensions to Discovery

   It is assumed that a Join Proxy as defined in
   [I-D.ietf-anima-constrained-join-proxy] seamlessly provides a
   (relayed) DTLS connection between the Pledge and the Registrar.  To
   use a Join Proxy, a Pledge needs to discover it.  For Pledge
   discovery of a Join Proxy, this section extends Section 4.1 of
   [RFC8995] for the constrained BRSKI case.

   In general, the Pledge may be one or more hops away from the
   Registrar, where one hop means the Registrar is a direct link-local
   neighbor of the Pledge.  The case of one hop away can be considered
   as a degenerate case, because a Join Proxy is not really needed then.

   The degenerate case would be unusual in constrained wireless network
   deployments, because a Registrar would typically not have a wireless
   network interface of the type used for constrained devices.  Rather,
   it would have a high-speed network interface.  Nevertheless, the
   situation where the Registrar is one hop away from the Pledge could
   occur in various cases like wired IoT networks or in wireless
   constrained networks where the Pledge is in radio range of a 6LoWPAN
   Border Router (6LBR) and the 6LBR happens to host a Registrar.

   In order to support the degenerate case, the Registrar SHOULD
   announce itself as if it were a Join Proxy -- though it would
   actually announce its real (stateful) Registrar CoAPS endpoint.  No
   actual Join Proxy functionality is then required on the Registrar.

   So, a Pledge only needs to discover a Join Proxy, regardless of
   whether it is one or more than one hop away from a relevant
   Registrar.  It first discovers the link-local address and the join-
   port of a Join Proxy.  The Pledge then follows the constrained BRSKI
   procedure of initiating a DTLS connection using the link-local
   address and join-port of the Join Proxy.

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   Once enrolled, a Pledge itself may function as a Join Proxy.  The
   decision whether or not to provide this functionality depends upon
   many factors and is out of scope for this document.  Such a decision
   might depend upon the amount of energy available to the device, the
   network bandwidth available, as well CPU and memory availability.

   The process by which a Pledge discovers the Join Proxy, and how a
   Join Proxy discovers the location of the Registrar, are the subject
   of the remainder of this section.  Further details on both these
   topics are provided in [I-D.ietf-anima-constrained-join-proxy].

10.1.  Discovery operations by Pledge

   The Pledge must discover the address/port and protocol with which to
   communicate.  The present document only defines coaps (CoAP over
   DTLS) as a protocol.

   Note that the identifying the format of the voucher-request and the
   voucher is not a required part of the Pledge's discovery operation.
   It is assumed that all Registrars support all relevant voucher(-
   request) formats, while the Pledge only supports a single format.  A
   Pledge that makes a voucher request to a Registrar that does not
   support that format will receive a CoAP 4.06 Not Acceptable status
   code and the bootstrap attempt will fail.

10.1.1.  GRASP discovery

   This section is normative for uses with an ANIMA ACP.  In the context
   of autonomic networks, the Join-Proxy uses the DULL GRASP M_FLOOD
   mechanism to announce itself.  Section 4.1.1 of [RFC8995] discusses
   this in more detail.

   The following changes are necessary with respect to figure 10 of
   [RFC8995]:

   *  The transport-proto is IPPROTO_UDP

   *  the objective is AN_Proxy

   The Registrar announces itself using ACP instance of GRASP using
   M_FLOOD messages.  Autonomic Network Join Proxies MUST support GRASP
   discovery of Registrar as described in section 4.3 of [RFC8995] .

   Here is an example M_FLOOD announcing the Join-Proxy at fe80::1, on
   standard coaps port 5684, using DTLS.

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        [M_FLOOD, 12340815, h'fe800000000000000000000000000001', 180000,
        [["AN_Proxy", 4, 1, "DTLS"],
        [O_IPv6_LOCATOR,
        h'fe800000000000000000000000000001', IPPROTO_UDP, 5684]]]

            Figure 7: Example of Join Proxy announcement message

   Note that a Join Proxy that supports also supports RFC8995 onboarding
   using HTTPS may announce more than one objective.  Objectives with an
   empty objective-value (whether CBOR NULL or an empty string) refer to
   [RFC8995] defaults.

   Here is an example of an announcement that offers both constrained
   and unconstrained onboarding:

        [M_FLOOD, 12340851, h'fe800000000000000000000000000001', 180000,
        [["AN_Proxy", 4, 1, ""],
         [O_IPv6_LOCATOR,
          h'fe800000000000000000000000000001', IPPROTO_TCP, 4443],
         ["AN_Proxy", 4, 1, "DTLS"],
         [O_IPv6_LOCATOR,
          h'fe800000000000000000000000000001', IPPROTO_UDP, 5684]]

      Figure 8: Example of Join Proxy announcing two bootstrap methods

10.1.2.  CoAP Discovery

   The details on CoAP discovery of a Join Proxy by a Pledge are
   provided in Section 5.2.1 of [I-D.ietf-anima-constrained-join-proxy].
   In this section some examples of CoAP discovery interactions are
   given.

   Below, a typical example is provided showing the Pledge's CoAP
   request and the Join Proxy's CoAP response.  The Join Proxy responds
   with a link-local source address, which is the same address as
   indicated in the URI-reference element ([RFC6690]) in the discovery
   response payload.  The Join Proxy has a dedicated port 8485 opened
   for DTLS connections of Pledges.

     REQ: GET coap://[ff02::fd]/.well-known/core?rt=brski.jp

     RES: 2.05 Content
     <coaps://[fe80::c78:e3c4:58a0:a4ad]:8485>;rt=brski.jp

   The next example shows a Join Proxy that uses the default CoAPS port
   5684 for DTLS connections of Pledges.  In this case, the Join Proxy
   host is not using port 5684 for any other purposes.

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     REQ: GET coap://[ff02::fd]/.well-known/core?rt=brski.jp

     RES: 2.05 Content
     <coaps://[fe80::c78:e3c4:58a0:a4ad]>;rt=brski.jp

   In the following example, two Join Proxies respond to the multicast
   query.  The Join Proxies use a slightly different CoRE Link Format
   encoding.  While the first encoding is more compact, both encodings
   are allowed per [RFC6690].  The Pledge may now select one of the two
   Join Proxies for initiating its DTLS connection.

     REQ: GET coap://[ff02::fd]/.well-known/core?rt=brski*

     RES: 2.05 Content
     <coaps://[fe80::c78:e3c4:58a0:a4ad]:8485>;rt=brski.jp

     RES: 2.05 Content
     <coaps://[fe80::d359:3813:f382:3b23]:63245>;rt="brski.jp"

10.2.  Discovery operations by Join Proxy

   The Join Proxy needs to discover a Registrar, at the moment it needs
   to relay data towards the Registrar or prior to that moment.

10.2.1.  GRASP Discovery

   This section is normative for uses with an ANIMA ACP.  In the context
   of autonomic networks, the Registrar announces itself to a stateful
   Join Proxy using ACP instance of GRASP using M_FLOOD messages.
   Section 4.3 of [RFC8995] discusses this in more detail.

   The following changes are necessary with respect to figure 10 of
   [RFC8995]:

   *  The transport-proto is IPPROTO_UDP

   *  the objective is AN_join_registrar, identical to [RFC8995].

   *  the objective name is "BRSKI_JP".

   The Registrar announces itself using ACP instance of GRASP using
   M_FLOOD messages.  Autonomic Network Join Proxies MUST support GRASP
   discovery of Registrar as described in section 4.3 of [RFC8995].

   Here is an example M_FLOOD announcing the Registrar on example port
   5684, which is the standard CoAPS port number.

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      [M_FLOOD, 51804321, h'fda379a6f6ee00000200000064000001', 180000,
      [["AN_join_registrar", 4, 255, "BRSKI_JP"],
       [O_IPv6_LOCATOR,
        h'fda379a6f6ee00000200000064000001', IPPROTO_UDP, 5684]]]

            Figure 9: Example of Registrar announcement message

   The Registrar uses a routable address that can be used by enrolled
   constrained Join Proxies.  The address will typically be a Unique
   Local Address (ULA) as in the example, but could also be a Global
   Unicast Address (GUA).

10.2.2.  CoAP discovery

   Further details on CoAP discovery of the Registrar by a Join Proxy
   are provided in Section 5.1.1 of
   [I-D.ietf-anima-constrained-join-proxy].

11.  Deployment-specific Discovery Considerations

   This section details how discovery is done in specific deployment
   scenarios.

11.1.  6TSCH Deployments

   In 6TISCH networks, the Constrained Join Proxy (CoJP) mechanism is
   described in [RFC9031].  Such networks are expected to use a
   [I-D.ietf-lake-edhoc] to do key management.  This is the subject of
   future work.  The Enhanced Beacon has been extended in [RFC9032] to
   allow for discovery of the Join Proxy.

11.2.  Generic networks using GRASP

   [RFC8995] defines a mechanism for the Pledge to discover a Join Proxy
   by listening for [RFC8990] GRASP messages.  This mechanism can be
   used on any network which does not have another more specific
   mechanism.  This mechanism supports mesh networks, and can also be
   used over unencrypted WIFI.

11.3.  Generic networks using mDNS

   [RFC8995] also defines a non-normative mechanism for the Pledge to
   discover a Join Proxy by doing mDNS queries.  This mechanism can be
   used on any network which does not have another recommended
   mechanism.  This mechanism does not easily support mesh networks.  It
   can be used over unencrypted WIFI.

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11.4.  Thread networks using Mesh Link Establishment (MLE)

   Thread [Thread] is a wireless mesh network protocol based on 6LoWPAN
   [RFC6282] and other IETF protocols.  In Thread, a new device
   discovers potential Thread networks and Thread routers to join by
   using the Mesh Link Establishment (MLE)
   [I-D.ietf-6lo-mesh-link-establishment] protocol.  MLE uses the UDP
   port number 19788.  The new device sends discovery requests on
   different IEEE 802.15.4 radio channels, to which routers (if any
   present) respond with a discovery response containing information
   about their respective network.  Once a suitable router is selected
   the new device initiates a DTLS transport-layer secured connection to
   the network's commissioning application, over a link-local single
   radio hop to the selected Thread router.  This link is not yet
   secured at the radio level: link-layer security will be set up once
   the new device is approved by the commissioning application to join
   the Thread network, and it gets provisioned with network access
   credentials.

   The Thread router acts here as a Join Proxy.  The MLE discovery
   response message contains UDP port information to signal the new
   device which port to use for its DTLS connection.

12.  Design Considerations

   The design considerations for the CBOR encoding of vouchers are much
   the same as for JSON vouchers in [RFC8366].  One key difference is
   that the names of the leaves in the YANG definition do not affect the
   size of the resulting CBOR, as the SID translation process assigns
   integers to the names.

   Any POST request to the Registrar with resource /vs or /es returns a
   2.04 Changed response with empty payload.  The client should be aware
   that the server may use a piggybacked CoAP response (ACK, 2.04) but
   may also respond with a separate CoAP response, i.e. first an (ACK,
   0.0) that is an acknowledgement of the request reception followed by
   a (CON, 2.04) response in a separate CoAP message.

13.  Raw Public Key Use Considerations

   This section explains techniques to reduce the data volume and
   complexity of the BRSKI bootstrap.  The use of a raw public key (RPK)
   in the pinning process can significantly reduce the number of bytes
   sent over the wire and round trips, and reduce the code footprint in
   a Pledge, but it comes with a few significant operational
   limitations.

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13.1.  The Registrar Trust Anchor

   When the Pledge first connects to the Registrar, the connection to
   the Registrar is provisional, as explained in Section 5.6.2 of
   [RFC8995].  The Registrar normally provides its public key in a
   TLSServerCertificate, and the Pledge uses that to validate that
   integrity of the (D)TLS connection, but it does not validate the
   identity of the provided certificate.

   As the TLSServerCertificate object is never verified directly by the
   Pledge, sending it can be considered superfluous.  So instead of
   using a (TLSServer)Certificate of type X509 (see section 4.4.2 of
   [RFC8446]), a RawPublicKey object (as defined by [RFC7250]) is used.

   A Registrar operating in a mixed environment can determine whether to
   send a Certificate or a Raw Public Key to the Pledge: this is
   signaled by the Pledge.  In the case it needs an RPK, it includes the
   server_certificate_type of RawPublicKey.  This is shown in section 5
   of [RFC7250].

   The Pledge always sends a client_certificate_type of X509 (not an
   RPK), so that the Registrar can properly identify the Pledge and
   distill the MASA URI information from its IDevID certificate.

13.2.  The Pledge Voucher Request

   The Pledge puts the Registrar's public key into the proximity-
   registrar-pubk field of the Pledge Voucher Request (PVR).  (The
   proximity-registrar-pubk-sha256 can also be used for efficiency, if
   the 32-bytes of a SHA256 hash turns out to be smaller than a typical
   ECDSA key.)

   As the format of this pubk field is identical to the TLS RawPublicKey
   data object, no manipulation at all is needed to insert this into the
   PVR.

13.3.  The Voucher Response

   A returned voucher will have a pinned-domain-pubk field with the
   identical key as was found in the proximity-registrar-pubk field
   above, as well as being identical to the Registrar's RPK in the
   currently active DTLS connection.

   Validation of this key by the Pledge is what takes the DTLS
   connection out of the provisional state; see Section 5.6.2 of
   [RFC8995] for more details.

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   The voucher needs to be validated first by the Pledge.  The Pledge
   needs to have a public key to validate the signature from the MASA on
   the voucher.

   The MASA's public key counterpart of the (private) MASA signing key
   MUST be already installed in the Pledge at manufacturing time.
   Otherwise, the Pledge cannot validate the voucher's signature.

14.  Use of constrained vouchers with HTTPS

   This specification contains two extensions to [RFC8995]: a
   constrained voucher format (COSE), and a constrained transfer
   protocol (CoAP).

   On constrained networks with constrained devices, it make senses to
   use both together.  However, this document does not mandate that this
   be the only way.

   A given constrained device design and software may be re-used for
   multiple device models, such as a model having only an IEEE 802.15.4
   radio, or a model having only an IEEE 802.11 (Wi-Fi) radio, or a
   model having both these radios.  A manufacturer of such device models
   may wish to have code only for the use of the constrained voucher
   format (COSE), and use it on all supported radios including the IEEE
   802.11 radio.  For this radio, the software stack to support HTTP/TLS
   may be already integrated into the radio module hence it is
   attractive for the manufacturer to reuse this.  This type of approach
   is supported by this document.  In the case that HTTPS is used, the
   regular long [RFC8995] resource names are used, together with the new
   "application/voucher-cose+cbor" media type described in this
   document.  For status telemetry requests, the Pledge may use either
   one or both of the formats defined in Section 6.2.1.  A Registrar
   MUST support both formats.

   Other combinations are possible, but they are not enumerated here.
   New work such as [I-D.ietf-anima-jws-voucher] provides new formats
   that may be useable over a number of different transports.  In
   general, sending larger payloads over constrained networks makes less
   sense, while sending smaller payloads over unconstrained networks is
   perfectly acceptable.

   The Pledge will in most cases support a single voucher format, which
   it uses without negotiation i.e. without discovery of formats
   supported.  The Registrar, being unconstrained, is expected to
   support all voucher formats.  There will be cases where a Registrar
   does not support a new format that a new Pledge uses, and this is an
   unfortunate situation that will result in lack of interoperation.

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   The responsability for supporting new formats is on the Registrar.

15.  Security Considerations

15.1.  Duplicate serial-numbers

   In the absense of correct use of idevid-issuer by the Registrar as
   detailed in Section 8.4, it would be possible for a malicious
   Registrar to use an unauthorized voucher for a device.  This would
   apply only to the case where a Manufacturer Authorized Signing
   Authority (MASA) is trusted by different products from the same
   manufacturer, and the manufacturer has duplicated serial numbers as a
   result of a merge, acquisition or mis-management.

   For example, imagine the same manufacturer makes light bulbs as well
   as gas centrifuges, and said manufacturer does not uniquely allocate
   product serial numbers.  This attack only works for nonceless
   vouchers.  The attacker has obtained a light bulb which happens to
   have the same serial-number as a gas centrofuge which it wishes to
   obtain access.  The attacker performs a normal BRSKI onboarding for
   the light bulb, but then uses the resulting voucher to onboard the
   gas centrofuge.  The attack requires that the gas centrofuge be
   returned to a state where it is willing to perform a new onboarding
   operation.

   This attack is prevented by the mechanism of having the Registrar
   include the idevid-issuer in the RVR, and the MASA including it in
   the resulting voucher.  The idevid-issuer is not included by default:
   a MASA needs to be aware if there are parts of the organization which
   duplicates serial numbers, and if so, include it.

15.2.  IDevID security in Pledge

   The security of this protocol depends upon the Pledge identifying
   itself to the Registrar using it's manufacturer installed
   certificate: the IDevID certificate.  Associated with this
   certificate is the IDevID private key, known only to the Pledge.
   Disclosure of this private key to an attacker would permit the
   attacker to impersonate the Pledge towards the Registrar, probably
   gaining access credentials to that Registrar's network.

   If the IDevID private key disclosure is known to the manufacturer,
   there is little recourse other than recall of the relevant part
   numbers.  The process for communicating this recall would be within
   the BRSKI-MASA protocol.  Neither this specification nor [RFC8995]
   provides for consultation of a Certification Revocation List (CRL) or
   Open Certificate Status Protocol (OCSP) by a Registrar when
   evaluating an IDevID certificate.  However, the BRSKI-MASA protocol

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   submits the IDevID from the Registrar to the manufacturer's MASA and
   a manufacturer would have an opportunity to decline to issue a
   voucher for a device which they believe has become compromised.

   It may be difficult for a manufacturer to determine when an IDevID
   private key has been disclosed.  Two situations present themselves:
   in the first situation a compromised private key might be reused in a
   counterfeit device, which is sold to another customer.  This would
   present itself as an onboarding of the same device in two different
   networks.  The manufacturer may become suspicious seeing two voucher
   requests for the same device from different Registrars.  Such
   activity could be indistinguishable from a device which has been
   resold from one operator to another, or re-deployed by an operator
   from one location to another.

   In the second situation, an attacker having compromised the IDevID
   private key of a device might then install malware into the same
   device and attempt to return it to service.  The device, now blank,
   would go through a second onboarding process with the original
   Registrar.  Such a Registrar could notice that the device has been
   "factory reset" and alert the operator to this situation.  One remedy
   against the presence of malware is through the use of Remote
   Attestation such as described in [I-D.ietf-rats-architecture].
   Future work will need to specify a background-check Attestation flow
   as part of the voucher-request/voucher-response process.  Attestation
   may still require access to a private key (e.g.  IDevID private key)
   in order to sign Evidence, so a primary goal should be to keep any
   private key safe within the Pledge.

   In larger, more expensive, systems there is budget (power, space, and
   bill of materials) to include more specific defenses for a private
   key.  For instance, this includes putting the IDevID private key in a
   Trusted Platform Module (TPM), or use of Trusted Execution
   Environments (TEE) for access to the key.  On smaller IoT devices,
   the cost and power budget for an extra part is often prohibitive.

   It is becoming more and more common for CPUs to have an internal set
   of one-time fuses that can be programmed (often they are "burnt" by a
   laser) at the factory.  This section of memory is only accessible in
   some priviledged CPU state.  The use of this kind of CPU is
   appropriate as it provides significant resistance against key
   disclosure even when the device can be disassembled by an attacker.

   In a number of industry verticals, there is increasing concern about
   counterfeit parts.  These may be look-alike parts created in a
   different factory, or parts which are created in the same factory
   during an illegal night-shift, but which are not subject to the
   appropriate level of quality control.  The use of a manufacturer-

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   signed IDevID certificate provides for discovery of the pedigree of
   each part, and this often justifies the cost of the security measures
   associated with storing the private key.

15.3.  Security of CoAP and UDP protocols

   Section 7.1 explains that no CoAPS version of the BRSKI-MASA protocol
   is proposed.  The connection from the Registrar to the MASA continues
   to be HTTPS as in [RFC8995].  This has been done to simplify the MASA
   deployment for the manufacturer, because no new protocol needs to be
   enabled on the server.

   The use of UDP protocols across the open Internet is sometimes
   fraught with security challenges.  Denial-of-service attacks against
   UDP based protocols are trivial as there is no three-way handshake as
   done for TCP.  The three-way handshake of TCP guarantees that the
   node sending the connection request is reachable using the origin IP
   address.  While DTLS contains an option to do a stateless challenge
   -- a process actually stronger than that done by TCP -- it is not yet
   common for this mechanism to be available in hardware at multigigabit
   speeds.  It is for this reason that this document defines using HTTPS
   for the Registrar to MASA connection.

15.4.  Registrar Certificate may be self-signed

   The provisional (D)TLS connection formed by the Pledge with the
   Registrar does not authenticate the Registrar's identity.  This
   Registrar's identity is validated by the [RFC8366] voucher that is
   issued by the MASA, signed with an anchor that was built-in to the
   Pledge.

   The Registrar may therefore use any certificate, including a self-
   signed one.  The only restrictions on the certificate is that it MUST
   have EKU bits set as detailed in Section 7.3.1.

15.5.  Use of RPK alternatives to proximity-registrar-cert

   In Section 9.1 two compact alternative fields for proximity-
   registrar-cert are defined that include an RPK: proximity-registrar-
   pubk and proximity-registrar-pubk-sha256.  The Pledge can use these
   fields in its PVR to identify the Registrar based on its public key
   only.  Since the full certificate of the proximate Registrar is not
   included, use of these fields by a Pledge implies that a Registrar
   could insert another certificate with the same public key identity
   into the RVR.  For example, an older or a newer version of its
   certificate.  The MASA will not be able to detect such act by the
   Registrar.  But since any 'other' certificate the Registrar could
   insert in this way still encodes its identity the additional risk of

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   using the RPK alternatives is neglible.

   When a Registrar sees a PVR that uses one of proximity-registrar-pubk
   or proximity-registrar-pubk-sha256 fields, this implies the Registrar
   must include the certificate identified by these fields into its RVR.
   Otherwise, the MASA is unable to verify proximity.  This requirement
   is already implied by the "MUST" requirement in Section 8.1.

15.6.  MASA support of CoAPS

   The use of CoAP for the BRSKI-MASA connection is not in scope of the
   current document.  The following security considerations have led to
   this choice of scope:

   *  the technology and experience to build secure Internet-scale HTTPS
      responders (which the MASA is) is common, while the experience in
      doing the same for CoAP is much less common.

   *  in many enterprise networks, outgoing UDP connections are often
      treated as suspicious, which could effectively block CoAP
      connections for some firewall configurations.

   *  reducing the complexity of MASA (i.e. less protocols supported)
      would also reduce its potential attack surface, which is relevant
      since the MASA is 24/7 exposed on the Internet and accepting
      (untrusted) incoming connections.

16.  IANA Considerations

16.1.  GRASP Discovery Registry

   IANA is asked to extend the registration of the "AN_Proxy" (without
   quotes) in the "GRASP Objective Names" table in the Grasp Parameter
   registry.  This document should also be cited for this existing
   registration, because Section 10.1.1 defines the new protocol value
   IPPROTO_UDP for the objective.

   IANA is asked to extend the registration of the "AN_join_registrar"
   (without quotes) in the "GRASP Objective Names" table in the Grasp
   Parameter registry.  This document should also be cited for this
   existing registration, because Section 10.2.1 adds the objective
   value "BRSKI_JP" to the objective.

16.2.  Resource Type Registry

   Additions to the sub-registry "Resource Type Link Target Attribute
   Values", within the "CoRE Parameters" IANA registry are specified
   below.

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   Reference: [This RFC]

         +===========+==========================================+
         | Attribute | Description                              |
         +===========+==========================================+
         | brski     | Root path of Bootstrapping Remote Secure |
         |           | Key Infrastructure (BRSKI) resources     |
         +-----------+------------------------------------------+
         | brski.rv  | BRSKI request voucher resource           |
         +-----------+------------------------------------------+
         | brski.vs  | BRSKI voucher status telemetry resource  |
         +-----------+------------------------------------------+
         | brski.es  | BRSKI enrollment status telemetry        |
         |           | resource                                 |
         +-----------+------------------------------------------+

            Table 4: Resource Type (rt) link target attribute
                       values for IANA registration

16.3.  The IETF XML Registry

   This document registers two URIs in the IETF XML registry [RFC3688].
   Following the format in [RFC3688], the following registration is
   requested:

     URI: urn:ietf:params:xml:ns:yang:ietf-voucher-constrained
     Registrant Contact: The ANIMA WG of the IETF.
     XML: N/A, the requested URI is an XML namespace.

     URI: urn:ietf:params:xml:ns:yang:ietf-voucher-request-constrained
     Registrant Contact: The ANIMA WG of the IETF.
     XML: N/A, the requested URI is an XML namespace.

16.4.  The YANG Module Names Registry

   This document registers two YANG modules in the YANG Module Names
   registry [RFC6020].  Following the format defined in [RFC6020], the
   the following registration is requested:

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     name:         ietf-voucher-constrained
     namespace:    urn:ietf:params:xml:ns:yang:ietf-voucher-constrained
     prefix:       vch
     reference:    RFC XXXX

     name:         ietf-voucher-request-constrained
     namespace:    urn:ietf:params:xml:ns:yang:ietf-voucher-
                                              request-constrained
     prefix:       vch
     reference:    RFC XXXX

16.5.  The RFC SID range assignment sub-registry

   ------------ ------ --------------------------- ------------
   Entry-point | Size | Module name               | RFC Number
   ------------ ------ --------------------------- ------------
   2450          50     ietf-voucher-constrained    [This RFC]
   2500          50     ietf-voucher-request        [This RFC]
                                    -constrained
   ----------- ------  --------------------------- ------------

   Warning: These SID values are defined in [I-D.ietf-core-sid], not as
   an Early Allocation.

   IANA: please update the names in the Registry to match these revised
   names, if they have not already been revised.

16.6.  Media Types Registry

   This section registers the 'application/voucher-cose+cbor' in the
   IANA "Media Types" registry.  This media type is used to indicate
   that the content is a CBOR voucher or voucher request signed with a
   COSE_Sign1 structure [RFC9052].

16.6.1.  application/voucher-cose+cbor

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   Type name:  application
   Subtype name:  voucher-cose+cbor
   Required parameters:  N/A
   Optional parameters:  N/A
   Encoding considerations:  binary (CBOR)
   Security considerations:  Security Considerations of [This RFC].
   Interoperability considerations:  The format is designed to be
     broadly interoperable.
   Published specification:  [This RFC]
   Applications that use this media type:  ANIMA, 6tisch, and other
     zero-touch onboarding systems
   Fragment identifier considerations:  The syntax and semantics of
     fragment identifiers specified for application/voucher-cose+cbor
     are as specified for application/cbor.  (At publication of this
     document, there is no fragment identification syntax defined for
     application/cbor.)
   Additional information:
     Deprecated alias names for this type: N/A
     Magic number(s):  N/A
     File extension(s):  .vch
     Macintosh file type code(s):  N/A
   Person & email address to contact for further information:  IETF
     ANIMA Working Group (anima@ietf.org) or IETF Operations and
     Management Area Working Group (opsawg@ietf.org)
   Intended usage:  COMMON
   Restrictions on usage:  N/A
   Author:  ANIMA WG
   Change controller:  IETF
   Provisional registration? (standards tree only):  NO

16.7.  CoAP Content-Format Registry

   IANA has allocated ID 836 from the sub-registry "CoAP Content-
   Formats".

   Media type                     Encoding   ID   Reference
   -----------------------------  --------- ----  ----------
   application/voucher-cose+cbor  -          836  [This RFC]

16.8.  Update to BRSKI Parameters Registry

   This section updates the BRSKI Well-Known URIs sub-registry of the
   IANA Bootstrapping Remote Secure Key Infrastructures (BRSKI)
   Parameters Registry by adding a new column "Short URI".  The contents
   of this field MUST be specified for any newly registered URI as
   follows:

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   Short URI: A short name for the "URI" resource that can be used by a
   Constrained BRSKI Pledge in a CoAP request to the Registrar.  In case
   the "URI" resource is only used between Registrar and MASA, the value
   "--" is registered denoting that a short name is not applicable.

   The initial contents of the sub-registry including the new column are
   as follows:

     +=================+=======+=======================+============+
     | URI             | Short | Description           | Reference  |
     |                 | URI   |                       |            |
     +=================+=======+=======================+============+
     | requestvoucher  | rv    | Request voucher:      | [RFC8995], |
     |                 |       | Pledge to Registrar,  | [This RFC] |
     |                 |       | and Registrar to MASA |            |
     +-----------------+-------+-----------------------+------------+
     | voucher_status  | vs    | Voucher status        | [RFC8995], |
     |                 |       | telemetry: Pledge to  | [This RFC] |
     |                 |       | Registrar             |            |
     +-----------------+-------+-----------------------+------------+
     | requestauditlog | --    | Request audit log:    | [RFC8995]  |
     |                 |       | Registrar to MASA     |            |
     +-----------------+-------+-----------------------+------------+
     | enrollstatus    | es    | Enrollment status     | [RFC8995], |
     |                 |       | telemetry: Pledge to  | [This RFC] |
     |                 |       | Registrar             |            |
     +-----------------+-------+-----------------------+------------+

         Table 5: Update of the BRSKI Well-Known URI Sub-Registry

17.  Acknowledgements

   We are very grateful to Jim Schaad for explaining COSE/CMS choices
   and for correcting early versions of the COSE_Sign1 objects.

   Michel Veillette did extensive work on _pyang_ to extend it to
   support the SID allocation process, and this document was among its
   first users.

   Russ Housley , Daniel Franke and Henk Birkholtz provided review
   feedback.

   The BRSKI design team has met on many Tuesdays and Thursdays for
   document review.  The team includes: Aurelio Schellenbaum , David von
   Oheimb , Steffen Fries , Thomas Werner and Toerless Eckert .

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18.  Changelog

   -11 to -19 (For change details see GitHub issues https://github.com/
   anima-wg/constrained-voucher/issues and related Pull Requests.)

   -10 Design considerations extended Examples made consistent

   -08 Examples for cose_sign1 are completed and improved.

   -06 New SID values assigned; regenerated examples

   -04 voucher and request-voucher MUST be signed examples for signed
   request are added in appendix IANA SID registration is updated SID
   values in examples are aligned signed cms examples aligned with new
   SIDs

   -03

   Examples are inverted.

   -02

   Example of requestvoucher with unsigned appllication/cbor is added
   attributes of voucher "refined" to optional
   CBOR serialization of vouchers improved
   Discovery port numbers are specified

   -01

   application/json is optional, application/cbor is compulsory
   Cms and cose mediatypes are introduced

19.  References

19.1.  Normative References

   [I-D.ietf-core-sid]
              Veillette, M., Pelov, A., Petrov, I., Bormann, C., and M.
              Richardson, "YANG Schema Item iDentifier (YANG SID)", Work
              in Progress, Internet-Draft, draft-ietf-core-sid-19, 26
              July 2022, <https://www.ietf.org/archive/id/draft-ietf-
              core-sid-19.txt>.

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   [I-D.ietf-cose-x509]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Header parameters for carrying and referencing X.509
              certificates", Work in Progress, Internet-Draft, draft-
              ietf-cose-x509-09, 13 October 2022,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              x509-09.txt>.

   [I-D.ietf-uta-rfc6125bis]
              Saint-Andre, P. and R. Salz, "Service Identity in TLS",
              Work in Progress, Internet-Draft, draft-ietf-uta-
              rfc6125bis-08, 12 October 2022,
              <https://www.ietf.org/archive/id/draft-ietf-uta-
              rfc6125bis-08.txt>.

   [ieee802-1AR]
              IEEE Standard, "IEEE 802.1AR Secure Device Identifier",
              2009, <http://standards.ieee.org/findstds/
              standard/802.1AR-2009.html>.

   [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>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,
              <https://www.rfc-editor.org/info/rfc4210>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

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   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [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>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [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>.

   [RFC8366]  Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
              "A Voucher Artifact for Bootstrapping Protocols",
              RFC 8366, DOI 10.17487/RFC8366, May 2018,
              <https://www.rfc-editor.org/info/rfc8366>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

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   [RFC8995]  Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
              May 2021, <https://www.rfc-editor.org/info/rfc8995>.

   [RFC9031]  Vučinić, M., Ed., Simon, J., Pister, K., and M.
              Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH",
              RFC 9031, DOI 10.17487/RFC9031, May 2021,
              <https://www.rfc-editor.org/info/rfc9031>.

   [RFC9032]  Dujovne, D., Ed. and M. Richardson, "Encapsulation of
              6TiSCH Join and Enrollment Information Elements",
              RFC 9032, DOI 10.17487/RFC9032, May 2021,
              <https://www.rfc-editor.org/info/rfc9032>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

   [RFC9148]  van der Stok, P., Kampanakis, P., Richardson, M., and S.
              Raza, "EST-coaps: Enrollment over Secure Transport with
              the Secure Constrained Application Protocol", RFC 9148,
              DOI 10.17487/RFC9148, April 2022,
              <https://www.rfc-editor.org/info/rfc9148>.

   [RFC9254]  Veillette, M., Ed., Petrov, I., Ed., Pelov, A., Bormann,
              C., and M. Richardson, "Encoding of Data Modeled with YANG
              in the Concise Binary Object Representation (CBOR)",
              RFC 9254, DOI 10.17487/RFC9254, July 2022,
              <https://www.rfc-editor.org/info/rfc9254>.

19.2.  Informative References

   [COSE-registry]
              IANA, "CBOR Object Signing and Encryption (COSE)
              registry", 2017,
              <https://www.iana.org/assignments/cose/cose.xhtml>.

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   [I-D.ietf-6lo-mesh-link-establishment]
              Kelsey, R., "Mesh Link Establishment", Work in Progress,
              Internet-Draft, draft-ietf-6lo-mesh-link-establishment-00,
              1 December 2015, <https://www.ietf.org/archive/id/draft-
              ietf-6lo-mesh-link-establishment-00.txt>.

   [I-D.ietf-anima-constrained-join-proxy]
              Richardson, M., Van der Stok, P., and P. Kampanakis,
              "Constrained Join Proxy for Bootstrapping Protocols", Work
              in Progress, Internet-Draft, draft-ietf-anima-constrained-
              join-proxy-13, 23 October 2022,
              <https://www.ietf.org/archive/id/draft-ietf-anima-
              constrained-join-proxy-13.txt>.

   [I-D.ietf-anima-jws-voucher]
              Werner, T. and M. Richardson, "JWS signed Voucher
              Artifacts for Bootstrapping Protocols", Work in Progress,
              Internet-Draft, draft-ietf-anima-jws-voucher-05, 24
              October 2022, <https://www.ietf.org/archive/id/draft-ietf-
              anima-jws-voucher-05.txt>.

   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J. P., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
              Progress, Internet-Draft, draft-ietf-lake-edhoc-18, 28
              November 2022, <https://www.ietf.org/archive/id/draft-
              ietf-lake-edhoc-18.txt>.

   [I-D.ietf-rats-architecture]
              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture", Work
              in Progress, Internet-Draft, draft-ietf-rats-architecture-
              22, 28 September 2022, <https://www.ietf.org/archive/id/
              draft-ietf-rats-architecture-22.txt>.

   [I-D.kuehlewind-update-tag]
              Kühlewind, M. and S. Krishnan, "Definition of new tags for
              relations between RFCs", Work in Progress, Internet-Draft,
              draft-kuehlewind-update-tag-04, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-kuehlewind-update-
              tag-04.txt>.

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   [I-D.richardson-anima-masa-considerations]
              Richardson, M. and W. Pan, "Operatonal Considerations for
              Voucher infrastructure for BRSKI MASA", Work in Progress,
              Internet-Draft, draft-richardson-anima-masa-
              considerations-07, 11 July 2022,
              <https://www.ietf.org/archive/id/draft-richardson-anima-
              masa-considerations-07.txt>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [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>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/info/rfc7030>.

   [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>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [RFC8990]  Bormann, C., Carpenter, B., Ed., and B. Liu, Ed., "GeneRic
              Autonomic Signaling Protocol (GRASP)", RFC 8990,
              DOI 10.17487/RFC8990, May 2021,
              <https://www.rfc-editor.org/info/rfc8990>.

   [RFC9053]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
              August 2022, <https://www.rfc-editor.org/info/rfc9053>.

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   [Thread]   Thread Group, Inc, "Thread support page, White Papers",
              November 2022,
              <https://www.threadgroup.org/support#Whitepapers>.

Appendix A.  Library Support for BRSKI

   For the implementation of BRSKI, the use of a software library to
   manipulate certificates and use crypto algorithms is often
   beneficial.  Two C-based examples are OpenSSL and mbedtls.  Others
   more targeted to specific platforms or languages exist.  It is
   important to realize that the library interfaces differ significantly
   between libraries.

   Libraries do not support all known crypto algorithms.  Before
   deciding on a library, it is important to look at their supported
   crypto algorithms and the roadmap for future support.  Apart from
   availability, the library footprint, and the required execution
   cycles should be investigated beforehand.

   The handling of certificates usually includes the checking of a
   certificate chain.  In some libraries, chains are constructed and
   verified on the basis of a set of certificates, the trust anchor
   (usually self signed root CA), and the target certificate.  In other
   libraries, the chain must be constructed beforehand and obey order
   criteria.  Verification always includes the checking of the
   signatures.  Less frequent is the checking the validity of the dates
   or checking the existence of a revoked certificate in the chain
   against a set of revoked certificates.  Checking the chain on the
   consistency of the certificate extensions which specify the use of
   the certificate usually needs to be programmed explicitly.

   A libary can be used to construct a (D)TLS connection.  It is useful
   to realize that differences beetween (D)TLS implementations will
   occur due to the differences in the certicate checks supported by the
   library.  On top of that, checks between client and server
   certificates enforced by (D)TLS are not always helpful for a BRSKI
   implementation.  For example, the certificates of Pledge and
   Registrar are usually not related when the BRSKI protocol is started.
   It must be verified that checks on the relation between client and
   server certificates do not hamper a succeful DTLS connection
   establishment.

A.1.  OpensSSL

   From openssl's apps/verify.c :

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   <CODE BEGINS>
   X509 *x = NULL;
   int i = 0, ret = 0;
   X509_STORE_CTX *csc;
   STACK_OF(X509) *chain = NULL;
   int num_untrusted;

   x = load_cert(file, "certificate file");
   if (x == NULL)
       goto end;

   csc = X509_STORE_CTX_new();
   if (csc == NULL) {
       BIO_printf(bio_err, "error %s: X.509 store context"
                  "allocation failed\n",
                  (file == NULL) ? "stdin" : file);
       goto end;
   }

   X509_STORE_set_flags(ctx, vflags);
   if (!X509_STORE_CTX_init(csc, ctx, x, uchain)) {
       X509_STORE_CTX_free(csc);
       BIO_printf(bio_err,
                  "error %s: X.509 store context"
                  "initialization failed\n",
                  (file == NULL) ? "stdin" : file);
       goto end;
   }
   if (tchain != NULL)
       X509_STORE_CTX_set0_trusted_stack(csc, tchain);
   if (crls != NULL)
       X509_STORE_CTX_set0_crls(csc, crls);

   i = X509_verify_cert(csc);
   X509_STORE_CTX_free(csc);

   <CODE ENDS>

A.2.  mbedTLS

   <CODE BEGINS>
   mbedtls_x509_crt cert;
   mbedtls_x509_crt caCert;
   uint32_t         certVerifyResultFlags;
   ...
   int result = mbedtls_x509_crt_verify(&cert, &caCert, NULL, NULL,
                                &certVerifyResultFlags, NULL, NULL);
   <CODE ENDS>

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Appendix B.  Constrained BRSKI-EST Message Examples

   This appendix extends the message examples from Appendix A of
   [RFC9148] with constrained BRSKI messages.  The CoAP headers are only
   fully worked out for the first example, enrollstatus.

B.1.  enrollstatus

   A coaps enrollstatus message from Pledge to Registrar can be as
   follows:

     REQ: POST coaps://192.0.2.1:8085/b/es
     Content-Format: 60
     Payload: <binary CBOR encoding of an enrollstatus map>

   The corresponding CoAP header fields for this request are shown
   below.

     Ver = 1
     T = 0 (CON)
     TKL = 1
     Code = 0x02 (0.02 is POST method)
     Message ID = 0xab0f
     Token = 0x4d
     Options
      Option  (Uri-Path)
        Option Delta = 0xb   (option nr = 11)
        Option Length = 0x1
        Option Value = "b"
      Option  (Uri-Path)
        Option Delta = 0x0   (option nr = 11)
        Option Length = 0x2
        Option Value = "es"
      Option  (Content-Format)
        Option Delta = 0x1   (option nr = 12)
        Option Length = 0x1
        Option Value = 60    (application/cbor)
     Payload Marker = 0xFF
     Payload = A26776657273696F6E0166737461747573F5 (18 bytes binary)

   The Uri-Host and Uri-Port Options are omitted because they coincide
   with the transport protocol (UDP) destination address and port
   respectively.

   The above binary CBOR enrollstatus payload looks as follows in CBOR
   diagnostic notation, for the case of enrollment success:

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     {
       "version": 1,
       "status": true
      }

   Alternatively the payload could look as follows in case of enrollment
   failure, using the reason field to describe the failure:

     Payload = A36776657273696F6E0166737461747573F466726561736F6E782A3C
               496E666F726D61746976652068756D616E207265616461626C652065
               72726F72206D6573736167653E    (69 bytes binary)

     {
       "version": 1,
       "status": false,
       "reason": "<Informative human readable error message>"
     }

   To indicate successful reception of the enrollmentstatus telemetry
   report, a response from the Registrar may then be:

     2.04 Changed

   Which in case of a piggybacked response has the following CoAP header
   fields:

     Ver=1
     T=2 (ACK)
     TKL=1
     Code = 0x44 (2.04 Changed)
     Message ID = 0xab0f
     Token = 0x4d

B.2.  voucher_status

   A coaps voucher_status message from Pledge to Registrar can be as
   follows:

     REQ: POST coaps://[2001:db8::2:1]/.well-known/brski/vs
     Content-Format: 60 (application/cbor)
     Payload =
       A46776657273696F6E0166737461747573F466726561736F6E7828496E66
       6F726D61746976652068756D616E2D7265616461626C65206572726F7220
       6D6573736167656E726561736F6E2D636F6E74657874A100764164646974
       696F6E616C20696E666F726D6174696F6E

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   The request payload above is binary CBOR but represented here in
   hexadecimal for readability.  Below is the equivalent CBOR diagnostic
   format.

     {
       "version": 1,
       "status": false,
       "reason": "Informative human-readable error message",
       "reason-context": { 0: "Additional information" }
     }

   A success response without payload will then be sent by the Registrar
   back to the Pledge to indicate reception of the telemetry report:

     2.04 Changed

Appendix C.  COSE-signed Voucher (Request) Examples

   This appendix provides examples of COSE-signed voucher requests and
   vouchers.  First, the used test keys and certificates are described,
   followed by examples of a constrained PVR, RVR and voucher.

C.1.  Pledge, Registrar and MASA Keys

   This section documents the public and private keys used for all
   examples in this appendix.  These keys are not used in any production
   system, and must only be used for testing purposes.

C.1.1.  Pledge IDevID private key

   <CODE BEGINS>
   -----BEGIN EC PRIVATE KEY-----
   MHcCAQEEIMv+C4dbzeyrEH20qkpFlWIH2FFACGZv9kW7rNWtSlYtoAoGCCqGSM49
   AwEHoUQDQgAESH6OUiYFRhfIgWl4GG8jHoj8a+8rf6t5s1mZ/4SePlKom39GQ34p
   VYryJ9aHmboLLfz69bzICQFKbkoQ5oaiew==
   -----END EC PRIVATE KEY-----
   <CODE ENDS>

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   <CODE BEGINS>
   Private-Key: (256 bit)
   priv:
       cb:fe:0b:87:5b:cd:ec:ab:10:7d:b4:aa:4a:45:95:
       62:07:d8:51:40:08:66:6f:f6:45:bb:ac:d5:ad:4a:
       56:2d
   pub:
       04:48:7e:8e:52:26:05:46:17:c8:81:69:78:18:6f:
       23:1e:88:fc:6b:ef:2b:7f:ab:79:b3:59:99:ff:84:
       9e:3e:52:a8:9b:7f:46:43:7e:29:55:8a:f2:27:d6:
       87:99:ba:0b:2d:fc:fa:f5:bc:c8:09:01:4a:6e:4a:
       10:e6:86:a2:7b
   ASN1 OID: prime256v1
   NIST CURVE: P-256

   <CODE ENDS>

C.1.2.  Registrar private key

   <CODE BEGINS>
   -----BEGIN PRIVATE KEY-----
   MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgYJ/MP0dWA9BkYd4W
   s6oRY62hDddaEmrAVm5dtAXE/UGhRANCAAQgMIVb6EaRCz7LFcr4Vy0+tWW9xlSh
   Xvr27euqi54WCMXJEMk6IIaPyFBNNw8bJvqXWfZ5g7t4hj7amsvqUST2
   -----END PRIVATE KEY-----
   <CODE ENDS>

   <CODE BEGINS>
   Private-Key: (256 bit)
   priv:
       60:9f:cc:3f:47:56:03:d0:64:61:de:16:b3:aa:11:
       63:ad:a1:0d:d7:5a:12:6a:c0:56:6e:5d:b4:05:c4:
       fd:41
   pub:
       04:20:30:85:5b:e8:46:91:0b:3e:cb:15:ca:f8:57:
       2d:3e:b5:65:bd:c6:54:a1:5e:fa:f6:ed:eb:aa:8b:
       9e:16:08:c5:c9:10:c9:3a:20:86:8f:c8:50:4d:37:
       0f:1b:26:fa:97:59:f6:79:83:bb:78:86:3e:da:9a:
       cb:ea:51:24:f6
   ASN1 OID: prime256v1
   NIST CURVE: P-256

   <CODE ENDS>

C.1.3.  MASA private key

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   <CODE BEGINS>
   -----BEGIN PRIVATE KEY-----
   MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgrbJ1oU+HIJ2SWYAk
   DkBTL+YNPxQG+gwsMsZB94N8mZ2hRANCAASS9NVlWJdztwNY81yPlH2UODYWhlYA
   ZfsqnEPSFZKnq8mq8gF78ZVbYi6q2FEg8kkORY/rpIU/X7SQsRuD+wMW
   -----END PRIVATE KEY-----
   <CODE ENDS>

   <CODE BEGINS>
   Private-Key: (256 bit)
   priv:
       ad:b2:75:a1:4f:87:20:9d:92:59:80:24:0e:40:53:
       2f:e6:0d:3f:14:06:fa:0c:2c:32:c6:41:f7:83:7c:
       99:9d
   pub:
       04:92:f4:d5:65:58:97:73:b7:03:58:f3:5c:8f:94:
       7d:94:38:36:16:86:56:00:65:fb:2a:9c:43:d2:15:
       92:a7:ab:c9:aa:f2:01:7b:f1:95:5b:62:2e:aa:d8:
       51:20:f2:49:0e:45:8f:eb:a4:85:3f:5f:b4:90:b1:
       1b:83:fb:03:16
   ASN1 OID: prime256v1
   NIST CURVE: P-256

   <CODE ENDS>

C.2.  Pledge, Registrar, Domain CA and MASA Certificates

   All keys and certificates used for the examples have been generated
   with OpenSSL - see Appendix D for more details on certificate
   generation.  Below the certificates are listed that accompany the
   keys shown above.  Each certificate description is followed by the
   hexadecimal representation of the X.509 ASN.1 DER encoded
   certificate.  This representation can be for example decoded using an
   online ASN.1 decoder.

C.2.1.  Pledge IDevID Certificate

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   <CODE BEGINS>
   Certificate:
   Data:
    Version: 3 (0x2)
    Serial Number: 32429 (0x7ead)
    Signature Algorithm: ecdsa-with-SHA256
    Issuer: CN = masa.stok.nl, O = vanderstok, L = Helmond,
            C = NL
    Validity
      Not Before: Dec  9 12:50:47 2022 GMT
      Not After : Dec 31 12:50:47 9999 GMT
    Subject: CN = Stok IoT sensor Y-42, serialNumber = JADA123456789
    Subject Public Key Info:
      Public Key Algorithm: id-ecPublicKey
        Public-Key: (256 bit)
        pub:
          04:48:7e:8e:52:26:05:46:17:c8:81:69:78:18:6f:
          23:1e:88:fc:6b:ef:2b:7f:ab:79:b3:59:99:ff:84:
          9e:3e:52:a8:9b:7f:46:43:7e:29:55:8a:f2:27:d6:
          87:99:ba:0b:2d:fc:fa:f5:bc:c8:09:01:4a:6e:4a:
          10:e6:86:a2:7b
        ASN1 OID: prime256v1
        NIST CURVE: P-256
    X509v3 extensions:
      X509v3 Key Usage: critical
        Digital Signature, Non Repudiation, Key Encipherment,
                Data Encipherment
      X509v3 Basic Constraints:
        CA:FALSE
      X509v3 Authority Key Identifier:
        CB:8D:98:CA:74:C5:1B:58:DD:E7:AC:EF:86:9A:94:43:A8:D6:66:A6
      1.3.6.1.5.5.7.1.32:
         hl=2 l=  12 prim: IA5STRING     :masa.stok.nl

   Signature Algorithm: ecdsa-with-SHA256
   Signature Value:
    30:45:02:20:4d:89:90:7e:03:fb:52:56:42:0c:3f:c1:b1:f1:
    47:b5:b3:93:65:45:2e:be:50:db:67:85:8f:23:89:a2:3f:9e:
    02:21:00:95:33:69:d1:c6:db:f0:f1:f6:52:24:59:d3:0a:95:
    4e:b2:f4:96:a1:31:3c:7b:d9:2f:28:b3:29:71:bb:60:df

   <CODE ENDS>

   Below is the hexadecimal representation of the binary X.509 DER-
   encoded certificate:

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   <CODE BEGINS>
   308201CE30820174A00302010202027EAD300A06082A8648CE3D040302304B31
   15301306035504030C0C6D6173612E73746F6B2E6E6C31133011060355040A0C
   0A76616E64657273746F6B3110300E06035504070C0748656C6D6F6E64310B30
   09060355040613024E4C3020170D3232313230393132353034375A180F393939
   39313233313132353034375A3037311D301B06035504030C1453746F6B20496F
   542073656E736F7220592D3432311630140603550405130D4A41444131323334
   35363738393059301306072A8648CE3D020106082A8648CE3D03010703420004
   487E8E5226054617C8816978186F231E88FC6BEF2B7FAB79B35999FF849E3E52
   A89B7F46437E29558AF227D68799BA0B2DFCFAF5BCC809014A6E4A10E686A27B
   A35A3058300E0603551D0F0101FF0404030204F030090603551D130402300030
   1F0603551D23041830168014CB8D98CA74C51B58DDE7ACEF869A9443A8D666A6
   301A06082B06010505070120040E160C6D6173612E73746F6B2E6E6C300A0608
   2A8648CE3D040302034800304502204D89907E03FB5256420C3FC1B1F147B5B3
   9365452EBE50DB67858F2389A23F9E022100953369D1C6DBF0F1F6522459D30A
   954EB2F496A1313C7BD92F28B32971BB60DF

   <CODE ENDS>

C.2.2.  Registrar Certificate

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   <CODE BEGINS>
   Certificate:
   Data:
    Version: 3 (0x2)
    Serial Number:
      c3:f6:21:49:b2:e3:0e:3e
    Signature Algorithm: ecdsa-with-SHA256
    Issuer: CN = Custom-ER Global CA, OU = IT, O = "Custom-ER, Inc.",
            L = San Jose, ST = CA, C = US
    Validity
      Not Before: Dec  9 12:50:47 2022 GMT
      Not After : Dec  8 12:50:47 2025 GMT
    Subject: CN = Custom-ER Registrar, OU = Office dept, O = "Custom-ER,
            Inc.", L = Ottowa, ST = ON, C = CA
    Subject Public Key Info:
      Public Key Algorithm: id-ecPublicKey
        Public-Key: (256 bit)
        pub:
          04:20:30:85:5b:e8:46:91:0b:3e:cb:15:ca:f8:57:
          2d:3e:b5:65:bd:c6:54:a1:5e:fa:f6:ed:eb:aa:8b:
          9e:16:08:c5:c9:10:c9:3a:20:86:8f:c8:50:4d:37:
          0f:1b:26:fa:97:59:f6:79:83:bb:78:86:3e:da:9a:
          cb:ea:51:24:f6
        ASN1 OID: prime256v1
        NIST CURVE: P-256
    X509v3 extensions:
      X509v3 Key Usage: critical
        Digital Signature, Non Repudiation, Key Encipherment,
                Data Encipherment
      X509v3 Basic Constraints:
        CA:FALSE
      X509v3 Subject Key Identifier:
        C9:08:0B:38:7D:8D:D8:5B:3A:59:E7:EC:10:0B:86:63:93:A9:CA:4C
      X509v3 Authority Key Identifier:
        92:EA:76:40:40:4A:8F:AB:4F:27:0B:F3:BC:37:9D:86:CD:72:80:F8
      X509v3 Extended Key Usage: critical
        CMC Registration Authority, TLS Web Server Authentication,
                TLS Web Client Authentication
   Signature Algorithm: ecdsa-with-SHA256
   Signature Value:
    30:45:02:21:00:d8:4a:7c:69:2f:f9:58:6e:82:22:87:18:f6:
    3b:c3:05:f0:ae:b8:ae:ec:42:78:82:38:79:81:2a:5d:15:61:
    64:02:20:08:f2:3c:13:69:13:b0:2c:e2:63:09:d5:99:4f:eb:
    75:70:af:af:ed:98:cd:f1:12:11:c0:37:f7:18:4d:c1:9d

   <CODE ENDS>

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   Below is the hexadecimal representation of the binary X.509 DER-
   encoded certificate:

   <CODE BEGINS>
   3082026D30820213A003020102020900C3F62149B2E30E3E300A06082A8648CE
   3D0403023072311C301A06035504030C13437573746F6D2D455220476C6F6261
   6C204341310B3009060355040B0C02495431183016060355040A0C0F43757374
   6F6D2D45522C20496E632E3111300F06035504070C0853616E204A6F7365310B
   300906035504080C024341310B3009060355040613025553301E170D32323132
   30393132353034375A170D3235313230383132353034375A3079311C301A0603
   5504030C13437573746F6D2D4552205265676973747261723114301206035504
   0B0C0B4F6666696365206465707431183016060355040A0C0F437573746F6D2D
   45522C20496E632E310F300D06035504070C064F74746F7761310B3009060355
   04080C024F4E310B30090603550406130243413059301306072A8648CE3D0201
   06082A8648CE3D030107034200042030855BE846910B3ECB15CAF8572D3EB565
   BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868FC8504D370F1B26FA9759
   F67983BB78863EDA9ACBEA5124F6A3818A308187300E0603551D0F0101FF0404
   030204F030090603551D1304023000301D0603551D0E04160414C9080B387D8D
   D85B3A59E7EC100B866393A9CA4C301F0603551D2304183016801492EA764040
   4A8FAB4F270BF3BC379D86CD7280F8302A0603551D250101FF0420301E06082B
   0601050507031C06082B0601050507030106082B06010505070302300A06082A
   8648CE3D0403020348003045022100D84A7C692FF9586E82228718F63BC305F0
   AEB8AEEC4278823879812A5D156164022008F23C136913B02CE26309D5994FEB
   7570AFAFED98CDF11211C037F7184DC19D

   <CODE ENDS>

C.2.3.  Domain CA Certificate

   The Domain CA certificate is the CA of the customer's domain.  It has
   signed the Registrar (RA) certificate.

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   <CODE BEGINS>
   Certificate:
   Data:
    Version: 3 (0x2)
    Serial Number: 3092288576548618702 (0x2aea0413a42dc1ce)
    Signature Algorithm: ecdsa-with-SHA256
    Issuer: CN = Custom-ER Global CA, OU = IT, O = "Custom-ER, Inc.",
            L = San Jose, ST = CA, C = US
    Validity
      Not Before: Dec  9 12:50:47 2022 GMT
      Not After : Dec  6 12:50:47 2032 GMT
    Subject: CN = Custom-ER Global CA, OU = IT, O = "Custom-ER, Inc.",
            L = San Jose, ST = CA, C = US
    Subject Public Key Info:
      Public Key Algorithm: id-ecPublicKey
        Public-Key: (256 bit)
        pub:
          04:97:b1:ed:96:91:64:93:09:85:bb:b8:ac:9a:2a:
          f9:45:5c:df:ee:a4:b1:1d:e2:e7:9d:06:8b:fa:80:
          39:26:b4:00:52:51:b3:4f:1c:08:15:a4:cb:e0:3f:
          bd:1b:bc:b6:35:f6:43:1a:22:de:78:65:3b:87:b9:
          95:37:ec:e1:6c
        ASN1 OID: prime256v1
        NIST CURVE: P-256
    X509v3 extensions:
      X509v3 Subject Alternative Name:
        email:help@custom-er.example.com
      X509v3 Key Usage: critical
        Digital Signature, Certificate Sign, CRL Sign
      X509v3 Basic Constraints: critical
        CA:TRUE
      X509v3 Subject Key Identifier:
        92:EA:76:40:40:4A:8F:AB:4F:27:0B:F3:BC:37:9D:86:CD:72:80:F8
   Signature Algorithm: ecdsa-with-SHA256
   Signature Value:
    30:44:02:20:66:15:df:c3:70:11:f6:73:78:d8:fd:1c:2a:3f:
    bd:d1:3f:51:f6:b6:6f:2d:7c:e2:7a:13:18:21:bb:70:f0:c0:
    02:20:69:86:d8:d2:28:b2:92:6e:23:9e:19:0b:8f:18:25:c9:
    c1:4c:67:95:ff:a0:b3:24:bd:4d:ac:2e:cb:68:d7:13

   <CODE ENDS>

   Below is the hexadecimal representation of the binary X.509 DER-
   encoded certificate:

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   <CODE BEGINS>
   30820242308201E9A00302010202082AEA0413A42DC1CE300A06082A8648CE3D
   0403023072311C301A06035504030C13437573746F6D2D455220476C6F62616C
   204341310B3009060355040B0C02495431183016060355040A0C0F437573746F
   6D2D45522C20496E632E3111300F06035504070C0853616E204A6F7365310B30
   0906035504080C024341310B3009060355040613025553301E170D3232313230
   393132353034375A170D3332313230363132353034375A3072311C301A060355
   04030C13437573746F6D2D455220476C6F62616C204341310B3009060355040B
   0C02495431183016060355040A0C0F437573746F6D2D45522C20496E632E3111
   300F06035504070C0853616E204A6F7365310B300906035504080C024341310B
   30090603550406130255533059301306072A8648CE3D020106082A8648CE3D03
   01070342000497B1ED969164930985BBB8AC9A2AF9455CDFEEA4B11DE2E79D06
   8BFA803926B4005251B34F1C0815A4CBE03FBD1BBCB635F6431A22DE78653B87
   B99537ECE16CA369306730250603551D11041E301C811A68656C704063757374
   6F6D2D65722E6578616D706C652E636F6D300E0603551D0F0101FF0404030201
   86300F0603551D130101FF040530030101FF301D0603551D0E0416041492EA76
   40404A8FAB4F270BF3BC379D86CD7280F8300A06082A8648CE3D040302034700
   304402206615DFC37011F67378D8FD1C2A3FBDD13F51F6B66F2D7CE27A131821
   BB70F0C002206986D8D228B2926E239E190B8F1825C9C14C6795FFA0B324BD4D
   AC2ECB68D713

   <CODE ENDS>

C.2.4.  MASA Certificate

   The MASA CA certificate is the CA that signed the Pledge's IDevID
   certificate.

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   <CODE BEGINS>
   Certificate:
   Data:
    Version: 3 (0x2)
    Serial Number:
      e3:9c:da:17:e1:38:6a:0a
    Signature Algorithm: ecdsa-with-SHA256
    Issuer: CN = masa.stok.nl, O = vanderstok, L = Helmond,
            C = NL
    Validity
      Not Before: Dec  9 12:50:47 2022 GMT
      Not After : Dec  6 12:50:47 2032 GMT
    Subject: CN = masa.stok.nl, O = vanderstok, L = Helmond,
            C = NL
    Subject Public Key Info:
      Public Key Algorithm: id-ecPublicKey
        Public-Key: (256 bit)
        pub:
          04:92:f4:d5:65:58:97:73:b7:03:58:f3:5c:8f:94:
          7d:94:38:36:16:86:56:00:65:fb:2a:9c:43:d2:15:
          92:a7:ab:c9:aa:f2:01:7b:f1:95:5b:62:2e:aa:d8:
          51:20:f2:49:0e:45:8f:eb:a4:85:3f:5f:b4:90:b1:
          1b:83:fb:03:16
        ASN1 OID: prime256v1
        NIST CURVE: P-256
    X509v3 extensions:
      X509v3 Subject Alternative Name:
        email:info@masa.stok.nl
      X509v3 Key Usage: critical
        Digital Signature, Certificate Sign, CRL Sign
      X509v3 Basic Constraints: critical
        CA:TRUE, pathlen:3
      X509v3 Subject Key Identifier:
        CB:8D:98:CA:74:C5:1B:58:DD:E7:AC:EF:86:9A:94:43:A8:D6:66:A6
   Signature Algorithm: ecdsa-with-SHA256
   Signature Value:
    30:46:02:21:00:94:3f:a5:26:51:68:16:38:5b:78:9a:d8:c3:
    af:8e:49:28:22:60:56:26:43:4a:14:98:3e:e1:e4:81:ad:ca:
    1b:02:21:00:ba:4d:aa:fd:fa:68:42:74:03:2b:a8:41:6b:e2:
    90:0c:9e:7b:b8:c0:9c:f7:0e:3f:b4:36:8a:b3:9c:3e:31:0e

   <CODE ENDS>

   Below is the hexadecimal representation of the binary X.509 DER-
   encoded certificate:

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   <CODE BEGINS>
   308201F130820196A003020102020900E39CDA17E1386A0A300A06082A8648CE
   3D040302304B3115301306035504030C0C6D6173612E73746F6B2E6E6C311330
   11060355040A0C0A76616E64657273746F6B3110300E06035504070C0748656C
   6D6F6E64310B3009060355040613024E4C301E170D3232313230393132353034
   375A170D3332313230363132353034375A304B3115301306035504030C0C6D61
   73612E73746F6B2E6E6C31133011060355040A0C0A76616E64657273746F6B31
   10300E06035504070C0748656C6D6F6E64310B3009060355040613024E4C3059
   301306072A8648CE3D020106082A8648CE3D0301070342000492F4D565589773
   B70358F35C8F947D9438361686560065FB2A9C43D21592A7ABC9AAF2017BF195
   5B622EAAD85120F2490E458FEBA4853F5FB490B11B83FB0316A3633061301C06
   03551D11041530138111696E666F406D6173612E73746F6B2E6E6C300E060355
   1D0F0101FF04040302018630120603551D130101FF040830060101FF02010330
   1D0603551D0E04160414CB8D98CA74C51B58DDE7ACEF869A9443A8D666A6300A
   06082A8648CE3D0403020349003046022100943FA526516816385B789AD8C3AF
   8E492822605626434A14983EE1E481ADCA1B022100BA4DAAFDFA684274032BA8
   416BE2900C9E7BB8C09CF70E3FB4368AB39C3E310E

   <CODE ENDS>

C.3.  COSE-signed Pledge Voucher Request (PVR)

   In this example, the voucher request (PVR) has been signed by the
   Pledge using the IDevID private key of Appendix C.1.1, and has been
   sent to the link-local constrained Join Proxy (JP) over CoAPS to the
   JP's join port.  The join port happens to use the default CoAPS UDP
   port 5684.

     REQ: POST coaps://[JP-link-local-address]/b/rv
     Content-Format: 836
     Payload: <signed_pvr>

   When the Join Proxy receives the DTLS handshake messages from the
   Pledge, it will relay these messages to the Registrar.  The payload
   signed_voucher_request is shown as hexadecimal dump (with lf added)
   below:

   <CODE BEGINS>
   D28443A10126A0587EA11909C5A40102074823BFBBC9C2BCF2130C585B305930
   1306072A8648CE3D020106082A8648CE3D030107034200042030855BE846910B
   3ECB15CAF8572D3EB565BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868F
   C8504D370F1B26FA9759F67983BB78863EDA9ACBEA5124F60D6D4A4144413132
   33343536373839584068987DE8B007F4E9416610BBE2D48E1D7EA1032092B8BF
   CE611421950F45B22F17E214820C07E777ADF86175E25D3205568404C25FCEEC
   1B817C7861A6104B3D

   <CODE ENDS>

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   The representiation of signed_pvr in CBOR diagnostic format (with lf
   added) is:

   <CODE BEGINS>
   18([h'A10126', {}, h'A11909C5A40102074823BFBBC9C2BCF2130C585B3059301
   306072A8648CE3D020106082A8648CE3D030107034200042030855BE846910B3ECB1
   5CAF8572D3EB565BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868FC8504D370
   F1B26FA9759F67983BB78863EDA9ACBEA5124F60D6D4A41444131323334353637383
   9', h'68987DE8B007F4E9416610BBE2D48E1D7EA1032092B8BFCE611421950F45B2
   2F17E214820C07E777ADF86175E25D3205568404C25FCEEC1B817C7861A6104B3D']
   )

   <CODE ENDS>

   The COSE payload is the PVR, encoded as a CBOR byte string.  The
   diagnostic representation of it is shown below:

   <CODE BEGINS>
   {2501: {1: 2, 7: h'23BFBBC9C2BCF213', 12: h'3059301306072A8648CE3D02
   0106082A8648CE3D030107034200042030855BE846910B3ECB15CAF8572D3EB565BD
   C654A15EFAF6EDEBAA8B9E1608C5C910C93A20868FC8504D370F1B26FA9759F67983
   BB78863EDA9ACBEA5124F6', 13: "JADA123456789"}}

   <CODE ENDS>

   The Pledge uses the "proximity" (key '1', SID key 2502, enum value 2)
   assertion together with an included proximity-registrar-pubk field
   (key '12', SID 2513) to inform MASA about its proximity to the
   specific Registrar.

C.4.  COSE-signed Registrar Voucher Request (RVR)

   In this example the Registrar's voucher request has been signed by
   the JRC (Registrar) using the private key from Appendix C.1.2.
   Contained within this voucher request is the voucher request PVR that
   was made by the Pledge to JRC.  Note that the RVR uses the HTTPS
   protocol (not CoAP) and corresponding long URI path names as defined
   in [RFC8995].  The Content-Type and Accept headers indicate the
   constrained voucher format that is defined in the present document.
   Because the Pledge used this format in the PVR, the JRC must also use
   this format in the RVR.

     REQ: POST https://masa.stok.nl/.well-known/brski/requestvoucher
     Content-Type: application/voucher-cose+cbor
     Accept: application/voucher-cose+cbor
     Body: <signed_rvr>

   The payload signed_rvr is shown as hexadecimal dump (with lf added):

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   <CODE BEGINS>
   D28443A10126A11820825902843082028030820225A003020102020900C3F621
   49B2E30E3E300A06082A8648CE3D0403023072311C301A06035504030C134375
   73746F6D2D455220476C6F62616C204341310B3009060355040B0C0249543118
   3016060355040A0C0F437573746F6D2D45522C20496E632E3111300F06035504
   070C0853616E204A6F7365310B300906035504080C024341310B300906035504
   0613025553301E170D3232313230363131333735395A170D3235313230353131
   333735395A30818D3131302F06035504030C28437573746F6D2D455220436F6D
   6D65726369616C204275696C64696E6773205265676973747261723113301106
   0355040B0C0A4F6666696365206F707331183016060355040A0C0F437573746F
   6D2D45522C20496E632E310F300D06035504070C064F74746F7761310B300906
   035504080C024F4E310B30090603550406130243413059301306072A8648CE3D
   020106082A8648CE3D030107034200042030855BE846910B3ECB15CAF8572D3E
   B565BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868FC8504D370F1B26FA
   9759F67983BB78863EDA9ACBEA5124F6A3818730818430090603551D13040230
   00300B0603551D0F0404030204F0301D0603551D0E04160414C9080B387D8DD8
   5B3A59E7EC100B866393A9CA4C301F0603551D2304183016801492EA7640404A
   8FAB4F270BF3BC379D86CD7280F8302A0603551D250101FF0420301E06082B06
   01050507031C06082B0601050507030106082B06010505070302300A06082A86
   48CE3D040302034900304602210091A2033692EB81503D53505FFC8DA326B1EE
   7DEA96F29174F0B3341A07812201022100FF7339288108B712F418530A18025A
   895408CC45E0BB678B46FBAB37DDB4D36B59024730820243308201E9A0030201
   0202082AEA0413A42DC1CE300A06082A8648CE3D0403023072311C301A060355
   04030C13437573746F6D2D455220476C6F62616C204341310B3009060355040B
   0C02495431183016060355040A0C0F437573746F6D2D45522C20496E632E3111
   300F06035504070C0853616E204A6F7365310B300906035504080C024341310B
   3009060355040613025553301E170D3232313230363131333735395A170D3332
   313230333131333735395A3072311C301A06035504030C13437573746F6D2D45
   5220476C6F62616C204341310B3009060355040B0C0249543118301606035504
   0A0C0F437573746F6D2D45522C20496E632E3111300F06035504070C0853616E
   204A6F7365310B300906035504080C024341310B300906035504061302555330
   59301306072A8648CE3D020106082A8648CE3D0301070342000497B1ED969164
   930985BBB8AC9A2AF9455CDFEEA4B11DE2E79D068BFA803926B4005251B34F1C
   0815A4CBE03FBD1BBCB635F6431A22DE78653B87B99537ECE16CA3693067300F
   0603551D130101FF040530030101FF30250603551D11041E301C811A68656C70
   40637573746F6D2D65722E6578616D706C652E636F6D300E0603551D0F0101FF
   040403020186301D0603551D0E0416041492EA7640404A8FAB4F270BF3BC379D
   86CD7280F8300A06082A8648CE3D0403020348003045022100D6D813B390BD3A
   7B4E85424BCB1ED933AD1E981F2817B59083DD6EC1C5E3FADF02202CEE440619
   2BC767E98D7CFAE044C6807481AD8564A7D569DCA3D1CDF1E5E843590124A119
   09C5A60102027818323032322D31322D30365432303A30343A31352E3735345A
   05581A041830168014CB8D98CA74C51B58DDE7ACEF869A9443A8D666A6074823
   BFBBC9C2BCF2130958C9D28443A10126A0587EA11909C5A40102074823BFBBC9
   C2BCF2130C585B3059301306072A8648CE3D020106082A8648CE3D0301070342
   00042030855BE846910B3ECB15CAF8572D3EB565BDC654A15EFAF6EDEBAA8B9E
   1608C5C910C93A20868FC8504D370F1B26FA9759F67983BB78863EDA9ACBEA51
   24F60D6D4A414441313233343536373839584068987DE8B007F4E9416610BBE2
   D48E1D7EA1032092B8BFCE611421950F45B22F17E214820C07E777ADF86175E2

Richardson, et al.         Expires 6 July 2023                 [Page 82]
Internet-Draft              Constrained BRSKI               January 2023

   5D3205568404C25FCEEC1B817C7861A6104B3D0D6D4A41444131323334353637
   38395840B1DD40B10787437588AEAC9036899191C16CCDBECA31C197855CCB6B
   BA142D709FE329CBC3F76297D6063ACB6759EAB98E96EA4C4AA2135AA48A247B
   AC1D6A3F

   <CODE ENDS>

   The representiation of signed_rvr in CBOR diagnostic format (with lf
   added) is:

   <CODE BEGINS>
   18([h'A10126', {32: [h'3082028030820225A003020102020900C3F62149B2E30
   E3E300A06082A8648CE3D0403023072311C301A06035504030C13437573746F6D2D4
   55220476C6F62616C204341310B3009060355040B0C02495431183016060355040A0
   C0F437573746F6D2D45522C20496E632E3111300F06035504070C0853616E204A6F7
   365310B300906035504080C024341310B3009060355040613025553301E170D32323
   13230363131333735395A170D3235313230353131333735395A30818D3131302F060
   35504030C28437573746F6D2D455220436F6D6D65726369616C204275696C64696E6
   7732052656769737472617231133011060355040B0C0A4F6666696365206F7073311
   83016060355040A0C0F437573746F6D2D45522C20496E632E310F300D06035504070
   C064F74746F7761310B300906035504080C024F4E310B30090603550406130243413
   059301306072A8648CE3D020106082A8648CE3D030107034200042030855BE846910
   B3ECB15CAF8572D3EB565BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868FC85
   04D370F1B26FA9759F67983BB78863EDA9ACBEA5124F6A3818730818430090603551
   D1304023000300B0603551D0F0404030204F0301D0603551D0E04160414C9080B387
   D8DD85B3A59E7EC100B866393A9CA4C301F0603551D2304183016801492EA7640404
   A8FAB4F270BF3BC379D86CD7280F8302A0603551D250101FF0420301E06082B06010
   50507031C06082B0601050507030106082B06010505070302300A06082A8648CE3D0
   40302034900304602210091A2033692EB81503D53505FFC8DA326B1EE7DEA96F2917
   4F0B3341A07812201022100FF7339288108B712F418530A18025A895408CC45E0BB6
   78B46FBAB37DDB4D36B', h'30820243308201E9A00302010202082AEA0413A42DC1
   CE300A06082A8648CE3D0403023072311C301A06035504030C13437573746F6D2D45
   5220476C6F62616C204341310B3009060355040B0C02495431183016060355040A0C
   0F437573746F6D2D45522C20496E632E3111300F06035504070C0853616E204A6F73
   65310B300906035504080C024341310B3009060355040613025553301E170D323231
   3230363131333735395A170D3332313230333131333735395A3072311C301A060355
   04030C13437573746F6D2D455220476C6F62616C204341310B3009060355040B0C02
   495431183016060355040A0C0F437573746F6D2D45522C20496E632E3111300F0603
   5504070C0853616E204A6F7365310B300906035504080C024341310B300906035504
   06130255533059301306072A8648CE3D020106082A8648CE3D0301070342000497B1
   ED969164930985BBB8AC9A2AF9455CDFEEA4B11DE2E79D068BFA803926B4005251B3
   4F1C0815A4CBE03FBD1BBCB635F6431A22DE78653B87B99537ECE16CA3693067300F
   0603551D130101FF040530030101FF30250603551D11041E301C811A68656C704063
   7573746F6D2D65722E6578616D706C652E636F6D300E0603551D0F0101FF04040302
   0186301D0603551D0E0416041492EA7640404A8FAB4F270BF3BC379D86CD7280F830
   0A06082A8648CE3D0403020348003045022100D6D813B390BD3A7B4E85424BCB1ED9
   33AD1E981F2817B59083DD6EC1C5E3FADF02202CEE4406192BC767E98D7CFAE044C6
   807481AD8564A7D569DCA3D1CDF1E5E843']}, h'A11909C5A601020278183230323

Richardson, et al.         Expires 6 July 2023                 [Page 83]
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   22D31322D30365432303A30343A31352E3735345A05581A041830168014CB8D98CA7
   4C51B58DDE7ACEF869A9443A8D666A6074823BFBBC9C2BCF2130958C9D28443A1012
   6A0587EA11909C5A40102074823BFBBC9C2BCF2130C585B3059301306072A8648CE3
   D020106082A8648CE3D030107034200042030855BE846910B3ECB15CAF8572D3EB56
   5BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868FC8504D370F1B26FA9759F67
   983BB78863EDA9ACBEA5124F60D6D4A414441313233343536373839584068987DE8B
   007F4E9416610BBE2D48E1D7EA1032092B8BFCE611421950F45B22F17E214820C07E
   777ADF86175E25D3205568404C25FCEEC1B817C7861A6104B3D0D6D4A41444131323
   3343536373839', h'B1DD40B10787437588AEAC9036899191C16CCDBECA31C19785
   5CCB6BBA142D709FE329CBC3F76297D6063ACB6759EAB98E96EA4C4AA2135AA48A24
   7BAC1D6A3F'])

   <CODE ENDS>

C.5.  COSE-signed Voucher from MASA

   The resulting voucher is created by the MASA and returned to the
   Registrar:

     RES: 200 OK
     Content-Type: application/voucher-cose+cbor
     Body: <signed_voucher>

   The Registrar then returns the voucher to the Pledge:

     RES: 2.04 Changed
     Content-Format: 836
     Body: <signed_voucher>

   It is signed by the MASA's private key (see Appendix C.1.3) and can
   be verified by the Pledge using the MASA's public key that it stores.

   Below is the binary signed_voucher, encoded in hexadecimal (with lf
   added):

Richardson, et al.         Expires 6 July 2023                 [Page 84]
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   <CODE BEGINS>
   D28443A10126A0590288A1190993A60102027818323032322D31322D30365432
   303A32333A33302E3730385A03F4074857EED786AD4049070859024730820243
   308201E9A00302010202082AEA0413A42DC1CE300A06082A8648CE3D04030230
   72311C301A06035504030C13437573746F6D2D455220476C6F62616C20434131
   0B3009060355040B0C02495431183016060355040A0C0F437573746F6D2D4552
   2C20496E632E3111300F06035504070C0853616E204A6F7365310B3009060355
   04080C024341310B3009060355040613025553301E170D323231323036313133
   3735395A170D3332313230333131333735395A3072311C301A06035504030C13
   437573746F6D2D455220476C6F62616C204341310B3009060355040B0C024954
   31183016060355040A0C0F437573746F6D2D45522C20496E632E3111300F0603
   5504070C0853616E204A6F7365310B300906035504080C024341310B30090603
   550406130255533059301306072A8648CE3D020106082A8648CE3D0301070342
   000497B1ED969164930985BBB8AC9A2AF9455CDFEEA4B11DE2E79D068BFA8039
   26B4005251B34F1C0815A4CBE03FBD1BBCB635F6431A22DE78653B87B99537EC
   E16CA3693067300F0603551D130101FF040530030101FF30250603551D11041E
   301C811A68656C7040637573746F6D2D65722E6578616D706C652E636F6D300E
   0603551D0F0101FF040403020186301D0603551D0E0416041492EA7640404A8F
   AB4F270BF3BC379D86CD7280F8300A06082A8648CE3D04030203480030450221
   00D6D813B390BD3A7B4E85424BCB1ED933AD1E981F2817B59083DD6EC1C5E3FA
   DF02202CEE4406192BC767E98D7CFAE044C6807481AD8564A7D569DCA3D1CDF1
   E5E8430B6D4A4144413132333435363738395840DF31B21A6AD3F5AC7F4C8B02
   6F551BD28FBCE62330D3E262AC170F6BFEDDBA5F2E8FBAA2CAACFED9E8614EAC
   5BF2450DADC53AC29DFA30E8787A1400B2E7C832

   <CODE ENDS>

   The representiation of signed_voucher in CBOR diagnostic format (with
   lf added) is:

Richardson, et al.         Expires 6 July 2023                 [Page 85]
Internet-Draft              Constrained BRSKI               January 2023

   <CODE BEGINS>
   18([h'A10126', {}, h'A1190993A60102027818323032322D31322D30365432303
   A32333A33302E3730385A03F4074857EED786AD4049070859024730820243308201E
   9A00302010202082AEA0413A42DC1CE300A06082A8648CE3D0403023072311C301A0
   6035504030C13437573746F6D2D455220476C6F62616C204341310B3009060355040
   B0C02495431183016060355040A0C0F437573746F6D2D45522C20496E632E3111300
   F06035504070C0853616E204A6F7365310B300906035504080C024341310B3009060
   355040613025553301E170D3232313230363131333735395A170D333231323033313
   1333735395A3072311C301A06035504030C13437573746F6D2D455220476C6F62616
   C204341310B3009060355040B0C02495431183016060355040A0C0F437573746F6D2
   D45522C20496E632E3111300F06035504070C0853616E204A6F7365310B300906035
   504080C024341310B30090603550406130255533059301306072A8648CE3D0201060
   82A8648CE3D0301070342000497B1ED969164930985BBB8AC9A2AF9455CDFEEA4B11
   DE2E79D068BFA803926B4005251B34F1C0815A4CBE03FBD1BBCB635F6431A22DE786
   53B87B99537ECE16CA3693067300F0603551D130101FF040530030101FF302506035
   51D11041E301C811A68656C7040637573746F6D2D65722E6578616D706C652E636F6
   D300E0603551D0F0101FF040403020186301D0603551D0E0416041492EA7640404A8
   FAB4F270BF3BC379D86CD7280F8300A06082A8648CE3D0403020348003045022100D
   6D813B390BD3A7B4E85424BCB1ED933AD1E981F2817B59083DD6EC1C5E3FADF02202
   CEE4406192BC767E98D7CFAE044C6807481AD8564A7D569DCA3D1CDF1E5E8430B6D4
   A414441313233343536373839', h'DF31B21A6AD3F5AC7F4C8B026F551BD28FBCE6
   2330D3E262AC170F6BFEDDBA5F2E8FBAA2CAACFED9E8614EAC5BF2450DADC53AC29D
   FA30E8787A1400B2E7C832'])

   <CODE ENDS>

   In the above, the third element in the array is the plain CBOR
   voucher encoded as a CBOR byte string.  When decoded, it can be
   represented by the following CBOR diagnostic notation:

Richardson, et al.         Expires 6 July 2023                 [Page 86]
Internet-Draft              Constrained BRSKI               January 2023

   <CODE BEGINS>
   {2451: {1: 2, 2: "2022-12-06T20:23:30.708Z", 3: false, 7: h'57EED786
   AD404907', 8: h'30820243308201E9A00302010202082AEA0413A42DC1CE300A06
   082A8648CE3D0403023072311C301A06035504030C13437573746F6D2D455220476C
   6F62616C204341310B3009060355040B0C02495431183016060355040A0C0F437573
   746F6D2D45522C20496E632E3111300F06035504070C0853616E204A6F7365310B30
   0906035504080C024341310B3009060355040613025553301E170D32323132303631
   31333735395A170D3332313230333131333735395A3072311C301A06035504030C13
   437573746F6D2D455220476C6F62616C204341310B3009060355040B0C0249543118
   3016060355040A0C0F437573746F6D2D45522C20496E632E3111300F06035504070C
   0853616E204A6F7365310B300906035504080C024341310B30090603550406130255
   533059301306072A8648CE3D020106082A8648CE3D0301070342000497B1ED969164
   930985BBB8AC9A2AF9455CDFEEA4B11DE2E79D068BFA803926B4005251B34F1C0815
   A4CBE03FBD1BBCB635F6431A22DE78653B87B99537ECE16CA3693067300F0603551D
   130101FF040530030101FF30250603551D11041E301C811A68656C7040637573746F
   6D2D65722E6578616D706C652E636F6D300E0603551D0F0101FF040403020186301D
   0603551D0E0416041492EA7640404A8FAB4F270BF3BC379D86CD7280F8300A06082A
   8648CE3D0403020348003045022100D6D813B390BD3A7B4E85424BCB1ED933AD1E98
   1F2817B59083DD6EC1C5E3FADF02202CEE4406192BC767E98D7CFAE044C6807481AD
   8564A7D569DCA3D1CDF1E5E843', 11: "JADA123456789"}}

   <CODE ENDS>

   The largest element in the voucher is identified by key 8, which
   decodes to SID key 2459 (pinned-domain-cert).  It contains the
   complete DER-encoded X.509 certificate of the Registrar's domain CA.
   This certificate is shown in Appendix C.2.3.

Appendix D.  Generating Certificates with OpenSSL

   This informative appendix shows example Bash shell scripts to
   generate test certificates for the Pledge IDevID, the Registrar and
   the MASA.  The shell scripts cannot be run stand-alone because they
   depend on input files which are not all included in this appendix.
   Nevertheless, these scripts may provide guidance on how OpenSSL can
   be configured for generating Constrained BRSKI certificates.

   The scripts were tested with OpenSSL 3.0.2.  Older versions may not
   work -- OpenSSL 1.1.1 for example does not support all extensions
   used.

Richardson, et al.         Expires 6 July 2023                 [Page 87]
Internet-Draft              Constrained BRSKI               January 2023

   <CODE BEGINS>
   #!/bin/bash
   # File: create-cert-Pledge.sh
   # Create new cert for: Pledge IDevID

   # days certificate is valid - try to get close to the 802.1AR
   # specified 9999-12-31 end date.
   SECONDS1=`date +%s` # time now
   SECONDS2=`date --date="9999-12-31 23:59:59Z" +%s` # target end time
   let VALIDITY="(${SECONDS2}-${SECONDS1})/(24*3600)"
   echo "Using validity param -days ${VALIDITY}"

   NAME=pledge

   # create csr for device
   # conform to 802.1AR guidelines, using only CN + serialNumber when
   # manufacturer is already present as CA.
   # CN is not even mandatory, but just good practice.
   openssl req -new -key keys/privkey_pledge.pem -out $NAME.csr -subj \
                "/CN=Stok IoT sensor Y-42/serialNumber=JADA123456789"

   # sign csr
   openssl x509 -set_serial 32429 -CAform PEM -CA output/masa_ca.pem \
     -CAkey keys/privkey_masa_ca.pem -extfile x509v3.ext -extensions \
     pledge_ext -req -in $NAME.csr -out output/$NAME.pem \
     -days $VALIDITY -sha256

   # delete temp files
   rm -f $NAME.csr

   # convert to .der format
   openssl x509 -in output/$NAME.pem -inform PEM -out output/$NAME.der \
                -outform DER

   <CODE ENDS>

   <CODE BEGINS>
   # File: x509v3.ext
   # This file contains all X509v3 extension definitions for OpenSSL
   # certificate generation. Each certificate has its own _ext
   # section below.

   [ req ]
   prompt = no

   [ masa_ca_ext ]
   subjectAltName=email:info@masa.stok.nl
   keyUsage = critical,digitalSignature, keyCertSign, cRLSign

Richardson, et al.         Expires 6 July 2023                 [Page 88]
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   basicConstraints = critical,CA:TRUE,pathlen:3
   subjectKeyIdentifier=hash
   authorityKeyIdentifier=keyid

   [ pledge_ext ]
   keyUsage = critical,digitalSignature, nonRepudiation, \
              keyEncipherment, dataEncipherment
   # basicConstraints for a non-CA cert MAY be marked either
   # non-critical or critical.
   basicConstraints = CA:FALSE
   # Don't include subjectKeyIdentifier (SKI) - see 802.1AR-2018
   subjectKeyIdentifier = none
   authorityKeyIdentifier=keyid
   # Include the MASA URI
   1.3.6.1.5.5.7.1.32 = ASN1:IA5STRING:masa.stok.nl

   [ domain_ca_ext ]
   subjectAltName=email:help@custom-er.example.com
   keyUsage = critical, keyCertSign, digitalSignature, cRLSign
   basicConstraints=critical,CA:TRUE
   # RFC 5280 4.2.1.1 : AKI MAY be omitted, and MUST be non-critical;
   # SKI MUST be non-critical
   subjectKeyIdentifier=hash

   [ registrar_ext ]
   keyUsage = critical, digitalSignature, nonRepudiation, \
              keyEncipherment, dataEncipherment
   basicConstraints=CA:FALSE
   subjectKeyIdentifier=hash
   authorityKeyIdentifier=keyid
   # Set Registrar 'RA' flag along with TLS client/server usage
   #  see draft-ietf-anima-constrained-voucher#section-7.3
   #  see tools.ietf.org/html/rfc6402#section-2.10
   #  see www.openssl.org/docs/man1.1.1/man5/x509v3_config.html
   extendedKeyUsage = critical,1.3.6.1.5.5.7.3.28, serverAuth, \
                      clientAuth

   <CODE ENDS>

Richardson, et al.         Expires 6 July 2023                 [Page 89]
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   <CODE BEGINS>
   #!/bin/bash
   # File: create-cert-Registrar.sh
   # Create new cert for: Registrar in a company domain

   # days certificate is valid
   VALIDITY=1095

   # cert filename
   NAME=registrar

   # create csr
   openssl req -new -key keys/privkey_registrar.pem -out $NAME.csr \
    -subj "/CN=Custom-ER Registrar/OU=Office dept/O=Custom-ER, Inc./\
   L=Ottowa/ST=ON/C=CA"

   # sign csr
   openssl x509 -set_serial 0xC3F62149B2E30E3E -CAform PEM -CA \
    output/domain_ca.pem -extfile x509v3.ext -extensions registrar_ext \
    -req -in $NAME.csr -CAkey keys/privkey_domain_ca.pem \
    -out output/$NAME.pem -days $VALIDITY -sha256

   # delete temp files
   rm -f $NAME.csr

   # convert to .der format
   openssl x509 -in output/$NAME.pem -inform PEM -out output/$NAME.der \
                -outform DER

   <CODE ENDS>

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   <CODE BEGINS>
   #!/bin/bash
   # File: create-cert-MASA.sh
   # Create new cert for: MASA CA, self-signed CA certificate

   # days certificate is valid
   VALIDITY=3650

   NAME=masa_ca

   # create csr
   openssl req -new -key keys/privkey_masa_ca.pem -out $NAME.csr \
               -subj "/CN=masa.stok.nl/O=vanderstok/L=Helmond/C=NL"

   # sign csr
   mkdir output >& /dev/null
   openssl x509 -set_serial 0xE39CDA17E1386A0A  -extfile x509v3.ext \
    -extensions masa_ca_ext -req -in $NAME.csr \
    -signkey keys/privkey_masa_ca.pem -out output/$NAME.pem \
    -days $VALIDITY -sha256

   # delete temp files
   rm -f $NAME.csr

   # convert to .der format
   openssl x509 -in output/$NAME.pem -inform PEM -out output/$NAME.der \
                -outform DER

   <CODE ENDS>

Appendix E.  Pledge Device Class Profiles

   This specification allows implementers to select between various
   functional options for the Pledge, yielding different code size
   footprints and different requirements on Pledge hardware.  Thus for
   each product an optimal trade-off between functionality, development/
   maintenance cost and hardware cost can be made.

   This appendix illustrates different selection outcomes by means of
   defining different example "profiles" of constrained Pledges.  In the
   following subsections, these profiles are defined and a comparison is
   provided.

E.1.  Minimal Pledge

   The Minimal Pledge profile (Min) aims to reduce code size and
   hardware cost to a minimum.  This comes with some severe functional
   restrictions, in particular:

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   *  No support for EST re-enrollment: whenever this would be needed, a
      factory reset followed by a new bootstrap process is required.

   *  No support for change of Registrar: for this case, a factory reset
      followed by a new bootstrap process is required.

   This profile would be appropriate for single-use devices which must
   be replaced rather than re-deployed.  That might include medical
   devices, but also sensors used during construction, such as concrete
   temperature sensors.

E.2.  Typical Pledge

   The Typical Pledge profile (Typ) aims to support a typical
   Constrained BRSKI feature set including EST re-enrollment support and
   Registrar changes.

E.3.  Full-featured Pledge

   The Full-featured Pledge profile (Full) illustrates a Pledge category
   that supports multiple bootstrap methods, hardware real-time clock,
   BRSKI/EST resource discovery, and CSR Attributes request/response.
   It also supports most of the optional features defined in this
   specification.

E.4.  Comparison Chart of Pledge Classes

   The below table specifies the functions implemented in the three
   example Pledge classes Min, Typ and Full.

   +==============================================+=======+=====+======+
   | Function |====================| Profiles ->  |  Min  | Typ | Full |
   +==============================================+=======+=====+======+
   | *General*                                    |  ===  | === | ==== |
   +----------------------------------------------+-------+-----+------+
   | Support Constrained BRSKI bootstrap          |   Y   |  Y  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | Support other bootstrap method(s)            |   -   |  -  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | Real-time clock and cert time checks         |   -   |  -  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | *Constrained BRSKI*                          |  ===  | === | ==== |
   +----------------------------------------------+-------+-----+------+
   | Discovery for rt=brski*                      |   -   |  -  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | Support pinned Registrar public key (RPK)    |   Y   |  -  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | Support pinned Registrar certificate         |   -   |  Y  |  Y   |

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   +----------------------------------------------+-------+-----+------+
   | Support pinned Domain CA                     |   -   |  Y  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | *Constrained EST*                            |  ===  | === | ==== |
   +----------------------------------------------+-------+-----+------+
   | Discovery for rt=ace.est*                    |   -   |  -  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | GET /att and response parsing                |   -   |  -  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | GET /crts format 281 (multiple CA certs)     |   -   |  -  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | GET /crts only format 287 (one CA cert only) |   Y   |  Y  |  -   |
   +----------------------------------------------+-------+-----+------+
   | ETag handling support for GET /crts          |   -   |  Y  |  Y   |
   +----------------------------------------------+-------+-----+------+
   | Re-enrollment supported                      |   -   |  Y  |  Y   |
   |                                              |  (1)  |     |      |
   +----------------------------------------------+-------+-----+------+
   | 6.6.1 optimized procedure                    |   Y   |  Y  |  -   |
   +----------------------------------------------+-------+-----+------+
   | Pro-active cert re-enrollment at own         |  N/A  |  -  |  Y   |
   | initiative                                   |       |     |      |
   +----------------------------------------------+-------+-----+------+
   | Periodic trust anchor retrieval GET /crts    |   -   |  Y  |  Y   |
   |                                              |  (1)  |     |      |
   +----------------------------------------------+-------+-----+------+
   | Supports change of Registrar identity        |   -   |  Y  |  Y   |
   |                                              |  (1)  |     |      |
   +----------------------------------------------+-------+-----+------+

                                  Table 6

   Notes: (1) is possible only by doing a factory-reset followed by a
   new bootstrap procedure.

Authors' Addresses

   Michael Richardson
   Sandelman Software Works
   Email: mcr+ietf@sandelman.ca

   Peter van der Stok
   vanderstok consultancy
   Email: stokcons@bbhmail.nl

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   Panos Kampanakis
   Cisco Systems
   Email: pkampana@cisco.com

   Esko Dijk
   IoTconsultancy.nl
   Email: esko.dijk@iotconsultancy.nl

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