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Constrained Bootstrapping Remote Secure Key Infrastructure (BRSKI)
draft-ietf-anima-constrained-voucher-17

The information below is for an old version of the document.
Document Type This is an older version of an Internet-Draft whose latest revision is Active
Authors Michael Richardson , Peter Van der Stok , Panos Kampanakis , Esko Dijk
Last updated 2022-04-08 (Latest revision 2022-04-07)
Replaces draft-richardson-anima-ace-constrained-voucher
Stream Internet Engineering Task Force (IETF)
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draft-ietf-anima-constrained-voucher-17
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: 9 October 2022                                    P. Kampanakis
                                                           Cisco Systems
                                                                 E. Dijk
                                                       IoTconsultancy.nl
                                                            7 April 2022

   Constrained Bootstrapping Remote Secure Key Infrastructure (BRSKI)
                draft-ietf-anima-constrained-voucher-17

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.

Status of This Memo

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

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

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

   This Internet-Draft will expire on 9 October 2022.

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Copyright Notice

   Copyright (c) 2022 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   4.  Overview of Protocol  . . . . . . . . . . . . . . . . . . . .   6
   5.  Updates to RFC8366 and RFC8995  . . . . . . . . . . . . . . .   7
   6.  BRSKI-EST Protocol  . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Registrar and the Server Name Indicator (SNI) . . . . . .   8
     6.2.  TLS Client Certificates: IDevID authentication  . . . . .   9
     6.3.  Discovery, URIs and Content Formats . . . . . . . . . . .  10
       6.3.1.  RFC8995 Telemetry Returns . . . . . . . . . . . . . .  12
     6.4.  Join Proxy options  . . . . . . . . . . . . . . . . . . .  13
     6.5.  Extensions to BRSKI . . . . . . . . . . . . . . . . . . .  13
       6.5.1.  Discovery . . . . . . . . . . . . . . . . . . . . . .  13
       6.5.2.  CoAP responses  . . . . . . . . . . . . . . . . . . .  13
     6.6.  Extensions to EST-coaps . . . . . . . . . . . . . . . . .  14
       6.6.1.  Pledge Extensions . . . . . . . . . . . . . . . . . .  15
       6.6.2.  EST-client Extensions . . . . . . . . . . . . . . . .  16
       6.6.3.  Registrar Extensions  . . . . . . . . . . . . . . . .  18
     6.7.  DTLS handshake fragmentation Considerations . . . . . . .  19
   7.  BRSKI-MASA Protocol . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Protocol and Formats  . . . . . . . . . . . . . . . . . .  19
     7.2.  Registrar Voucher Request . . . . . . . . . . . . . . . .  20
     7.3.  MASA and the Server Name Indicator (SNI)  . . . . . . . .  20
       7.3.1.  Registrar Certificate Requirement . . . . . . . . . .  21
   8.  Pinning in Voucher Artifacts  . . . . . . . . . . . . . . . .  21
     8.1.  Registrar Identity Selection and Encoding . . . . . . . .  21
     8.2.  MASA Pinning Policy . . . . . . . . . . . . . . . . . . .  23
     8.3.  Pinning of Raw Public Keys  . . . . . . . . . . . . . . .  24
     8.4.  Considerations for use of IDevID-Issuer . . . . . . . . .  25
   9.  Artifacts . . . . . . . . . . . . . . . . . . . . . . . . . .  26
     9.1.  Voucher Request artifact  . . . . . . . . . . . . . . . .  26
       9.1.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . .  26

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       9.1.2.  SID values  . . . . . . . . . . . . . . . . . . . . .  27
       9.1.3.  YANG Module . . . . . . . . . . . . . . . . . . . . .  28
       9.1.4.  Example voucher request artifact  . . . . . . . . . .  32
     9.2.  Voucher artifact  . . . . . . . . . . . . . . . . . . . .  32
       9.2.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . .  32
       9.2.2.  SID values  . . . . . . . . . . . . . . . . . . . . .  33
       9.2.3.  YANG Module . . . . . . . . . . . . . . . . . . . . .  33
       9.2.4.  Example voucher artifacts . . . . . . . . . . . . . .  36
     9.3.  Signing voucher and voucher-request artifacts with
           COSE  . . . . . . . . . . . . . . . . . . . . . . . . . .  37
   10. Deployment-specific Discovery Considerations  . . . . . . . .  38
     10.1.  6TSCH Deployments  . . . . . . . . . . . . . . . . . . .  39
     10.2.  Generic networks using GRASP . . . . . . . . . . . . . .  39
     10.3.  Generic networks using mDNS  . . . . . . . . . . . . . .  39
     10.4.  Thread networks using Mesh Link Establishment (MLE)  . .  39
     10.5.  Non-mesh networks using CoAP Discovery . . . . . . . . .  40
   11. Design Considerations . . . . . . . . . . . . . . . . . . . .  40
   12. Raw Public Key Use Considerations . . . . . . . . . . . . . .  40
     12.1.  The Registrar Trust Anchor . . . . . . . . . . . . . . .  40
     12.2.  The Pledge Voucher Request . . . . . . . . . . . . . . .  41
     12.3.  The Voucher Response . . . . . . . . . . . . . . . . . .  41
   13. Use of constrained vouchers with HTTPS  . . . . . . . . . . .  41
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  42
     14.1.  Duplicate serial-numbers . . . . . . . . . . . . . . . .  42
     14.2.  IDevID security in Pledge  . . . . . . . . . . . . . . .  43
     14.3.  Security of CoAP and UDP protocols . . . . . . . . . . .  44
     14.4.  Registrar Certificate may be self-signed . . . . . . . .  45
     14.5.  Use of RPK alternatives to proximity-registrar-cert  . .  45
     14.6.  MASA support of CoAPS  . . . . . . . . . . . . . . . . .  46
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  46
     15.1.  Resource Type Registry . . . . . . . . . . . . . . . . .  46
     15.2.  The IETF XML Registry  . . . . . . . . . . . . . . . . .  47
     15.3.  The YANG Module Names Registry . . . . . . . . . . . . .  47
     15.4.  The RFC SID range assignment sub-registry  . . . . . . .  47
     15.5.  Media Types Registry . . . . . . . . . . . . . . . . . .  48
       15.5.1.  application/voucher-cose+cbor  . . . . . . . . . . .  48
     15.6.  CoAP Content-Format Registry . . . . . . . . . . . . . .  48
     15.7.  Update to BRSKI Parameters Registry  . . . . . . . . . .  49
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  49
   17. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  50
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     18.2.  Informative References . . . . . . . . . . . . . . . . .  54
   Appendix A.  Library Support for BRSKI  . . . . . . . . . . . . .  56
     A.1.  OpensSSL  . . . . . . . . . . . . . . . . . . . . . . . .  57
     A.2.  mbedTLS . . . . . . . . . . . . . . . . . . . . . . . . .  58
   Appendix B.  Constrained BRSKI-EST Message Examples . . . . . . .  59
     B.1.  enrollstatus  . . . . . . . . . . . . . . . . . . . . . .  59

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     B.2.  voucher_status  . . . . . . . . . . . . . . . . . . . . .  60
   Appendix C.  COSE-signed Voucher (Request) Examples . . . . . . .  61
     C.1.  Pledge, Registrar and MASA Keys . . . . . . . . . . . . .  61
       C.1.1.  Pledge IDevID private key . . . . . . . . . . . . . .  61
       C.1.2.  Registrar private key . . . . . . . . . . . . . . . .  61
       C.1.3.  MASA private key  . . . . . . . . . . . . . . . . . .  62
     C.2.  Pledge, Registrar and MASA Certificates . . . . . . . . .  62
       C.2.1.  Pledge IDevID Certificate . . . . . . . . . . . . . .  62
       C.2.2.  Registrar Certificate . . . . . . . . . . . . . . . .  64
       C.2.3.  MASA Certificate  . . . . . . . . . . . . . . . . . .  66
     C.3.  COSE-signed Pledge Voucher Request (PVR)  . . . . . . . .  68
     C.4.  COSE-signed Registrar Voucher Request (RVR) . . . . . . .  70
     C.5.  COSE-signed Voucher from MASA . . . . . . . . . . . . . .  72
   Appendix D.  Generating Certificates with OpenSSL . . . . . . . .  74
   Appendix E.  Pledge Device Class Profiles . . . . . . . . . . . .  77
     E.1.  Minimal Pledge  . . . . . . . . . . . . . . . . . . . . .  78
     E.2.  Typical Pledge  . . . . . . . . . . . . . . . . . . . . .  78
     E.3.  Full-featured Pledge  . . . . . . . . . . . . . . . . . .  78
     E.4.  Comparison Chart of Pledge Classes  . . . . . . . . . . .  78
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  79
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  80

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
   [I-D.ietf-ace-coap-est]) 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.

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   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
   [I-D.ietf-cose-rfc8152bis-struct].

   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 [I-D.ietf-ace-coap-est].

   The CBOR definitions for the constrained voucher format are defined
   using the mechanism described in [I-D.ietf-core-yang-cbor] 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.

   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.

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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.

4.  Overview of Protocol

   [RFC8366] provides for vouchers that assert proximity, authenticate
   the Registrar, and can offer varying levels of anti-replay
   protection.

   The proximity proof provided for in [RFC8366], is an assertion that
   the Pledge and the Registrar are believed to be close together, from
   a network topology point of view.  Like in [RFC8995], proximity is
   shown by making TLS connections between the Pledge and Registrar
   using IPv6 Link-Local addresses.

   The TLS connection is used to make a Voucher Request.  This request
   is verified by an agent of the Pledge's manufacturer, which then
   issues a voucher.  The voucher provides an authorization statement
   from the manufacturer indicating that the Registrar is the intended
   owner of the device.  The voucher refers to the Registrar through
   pinning of the Registrar's identity.

   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 X.509
   certificate, or if an IDevID is not available, then the private key
   associated with its manufacturer-installed raw public key (RPK).
   Section 12 provides additional details on PKIX-less operations.

   The constrained voucher MUST be signed by the MASA.

   For the constrained voucher request this document defines two
   distinct methods for the Pledge to identify the Registrar: using
   either the Registrar's X.509 certificate, or using a raw public key
   (RPK) of the Registrar.

   For the constrained voucher both methods are supported to indicate
   (pin) a trusted domain identity: using either a pinned domain X.509
   certificate, or a pinned raw public key (RPK).

   The BRSKI architectures mandates that the MASA be aware of the
   capabilities of the pledge.  This is not a drawback as the pledges
   are 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, PKIX
   format certificates, or if the pledge is limited to Raw Public Keys
   (RPK).  Based upon this, the MASA can select which attributes to use
   in the voucher for certain operations, like the pinning of the
   Registrar identity.  This is described in more detail in
   Section 9.2.3, Section 8 and Section 8.3 (for RPK specifically).

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 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 and Section 7.3

   *  clarifies when new trust anchors should be retrieved
      (Section 6.6.1),

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   *  clarified 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 6.6),

   *  provides the option to return trust anchors in a simpler format
      (Section 6.6.3)

   *  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
   [I-D.ietf-ace-coap-est] 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.  Registrar and the Server Name Indicator (SNI)

   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 6.4.  Regardless of the Join Proxy mechanism, the DTLS
   connection should operate identically.

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

   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 RFC6125 DNS-ID validation on the Registrar's
   certificate.  Instead, it must do validation using the RFC8366
   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].

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   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.

6.2.  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.

   Subsequently the Pledge will send a Pledge Voucher Request (PVR).

   As explained below in Section 8.1, the "x5bag" element may be used in
   the RVR to communicate identity of the Registrar to MASA.  The Pledge
   SHOULD NOT use the x5bag attribute in this way in the PVR.  A
   Registrar that processes a PVR with an x5bag attribute 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 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.

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6.3.  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 discovery.

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

   Similarly the constrained BRSKI server URIs differ from the BRSKI
   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
   prefix which requires no discovery.  This is the same "/.well-known/
   brski" prefix as defined in Section 5 of [RFC8995].

     coaps://server.example.com/brski/<short-name>
     coaps://server.example.com/b/<short-name>
     coaps://server.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
   [I-D.ietf-ace-coap-est] 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

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   Note that /requestvoucher indicated above 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
   [RFC6690] Section 4 by sending a discovery query to the Registrar.

   For example, if the Registrar supports a short BRSKI URL (/b) and
   supports the voucher format "application/voucher-cose+cbor" (TBD3),
   and status reporting in both CBOR and JSON formats:

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

     RES: 2.05 Content
     Content-Format: 40
     Payload:
     </b>;rt=brski,
     </b/rv>;rt=brski.rv;ct=TBD3,
     </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.

   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 and shorter URLs in any combination.

   When responding to a discovery request for BRSKI resources, the
   server 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 server.  The server responds with
   only the root path for the BRSKI resources (rt=brski, resource /b in
   above example) and no others in case the client queries for only
   rt=brski type resources.  (So, a query for rt=brski, without the
   wildcard character.)

   Without discovery, a longer well-known URL can only be used, such as:

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

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   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.  Note that Content-Format
   TBD3 ("application/voucher-cose+cbor") is defined in this document.

   Content-Format TBD3 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
   TBD3 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
   TBD3) for the voucher and for the voucher request.  The MASA needs to
   support all formats that the Pledge supports.

   Section 10 details how the Pledge discovers the Registrar and Join
   Proxy in different deployment scenarios.

6.3.1.  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.

   [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.

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6.4.  Join Proxy options

   [I-D.ietf-anima-constrained-join-proxy] specifies a constrained Join
   Proxy that is optionally placed between Pledge and Registrar.  This
   includes methods for discovery of the Join Proxy by the Pledge and
   discovery of the Registrar by the Join Proxy.

6.5.  Extensions to BRSKI

6.5.1.  Discovery

   The Pledge discovers an IP address and port number that connects to
   the Registrar (possibly 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 [I-D.ietf-ace-coap-est]), 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 discovery for BRSKI 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 doing any CoRE Link
   Format parsing, which is specified in [I-D.ietf-ace-coap-est],
   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.

   However, when discovery is being 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 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.  Using the same UDP server port for all
   resources allows the Pledge to continue via the same DTLS connection
   which is more efficient.

6.5.2.  CoAP responses

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

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   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 [I-D.ietf-ace-coap-est]
   process for Delayed Responses.

6.6.  Extensions to EST-coaps

   This document extends [I-D.ietf-ace-coap-est], 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 [I-D.ietf-ace-coap-est] results in the following shorter forms of
   URI paths for the commonly used resources:

       +=================+=========================+===============+
       | 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

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6.6.1.  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.

   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.

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   5.  When doing this /crts request, the Pledge MAY use a CoAP Accept
       Option with value TBD287 ("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 [I-D.ietf-ace-coap-est].

   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.

6.6.2.  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 TBD287 ("application/pkix-cert") in a /crts
   response, per Section 5.3 of [I-D.ietf-ace-coap-est].  In this case,
   it can only store one trust anchor of the domain.

   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.

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   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 server (Registrar) 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 server provides in the /crts response.

   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 Registrar:

   1.  The client connects with DTLS to the Registrar, and authenticates
       with its present domain certificate (LDevID certificate) as
       usual.  The Registrar 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 Registrar decides to re-enroll the client into a new
       domain.

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   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 report the error to
       the Registrar using enrollment status telemetry (/es).

   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.

6.6.3.  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 TBD3 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 TBD287 ("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.

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   If the Pledge included in its request an Accept Option for only the
   TBD287 ("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 TBD287 only.
   This signals the client that a new domain is assigned to it.  The
   client follows the procedure as defined in Section 6.6.2.

6.7.  DTLS handshake fragmentation Considerations

   DTLS includes a mechanism to fragment the handshake messages.  This
   is described in Section 4.4 of [I-D.ietf-tls-dtls13].  The protocol
   described in this document will often be used with a Join Proxy
   described in [I-D.ietf-anima-constrained-join-proxy].  The Join Proxy
   will need some overhead, while the maximum packet sized guaranteed on
   802.15.4 networks is 1280 bytes.  It is RECOMMENDED that a PMTU of
   1024 bytes be assumed for the DTLS handshake.  It is unlikely that
   any Packet Too Big indications [RFC4443] will be relayed by the Join
   Proxy.

   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.

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 reauqtes to the Registrar
   [RFC8995].

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   The MASA only needs to support formats for which there are 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 14.6.

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.

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   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 [RFC6125] DNS-ID checks 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.

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.

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   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

   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.

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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 6.6.1.

   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]).

   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.

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   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 an X.509 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 X.509
   certificates, 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.  A Pledge that does not support X.509 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 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

    WARNING, obsolete definitions

   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:   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
     grouping voucher-request-constrained-grouping {
       description

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         "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
                public key, but is only a minor savings compared to
                a 256-bit ECDSA public-key.

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                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 voucher request artifact

   Below is a CBOR serialization of an example constrained voucher
   request from a Pledge to a Registrar, shown in CBOR diagnostic
   notation.  The enum value of the assertion field is calculated to be
   2 by following the algorithm described in section 9.6.4.2 of
   [RFC7950].

  {
    2501: {
      +2 : "2016-10-07T19:31:42Z", / SID=2503, created-on /
      +4 : "2016-10-21T19:31:42Z", / SID=2505, expires-on /
      +1 : 2,                      / SID=2502, assertion "proximity" /
      +13: "JADA123456789",        / SID=2514, serial-number /
      +5 : h'08C2BF36....B3D2B3',  / SID=2506, idevid-issuer /
      +10: h'30820275....82c35f',  / SID=2511, proximity-registrar-cert/
      +3 : true,                   / SID=2504, domain-cert
                                                     -revocation-checks/
      +6 : "2017-10-07T19:31:42Z"  / SID=2507, last-renewal-date /
    }
  }

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.

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   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

         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

    WARNING, obsolete definitions

   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.

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   <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";

     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:   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.

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      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.";

       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.

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                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.";
           }
         }
       }
     }
   }
   <CODE ENDS>

9.2.4.  Example voucher artifacts

   Below the CBOR serialization of an example constrained voucher is
   shown in CBOR diagnostic notation.  The enum value of the assertion
   field is calculated to be zero by following the algorithm described
   in section 9.6.4.2 of [RFC7950].

   {
     2451: {
       +2 : "2016-10-07T19:31:42Z", / SID = 2453, created-on /
       +4 : "2016-10-21T19:31:42Z", / SID = 2455, expires-on /
       +1 : 0,                      / SID = 2452, assertion "verified" /
       +11: "JADA123456789",        / SID = 2462, serial-number /
       +5 : h'E40393B4....68A3',    / SID = 2456, idevid-issuer /
       +8 : h'30820275....C35F',    / SID = 2459, pinned-domain-cert/
       +3 : true,                   / SID = 2454, domain-cert /
                                    /               -revocation-checks /
       +6 : "2017-10-07T19:31:42Z"  / SID = 2457, last-renewal-date /
     }
   }

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9.3.  Signing voucher and voucher-request artifacts with COSE

   The COSE_Sign1 structure is discussed in Section 4.2 of
   [I-D.ietf-cose-rfc8152bis-struct].  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 [I-D.ietf-cose-rfc8152bis-algs].  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 [I-D.ietf-cose-rfc8152bis-algs].  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 occuring 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 is shown in
   Figure 3.

   18( / COSE_Sign1 /
     [
       h'A101382E',        / protected header encoding: {1: -47}     /
       {                   /       which means { "alg": ES256K }     /
         4 : h'7890A03F1234'  / 4 is the "kid" binary key identifier /
       },
       h'1234....5678', / voucher-request binary content (CBOR)      /
       h'4567....1234'  / voucher-request binary public signature    /
     ]
   )

          Figure 3: COSE_Sign1 example in CBOR diagnostic notation

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

   The "kid" field is optionally present: it is an unprotected field
   that identifies the public key of the key pair that was used to sign
   this message.  The value of the key identifier "kid" parameter is an
   example value.  Usually a hash of the public key is used to identify
   the public key, but a device serial number may also be used.  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.  For example, this context information may be a
   unique serial number encoded in the binary content (CBOR) field.

   A Registrar MAY use a "kid" parameter in its RVR to identify its
   signing key as used to sign the RVR.  The method of generating this
   "kid" 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 integrations, a Registrar MUST be
   configurable, on a per-manufacturer basis, to be able to configure
   the "kid" to a particular value.  Both binary and string values are
   to be supported, each being inserted using a CBOR bstr or tstr.  By
   default, a Registrar does not include a "kid" parameter in its RVR
   since the signing key is already identified by the included signing
   certificates in the COSE "x5bag" structure.

   A Pledge normally 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.  Still, where needed 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.  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.

   In Appendix C a binary COSE_Sign1 object is shown based on the
   voucher-request example of Section 9.1.4.

10.  Deployment-specific Discovery Considerations

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

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10.1.  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.

10.2.  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.

10.3.  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.

10.4.  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.

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10.5.  Non-mesh networks using CoAP Discovery

   On unencrypted constrained networks such as 802.15.4, CoAP discover
   may be done using the mechanism detailed in [I-D.ietf-ace-coap-est]
   section 5.1.

11.  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.

12.  Raw Public Key Use Considerations

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

12.1.  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 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.  Instead of using a
   (TLSServer)Certificate of type X509 (see section 4.4.2 of [RFC8446]),
   a RawPublicKey object is used.

   A Registrar operating in a mixed environment can determine whether to
   send a Certificate or a Raw Public key: this is determined by the
   pledge including the server_certificate_type of RawPublicKey.  This
   is shown in section 5 of [RFC7250].

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   The Pledge continues to send a client_certificate_type of X509, so
   that the Registrar can properly identify the pledge and distill the
   MASA URI information from its certificate.

12.2.  The Pledge Voucher Request

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

   As the format of the pubk field is identical to the TLS Certificate
   RawPublicKey, no manipulation at all is needed to insert this into a
   voucher-request.

12.3.  The Voucher Response

   A returned voucher will have a pinned-domain-subk field with the
   identical key as was found in the proximity-registrar-pubk field
   above, as well as in the TLS 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].

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

   In certain cases, the MASA's public key counterpart of the (private)
   signing key is already installed in the Pledge at manufacturing time.
   In other cases, if the MASA signing key is based upon a PKI (see
   [I-D.richardson-anima-masa-considerations] Section 2.3), then a
   certificate chain may need to be included with the voucher in order
   for the pledge to validate the signature.  In CMS signed artifacts,
   the CMS structure has a place for such certificates.

   In the COSE-signed Constrained Vouchers described in this document,
   the x5bag attribute from [I-D.ietf-cose-x509] is to be used for this.

13.  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.

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   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
   normal [RFC8995] resource names are used, together with the media
   types described in this document.

   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.

   The responsability for supporting new formats is on the Registrar.

14.  Security Considerations

14.1.  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

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   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.

14.2.  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
   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

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   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-
   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.

14.3.  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.

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   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.

14.4.  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.

14.5.  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
   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.

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14.6.  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.

15.  IANA Considerations

15.1.  Resource Type Registry

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

   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

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15.2.  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.

15.3.  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:

     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

15.4.  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.

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15.5.  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 [I-D.ietf-cose-rfc8152bis-struct].

15.5.1.  application/voucher-cose+cbor

   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

15.6.  CoAP Content-Format Registry

   One addition to the sub-registry "CoAP Content-Formats", within the
   "CoRE Parameters" registry is needed for a new content-format.  It
   can be registered in the Expert Review range (0-255) or the IETF
   Review range (256-9999).

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   Media type                     Encoding   ID   Reference
   -----------------------------  --------- ----  ----------
   application/voucher-cose+cbor  -         TBD3  [This RFC]

15.7.  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:

   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

16.  Acknowledgements

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

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

   Daniel Franke and Henk Birkholtz provided review feedback.

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

17.  Changelog

   -11 to -16 (For change details see GitHub issues https://github.com/
   anima-wg/constrained-voucher/issues)

   -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

18.  References

18.1.  Normative References

Richardson, et al.       Expires 9 October 2022                [Page 50]
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   [I-D.ietf-ace-coap-est]
              Stok, P. V. D., Kampanakis, P., Richardson, M. C., and S.
              Raza, "EST over secure CoAP (EST-coaps)", Work in
              Progress, Internet-Draft, draft-ietf-ace-coap-est-18, 6
              January 2020, <https://www.ietf.org/archive/id/draft-ietf-
              ace-coap-est-18.txt>.

   [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-18, 18
              November 2021, <https://www.ietf.org/archive/id/draft-
              ietf-core-sid-18.txt>.

   [I-D.ietf-core-yang-cbor]
              Veillette, M., Petrov, I., Pelov, A., Bormann, C., and M.
              Richardson, "CBOR Encoding of Data Modeled with YANG",
              Work in Progress, Internet-Draft, draft-ietf-core-yang-
              cbor-19, 20 March 2022, <https://www.ietf.org/archive/id/
              draft-ietf-core-yang-cbor-19.txt>.

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-algs-12.txt>.

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-struct-15.txt>.

   [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-x509-08, 14 December 2020,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              x509-08.txt>.

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   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-tls-
              dtls13-43.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>.

   [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>.

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   [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>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [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>.

18.2.  Informative References

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

   [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., Stok, P. V. D., and P. Kampanakis,
              "Constrained Join Proxy for Bootstrapping Protocols", Work
              in Progress, Internet-Draft, draft-ietf-anima-constrained-
              join-proxy-09, 25 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-anima-
              constrained-join-proxy-09.txt>.

   [I-D.ietf-anima-jws-voucher]
              Richardson, M. and T. Werner, "JWS signed Voucher
              Artifacts for Bootstrapping Protocols", Work in Progress,
              Internet-Draft, draft-ietf-anima-jws-voucher-03, 7 March
              2022, <https://www.ietf.org/archive/id/draft-ietf-anima-
              jws-voucher-03.txt>.

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   [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-12, 20
              October 2021, <https://www.ietf.org/archive/id/draft-ietf-
              lake-edhoc-12.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-
              15, 8 February 2022, <https://www.ietf.org/archive/id/
              draft-ietf-rats-architecture-15.txt>.

   [I-D.kuehlewind-update-tag]
              Kuehlewind, 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>.

   [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-06, 13 November 2021,
              <https://www.ietf.org/archive/id/draft-richardson-anima-
              masa-considerations-06.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>.

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   [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>.

   [Thread]   Thread Group, Inc, "Thread support page, White Papers",
              November 2021,
              <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

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   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
   [I-D.ietf-ace-coap-est] 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:

       POST coaps://192.0.2.1:8085/b/es
         Content-Format: 60
         Payload: <binary CBOR enrollstatus document>

   The corresponding CoAP header fields 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

     {
       "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

   With CoAP 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:

       POST coaps://[2001:db8::2:1]/.well-known/brski/vs
         Content-Format: 60 (application/cbor)
         Payload:
   a46776657273696f6e0166737461747573f466726561736f6e7828496e66
   6f726d61746976652068756d616e2d7265616461626c65206572726f7220
   6d6573736167656e726561736f6e2d636f6e74657874a100764164646974
   696f6e616c20696e666f726d6174696f6e

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

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   {
     "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,
   following 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>
   Private-Key: (256 bit)
   priv:
       9b:4d:43:b6:a9:e1:7c:04:93:45:c3:13:d9:b5:f0:
       41:a9:6a:9c:45:79:73:b8:62:f1:77:03:3a:fc:c2:
       9c:9a
   pub:
       04:d6:b7:6f:74:88:bd:80:ae:5f:28:41:2c:72:02:
       ef:5f:98:b4:81:e1:d9:10:4c:f8:1b:66:d4:3e:5c:
       ea:da:73:e6:a8:38:a9:f1:35:11:85:b6:cd:e2:04:
       10:be:fe:d5:0b:3b:14:69:2e:e1:b0:6a:bc:55:40:
       60:eb:95:5c:54
   ASN1 OID: prime256v1
   NIST CURVE: P-256
   <CODE ENDS>

C.1.2.  Registrar private key

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   <CODE BEGINS>
   Private-Key: (256 bit)
   priv:
       81:df:bb:50:a3:45:58:06:b5:56:3b:46:de:f3:e9:
       e9:00:ae:98:13:9e:2f:36:68:81:fc:d9:65:24:fb:
       21:7e
   pub:
       04:50:7a:c8:49:1a:8c:69:c7:b5:c3:1d:03:09:ed:
       35:ba:13:f5:88:4c:e6:2b:88:cf:30:18:15:4f:a0:
       59:b0:20:ec:6b:eb:b9:4e:02:b8:93:40:21:89:8d:
       a7:89:c7:11:ce:a7:13:39:f5:0e:34:8e:df:0d:92:
       3e:d0:2d:c7:b7
   ASN1 OID: prime256v1
   NIST CURVE: P-256
   <CODE ENDS>

C.1.3.  MASA private key

   <CODE BEGINS>
   Private-Key: (256 bit)
   priv:
       c6:bb:a5:8f:b6:d3:c4:75:28:d8:d3:d9:46:c3:31:
       83:6d:00:0a:9a:38:ce:22:5c:e9:d9:ea:3b:98:32:
       ec:31
   pub:
       04:59:80:94:66:14:94:20:30:3c:66:08:85:55:86:
       db:e7:d4:d1:d7:7a:d2:a3:1a:0c:73:6b:01:0d:02:
       12:15:d6:1f:f3:6e:c8:d4:84:60:43:3b:21:c5:83:
       80:1e:fc:e2:37:85:77:97:94:d4:aa:34:b5:b6:c6:
       ed:f3:17:5c:f1
   ASN1 OID: prime256v1
   NIST CURVE: P-256
   <CODE ENDS>

C.2.  Pledge, Registrar 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: 4822678189204992 (0x11223344556600)
         Signature Algorithm: ecdsa-with-SHA256
         Issuer: C=NL, ST=NB, L=Helmond, O=vanderstok, OU=manufacturer,
                                                       CN=masa.stok.nl
         Validity
             Not Before: Dec  9 10:02:36 2020 GMT
             Not After : Dec 31 23:59:59 9999 GMT
         Subject: C=NL, ST=NB, L=Helmond, O=vanderstok, OU=manufacturing,
                    CN=uuid:pledge.1.2.3.4/serialNumber=pledge.1.2.3.4
         Subject Public Key Info:
             Public Key Algorithm: id-ecPublicKey
                 Public-Key: (256 bit)
                 pub:
                     04:d6:b7:6f:74:88:bd:80:ae:5f:28:41:2c:72:02:
                     ef:5f:98:b4:81:e1:d9:10:4c:f8:1b:66:d4:3e:5c:
                     ea:da:73:e6:a8:38:a9:f1:35:11:85:b6:cd:e2:04:
                     10:be:fe:d5:0b:3b:14:69:2e:e1:b0:6a:bc:55:40:
                     60:eb:95:5c:54
                 ASN1 OID: prime256v1
                 NIST CURVE: P-256
         X509v3 extensions:
             X509v3 Basic Constraints:
                 CA:FALSE
             X509v3 Authority Key Identifier:
                 keyid:
         E4:03:93:B4:C3:D3:F4:2A:80:A4:77:18:F6:96:49:03:01:17:68:A3

     Signature Algorithm: ecdsa-with-SHA256
            30:46:02:21:00:d2:e6:45:3b:b0:c3:00:b3:25:8d:f1:83:fe:
            d9:37:c1:a2:49:65:69:7f:6b:b9:ef:2c:05:07:06:31:ac:17:
            bd:02:21:00:e2:ce:9e:7b:7f:74:50:33:ad:9e:ff:12:4e:e9:
            a6:f3:b8:36:65:ab:7d:80:bb:56:88:bc:03:1d:e5:1e:31:6f
   <CODE ENDS>

   Below is the hexadecimal representation:

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   <CODE BEGINS>
   30820226308201cba003020102020711223344556600300a06082a8648ce3d04
   0302306f310b3009060355040613024e4c310b300906035504080c024e423110
   300e06035504070c0748656c6d6f6e6431133011060355040a0c0a76616e6465
   7273746f6b31153013060355040b0c0c6d616e75666163747572657231153013
   06035504030c0c6d6173612e73746f6b2e6e6c3020170d323031323039313030
   3233365a180f39393939313233313233353935395a308190310b300906035504
   0613024e4c310b300906035504080c024e423110300e06035504070c0748656c
   6d6f6e6431133011060355040a0c0a76616e64657273746f6b31163014060355
   040b0c0d6d616e75666163747572696e67311c301a06035504030c1375756964
   3a706c656467652e312e322e332e34311730150603550405130e706c65646765
   2e312e322e332e343059301306072a8648ce3d020106082a8648ce3d03010703
   420004d6b76f7488bd80ae5f28412c7202ef5f98b481e1d9104cf81b66d43e5c
   eada73e6a838a9f1351185b6cde20410befed50b3b14692ee1b06abc554060eb
   955c54a32e302c30090603551d1304023000301f0603551d23041830168014e4
   0393b4c3d3f42a80a47718f6964903011768a3300a06082a8648ce3d04030203
   49003046022100d2e6453bb0c300b3258df183fed937c1a24965697f6bb9ef2c
   05070631ac17bd022100e2ce9e7b7f745033ad9eff124ee9a6f3b83665ab7d80
   bb5688bc031de51e316f
   <CODE ENDS>

C.2.2.  Registrar Certificate

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   <CODE BEGINS>
   Certificate:
     Data:
         Version: 3 (0x2)
         Serial Number:
           70:56:ea:aa:30:66:d8:82:6a:55:5b:90:88:d4:62:bf:9c:f2:8c:fd
         Signature Algorithm: ecdsa-with-SHA256
         Issuer: C=NL, ST=NB, L=Helmond, O=vanderstok, OU=consultancy,
                                                  CN=registrar.stok.nl
         Validity
             Not Before: Dec  9 10:02:36 2020 GMT
             Not After : Dec  9 10:02:36 2021 GMT
         Subject: C=NL, ST=NB, L=Helmond, O=vanderstok, OU=consultancy,
                                                   CN=registrar.stok.nl
         Subject Public Key Info:
             Public Key Algorithm: id-ecPublicKey
                 Public-Key: (256 bit)
                 pub:
                     04:50:7a:c8:49:1a:8c:69:c7:b5:c3:1d:03:09:ed:
                     35:ba:13:f5:88:4c:e6:2b:88:cf:30:18:15:4f:a0:
                     59:b0:20:ec:6b:eb:b9:4e:02:b8:93:40:21:89:8d:
                     a7:89:c7:11:ce:a7:13:39:f5:0e:34:8e:df:0d:92:
                     3e:d0:2d:c7:b7
                 ASN1 OID: prime256v1
                 NIST CURVE: P-256
         X509v3 extensions:
             X509v3 Subject Key Identifier:
       08:C2:BF:36:88:7F:79:41:21:85:87:2F:16:A7:AC:A6:EF:B3:D2:B3
             X509v3 Authority Key Identifier:
                 keyid:
       08:C2:BF:36:88:7F:79:41:21:85:87:2F:16:A7:AC:A6:EF:B3:D2:B3

             X509v3 Basic Constraints: critical
                 CA:TRUE
             X509v3 Extended Key Usage:
                 CMC Registration Authority, TLS Web Server
                 Authentication, TLS Web Client Authentication
             X509v3 Key Usage: critical
                 Digital Signature, Non Repudiation, Key Encipherment,
                 Data Encipherment, Certificate Sign, CRL Sign
     Signature Algorithm: ecdsa-with-SHA256
          30:44:02:20:74:4c:99:00:85:13:b2:f1:bc:fd:f9:02:1a:46:
          fb:17:4c:f8:83:a2:7c:a1:d9:3f:ae:ac:f3:1e:4e:dd:12:c6:
          02:20:11:47:14:db:f5:1a:5e:78:f5:81:b9:42:1c:6e:47:02:
          ab:53:72:70:c5:ba:fb:2d:16:c3:de:9a:a1:82:c3:5f
   <CODE ENDS>

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   Below is the hexadecimal representation which is in (request-)voucher
   examples referred to as regis-cert-hex:

   <CODE BEGINS>
   308202753082021ca00302010202147056eaaa3066d8826a555b9088d462bf9c
   f28cfd300a06082a8648ce3d0403023073310b3009060355040613024e4c310b
   300906035504080c024e423110300e06035504070c0748656c6d6f6e64311330
   11060355040a0c0a76616e64657273746f6b31143012060355040b0c0b636f6e
   73756c74616e6379311a301806035504030c117265676973747261722e73746f
   6b2e6e6c301e170d3230313230393130303233365a170d323131323039313030
   3233365a3073310b3009060355040613024e4c310b300906035504080c024e42
   3110300e06035504070c0748656c6d6f6e6431133011060355040a0c0a76616e
   64657273746f6b31143012060355040b0c0b636f6e73756c74616e6379311a30
   1806035504030c117265676973747261722e73746f6b2e6e6c3059301306072a
   8648ce3d020106082a8648ce3d03010703420004507ac8491a8c69c7b5c31d03
   09ed35ba13f5884ce62b88cf3018154fa059b020ec6bebb94e02b8934021898d
   a789c711cea71339f50e348edf0d923ed02dc7b7a3818d30818a301d0603551d
   0e0416041408c2bf36887f79412185872f16a7aca6efb3d2b3301f0603551d23
   04183016801408c2bf36887f79412185872f16a7aca6efb3d2b3300f0603551d
   130101ff040530030101ff30270603551d250420301e06082b0601050507031c
   06082b0601050507030106082b06010505070302300e0603551d0f0101ff0404
   030201f6300a06082a8648ce3d04030203470030440220744c99008513b2f1bc
   fdf9021a46fb174cf883a27ca1d93faeacf31e4edd12c60220114714dbf51a5e
   78f581b9421c6e4702ab537270c5bafb2d16c3de9aa182c35f
   <CODE ENDS>

C.2.3.  MASA Certificate

   <CODE BEGINS>
   Certificate:
     Data:
         Version: 3 (0x2)
         Serial Number:
           14:26:b8:1c:ce:d8:c3:e8:14:05:cb:87:67:0d:be:ef:d5:81:25:b4
         Signature Algorithm: ecdsa-with-SHA256
         Issuer: C=NL, ST=NB, L=Helmond, O=vanderstok,
             OU=manufacturer, CN=masa.stok.nl

         Validity
             Not Before: Dec  9 10:02:36 2020 GMT
             Not After : Sep  5 10:02:36 2023 GMT
         Subject: C=NL, ST=NB, L=Helmond, O=vanderstok,
             OU=manufacturer, CN=masa.stok.nl
         Subject Public Key Info:
             Public Key Algorithm: id-ecPublicKey
                 Public-Key: (256 bit)
                 pub:
                     04:59:80:94:66:14:94:20:30:3c:66:08:85:55:86:

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                     db:e7:d4:d1:d7:7a:d2:a3:1a:0c:73:6b:01:0d:02:
                     12:15:d6:1f:f3:6e:c8:d4:84:60:43:3b:21:c5:83:
                     80:1e:fc:e2:37:85:77:97:94:d4:aa:34:b5:b6:c6:
                     ed:f3:17:5c:f1
                 ASN1 OID: prime256v1
                 NIST CURVE: P-256
         X509v3 extensions:
             X509v3 Subject Key Identifier:
       E4:03:93:B4:C3:D3:F4:2A:80:A4:77:18:F6:96:49:03:01:17:68:A3
             X509v3 Authority Key Identifier:
                 keyid:
        E4:03:93:B4:C3:D3:F4:2A:80:A4:77:18:F6:96:49:03:01:17:68:A3

             X509v3 Basic Constraints: critical
                 CA:TRUE
             X509v3 Extended Key Usage:
                 CMC Registration Authority,
                 TLS Web Server Authentication,
                 TLS Web Client Authentication
             X509v3 Key Usage: critical
                 Digital Signature, Non Repudiation, Key Encipherment,
                       Data Encipherment, Certificate Sign, CRL Sign
     Signature Algorithm: ecdsa-with-SHA256
          30:44:02:20:2e:c5:f2:24:72:70:20:ea:6e:74:8b:13:93:67:
          8a:e6:fe:fb:8d:56:7f:f5:34:18:a9:ef:a5:0f:c3:99:ca:53:
          02:20:3d:dc:91:d0:e9:6a:69:20:01:fb:e4:20:40:de:7c:7d:
          98:ed:d8:84:53:61:84:a7:f9:13:06:4c:a9:b2:8f:5c
   <CODE ENDS>

   Below is the hexadecimal representation:

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   <CODE BEGINS>
   3082026d30820214a00302010202141426b81cced8c3e81405cb87670dbeefd5
   8125b4300a06082a8648ce3d040302306f310b3009060355040613024e4c310b
   300906035504080c024e423110300e06035504070c0748656c6d6f6e64311330
   11060355040a0c0a76616e64657273746f6b31153013060355040b0c0c6d616e
   7566616374757265723115301306035504030c0c6d6173612e73746f6b2e6e6c
   301e170d3230313230393130303233365a170d3233303930353130303233365a
   306f310b3009060355040613024e4c310b300906035504080c024e423110300e
   06035504070c0748656c6d6f6e6431133011060355040a0c0a76616e64657273
   746f6b31153013060355040b0c0c6d616e756661637475726572311530130603
   5504030c0c6d6173612e73746f6b2e6e6c3059301306072a8648ce3d02010608
   2a8648ce3d0301070342000459809466149420303c6608855586dbe7d4d1d77a
   d2a31a0c736b010d021215d61ff36ec8d48460433b21c583801efce237857797
   94d4aa34b5b6c6edf3175cf1a3818d30818a301d0603551d0e04160414e40393
   b4c3d3f42a80a47718f6964903011768a3301f0603551d23041830168014e403
   93b4c3d3f42a80a47718f6964903011768a3300f0603551d130101ff04053003
   0101ff30270603551d250420301e06082b0601050507031c06082b0601050507
   030106082b06010505070302300e0603551d0f0101ff0404030201f6300a0608
   2a8648ce3d040302034700304402202ec5f224727020ea6e748b1393678ae6fe
   fb8d567ff53418a9efa50fc399ca5302203ddc91d0e96a692001fbe42040de7c
   7d98edd884536184a7f913064ca9b28f5c
   <CODE ENDS>

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

   In this COSE example the voucher request has been signed by the
   Pledge using the private key of Appendix C.1.1, and has been sent to
   the link-local JRC (Registrar) over CoAPS.

       POST coaps://[JRC-link-local-address]/b/rv
       Content-Format: TBD3
       Payload: signed_request_voucher

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

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   <CODE BEGINS>
   d28444a101382ea104582097113db094eee8eae48683e7337875c0372164
   be89d023a5f3df52699c0fbfb55902d2a11909c5a60274323032302d3132
   2d32335431323a30353a32325a0474323032322d31322d32335431323a30
   353a32325a01020750684ca83e27230aff97630cf2c1ec409a0d6e706c65
   6467652e312e322e332e340a590279308202753082021ca0030201020214
   7056eaaa3066d8826a555b9088d462bf9cf28cfd300a06082a8648ce3d04
   03023073310b3009060355040613024e4c310b300906035504080c024e42
   3110300e06035504070c0748656c6d6f6e6431133011060355040a0c0a76
   616e64657273746f6b31143012060355040b0c0b636f6e73756c74616e63
   79311a301806035504030c117265676973747261722e73746f6b2e6e6c30
   1e170d3230313230393130303233365a170d323131323039313030323336
   5a3073310b3009060355040613024e4c310b300906035504080c024e4231
   10300e06035504070c0748656c6d6f6e6431133011060355040a0c0a7661
   6e64657273746f6b31143012060355040b0c0b636f6e73756c74616e6379
   311a301806035504030c117265676973747261722e73746f6b2e6e6c3059
   301306072a8648ce3d020106082a8648ce3d03010703420004507ac8491a
   8c69c7b5c31d0309ed35ba13f5884ce62b88cf3018154fa059b020ec6beb
   b94e02b8934021898da789c711cea71339f50e348edf0d923ed02dc7b7a3
   818d30818a301d0603551d0e0416041408c2bf36887f79412185872f16a7
   aca6efb3d2b3301f0603551d2304183016801408c2bf36887f7941218587
   2f16a7aca6efb3d2b3300f0603551d130101ff040530030101ff30270603
   551d250420301e06082b0601050507031c06082b0601050507030106082b
   06010505070302300e0603551d0f0101ff0404030201f6300a06082a8648
   ce3d04030203470030440220744c99008513b2f1bcfdf9021a46fb174cf8
   83a27ca1d93faeacf31e4edd12c60220114714dbf51a5e78f581b9421c6e
   4702ab537270c5bafb2d16c3de9aa182c35f58473045022063766c7bbd1b
   339dbc398e764af3563e93b25a69104befe9aac2b3336b8f56e1022100cd
   0419559ad960ccaed4dee3f436eca40b7570b25a52eb60332bc1f2991484
   e9
   <CODE ENDS>

   The Pledge uses the "proximity" (SID 2502, enum 2) assertion together
   with an included proximity-registrar-cert field (SID 2511) to inform
   MASA about its proximity to the specific Registrar.  The
   representiation of signed_voucher_request in CBOR diagnostic format
   is:

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   <CODE BEGINS>
   Diagnose(signed_request_voucher) =
   18([
   h'A101382E',     / {"alg": -47} /
   {4: h'97113DB094EEE8EAE48683E7337875C0372164B
         E89D023A5F3DF52699C0FBFB5'},
   h'<request_voucher>',  / byte string as detailed below /
   h'3045022063766C7BBD1B339DBC398E764AF3563E93B
   25A69104BEFE9AAC2B3336B8F56E1022100CD0419559A
   D960CCAED4DEE3F436ECA40B7570B25A52EB60332BC1F
   2991484E9'
   ])

   Diagnose(request_voucher) =
   {2501: {2: "2020-12-23T12:05:22Z",
           4: "2022-12-23T12:05:22Z",
           1: 2,
           7: h'684CA83E27230AFF97630CF2C1EC409A',
           13: "pledge.1.2.3.4",
           10: h'<regis-cert-hex>' / byte string as defined in C.2.2 /
   }}
   <CODE ENDS>

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.

       POST https://masa.example.com/.well-known/brski/requestvoucher
       Content-Type: application/voucher-cose+cbor
       Accept: application/voucher-cose+cbor
       Body: signed_masa_request_voucher

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

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   <CODE BEGINS>
   d28444a101382ea1045820e8735bc4b470c3aa6a7aa9aa8ee584c09c1113
   1b205efec5d0313bad84c5cd05590414a11909c5a60274323032302d3132
   2d32385431303a30333a33355a0474323032322d31322d32385431303a30
   333a33355a07501551631f6e0416bd162ba53ea00c2a050d6e706c656467
   652e312e322e332e3405587131322d32385431303a30333a33355a075015
   51631f6e0416bd162ba53ea00c2a050d6e706c656467652e312e322e332e
   3405587131322d32385431303a300000000000000000000000000416bd16
   2ba53ea00c2a050d6e706c656467652e312e322e332e3405587131322d32
   385431303a09590349d28444a101382ea104582097113db094eee8eae486
   83e7337875c0372164be89d023a5f3df52699c0fbfb55902d2a11909c5a6
   0274323032302d31322d32385431303a30333a33355a0474323032322d31
   322d32385431303a30333a33355a010207501551631f6e0416bd162ba53e
   a00c2a050d6e706c656467652e312e322e332e340a590279308202753082
   021ca00302010202147056eaaa3066d8826a555b9088d462bf9cf28cfd30
   0a06082a8648ce3d0403023073310b3009060355040613024e4c310b3009
   06035504080c024e423110300e06035504070c0748656c6d6f6e64311330
   11060355040a0c0a76616e64657273746f6b31143012060355040b0c0b63
   6f6e73756c74616e6379311a301806035504030c11726567697374726172
   2e73746f6b2e6e6c301e170d3230313230393130303233365a170d323131
   3230393130303233365a3073310b3009060355040613024e4c310b300906
   035504080c024e423110300e06035504070c0748656c6d6f6e6431133011
   060355040a0c0a76616e64657273746f6b31143012060355040b0c0b636f
   6e73756c74616e6379311a301806035504030c117265676973747261722e
   73746f6b2e6e6c3059301306072a8648ce3d020106082a8648ce3d030107
   03420004507ac8491a8c69c7b5c31d0309ed35ba13f5884ce62b88cf3018
   154fa059b020ec6bebb94e02b8934021898da789c711cea71339f50e348e
   df0d923ed02dc7b7a3818d30818a301d0603551d0e0416041408c2bf3688
   7f79412185872f16a7aca6efb3d2b3301f0603551d2304183016801408c2
   bf36887f79412185872f16a7aca6efb3d2b3300f0603551d130101ff0405
   30030101ff30270603551d250420301e06082b0601050507031c06082b06
   01050507030106082b06010505070302300e0603551d0f0101ff04040302
   01f6300a06082a8648ce3d04030203470030440220744c99008513b2f1bc
   fdf9021a46fb174cf883a27ca1d93faeacf31e4edd12c60220114714dbf5
   1a5e78f581b9421c6e4702ab537270c5bafb2d16c3de9aa182c35f584730
   45022063766c7bbd1b339dbc398e764af3563e93b25a69104befe9aac2b3
   336b8f56e1022100cd0419559ad960ccaed4dee3f436eca40b7570b25a52
   eb60332bc1f2991484e958473045022100e6b45558c1b806bba23f4ac626
   c9bdb6fd354ef4330d8dfb7c529f29cca934c802203c1f2ccbbac89733d1
   7ee7775bc2654c5f1cc96afba2741cc31532444aa8fed8
   <CODE ENDS>

   The representiation of signed_masa_voucher_request in CBOR diagnostic
   format is:

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   <CODE BEGINS>
   Diagnose(signed_registrar_request-voucher)
   18([
   h'A101382E',     / {"alg": -47} /
   h'E8735BC4B470C3AA6A7AA9AA8EE584C09C11131B205EFEC5D0313BAD84
   C5CD05'},
   h'<registrar_request_voucher>', / byte string as detailed below /
   h'3045022100E6B45558C1B806BBA23F4AC626C9BDB6FD354EF4330D8DFB
   7C529F29CCA934C802203C1F2CCBBAC89733D17EE7775BC2654C5F1CC96A
   FBA2741CC31532444AA8FED8'
   ])

   Diagnose(registrar_request_voucher)
   {2501:
       {2: "2020-12-28T10:03:35Z",
        4: "2022-12-28T10:03:35Z",
        7: h'1551631F6E0416BD162BA53EA00C2A05',
       13: "pledge.1.2.3.4",
        5: h'31322D32385431303A30333A33355A07501551631F6E0416BD
             162BA53EA00C2A050D6E706C656467652E312E322E332E3405
             587131322D32385431303A3000000000000000000000000004
             16BD162BA53EA00C2A050D6E706C656467652E312E322E332E
             3405587131322D32385431303A', / idevid-issuer /
        9: h'<prior-pvr>'  / prior-signed-voucher-request = PVR /
        }
   }
   <CODE ENDS>

C.5.  COSE-signed Voucher from MASA

   The resulting voucher is created by the MASA and returned via the JRC
   to the Pledge.  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):

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   <CODE BEGINS>
   d28444a101382ea104582039920a34ee92d3148ab3a729f58611193270c9
   029f7784daf112614b19445d5158cfa1190993a70274323032302d31322d
   32335431353a30333a31325a0474323032302d31322d32335431353a3233
   3a31325a010007506508e06b2959d5089d7a3169ea889a490b6e706c6564
   67652e312e322e332e340858753073310b3009060355040613024e4c310b
   300906035504080c024e423110300e06035504070c0748656c6d6f6e6431
   133011060355040a0c0a76616e64657273746f6b31143012060355040b0c
   0b636f6e73756c74616e6379311a301806035504030c1172656769737472
   61722e73746f6b2e6e6c03f458473045022022515d96cd12224ee5d3ac67
   3237163bba24ad84815699285d9618f463ee73fa022100a6bff9d8585c1c
   9256371ece94da3d26264a5dfec0a354fe7b3aef58344c512f
   <CODE ENDS>

   The representiation of signed_voucher in CBOR diagnostic format is:

   <CODE BEGINS>
   Diagnose(signed_voucher) =
   18([
   h'A101382E',     / {"alg": -47} /
   {4: h'39920A34EE92D3148AB3A729F58611193270C9029F7784DAF112614B194
   45D51'},
   h'<voucher>',    / byte string as detailed below /
   h'3045022022515D96CD12224EE5D3AC673237163BBA24AD84815699285D9618F
   463EE73FA022100A6BFF9D8585C1C9256371ECE94DA3D26264A5DFEC0A354FE7B
   3AEF58344C512F'
   ])

   Diagnose(voucher) =
   {2451:
      {2: "2020-12-23T15:03:12Z",
       4: "2020-12-23T15:23:12Z",
       1: 0,
       7: h'6508E06B2959D5089D7A3169EA889A49',
      11: "pledge.1.2.3.4",
       8: h'<regis-cert-hex>', / as detailed in C.2.2 /
       3: false}
   }
   <CODE ENDS>

   In above, regis-cert-hex represents the hexadecimal encoding of the
   Registrar certificate of Appendix C.2.2.

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Appendix D.  Generating Certificates with OpenSSL

   This informative appendix shows an example of a Bash shell script to
   generate test certificates for the Pledge IDevID, the Registrar and
   the MASA.  This shell script cannot be run stand-alone because it
   depends on particular input files which are not included in this
   appendix.  Nevertheless, this example script may provide guidance on
   how OpenSSL can be configured for generating Constrained BRSKI
   certificates.

   Note: the *-comb.crt certificate files combine the certificate with
   the private key.  These are generated to be used by libcoap for DTLS
   connection establishment.

   <CODE BEGINS>
   #!/bin/bash
   #try-cert.sh
   export dir=./brski/intermediate
   export cadir=./brski
   export cnfdir=./conf
   export format=pem
   export default_crl_days=30
   sn=8

   DevID=pledge.1.2.3.4
   serialNumber="serialNumber=$DevID"
   export hwType=1.3.6.1.4.1.6715.10.1
   export hwSerialNum=01020304 # Some hex
   export subjectAltName="otherName:1.3.6.1.5.5.7.8.4;SEQ:hmodname"
   echo  $hwType - $hwSerialNum
   echo $serialNumber
   OPENSSL_BIN="openssl"

   # remove all files
   rm -r ./brski/*
   #
   # initialize file structure
   # root level
   cd $cadir
   mkdir certs crl csr newcerts private
   chmod 700 private
   touch index.txt
   touch serial
   echo 11223344556600 >serial
   echo 1000 > crlnumber
   # intermediate level
   mkdir intermediate
   cd intermediate

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   mkdir certs crl csr newcerts private
   chmod 700 private
   touch index.txt
   echo 11223344556600 >serial
   echo 1000 > crlnumber
   cd ../..

   # file structure is cleaned start filling

   echo "#############################"
   echo "create registrar keys and certificates "
   echo "#############################"

   echo "create root registrar certificate using ecdsa with sha 256 key"
   $OPENSSL_BIN ecparam -name prime256v1 -genkey \
      -noout -out $cadir/private/ca-regis.key

   $OPENSSL_BIN req -new -x509 \
    -config $cnfdir/openssl-regis.cnf \
    -key $cadir/private/ca-regis.key \
    -out $cadir/certs/ca-regis.crt \
    -extensions v3_ca\
    -days 365 \
    -subj "/C=NL/ST=NB/L=Helmond/O=vanderstok/OU=consultancy \
   /CN=registrar.stok.nl"

   # Combine authority certificate and key
   echo "Combine authority certificate and key"
   $OPENSSL_BIN pkcs12 -passin pass:watnietWT -passout pass:watnietWT\
      -inkey $cadir/private/ca-regis.key \
      -in $cadir/certs/ca-regis.crt -export \
      -out $cadir/certs/ca-regis-comb.pfx

   # converteer authority pkcs12 file to pem
   echo "converteer authority pkcs12 file to pem"
   $OPENSSL_BIN pkcs12 -passin pass:watnietWT -passout pass:watnietWT\
      -in $cadir/certs/ca-regis-comb.pfx \
      -out $cadir/certs/ca-regis-comb.crt -nodes

   #show certificate in registrar combined certificate
   $OPENSSL_BIN  x509 -in $cadir/certs/ca-regis-comb.crt -text

   #
   # Certificate Authority for MASA
   #

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   echo "#############################"
   echo "create MASA keys and certificates "
   echo "#############################"

   echo "create root MASA certificate using ecdsa with sha 256 key"
   $OPENSSL_BIN ecparam -name prime256v1 -genkey -noout \
      -out $cadir/private/ca-masa.key

   $OPENSSL_BIN req -new -x509 \
    -config $cnfdir/openssl-masa.cnf \
    -days 1000 -key $cadir/private/ca-masa.key \
     -out $cadir/certs/ca-masa.crt \
    -extensions v3_ca\
    -subj "/C=NL/ST=NB/L=Helmond/O=vanderstok/OU=manufacturer\
   /CN=masa.stok.nl"

   # Combine authority certificate and key
   echo "Combine authority certificate and key for masa"
   $OPENSSL_BIN pkcs12 -passin pass:watnietWT -passout pass:watnietWT\
      -inkey $cadir/private/ca-masa.key \
      -in $cadir/certs/ca-masa.crt -export \
      -out $cadir/certs/ca-masa-comb.pfx

   # converteer authority pkcs12 file to pem for masa
   echo "converteer authority pkcs12 file to pem for masa"
   $OPENSSL_BIN pkcs12 -passin pass:watnietWT -passout pass:watnietWT\
      -in $cadir/certs/ca-masa-comb.pfx \
      -out $cadir/certs/ca-masa-comb.crt -nodes

   #show certificate in pledge combined certificate
   $OPENSSL_BIN  x509 -in $cadir/certs/ca-masa-comb.crt -text

   #
   # Certificate for Pledge derived from MASA certificate
   #
   echo "#############################"
   echo "create pledge keys and certificates "
   echo "#############################"

   # Pledge derived Certificate

   echo "create pledge derived certificate using ecdsa with sha 256 key"
   $OPENSSL_BIN ecparam -name prime256v1 -genkey -noout \
      -out $dir/private/pledge.key

   echo "create pledge certificate request"

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   $OPENSSL_BIN req -nodes -new -sha256 \
      -key $dir/private/pledge.key -out $dir/csr/pledge.csr \
     -subj "/C=NL/ST=NB/L=Helmond/O=vanderstok/OU=manufacturing\
    /CN=uuid:$DevID/$serialNumber"

   # Sign pledge derived Certificate
   echo "sign pledge derived certificate "
   $OPENSSL_BIN ca -config $cnfdir/openssl-pledge.cnf \
    -extensions 8021ar_idevid\
    -days 365 -in $dir/csr/pledge.csr \
    -out $dir/certs/pledge.crt

   # Add pledge key and pledge certificate to pkcs12 file
   echo "Add derived pledge key and derived pledge \
    certificate to pkcs12 file"
   $OPENSSL_BIN pkcs12  -passin pass:watnietWT -passout pass:watnietWT\
      -inkey $dir/private/pledge.key \
      -in $dir/certs/pledge.crt -export \
      -out $dir/certs/pledge-comb.pfx

   # converteer pledge pkcs12 file to pem
   echo "converteer pledge pkcs12 file to pem"
   $OPENSSL_BIN pkcs12 -passin pass:watnietWT -passout pass:watnietWT\
      -in $dir/certs/pledge-comb.pfx \
      -out $dir/certs/pledge-comb.crt -nodes

   #show certificate in pledge-comb.crt
   $OPENSSL_BIN  x509 -in $dir/certs/pledge-comb.crt -text

   #show private key in pledge-comb.crt
   $OPENSSL_BIN ecparam -name prime256v1\
     -in $dir/certs/pledge-comb.crt -text

   <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.

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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:

   *  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*                         | === | === | ==== |

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    +---------------------------------------------+-----+-----+------+
    | Discovery for rt=brski*                     |  -  |  -  |  Y   |
    +---------------------------------------------+-----+-----+------+
    | Support pinned Registrar public key (RPK)   |  Y  |  -  |  Y   |
    +---------------------------------------------+-----+-----+------+
    | Support pinned Registrar certificate        |  -  |  Y  |  Y   |
    +---------------------------------------------+-----+-----+------+
    | 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 TBD287 (one CA cert   |  Y  |  Y  |  -   |
    | only)                                       |     |     |      |
    +---------------------------------------------+-----+-----+------+
    | 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.

Contributors

   Russ Housley
   Email: housley@vigilsec.com

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Authors' Addresses

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

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

   Panos Kampanakis
   Cisco Systems
   Email: pkampana@cisco.com

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

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