constrained-voucher-25.txt

anima Working Group                                        M. Richardson
Internet-Draft                                  Sandelman Software Works
Updates: 8995, 9148 (if approved)                        P. van der Stok
Intended status: Standards Track                  vanderstok consultancy
Expires: 9 January 2025                                    P. Kampanakis
                                                           Cisco Systems
                                                                 E. Dijk
                                                       IoTconsultancy.nl
                                                             8 July 2024


  Constrained Bootstrapping Remote Secure Key Infrastructure (cBRSKI)
                draft-ietf-anima-constrained-voucher-25

Abstract

   This document defines the Constrained Bootstrapping Remote Secure Key
   Infrastructure (cBRSKI) protocol, which provides a solution for
   secure zero-touch onboarding 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. cBRSKI 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 data is
   encoded in JSON, cBRSKI uses a compact CBOR-encoded voucher.  The
   BRSKI voucher data definition 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 DTLS-secured CoAP
   (CoAPS).  This document Updates RFC 8995 and RFC 9148.

About This Document

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

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

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

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




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Status of This Memo

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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 9 January 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   7
   4.  Overview of Protocol  . . . . . . . . . . . . . . . . . . . .   7
   5.  Updates to RFC 8995 and RFC 9148  . . . . . . . . . . . . . .   8
   6.  BRSKI-EST Protocol  . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  DTLS Connection . . . . . . . . . . . . . . . . . . . . .  10
       6.1.1.  DTLS Version  . . . . . . . . . . . . . . . . . . . .  10
       6.1.2.  TLS Client Certificates: IDevID authentication  . . .  10
       6.1.3.  DTLS Handshake Fragmentation Considerations . . . . .  10
       6.1.4.  Registrar and the Server Name Indicator (SNI) . . . .  11
       6.1.5.  Registrar Server Certificate Requirements . . . . . .  12
     6.2.  cBRSKI Join Proxy . . . . . . . . . . . . . . . . . . . .  12
     6.3.  Request URIs, Resource Discovery and Content Formats  . .  12
       6.3.1.  RFC8995 Telemetry Returns . . . . . . . . . . . . . .  14



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       6.3.2.  CoAP Resources Table  . . . . . . . . . . . . . . . .  14
     6.4.  CoAP Responses  . . . . . . . . . . . . . . . . . . . . .  15
     6.5.  Extensions to EST-coaps . . . . . . . . . . . . . . . . .  15
       6.5.1.  Pledge enrollment procedure . . . . . . . . . . . . .  16
       6.5.2.  Renewal of CA certificates  . . . . . . . . . . . . .  17
       6.5.3.  Change of domain trust anchor(s)  . . . . . . . . . .  17
       6.5.4.  Re-enrollment procedure . . . . . . . . . . . . . . .  17
       6.5.5.  Multipart Content Format for CA certificates (/crts)
               Resource  . . . . . . . . . . . . . . . . . . . . . .  19
     6.6.  Registrar Extensions  . . . . . . . . . . . . . . . . . .  20
   7.  BRSKI-MASA Protocol . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Protocol and Formats  . . . . . . . . . . . . . . . . . .  20
     7.2.  Registrar Voucher Request . . . . . . . . . . . . . . . .  21
     7.3.  MASA and the Server Name Indicator (SNI)  . . . . . . . .  21
     7.4.  Registrar Client Certificate Requirement  . . . . . . . .  22
   8.  Pinning in Voucher Artifacts  . . . . . . . . . . . . . . . .  22
     8.1.  Registrar Identity Selection and Encoding . . . . . . . .  22
     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.  Example Artifacts . . . . . . . . . . . . . . . . . . . .  27
       9.1.1.  Example Pledge voucher request (PVR) artifact . . . .  27
       9.1.2.  Example Registrar voucher request (RVR) artifact  . .  27
       9.1.3.  Example voucher artifacts . . . . . . . . . . . . . .  28
     9.2.  Signing voucher and voucher request artifacts with
           COSE  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
       9.2.1.  Signing of Registrar Voucher Request (RVR)  . . . . .  30
       9.2.2.  Signing of Pledge Voucher Request (PVR) . . . . . . .  31
       9.2.3.  Signing of voucher by MASA  . . . . . . . . . . . . .  32
   10. Extensions to Discovery . . . . . . . . . . . . . . . . . . .  33
     10.1.  Discovery Operations by a Pledge . . . . . . . . . . . .  34
       10.1.1.  Examples . . . . . . . . . . . . . . . . . . . . . .  35
     10.2.  Discovery Operations by a Join Proxy . . . . . . . . . .  36
   11. Deployment-specific Discovery Considerations  . . . . . . . .  36
     11.1.  6TiSCH Deployments . . . . . . . . . . . . . . . . . . .  36
     11.2.  IP networks using GRASP  . . . . . . . . . . . . . . . .  36
     11.3.  IP networks using mDNS . . . . . . . . . . . . . . . . .  37
     11.4.  Thread networks using Mesh Link Establishment (MLE)  . .  37
   12. Design and Implementation Considerations  . . . . . . . . . .  37
     12.1.  Voucher Format and Encoding  . . . . . . . . . . . . . .  38
     12.2.  Use of cBRSKI with HTTPS . . . . . . . . . . . . . . . .  38
   13. Raw Public Key Variant  . . . . . . . . . . . . . . . . . . .  39
     13.1.  Introduction and Scope . . . . . . . . . . . . . . . . .  39
     13.2.  The Registrar Trust Anchor . . . . . . . . . . . . . . .  40
     13.3.  The Pledge Voucher Request . . . . . . . . . . . . . . .  40
     13.4.  The Voucher Response . . . . . . . . . . . . . . . . . .  40
     13.5.  The Enrollment Phase . . . . . . . . . . . . . . . . . .  41



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   14. Security Considerations . . . . . . . . . . . . . . . . . . .  41
     14.1.  Duplicate serial-numbers . . . . . . . . . . . . . . . .  41
     14.2.  IDevID security in Pledge  . . . . . . . . . . . . . . .  42
     14.3.  Security of CoAP and UDP protocols . . . . . . . . . . .  43
     14.4.  Registrar Certificate may be self-signed . . . . . . . .  44
     14.5.  Use of RPK alternatives to proximity-registrar-cert  . .  44
     14.6.  MASA support of CoAPS  . . . . . . . . . . . . . . . . .  44
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  45
     15.1.  Resource Type Link Target Attribute Values Registry  . .  45
     15.2.  Media Types Registry . . . . . . . . . . . . . . . . . .  45
       15.2.1.  application/voucher+cose . . . . . . . . . . . . . .  45
     15.3.  CoAP Content-Format Registry . . . . . . . . . . . . . .  46
     15.4.  Update to BRSKI Parameters Registry  . . . . . . . . . .  46
     15.5.  Structured Syntax Suffixes Registry  . . . . . . . . . .  47
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  48
   17. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  49
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     18.2.  Informative References . . . . . . . . . . . . . . . . .  53
   Appendix A.  Software and Library Support for cBRSKI  . . . . . .  56
     A.1.  Open Source cBRSKI Implementations  . . . . . . . . . . .  56
     A.2.  Security Library Support  . . . . . . . . . . . . . . . .  56
       A.2.1.  OpensSSL Example Code . . . . . . . . . . . . . . . .  57
       A.2.2.  mbedTLS Example Code  . . . . . . . . . . . . . . . .  58
     A.3.  Generating Certificates with OpenSSL  . . . . . . . . . .  59
   Appendix B.  cBRSKI Message Examples  . . . . . . . . . . . . . .  63
     B.1.  enrollstatus  . . . . . . . . . . . . . . . . . . . . . .  63
     B.2.  voucher_status  . . . . . . . . . . . . . . . . . . . . .  65
   Appendix C.  COSE-signed Voucher (Request) Examples . . . . . . .  66
     C.1.  Pledge, Registrar and MASA Keys . . . . . . . . . . . . .  66
       C.1.1.  Pledge IDevID private key . . . . . . . . . . . . . .  66
       C.1.2.  Registrar private key . . . . . . . . . . . . . . . .  66
       C.1.3.  MASA private key  . . . . . . . . . . . . . . . . . .  67
     C.2.  Pledge, Registrar, Domain CA and MASA Certificates  . . .  67
       C.2.1.  Pledge IDevID Certificate . . . . . . . . . . . . . .  67
       C.2.2.  Registrar Certificate . . . . . . . . . . . . . . . .  69
       C.2.3.  Domain CA Certificate . . . . . . . . . . . . . . . .  71
       C.2.4.  MASA Certificate  . . . . . . . . . . . . . . . . . .  73
     C.3.  COSE-signed Pledge Voucher Request (PVR)  . . . . . . . .  75
     C.4.  COSE-signed Registrar Voucher Request (RVR) . . . . . . .  76
     C.5.  COSE-signed Voucher from MASA . . . . . . . . . . . . . .  79
   Appendix D.  Pledge Device Class Profiles . . . . . . . . . . . .  81
     D.1.  Minimal Pledge  . . . . . . . . . . . . . . . . . . . . .  81
     D.2.  Typical Pledge  . . . . . . . . . . . . . . . . . . . . .  81
     D.3.  Full-featured Pledge  . . . . . . . . . . . . . . . . . .  81
     D.4.  Comparison Chart of Pledge Classes  . . . . . . . . . . .  82
   Appendix E.  Pledge Discovery of Onboarding and Enrollment
           Options . . . . . . . . . . . . . . . . . . . . . . . . .  84



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     E.1.  Pledge Discovery Query for All cBRSKI Resources . . . . .  84
     E.2.  Pledge Discovery Query for the Root cBRSKI Resource . . .  85
     E.3.  Usage of ct Attribute . . . . . . . . . . . . . . . . . .  86
     E.4.  EST-coaps Resource Discovery  . . . . . . . . . . . . . .  87
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  88

1.  Introduction

   Secure enrollment of new nodes into constrained networks with
   constrained nodes presents unique challenges.  As explained in
   [RFC7228], such networks may have limited data throughput or may
   experience frequent packet loss.  In addition, its nodes may be
   constrained by energy availability, memory space, and code size.

   The Bootstrapping Remote Secure Key Infrastructure (BRSKI) protocol
   described in [RFC8995] provides a solution for secure zero-touch
   (automated) onboarding of new (unconfigured) devices.  In it, these
   new devices are called "pledges", equipped with a factory-installed
   Initial Device Identifier (IDevID) (see [ieee802-1AR]).  Using the
   IDevID the pledges are securely 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 uses the constrained voucher artifact and
   voucher request artifact defined in [RFC8366bis] and specifies a
   constrained version of the BRSKI protocol: cBRSKI.  The cBRSKI
   protocol uses the CoAP-based version of EST (EST-coaps from
   [RFC9148]) rather than the EST over HTTPS [RFC7030].  cBRSKI is
   itself scalable to multiple resource levels through the definition of
   optional functions.  Appendix D illustrates this.

   In BRSKI, the [RFC8366] voucher data is by default serialized to JSON
   with a signature in CMS [RFC5652].  This document uses the new CBOR
   [RFC8949] voucher data serialization, as defined by [RFC8366bis], and
   applies a new COSE [RFC9052] signature format as defined in
   Section 9.

   This COSE-signed CBOR-encoded voucher is transported using both
   secured CoAP [RFC7252] and HTTPS.  The CoAP connection (between
   Pledge and Registrar) is to be protected by DTLS (CoAPS).  The HTTP
   connection (between Registrar and MASA) is to be protected using TLS
   (HTTPS).

   Section 4 to Section 10 define the default cBRSKI protocol, by means
   of additions to and modifications of regular BRSKI.  Section 11
   considers some variations of the protocol, specific to particular



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   deployments or IoT networking technologies.  Next in Section 12, some
   considerations for the design and implementation of cBRSKI components
   are provided.

   Section 13 introduces a variant of cBRSKI for the most-constrained
   Pledges: the use of Raw Public Keys (RPK).  This variant achieves
   smaller sizes of data objects and avoids doing certain costly PKIX
   verification operations on the Pledge.

   Appendix E provides more details on how a Pledge may discover the
   various onboarding/enrollment options that a Registrar provides.
   Implementing these methods is optional for a Pledge.

2.  Terminology

   The following terms are defined in [RFC8366bis], and are used
   identically as in that document: artifact, domain, Join Registrar/
   Coordinator (JRC), malicious Registrar, Manufacturer Authorized
   Signing Authority (MASA), Pledge, Registrar, Onboarding, Owner,
   Voucher Data and Voucher.

   The protocol described in this document is referred to as cBRSKI
   where the constrained version must be constrasted with the non-
   constrained version described in [RFC8995].

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

   The following terms from [RFC7030] are used identically as in that
   document: Explicit Trust Anchor (TA), Explicit TA database, Third-
   party TA.

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

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

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

   In code examples, the string "<CODE BEGINS>" denotes the start of a
   code example and "<CODE ENDS>" the end of the code example.  The
   ellipsis ("...") in a CBOR diagnostic notation byte string denotes a
   further sequence of bytes that is not shown for brevity.  This
   notation is defined in [I-D.ietf-cbor-edn-literals].




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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

   [RFC8366bis] defines a voucher that can assert proximity,
   authenticates the Registrar, and can offer varying levels of anti-
   replay protection.  The proximity proof provided by a voucher is an
   assertion that the Pledge and the Registrar are believed to be close
   together, from a network topology point of view.  Similar to BRSKI
   [RFC8995], proximity is proven by making a DTLS connection between a
   Pledge and a Registrar.  The Pledge initiates this connection using a
   link-local source address.

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

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

   This document does not make any extensions to the semantic meaning of
   vouchers, though a new signature method based on COSE [RFC9052] is
   defined to optimize for constrained devices and networks.

   The two main parts of the BRSKI protocol are named separately in this
   document: BRSKI-EST (Section 6) for the protocol between Pledge and
   Registrar, and BRSKI-MASA (Section 7) for the protocol between the
   Registrar and the MASA.






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   Time-based vouchers are supported, but given that constrained devices
   are 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.

   [RFC8366bis] defines the CBOR voucher data encoding for the
   constrained voucher and the constrained voucher request, which are
   used by cBRSKI.

   The constrained voucher request MUST be signed by the Pledge.  COSE
   [RFC9052] is used for signing as defined in Section 9.2.  It signs
   using the private key associated with its IDevID certificate.

   The constrained voucher MUST be signed by the MASA.  Also in this
   case, COSE is used for signing.

   For the constrained voucher request (PVR) the default method for the
   Pledge to identify the Registrar is using the Registrar's full PKIX
   certificate.  But when operating PKIX-less as described in
   Section 13, the Registrar's Raw Public Key (RPK) is used for this.

   For the constrained voucher the default method to indicate ("pin") a
   trusted domain identity is the domain's PKIX CA certificate, but when
   operating PKIX-less instead the RPK of the Registrar is pinned.

   For certificates, cBRSKI currently uses the X.509 format, like BRSKI.
   The protocol and data formats are defined such that future extension
   to other certificate formats is enabled.  For example, CBOR-encoded
   and COSE-signed C509 certificates ([I-D.ietf-cose-cbor-encoded-cert])
   may provide data size savings as well as code sharing benefits with
   CBOR/COSE libraries, when applied to cBRSKI.

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

5.  Updates to RFC 8995 and RFC 9148

   This section details the ways in which this document updates other
   RFCs.

   This document Updates [RFC8995] because it:



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   *  clarifies how pinning in vouchers is done (Section 8),

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

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

   *  clarifies what kinds of Extended Key Usage attributes are
      appropriate for each certificate (Section 6.1.5, Section 7.4).

   *  extends BRSKI with the use of CoAP,

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

   *  extends the BRSKI-EST protocol (Section 6, Section 9.2) to carry
      the new "application/voucher+cose" format.

   *  extends the BRSKI-MASA protocol (Section 7, Section 9.2) to carry
      the new "application/voucher+cose" format.

   This document Updates [RFC9148] because it:

   *  defines stricter DTLS requirements (Section 6.1)),

   *  details how an EST-coaps client handles certificate renewal and
      re-enrollment (Section 6.5),

   *  details how an EST-coaps server processes a "CA certificates"
      request for content format 287 ("application/pkix-cert")
      (Section 6.6).

   *  adds enrollment status telemetry to the certificate renewal
      procedure (Section 6.5.4),

   *  adds a new media type for the CA certificates (/crts) resource
      (Section 6.5.5).

6.  BRSKI-EST Protocol

   This section describes the extensions to both BRSKI [RFC8995] and
   EST-coaps [RFC9148] protocol operations between Pledge and Registrar.
   The extensions are targeting low-resource networks with small
   packets, based on CoAP and DTLS.






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6.1.  DTLS Connection

   A DTLS connection is established between the Pledge and the
   Registrar, similar to the TLS connection described in Section 5.1 of
   [RFC8995].  This may occur via a Join Proxy as described in
   Section 6.2.  Regardless of the Join Proxy presence or particular
   mechanism used, the DTLS connection should operate identically.  The
   cBRSKI and EST-coaps requests and responses for onboarding are
   carried over this DTLS connection.

6.1.1.  DTLS Version

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

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

6.1.2.  TLS Client Certificates: IDevID authentication

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

   Subsequently the Pledge will send a Pledge Voucher Request (PVR).
   Further elements of Pledge authentication may be present in the PVR,
   as detailed in Section 9.2.

6.1.3.  DTLS Handshake Fragmentation Considerations

   DTLS includes a mechanism to fragment handshake messages.  This is
   described in Section 4.4 of [RFC9147]. cBRSKI will often be used with
   a Join Proxy, described in Section 6.2, which relays each DTLS
   message to the Registrar.  A stateless Join Proxy will need some
   additional space to wrap each DTLS message inside a CoAP request,
   while the wrapped result needs to fit in the maximum IPv6 MTU
   guaranteed on 6LoWPAN networks, which is 1280 bytes.




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

   During the operation of the EST-coaps protocol, the CoAP Block-wise
   transfer mechanism [RFC7959] will be automatically 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, when
   operating as a DTLS 1.2 client.

   A Pledge/EST-client operating as DTLS 1.3 client, MUST use the (D)TLS
   record size limit extensions ("record_size_limit") defined in
   Section 4 of [RFC8449], with RecordSizeLimit set to a value between
   512 and 1024.

6.1.4.  Registrar and the Server Name Indicator (SNI)

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

   As the Registrar is discovered by IP address, and typically connected
   via a Join Proxy, the name of the Registrar is not known to the
   Pledge.  The Pledge will not know what the hostname for the Registrar
   is, so it cannot do DNS-ID validation ([RFC9525]) on the Registrar's
   certificate.  Instead, it must do validation using the voucher.

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

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

   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




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   *  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.

6.1.5.  Registrar Server Certificate Requirements

   As per Section 3.6.1 of [RFC7030], the Registrar certificate MUST
   have the Extended Key Usage (EKU) id-kp-cmcRA.  This certificate is
   also used as a TLS Server Certificate, so it MUST also have the EKU
   id-kp-serverAuth.

   See Appendix C.2.2 for an example of a Registrar certificate with
   these EKUs set.  See Section 6.1.5 for Registrar client certificate
   requirements.

6.2.  cBRSKI Join Proxy

   [I-D.ietf-anima-constrained-join-proxy] specifies the details for a
   stateful and stateless constrained Join Proxy which is equivalent to
   the Proxy defined in [RFC8995], Section 4.  See also Section 10 for
   more details on discovery of a Join Proxy by a Pledge, and discovery
   of a Registrar by a Join Proxy.

6.3.  Request URIs, Resource Discovery and Content Formats

   cBRSKI operates using CoAP over DTLS, with request URIs using the
   coaps scheme.  The Pledge operates in CoAP client role.  To keep the
   protocol messages small the EST-coaps and cBRSKI request URIs are
   shorter than the respective EST and BRSKI URIs.

   During the cBRSKI onboarding on an IPv6 network these request URIs
   have the following form:

     coaps://[<link-local-ipv6>]:<port>/.well-known/brski/<short-name>
     coaps://[<link-local-ipv6>]:<port>/.well-known/est/<short-name>

   where <link-local-ipv6> is the discovered link-local IPv6 address of
   a Join Proxy, and <port> is the discovered port of the Join Proxy
   that is used to offer the cBRSKI proxy functionality.

   <short-name> is the short resource name for the cBRSKI and EST-coaps
   resources.  For EST-coaps, Section 5.1 of [RFC9148] defines the CoAP
   <short-name> resource names.  For cBRSKI, this document defines the
   short resource names based on the [RFC8995] long HTTP resource names.
   See Table 1 for a summary of these resource names.




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   Section 11 details how the Pledge discovers a Join Proxy link-local
   address and port in different deployment scenarios.

   The request URI formats defined here enable the Pledge to perform
   onboarding/enrollment without requiring discovery of the available
   onboarding options, voucher formats, BRSKI/EST resources, enrollment
   protocols, and so on.  This is helpful for the majority of
   constrained Pledges that would support only a single set of these
   options.  However, for Pledges that do support multiple options,
   [I-D.eckert-anima-brski-discovery] will define discovery methods so
   that a Pledge can select the optimal set of options for the current
   onboarding operation.  Alternatively, a Pledge could also send CoAP
   discovery queries (Section 7 of [RFC7252]) to the Registrar to
   discover detailed options for onboarding and/or enrollment functions.
   Supporting these queries is OPTIONAL for both the Pledge and the
   Registrar.  To clarify which options in particular can be discovered,
   Appendix E provides an informative overview of what can be discovered
   and how to discover it.

   Because a Pledge only has indirect access to the Registrar via a
   single port on the Join Proxy, the Registrar MUST host all cBRSKI/
   EST-coaps resources on the same (UDP) server IP address and port.
   This is the address and port where a Join Proxy would relay DTLS
   records from the Pledge to.

   Although the request URI templates include IP address, scheme and
   port, in practice the CoAP request sent over the secure DTLS
   connection only encodes the request URI.  For example, a Pledge that
   skips resource discovery operations just sends the initial CoAP
   voucher request as follows:

     REQ: POST /.well-known/brski/rv
       Content-Format: 836
       Payload       : (COSE-signed Pledge Voucher Request, PVR)

   Note that only Content-Format 836 ("application/voucher+cose") is
   defined in this document for the payload sent to the voucher request
   resource (/rv).  Content-Format 836 MUST be supported by the
   Registrar for the /rv resource and it MAY support additional formats.
   The Pledge MAY also indicate in the request the desired format of the
   (voucher) response, using the Accept Option.  An example of using
   this option in the request is as follows:

     REQ: POST /.well-known/brski/rv
       Content-Format: 836
       Accept        : 836
       Payload       : (COSE-signed Pledge Voucher Request, PVR)




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   If the Accept Option is omitted in the request, the response format
   follows from the request payload format (which is 836).

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

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

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 CoAP POST request 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 this with an
   additional CBOR format, derived using the CDDL rules from [RFC8610].

   The new CBOR format has CoAP Content-Format 60 ("application/cbor")
   and MUST be supported by the Registrar for both the /vs and /es
   resources.  The existing JSON format has CoAP Content-Format 50
   ("application/json") and also MUST be supported by the Registrar.  A
   Pledge MUST support at least the new CBOR format and it MAY support
   the JSON format.

6.3.2.  CoAP Resources Table

   This document inherits EST-coaps [RFC9148] functions: 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-coaps server.

   Table 1 summarizes the resources used in cBRSKI.  It includes both
   the short-name cBRSKI resources and the EST-coaps resources.









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   +=================+====================+===============+============+
   | BRSKI + EST     | cBRSKI + EST-coaps | Well-known    | Required   |
   |                 | <short-name>       | URI           | for        |
   |                 |                    | namespace     | Registrar? |
   +=================+====================+===============+============+
   | /enrollstatus   | /es                | brski         | MUST       |
   +-----------------+--------------------+---------------+------------+
   | /requestvoucher | /rv                | brski         | MUST       |
   +-----------------+--------------------+---------------+------------+
   | /voucher_status | /vs                | brski         | MUST       |
   +-----------------+--------------------+---------------+------------+
   | /cacerts        | /crts              | est           | MUST       |
   +-----------------+--------------------+---------------+------------+
   | /csrattrs       | /att               | est           | MAY        |
   +-----------------+--------------------+---------------+------------+
   | /simpleenroll   | /sen               | est           | MUST       |
   +-----------------+--------------------+---------------+------------+
   | /simplereenroll | /sren              | est           | MUST       |
   +-----------------+--------------------+---------------+------------+
   | /serverkeygen   | /skg               | est           | MAY        |
   +-----------------+--------------------+---------------+------------+
   | /serverkeygen   | /skc               | est           | MAY        |
   +-----------------+--------------------+---------------+------------+

        Table 1: BRSKI/EST resource name mapping to cBRSKI/EST-coaps
                            short resource name

6.4.  CoAP Responses

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

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

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

6.5.  Extensions to EST-coaps

   This section defines extensions to EST-coaps for Pledges (during
   initial onboarding), EST-coaps clients (after initial onboarding) and
   Registrars (that implement an EST-coaps server).  Note that a device
   that is already onboarded is not called "Pledge" in this section: it
   now acts in the role of an EST-coaps client.





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6.5.1.  Pledge enrollment procedure

   This section defines optimizations for the EST-coaps protocol as used
   by a Pledge.  These aim to reduce payload sizes and the number of
   messages (round-trips) required for the initial EST enrollment.

   A Pledge SHOULD NOT perform the optional EST-coaps "CSR attributes
   request" (/att).  Instead, the Pledge selects the attributes to
   include in the CSR as specified below.

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

   A Pledge uses the following optimized EST-coaps procedure:

   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 a "CA certificates request" (/crts) to the
       Registrar.

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

   3.  If the Pledge determines that the pinned domain CA is (1) a root
       CA certificate and (2) signer of the LDevID certificate, the
       Pledge accepts the pinned domain CA certificate as the legitimate
       trust anchor root CA for the Registrar's domain.  It also accepts
       the LDevID certificate as its new LDevID identity.  And steps 4
       and 5 are skipped.

   4.  Otherwise, if the step 3 condition was not met, the Pledge MUST
       perform a "CA certificates request" (/crts) to the EST server to
       obtain the full set of EST CA trust anchors.  It then MUST
       attempt to chain the LDevID certificate to one of the CAs in the
       set.

   5.  If the Pledge cannot obtain the set of CA certificates, or it is
       unable to create the chain as defined in step 4, the Pledge MUST
       abort the enrollment process and report the error using the
       enrollment status telemetry (/es).




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6.5.2.  Renewal of CA certificates

   An EST-coaps client that has an idea of the current time (internally,
   or via Network Time Protocol) SHOULD consider the validity time of
   the trust anchor CA(s), and MAY begin requesting new trust anchor
   certificates(s) 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 trust anchor CA(s) have expired,
   and SHOULD poll periodically for a new trust anchor certificate(s)
   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.

6.5.3.  Change of domain trust anchor(s)

   The domain trust anchor(s) may change over time.  Such a change may
   happen due to relocation of the client device to a new domain, a new
   subdomain, or due to a key update of a trust anchor as described in
   [RFC4210], Section 4.4.

   From the client's viewpoint, a trust anchor change happens during
   EST-coaps re-enrollment: since a change of domain CA requires all
   devices operating under the old domain CA to acquire a new LDevID
   certificate 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, or other.  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 [RFC7030], Section 4.1.3 and [RFC4210],
   Section 4.4 for Root CA key update requires four certificates:
   OldWithOld, OldWithNew, NewWithOld, and NewWithNew.  Of these four,
   the OldWithOld certificate is already stored in the client's Explicit
   TA database.  The other certificates will be provided to the client
   in a /crts response, during the EST-coaps re-enrollment procedure.

6.5.4.  Re-enrollment procedure

   For re-enrollment, the EST-coaps client MUST support the following
   EST-coaps procedure.  During this procedure the EST-coaps server MAY
   re-enroll the client into a new domain or into a new sub-CA within a
   domain.




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   1.  The client connects with DTLS to the EST-coaps server, and
       authenticates with its present domain certificate (LDevID) as
       usual.  The EST-coaps server authenticates itself with its domain
       RA certificate that is currently trusted by the client, i.e. it
       chains to a trust anchor CA that the client has stored in its
       Explicit TA database.  This is the OldWithOld trust anchor.  The
       client checks that the server is a Registration Authority (RA) of
       the domain as required by Section 3.6.1 of [RFC7030] before
       proceeding.

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

   3.  The client verifies the new LDevID certificate against its
       Explicit TA database.  If the new LDevID chains successfully to a
       TA, this means trust anchors did not significantly change and the
       client MAY skip retrieving the current CA certificates using the
       "CA certificates request" (/crts).  If it does not chain
       successfully, it means trust anchor(s) were changed significantly
       and the client MUST retrieve the new domain trust anchors using
       the "CA certificates request" (/crts).

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

   Note that even though the EST-coaps client may skip the /crts request
   in step 3 at this time, it SHOULD still support retrieval of the
   trust anchors periodically as detailed in Section 6.5.2.

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










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6.5.5.  Multipart Content Format for CA certificates (/crts) Resource

   In EST-coaps [RFC9148] the PKCS#7 container format is used for CA
   certificates distribution.  Because the PKCS#7 format is only used as
   a certificate container and no additional security is applied on the
   container, it becomes attractive to replace this format by something
   simpler, on a constrained Pledge: so that additional PKCS#7 code is
   avoided.  Therefore, this document defines a container format using
   the [RFC8710] "application/multipart-core" media type (CoAP Content-
   Format 62).  This is beneficial since a Pledge necessarily already
   supports CBOR parsing, so there is little code overhear to support
   this CBOR-based container format.

   A Registrar or EST-coaps server MUST support Content-Format 62 for
   the /crts resource.  The multipart collection MUST contain the
   individual CA certificates, each encoded as an "application/pkix-
   cert" (287) representation.  Future documents may define other
   certificate formats: the multipart collection can handle any future
   types.  The order of CA certificates MUST be in the CA hierarchy
   order starting from the issuer of the client's LDevID first, up to
   the highest-level domain CA, then optionally followed by any further
   CA certificates that are not part of this hierarchy.  These further
   CA certificates may be Third-party TAs as defined in [RFC7030].  The
   highest-level domain CA may or may not be a root CA certificate.

   As an example, for the two-level CA domain PKI of Figure 1 the
   multipart container will contain two representations:

   [ <domain sub-CA cert (2)> , <domain CA cert (1)> ]

   Encoded as an "application/multipart-core" CBOR array this is (shown
   in CBOR diagnostic notation):

   [ 287, h'3082' ... 'd713', 287, h'3082' ... 'a034' ]

   The total number of CA certificates SHOULD be 1, 2 or 3 and not
   higher to prevent constrained Pledges from running out of memory for
   the trust anchor storage (Explicit TA database).  However if a domain
   operator can guarantee that any Pledges enrolled in its network can
   support larger sets of CA certificates, the total number MAY be
   configured as higher than 3.  To facilitate a reliable transfer of
   large payloads over constrained networks, the server MUST support
   CoAP Block-wise transfer for the /crts response.  The server MUST
   also support the Size2 Option [RFC7959] to provide the total resource
   length in bytes, when requested by a client.






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   Implementation notes: a client that receives the first block of
   payload data from the server, can already inspect the total number of
   CA certificates by decoding the first byte of the payload.  In CBOR
   encoding, the respective first bytes 0x81-0x97 represent an array
   with length 1-23, respectively.  Furthermore, the length in bytes of
   the first CA certificate can be already determined by decoding the
   first bytes of the second element, because the CBOR encoding for
   binary string includes the length of this string.  A client that
   requires an estimate of the total resource size (to be returned as
   part of the first Block2 response from the server) can use a Size2
   Option with value 0 in its request.  Knowing the overall progress of
   the data transfer may be helpful in certain cases, e.g. when a Pledge
   provides visual progress information on the onboarding progress.

6.6.  Registrar Extensions

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

   Content-Format 836 ("application/voucher+cose") MUST be supported by
   the Registrar for the /rv resource for CoAP POST requests, both as
   request payload and as response payload.

   Content-Format 287 ("application/pkix-cert") MUST be supported by the
   Registrar as a response payload for the /sen and /sren resources.

   When a Registrar receives a "CA certificates request" (/crts) request
   with a CoAP Accept Option with value 287 ("application/pkix-cert") it
   MUST return only the single CA certificate that is the envisioned or
   actual CA authority for the current, authenticated Pledge making the
   request.  An exception to this rule is when 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 to the client
   that it needs to request another Content Format that supports
   retrieval of multiple CA certificates.

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" in its voucher request to MASA, when the
   Pledge uses this format in its request to the Registrar.



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

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

   At the moment of writing the creation of coaps based MASAs is deemed
   unrealistic.  The use of CoAP for the BRSKI-MASA connection is out of
   scope but 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.

   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.



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

   The Registrar MUST do DNS-ID checks ([RFC9525]) 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).

7.4.  Registrar Client Certificate Requirement

   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.

   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.

   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



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   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 as defined in [RFC9360].

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

8.2.  MASA Pinning Policy

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





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

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

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

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

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

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

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

8.3.  Pinning of Raw Public Keys

   Specifically for the most-constrained use cases, the pinning of the
   raw public key (RPK) of the Registrar is also supported in the
   constrained voucher, instead of a PKIX certificate.  This is used by
   the RPK variant of cBRSKI described in Section 13, but it can also be
   used in the default cBRSKI flow as a means to reduce voucher size.




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   For both cases, if an RPK is pinned, it MUST be the RPK of the
   Registrar.

   When the Pledge is known by MASA to support the RPK variant only, 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 the
   voucher data.  This is described in more detail in [RFC8366bis] and
   Section 13.

   When the Pledge is known by MASA to support PKIX certificates, the
   "pinned-domain-cert" field present in a voucher normally 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
   by MASA to also support RPK pinning and the MASA intends to pin the
   Registrar in the voucher (and not the CA), then MASA SHOULD pin the
   RPK (RPK3 in Figure 2) of the Registrar instead of the Registrar's
   End-Entity certificate to save space in the voucher.

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

              Figure 2: Raw Public Key (RPK) pinning examples

8.4.  Considerations for use of IDevID-Issuer

   [RFC8366bis] and [RFC8995] define 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



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   the IDevID.  In that case, the serial-number in the voucher data must
   refer to the same device as the serial-number that is in the 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
   arises as to what the format of the idevid-issuer contents are.

   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 IDevID certificate.  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, [RFC8366bis] 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

   The YANG ([RFC7950]) module and CBOR serialization for the
   constrained voucher as used by cBRSKI are described in [RFC8366bis].
   That document also assigns SID values to YANG elements in accordance
   with [I-D.ietf-core-sid].  The present section provides some examples
   of these artifacts and defines a new signature format for these,
   using COSE.

   Compared to the first voucher request definition done in [RFC8995],
   the constrained voucher request adds the fields proximity-registrar-
   pubk and proximity-registrar-pubk-sha256.  One of these is optionally
   used to replace the proximity-registrar-cert field, for a smaller
   voucher data size - useful for the most constrained cases.



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   The constrained voucher adds the fields pinned-domain-pubk and
   pinned-domain-pubk-sha256.  One of these is optionally used instead
   of the pinned-domain-cert field, for a smaller voucher data size.

9.1.  Example Artifacts

9.1.1.  Example Pledge voucher request (PVR) artifact

   Below, example voucher data from a constrained voucher request (PVR)
   from a Pledge to a Registrar is shown in CBOR diagnostic notation.
   Long CBOR byte strings have been shortened for readability, using the
   ellipsis ("...") to indicate elided bytes.  This notation is defined
   in [I-D.ietf-cbor-edn-literals].  The enum value of the assertion
   field is 2 for the "proximity" assertion as defined in Section 6.3 of
   [RFC8366bis].

   {
    2501: {          / SID=2501, ietf-voucher-request:voucher|voucher /
      1: 2,                      / SID=2502, assertion 2 = "proximity"/
      7: h'831D5198A6CA2C7F',    / SID=2508, nonce                    /
     12: h'30593013' ... '9A54', / SID=2513, proximity-registrar-pubk /
     13: "JADA123456789"         / SID=2514, serial-number            /
    }
   }

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

9.1.2.  Example Registrar voucher request (RVR) artifact

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











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

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

9.1.3.  Example voucher artifacts

   Below, an example of constrained voucher data is shown in CBOR
   diagnostic notation.  It was created by a MASA in response to
   receiving the Registrar Voucher Request (RVR) shown in Section 9.1.2.
   The enum value of the assertion field is set to 2, to acknowledge to
   both the Pledge and the Registrar that the proximity of the Pledge to
   the Registrar is considered proven.

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

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



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

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

9.2.  Signing voucher and voucher request artifacts with COSE

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

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

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

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

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





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

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

        Figure 3: COSE_Sign1 PVR example in CBOR diagnostic notation

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

9.2.1.  Signing of Registrar Voucher Request (RVR)

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

   18(                    / tag for COSE_Sign1                      /
    [
      h'A10126',          / protected COSE header encoding: {1: -7} /
                          /            which means { "alg": ES256 } /
      {
        32: [h'308202' ... '20AE', h'308201' ... '8CFF']  / x5bag   /
      },
      h'A178' ... '7FED', / voucher-request binary content (in CBOR)/
      h'E1B7' ... '2925'  / voucher-request binary Sign1 signature  /
    ]
   )

        Figure 4: COSE_Sign1 RVR example in CBOR diagnostic notation

   A "kid" (key ID) field is optionally present in the unprotected COSE
   header parameters map of a COSE object.  If present, it identifies
   the public key of the key pair that was used to sign the COSE
   message.  The value of the key identifier "kid" parameter may be in
   any format agreed between signer and verifier.  Usually a hash of the
   public key is used to identify the public key; but the choice of key



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   identifier method is vendor-specific.  If "kid" is not present, then
   a verifying party needs to use other context information to retrieve
   the right public key to verify the COSE_Sign1 object against.

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

   The method of generating such "kid" value is vendor-specific and
   SHOULD be configurable in the Registrar to support commonly used
   methods.  In order to support future business cases and supply chain
   integrations, a Registrar using the "kid" field MUST be configurable,
   on a per-manufacturer basis, to select a particular method for
   generating the "kid" value such that it is compatible with the method
   that the manufacturer expects.  Note that the "kid" field always has
   a CBOR byte string (bstr) format.

9.2.2.  Signing of Pledge Voucher Request (PVR)

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

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

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









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

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

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

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

   In Appendix C further examples of signed PVRs are shown.

9.2.3.  Signing of voucher by MASA

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

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

   If the MASA signing key is based upon a PKI (see
   [I-D.richardson-anima-masa-considerations] Section 2.3), and the
   Pledge only pre-stores a manufacturer (root) CA identity in its Trust
   Store which is not the identity that signs the voucher, then a



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   certificate chain needs to be included with the voucher in order for
   the Pledge to validate the MASA signing CA's signature by validating
   the chain up to the CA in its Trust Store.

   In BRSKI CMS signed vouchers [RFC8995], the CMS structure has a place
   for such certificates.  In the COSE-signed constrained vouchers
   described in this document, the x5bag attribute [RFC9360] is used to
   contain the needed certificates to form the chain.  A Registrar MUST
   NOT remove the x5bag attribute from the unprotected COSE header
   parameters when sending the voucher back to the Pledge.

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

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

      Figure 6: COSE_Sign1 signed voucher in CBOR diagnostic notation

10.  Extensions to Discovery

   It is assumed that a Join Proxy (Section 6.2) seamlessly provides a
   relayed DTLS connection between the Pledge and the Registrar.  To use
   a Join Proxy, a Pledge needs to discover it.  For Pledge discovery of
   a Join Proxy, this section extends Section 4.1 of [RFC8995] for the
   cBRSKI case.

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

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



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

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

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

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

10.1.  Discovery Operations by a Pledge

   The Pledge must discover the address/port and optionally the protocol
   with which to communicate.  The present document only defines coaps
   (CoAP over DTLS) as the default protocol for cBRSKI, therefore
   protocol discovery is out of scope.

   For the discovery method, this section only defines unsecured CoAP
   discovery per Section 7 of [RFC7252] as the default method.  This
   uses CoRE Link Format [RFC6690] payloads.

   Section 11 briefly mentions other methods that apply to specific
   deployment types or technologies.  Details about these deployment-
   specific methods, or yet other methods, new payload formats, or more
   elaborate CoAP-based methods, may be defined in future documents such
   as [I-D.eckert-anima-brski-discovery].  The more elaborate methods
   for example may include discovering only Join Proxies that support a
   particular desired onboarding protocol, voucher format, or cBRSKI
   variant.









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

   Using CoAP discovery, a Pledge can discover a Join Proxy by sending a
   link-local multicast discovery message to the All CoAP Nodes address
   FF02::FD.  Zero, one, or multiple Constrained Join Proxies may
   respond.  The handling of multiple responses and absence of responses
   cases follow the guidelines of Section 4 of [RFC8995].  The discovery
   message is a CoAP GET request on the URI path "/.well-known/core"
   using a URI query "rt=brski.jp".  This resource type (rt) is defined
   in Section 8.3 of [I-D.ietf-anima-constrained-join-proxy].

10.1.1.  Examples

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

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

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

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

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

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

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




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

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

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

10.2.  Discovery Operations by a Join Proxy

   A Join Proxy needs to discover a Registrar, either at the moment it
   needs to relay data (of a Pledge) towards the Registrar, or prior to
   that moment.  For example, it may start Registrar discovery as soon
   as it is requested to be enabled in a Join Proxy role.  It may
   periodically redo this discovery, or periodically or on-demand check
   that the Registrar is still available in the network at the
   discovered IP address.

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

11.  Deployment-specific Discovery Considerations

   This section details how discovery of a Join Proxy is done by the
   Pledge in specific deployment scenarios.  Future work such as
   [I-D.eckert-anima-brski-discovery] may define more details on
   discovery operations in the specific deployments listed here.

11.1.  6TiSCH Deployments

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

11.2.  IP networks using GRASP

   In IP networks that support GRASP [RFC8990], a Pledge can discover a
   Join Proxy by listening for GRASP messages.  GRASP supports mesh
   networks, and can also be used over unencrypted Wi-Fi.

   Details of GRASP discovery of Constrained Join Proxies are out of
   scope of this document and may be defined in future work.






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11.3.  IP networks using mDNS

   [RFC8995] defines a mechanism for the Pledge to discover a Join Proxy
   by sending mDNS [RFC6762] queries.  This mechanism can be used on any
   IP network which does not have another recommended mechanism.  It can
   be used over unencrypted Wi-Fi.  This mechanism does support link-
   local Join Proxy discovery in mesh networks.  However, it does not
   support Registrar discovery by Join Proxies in mesh networks, because
   the Registrar is typically not reachable by link-local communication
   in that case.  For this, another mechanism is needed, which is out of
   scope of this document and may be defined in future work.

   A Pledge uses an mDNS PTR query for the name "_brski-
   proxy._udp.local." to discover link-local Constrained Join Proxies.
   The label "_udp" here indicates a query for cBRSKI Constrained Join
   Proxies, as opposed to "_tcp" defined in [RFC8995] which is for
   discovering BRSKI Join Proxies.

11.4.  Thread networks using Mesh Link Establishment (MLE)

   Thread [Thread] is a wireless mesh network protocol based on 6LoWPAN
   [RFC6282] and other IETF protocols.  In Thread, a new device
   discovers potential Thread networks and Thread routers to join by
   using the Mesh Link Establishment (MLE)
   [I-D.ietf-6lo-mesh-link-establishment] protocol.  MLE uses the UDP
   port number 19788.  The new device sends discovery requests on
   different IEEE 802.15.4 radio channels, to which routers (if any
   present) respond with a discovery response containing information
   about their respective network.  Once a suitable router is selected
   the new device initiates a DTLS transport-layer secured connection to
   the network's commissioning application, over a link-local single
   radio hop to the selected Thread router.  This link is not yet
   secured at the radio/MAC link layer: 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 to the Join Proxy
   function.  The link-local IPv6 source address of the MLE response
   message indicates the address of the Join Proxy.

12.  Design and Implementation Considerations







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12.1.  Voucher Format and Encoding

   The design considerations for vouchers from Section 8 of [RFC8366bis]
   apply.  Specifically for CBOR encoding of voucher data, one key
   difference with JSON encoding is that the names of the leaves in the
   YANG definition do not affect the size of the resulting CBOR, as the
   SID ([I-D.ietf-core-sid]) translation process assigns integers to the
   names.

   To obtain the lowest code size and RAM use on the Pledge, it is
   recommended that a Pledge is designed to only process/generate these
   SID integers and not the lengthy strings.  The MASA in that case is
   required to generate the voucher data for that Pledge using only SID
   integers.  Yet, this MASA implementation MUST still support both SID
   integers and strings, to be able to process the field names in the
   RVR.

   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.  See [RFC7252] for
   details.

12.2.  Use of cBRSKI 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
   regular long [RFC8995] resource names are used, together with the new
   "application/voucher+cose" media type described in this document.
   For status telemetry requests, the Pledge may use either one or both
   of the formats defined in Section 6.3.1.  A Registrar MUST support
   both formats.

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

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

   The responsability for supporting new formats is on the Registrar.

13.  Raw Public Key Variant

13.1.  Introduction and Scope

   This section introduces a cBRSKI variant to further reduce the data
   volume and complexity of the cBRSKI onboarding.  The use of a raw
   public key (RPK) in the pinning process can significantly reduce the
   number of bytes sent over the wire and the number of round trips, and
   reduce the code footprint in a Pledge.  But it comes with a few
   significant operational limitations.

   One simplification that comes with RPK use is that a Pledge can avoid
   doing PKIX certificate operations, such as certificate chain
   validation.





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13.2.  The Registrar Trust Anchor

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

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

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

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

13.3.  The Pledge Voucher Request

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

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

13.4.  The Voucher Response

   A returned voucher will have a pinned-domain-pubk field with the
   identical key as was found in the proximity-registrar-pubk field
   above, as well as being identical to the Registrar's RPK in the
   currently active DTLS connection.  (Or alternatively the MASA may
   include the "pinned-domain-pubk-sha256" field if it knows the Pledge
   supports this field.)






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

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

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

13.5.  The Enrollment Phase

   A Pledge that does not support PKIX operations cannot use EST to
   enroll; it has to use another method for enrollment without
   certificates and the Registrar has to support this method also.  For
   example, an enrollment process that records an RPK owned by the
   Pledge as a legitimate entity that is part of the domain.

   It is possible that the Pledge will not enroll after obtaining a
   valid voucher, but instead will do only a network join operation (see
   for example [RFC9031]).  How the Pledge discovers this method and
   details of such enrollment methods are out of scope of this document.

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 centrifuge which it wishes to
   obtain access.  The attacker performs a normal BRSKI onboarding for
   the light bulb, but then uses the resulting voucher to onboard the
   gas centrifuge.  The attack requires that the gas centrifuge be
   returned to a state where it is willing to perform a new onboarding
   operation.




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   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
   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



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   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.

   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.






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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 [RFC8366bis] 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 6.1.5 and Section 7.4.

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

   In [RFC8366bis], Part voucher-request-artifact 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.

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.




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   *  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 Link Target Attribute Values 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 2: Resource Type (rt) link target attribute
                       values for IANA registration

15.2.  Media Types Registry

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

15.2.1.  application/voucher+cose











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   Type name:  application
   Subtype name:  voucher+cose
   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: N/A
   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

15.3.  CoAP Content-Format Registry

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

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

15.4.  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
   cBRSKI ([This RFC]) 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.



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   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 3: Update of the BRSKI Well-Known URI Sub-Registry

15.5.  Structured Syntax Suffixes Registry

   This section registers the "+cose" suffix in the IANA Structured
   Syntax Suffixes Registry based on the [RFC6838] procedure.






















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   Name:       CBOR Object Signing and Encryption (COSE) object
   +suffix:    +cose
   References: the "application/cose" media type [RFC9052]
   Encoding considerations: binary (CBOR)
   Interoperability considerations:
     the "application/cose" media type has an optional parameter
     "cose-type". Any new media type that uses the +cose suffix
     and allows use of this parameter MUST specify this
     explicitly, per Section 4.3 of [RFC6838]. If the parameter
     "cose-type" is allowed, its usage MUST be identical to the
     usage defined for the "application/cose" media type in
     Section 2 of [RFC9052].
     A COSE processor handling a media type "foo+cose" and which
     does not know the specific type "foo" SHOULD use the
     cose-type tag, if present, or cose-type parameter, if
     present, to determine the specific COSE object type during
     processing. If the specific type cannot be determined,
     it MUST assume only the generic COSE object structure and
     it MUST NOT perform security-critical operations using the
     COSE object.
   Fragment identifier considerations: N/A
   Security considerations: see [RFC9052]
   Contact:
     IETF COSE Working Group (cose@ietf.org) or IESG
     (iesg@ietf.org)
   Author/Change controller:
     IETF ANIMA Working Group (anima@ietf.org).
     IESG has change control over this registration.

16.  Acknowledgements

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

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

   Russ Housley , Daniel Franke , Henk Birkholtz , Kathleen Moriarty ,
   Xufeng Liu and Karl Moberg provided review feedback.

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

   Darrel Miller , Orie Steele and Manu Sporny provided review feedback
   on the registration of the +cose structured syntax suffix.



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17.  Changelog

   -25: Moved all software/library support info into Appendix A and
   added "open source" section; Removed use of formal Extends/Amends
   Update-tags (#303, #304); Moved Section 14 to Appendix E (#302);
   Editorial improvements.

   -24: Rephrased well-known URL requirement in 14.1 (#292, #293); Added
   paragraph on future certificate formats like C509 (#281, #294); Add
   formal specification for CoAP discovery of Join Proxy by Pledge,
   instead of only showing examples (#296, #300); Enable mDNS discovery
   of Join Proxy by Pledge (also in mesh networks) and list service name
   to use (#297, #299); Add requirement to support Content-Format 287 in
   /sen and /sren response (#295, #298).

   -23:
   Removed Update tag for RFC 8366 (#285, #288); Introduced cBRSKI
   acronym (#284, #286); Added Update tag for RFC 9148 (#283, #289);
   Keep CoAP discovery as only mechanism and refer to future discovery
   work (#279, #282, #290); Introduce formal CBOR diagnostics ellipsis
   elision syntax (#281, #287); Support for multi-tier CAs by
   introducing multipart-core /crts format (#275, #291); Terminology
   updated for consistency with RFC 8366-bis (#274, #280); Rename
   voucher media type to application/voucher+cose and register +cose SSS
   (#264, #277); Editorial changes including section restructuring.

   -22:
   Streamlined text to focus mostly on the default flow, with optional
   functions moved to their own sections (#269, #273); For DTLS 1.3
   client, use the record_size_limit extensions RFC 8449 (#270);
   Editorial updates; Reference rfc6125bis updated to RFC 9525.

   -11 to -21:
   (For change details see GitHub issues https://github.com/anima-wg/
   constrained-voucher/issues , related Pull Requests and commits.)

   -10:
   Design considerations extended; Examples made consistent.

   -08:
   Examples for cose_sign1 are completed and improved.

   -06:
   New SID values assigned; regenerated examples.







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   -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.

   -00:
   Initial version.

18.  References

18.1.  Normative References

   [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/rfc/rfc2119>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc4210>.







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   [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/rfc/rfc5280>.

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

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

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

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/rfc/rfc6762>.

   [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/rfc/rfc7250>.

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

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

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

   [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/rfc/rfc8174>.





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   [RFC8366bis]
              Watsen, K., Richardson, M., Pritikin, M., Eckert, T. T.,
              and Q. Ma, "A Voucher Artifact for Bootstrapping
              Protocols", Work in Progress, Internet-Draft, draft-ietf-
              anima-rfc8366bis-11, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-anima-
              rfc8366bis-11>.

   [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/rfc/rfc8446>.

   [RFC8449]  Thomson, M., "Record Size Limit Extension for TLS",
              RFC 8449, DOI 10.17487/RFC8449, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8449>.

   [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/rfc/rfc8610>.

   [RFC8710]  Fossati, T., Hartke, K., and C. Bormann, "Multipart
              Content-Format for the Constrained Application Protocol
              (CoAP)", RFC 8710, DOI 10.17487/RFC8710, February 2020,
              <https://www.rfc-editor.org/rfc/rfc8710>.

   [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/rfc/rfc8949>.

   [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/rfc/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/rfc/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/rfc/rfc9032>.





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

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

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

   [RFC9360]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Header Parameters for Carrying and Referencing X.509
              Certificates", RFC 9360, DOI 10.17487/RFC9360, February
              2023, <https://www.rfc-editor.org/rfc/rfc9360>.

   [RFC9525]  Saint-Andre, P. and R. Salz, "Service Identity in TLS",
              RFC 9525, DOI 10.17487/RFC9525, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9525>.

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.eckert-anima-brski-discovery]
              Eckert, T. T., von Oheimb, D., and E. Dijk, "Discovery for
              BRSKI variations", Work in Progress, Internet-Draft,
              draft-eckert-anima-brski-discovery-01, 23 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-eckert-anima-
              brski-discovery-01>.

   [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://datatracker.ietf.org/doc/html/
              draft-ietf-6lo-mesh-link-establishment-00>.

   [I-D.ietf-anima-constrained-join-proxy]
              Richardson, M., Van der Stok, P., and P. Kampanakis, "Join
              Proxy for Bootstrapping of Constrained Network Elements",



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              Work in Progress, Internet-Draft, draft-ietf-anima-
              constrained-join-proxy-15, 6 November 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-anima-
              constrained-join-proxy-15>.

   [I-D.ietf-anima-jws-voucher]
              Werner, T. and M. Richardson, "JWS signed Voucher
              Artifacts for Bootstrapping Protocols", Work in Progress,
              Internet-Draft, draft-ietf-anima-jws-voucher-10, 18 June
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              anima-jws-voucher-10>.

   [I-D.ietf-cbor-edn-literals]
              Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
              Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
              literals-10, 4 July 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
              edn-literals-10>.

   [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-24, 22
              December 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-core-sid-24>.

   [I-D.ietf-cose-cbor-encoded-cert]
              Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
              M. Furuhed, "CBOR Encoded X.509 Certificates (C509
              Certificates)", Work in Progress, Internet-Draft, draft-
              ietf-cose-cbor-encoded-cert-09, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              cbor-encoded-cert-09>.

   [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-23, 22
              January 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-lake-edhoc-23>.

   [I-D.ietf-rats-architecture]
              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote ATtestation procedureS (RATS)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-rats-architecture-22, 28 September 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-rats-
              architecture-22>.



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   [I-D.richardson-anima-masa-considerations]
              Richardson, M. and W. Pan, "Operational Considerations for
              Voucher infrastructure for BRSKI MASA", Work in Progress,
              Internet-Draft, draft-richardson-anima-masa-
              considerations-08, 9 May 2023,
              <https://datatracker.ietf.org/doc/html/draft-richardson-
              anima-masa-considerations-08>.

   [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/rfc/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/rfc/rfc6282>.

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

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/rfc/rfc6838>.

   [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/rfc/rfc7030>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/rfc/rfc7228>.

   [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/rfc/rfc8366>.

   [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/rfc/rfc8990>.




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

   [Thread]   Thread Group, Inc, "Thread support page, White Papers",
              November 2023,
              <https://www.threadgroup.org/support#Whitepapers>.

Appendix A.  Software and Library Support for cBRSKI

   This appendix lists software and security libraries that may be
   useful for implementing cBRSKI functionality.

A.1.  Open Source cBRSKI Implementations

   There are a few ongoing open source projects to support cBRSKI
   development and testing.  These include:

   *  OpenThread Registrar (OT Registrar) - a cBRSKI Registrar, test
      MASA server, and test Pledge written in Java.
      Link: https://github.com/EskoDijk/ot-registrar

   *  OpenThread CCM (pre-alpha) - a cBRSKI Pledge and Join Proxy for
      OpenThread-based IoT nodes, written in C/C++.
      Link: https://github.com/EskoDijk/openthread/pull/7

   *  OpenThread Network Simulator v2 (OTNS2) - a CLI + GUI simulator
      for OpenThread IoT nodes in 6LoWPAN mesh networks, able to
      accurately simulate cBRSKI Pledges onboarding (pre-alpha
      functionality) to a Thread mesh network via an OT Registrar.
      Link: https://github.com/EskoDijk/ot-ns/pull/165

   *  Fountain - a BRSKI/6TiSCH Registrar with support for COSE-signed
      vouchers, written in Ruby.
      Link: https://github.com/AnimaGUS-minerva/fountain

A.2.  Security Library Support

   For the implementation of BRSKI/cBRSKI, the use of a software library
   to manipulate PKIX certificates, estabilish secure (D)TLS
   connections, 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.







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

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

A.2.1.  OpensSSL Example Code

   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.2.2.  mbedTLS Example Code












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   <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>

A.3.  Generating Certificates with OpenSSL

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

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





























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

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

   NAME=pledge

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

   # sign csr - it uses faketime only to get endtime to 23:59:59Z
   faketime '23:59:59Z' \
   openssl x509 -set_serial 32429 -CAform PEM -CA output/masa_ca.pem \
     -CAkey keys/privkey_masa_ca.pem -extfile x509v3.ext -extensions \
     pledge_ext -req -in $NAME.csr -out output/$NAME.pem \
     -days $VALIDITY -sha256

   # Note: alternative method using 'ca' command. Currently
   # doesn't work without 'country' subject field.
   # openssl ca -rand_serial -enddate 99991231235959Z -certform PEM \
   #  -cert output/masa_ca.pem -keyfile keys/privkey_masa_ca.pem \
   #  -extfile x509v3.ext -extensions pledge_ext -in $NAME.csr \
   #  -out $NAME.pem -outdir output

   # delete temp files
   rm -f $NAME.csr

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

   <CODE ENDS>








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

   [ req ]
   prompt = no

   [ masa_ca_ext ]
   subjectAltName=email:info@masa.stok.nl
   keyUsage = critical,digitalSignature, keyCertSign, cRLSign
   basicConstraints = critical,CA:TRUE,pathlen:3
   subjectKeyIdentifier=hash
   authorityKeyIdentifier=keyid

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

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

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



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   <CODE ENDS>

   <CODE BEGINS>
   #!/bin/bash
   # File: create-cert-Registrar.sh
   # Create new cert for: Registrar in a company domain

   # days certificate is valid
   VALIDITY=1095

   # cert filename
   NAME=registrar

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

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

   # delete temp files
   rm -f $NAME.csr

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

   <CODE ENDS>



















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

   # days certificate is valid
   VALIDITY=3650

   NAME=masa_ca

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

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

   # delete temp files
   rm -f $NAME.csr

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

   <CODE ENDS>

Appendix B.  cBRSKI Message Examples

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

B.1.  enrollstatus

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

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

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




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     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:

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

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

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

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





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

     2.04 Changed

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

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

B.2.  voucher_status

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

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

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

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

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

     2.04 Changed








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Appendix C.  COSE-signed Voucher (Request) Examples

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

C.1.  Pledge, Registrar and MASA Keys

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

C.1.1.  Pledge IDevID private key

   -----BEGIN EC PRIVATE KEY-----
   MHcCAQEEIMv+C4dbzeyrEH20qkpFlWIH2FFACGZv9kW7rNWtSlYtoAoGCCqGSM49
   AwEHoUQDQgAESH6OUiYFRhfIgWl4GG8jHoj8a+8rf6t5s1mZ/4SePlKom39GQ34p
   VYryJ9aHmboLLfz69bzICQFKbkoQ5oaiew==
   -----END EC PRIVATE KEY-----

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

C.1.2.  Registrar private key

   -----BEGIN PRIVATE KEY-----
   MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgYJ/MP0dWA9BkYd4W
   s6oRY62hDddaEmrAVm5dtAXE/UGhRANCAAQgMIVb6EaRCz7LFcr4Vy0+tWW9xlSh
   Xvr27euqi54WCMXJEMk6IIaPyFBNNw8bJvqXWfZ5g7t4hj7amsvqUST2
   -----END PRIVATE KEY-----









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

C.1.3.  MASA private key

   -----BEGIN PRIVATE KEY-----
   MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgrbJ1oU+HIJ2SWYAk
   DkBTL+YNPxQG+gwsMsZB94N8mZ2hRANCAASS9NVlWJdztwNY81yPlH2UODYWhlYA
   ZfsqnEPSFZKnq8mq8gF78ZVbYi6q2FEg8kkORY/rpIU/X7SQsRuD+wMW
   -----END PRIVATE KEY-----

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

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

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

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

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









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

C.2.2.  Registrar Certificate


































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

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





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Internet-Draft         Constrained BRSKI (cBRSKI)              July 2024


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

C.2.3.  Domain CA Certificate

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


























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

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










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Internet-Draft         Constrained BRSKI (cBRSKI)              July 2024


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

C.2.4.  MASA Certificate

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



























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

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









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Internet-Draft         Constrained BRSKI (cBRSKI)              July 2024


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

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

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

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

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

   D28443A10126A0587EA11909C5A40102074823BFBBC9C2BCF2130C585B305930
   1306072A8648CE3D020106082A8648CE3D030107034200042030855BE846910B
   3ECB15CAF8572D3EB565BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868F
   C8504D370F1B26FA9759F67983BB78863EDA9ACBEA5124F60D6D4A4144413132
   33343536373839584068987DE8B007F4E9416610BBE2D48E1D7EA1032092B8BF
   CE611421950F45B22F17E214820C07E777ADF86175E25D3205568404C25FCEEC
   1B817C7861A6104B3D

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







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Internet-Draft         Constrained BRSKI (cBRSKI)              July 2024


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

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

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

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

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

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

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

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

   D28443A10126A11820825902843082028030820225A003020102020900C3F621
   49B2E30E3E300A06082A8648CE3D0403023072311C301A06035504030C134375
   73746F6D2D455220476C6F62616C204341310B3009060355040B0C0249543118
   3016060355040A0C0F437573746F6D2D45522C20496E632E3111300F06035504
   070C0853616E204A6F7365310B300906035504080C024341310B300906035504
   0613025553301E170D3232313230363131333735395A170D3235313230353131
   333735395A30818D3131302F06035504030C28437573746F6D2D455220436F6D
   6D65726369616C204275696C64696E6773205265676973747261723113301106



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Internet-Draft         Constrained BRSKI (cBRSKI)              July 2024


   0355040B0C0A4F6666696365206F707331183016060355040A0C0F437573746F
   6D2D45522C20496E632E310F300D06035504070C064F74746F7761310B300906
   035504080C024F4E310B30090603550406130243413059301306072A8648CE3D
   020106082A8648CE3D030107034200042030855BE846910B3ECB15CAF8572D3E
   B565BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868FC8504D370F1B26FA
   9759F67983BB78863EDA9ACBEA5124F6A3818730818430090603551D13040230
   00300B0603551D0F0404030204F0301D0603551D0E04160414C9080B387D8DD8
   5B3A59E7EC100B866393A9CA4C301F0603551D2304183016801492EA7640404A
   8FAB4F270BF3BC379D86CD7280F8302A0603551D250101FF0420301E06082B06
   01050507031C06082B0601050507030106082B06010505070302300A06082A86
   48CE3D040302034900304602210091A2033692EB81503D53505FFC8DA326B1EE
   7DEA96F29174F0B3341A07812201022100FF7339288108B712F418530A18025A
   895408CC45E0BB678B46FBAB37DDB4D36B59024730820243308201E9A0030201
   0202082AEA0413A42DC1CE300A06082A8648CE3D0403023072311C301A060355
   04030C13437573746F6D2D455220476C6F62616C204341310B3009060355040B
   0C02495431183016060355040A0C0F437573746F6D2D45522C20496E632E3111
   300F06035504070C0853616E204A6F7365310B300906035504080C024341310B
   3009060355040613025553301E170D3232313230363131333735395A170D3332
   313230333131333735395A3072311C301A06035504030C13437573746F6D2D45
   5220476C6F62616C204341310B3009060355040B0C0249543118301606035504
   0A0C0F437573746F6D2D45522C20496E632E3111300F06035504070C0853616E
   204A6F7365310B300906035504080C024341310B300906035504061302555330
   59301306072A8648CE3D020106082A8648CE3D0301070342000497B1ED969164
   930985BBB8AC9A2AF9455CDFEEA4B11DE2E79D068BFA803926B4005251B34F1C
   0815A4CBE03FBD1BBCB635F6431A22DE78653B87B99537ECE16CA3693067300F
   0603551D130101FF040530030101FF30250603551D11041E301C811A68656C70
   40637573746F6D2D65722E6578616D706C652E636F6D300E0603551D0F0101FF
   040403020186301D0603551D0E0416041492EA7640404A8FAB4F270BF3BC379D
   86CD7280F8300A06082A8648CE3D0403020348003045022100D6D813B390BD3A
   7B4E85424BCB1ED933AD1E981F2817B59083DD6EC1C5E3FADF02202CEE440619
   2BC767E98D7CFAE044C6807481AD8564A7D569DCA3D1CDF1E5E843590124A119
   09C5A60102027818323032322D31322D30365432303A30343A31352E3735345A
   05581A041830168014CB8D98CA74C51B58DDE7ACEF869A9443A8D666A6074823
   BFBBC9C2BCF2130958C9D28443A10126A0587EA11909C5A40102074823BFBBC9
   C2BCF2130C585B3059301306072A8648CE3D020106082A8648CE3D0301070342
   00042030855BE846910B3ECB15CAF8572D3EB565BDC654A15EFAF6EDEBAA8B9E
   1608C5C910C93A20868FC8504D370F1B26FA9759F67983BB78863EDA9ACBEA51
   24F60D6D4A414441313233343536373839584068987DE8B007F4E9416610BBE2
   D48E1D7EA1032092B8BFCE611421950F45B22F17E214820C07E777ADF86175E2
   5D3205568404C25FCEEC1B817C7861A6104B3D0D6D4A41444131323334353637
   38395840B1DD40B10787437588AEAC9036899191C16CCDBECA31C197855CCB6B
   BA142D709FE329CBC3F76297D6063ACB6759EAB98E96EA4C4AA2135AA48A247B
   AC1D6A3F

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





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Internet-Draft         Constrained BRSKI (cBRSKI)              July 2024


   18([h'A10126', {32: [h'3082028030820225A003020102020900C3F62149B2E30
   E3E300A06082A8648CE3D0403023072311C301A06035504030C13437573746F6D2D4
   55220476C6F62616C204341310B3009060355040B0C02495431183016060355040A0
   C0F437573746F6D2D45522C20496E632E3111300F06035504070C0853616E204A6F7
   365310B300906035504080C024341310B3009060355040613025553301E170D32323
   13230363131333735395A170D3235313230353131333735395A30818D3131302F060
   35504030C28437573746F6D2D455220436F6D6D65726369616C204275696C64696E6
   7732052656769737472617231133011060355040B0C0A4F6666696365206F7073311
   83016060355040A0C0F437573746F6D2D45522C20496E632E310F300D06035504070
   C064F74746F7761310B300906035504080C024F4E310B30090603550406130243413
   059301306072A8648CE3D020106082A8648CE3D030107034200042030855BE846910
   B3ECB15CAF8572D3EB565BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868FC85
   04D370F1B26FA9759F67983BB78863EDA9ACBEA5124F6A3818730818430090603551
   D1304023000300B0603551D0F0404030204F0301D0603551D0E04160414C9080B387
   D8DD85B3A59E7EC100B866393A9CA4C301F0603551D2304183016801492EA7640404
   A8FAB4F270BF3BC379D86CD7280F8302A0603551D250101FF0420301E06082B06010
   50507031C06082B0601050507030106082B06010505070302300A06082A8648CE3D0
   40302034900304602210091A2033692EB81503D53505FFC8DA326B1EE7DEA96F2917
   4F0B3341A07812201022100FF7339288108B712F418530A18025A895408CC45E0BB6
   78B46FBAB37DDB4D36B', h'30820243308201E9A00302010202082AEA0413A42DC1
   CE300A06082A8648CE3D0403023072311C301A06035504030C13437573746F6D2D45
   5220476C6F62616C204341310B3009060355040B0C02495431183016060355040A0C
   0F437573746F6D2D45522C20496E632E3111300F06035504070C0853616E204A6F73
   65310B300906035504080C024341310B3009060355040613025553301E170D323231
   3230363131333735395A170D3332313230333131333735395A3072311C301A060355
   04030C13437573746F6D2D455220476C6F62616C204341310B3009060355040B0C02
   495431183016060355040A0C0F437573746F6D2D45522C20496E632E3111300F0603
   5504070C0853616E204A6F7365310B300906035504080C024341310B300906035504
   06130255533059301306072A8648CE3D020106082A8648CE3D0301070342000497B1
   ED969164930985BBB8AC9A2AF9455CDFEEA4B11DE2E79D068BFA803926B4005251B3
   4F1C0815A4CBE03FBD1BBCB635F6431A22DE78653B87B99537ECE16CA3693067300F
   0603551D130101FF040530030101FF30250603551D11041E301C811A68656C704063
   7573746F6D2D65722E6578616D706C652E636F6D300E0603551D0F0101FF04040302
   0186301D0603551D0E0416041492EA7640404A8FAB4F270BF3BC379D86CD7280F830
   0A06082A8648CE3D0403020348003045022100D6D813B390BD3A7B4E85424BCB1ED9
   33AD1E981F2817B59083DD6EC1C5E3FADF02202CEE4406192BC767E98D7CFAE044C6
   807481AD8564A7D569DCA3D1CDF1E5E843']}, h'A11909C5A601020278183230323
   22D31322D30365432303A30343A31352E3735345A05581A041830168014CB8D98CA7
   4C51B58DDE7ACEF869A9443A8D666A6074823BFBBC9C2BCF2130958C9D28443A1012
   6A0587EA11909C5A40102074823BFBBC9C2BCF2130C585B3059301306072A8648CE3
   D020106082A8648CE3D030107034200042030855BE846910B3ECB15CAF8572D3EB56
   5BDC654A15EFAF6EDEBAA8B9E1608C5C910C93A20868FC8504D370F1B26FA9759F67
   983BB78863EDA9ACBEA5124F60D6D4A414441313233343536373839584068987DE8B
   007F4E9416610BBE2D48E1D7EA1032092B8BFCE611421950F45B22F17E214820C07E
   777ADF86175E25D3205568404C25FCEEC1B817C7861A6104B3D0D6D4A41444131323
   3343536373839', h'B1DD40B10787437588AEAC9036899191C16CCDBECA31C19785
   5CCB6BBA142D709FE329CBC3F76297D6063ACB6759EAB98E96EA4C4AA2135AA48A24
   7BAC1D6A3F'])



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C.5.  COSE-signed Voucher from MASA

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

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

   The Registrar then returns the voucher to the Pledge:

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

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

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

   D28443A10126A0590288A1190993A60102027818323032322D31322D30365432
   303A32333A33302E3730385A03F4074857EED786AD4049070859024730820243
   308201E9A00302010202082AEA0413A42DC1CE300A06082A8648CE3D04030230
   72311C301A06035504030C13437573746F6D2D455220476C6F62616C20434131
   0B3009060355040B0C02495431183016060355040A0C0F437573746F6D2D4552
   2C20496E632E3111300F06035504070C0853616E204A6F7365310B3009060355
   04080C024341310B3009060355040613025553301E170D323231323036313133
   3735395A170D3332313230333131333735395A3072311C301A06035504030C13
   437573746F6D2D455220476C6F62616C204341310B3009060355040B0C024954
   31183016060355040A0C0F437573746F6D2D45522C20496E632E3111300F0603
   5504070C0853616E204A6F7365310B300906035504080C024341310B30090603
   550406130255533059301306072A8648CE3D020106082A8648CE3D0301070342
   000497B1ED969164930985BBB8AC9A2AF9455CDFEEA4B11DE2E79D068BFA8039
   26B4005251B34F1C0815A4CBE03FBD1BBCB635F6431A22DE78653B87B99537EC
   E16CA3693067300F0603551D130101FF040530030101FF30250603551D11041E
   301C811A68656C7040637573746F6D2D65722E6578616D706C652E636F6D300E
   0603551D0F0101FF040403020186301D0603551D0E0416041492EA7640404A8F
   AB4F270BF3BC379D86CD7280F8300A06082A8648CE3D04030203480030450221
   00D6D813B390BD3A7B4E85424BCB1ED933AD1E981F2817B59083DD6EC1C5E3FA
   DF02202CEE4406192BC767E98D7CFAE044C6807481AD8564A7D569DCA3D1CDF1
   E5E8430B6D4A4144413132333435363738395840DF31B21A6AD3F5AC7F4C8B02
   6F551BD28FBCE62330D3E262AC170F6BFEDDBA5F2E8FBAA2CAACFED9E8614EAC
   5BF2450DADC53AC29DFA30E8787A1400B2E7C832

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




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

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

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





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   The largest element in the voucher is identified by key 8, which
   decodes to SID 2459 (pinned-domain-cert).  It contains the complete
   PKIX (DER-encoded X.509v3) certificate of the Registrar's domain CA.
   This certificate is shown in Appendix C.2.3.

Appendix D.  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.

D.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 onboarding process is required.

   *  No support for change of Registrar: for this case, a factory reset
      followed by a new onboarding 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.

D.2.  Typical Pledge

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

D.3.  Full-featured Pledge

   The Full-featured Pledge profile (Full) illustrates a Pledge category
   that supports multiple onboarding 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.



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D.4.  Comparison Chart of Pledge Classes

   The below table specifies the functions implemented in the three
   example Pledge classes Min (Appendix D.1), Typ (Appendix D.2) and
   Full (Appendix D.3).














































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    +============================================+=======+=====+======+
    | Functions Implemented                      |  Min  | Typ | Full |
    +============================================+=======+=====+======+
    | *General*                                  |       |     |      |
    +--------------------------------------------+-------+-----+------+
    | Support cBRSKI onboarding                  |   Y   |  Y  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | Support other onboarding method(s)         |   -   |  -  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | Real-time clock and cert time checks       |   -   |  -  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | *cBRSKI*                                   |       |     |      |
    +--------------------------------------------+-------+-----+------+
    | 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   |
    +--------------------------------------------+-------+-----+------+
    | *EST-coaps*                                |       |     |      |
    +--------------------------------------------+-------+-----+------+
    | Explicit TA database size (#certs)         |   0   |  3  |  8   |
    +--------------------------------------------+-------+-----+------+
    | Discovery for rt=ace.est*                  |   -   |  -  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | GET /att and response parsing              |   -   |  -  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | GET /crts format 62 (multiple CA certs)    |   -   |  Y  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | GET /crts format 281 (multiple CA certs)   |   -   |  -  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | ETag handling support for GET /crts        |   -   |  Y  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | Re-enrollment supported                    | - (*) |  Y  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | 6.6.1 optimized procedure                  |   Y   |  Y  |  -   |
    +--------------------------------------------+-------+-----+------+
    | Pro-active re-enrollment at own initiative |   -   |  -  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | Periodic trust anchor retrieval GET /crts  | - (*) |  Y  |  Y   |
    +--------------------------------------------+-------+-----+------+
    | Supports change of Registrar identity      | - (*) |  Y  |  Y   |
    +--------------------------------------------+-------+-----+------+

                                  Table 4




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   Notes: (*) means only possible via a factory-reset followed by a new
   cBRSKI onboarding procedure.

Appendix E.  Pledge Discovery of Onboarding and Enrollment Options

   The functionality described in this section is informative only.  In
   typical cases, for a constrained Pledge that only supports a single
   onboarding and enrollment method, this functionality is not needed.

E.1.  Pledge Discovery Query for All cBRSKI Resources

   A Pledge that wishes to discover the available cBRSKI onboarding
   options/formats can do a discovery operation using CoAP discovery per
   Section 7 of [RFC7252] and Section 4 of [RFC6690].  It first sends a
   CoAP discovery query to the Registrar over the secured DTLS
   connection.  The Registrar then responds with a CoRE Link Format
   payload containing the requested resources, if any.

   For example, if the Registrar supports a short cBRSKI URL (/b)
   instead of just the longer "/.well-known" resources, and supports
   only the voucher format "application/voucher+cose" (836), and status
   reporting in both CBOR and JSON formats, a CoAP resource discovery
   request and response may look as follows:

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

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

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

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

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



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   When responding to a discovery request for cBRSKI resources, the
   Registrar may return the full resource paths for all <short-name>
   resources and the content types which are supported by these
   resources (using ct attributes) as shown in the above examples.  This
   is useful when multiple content types are specified for a particular
   resource on the Registrar and the discovering Pledge also supports
   multiple.

   Registrars that have implemented shorter URLs must process a request
   on the corresponding "/.well-known/brski/<short-name>" URL
   identically.  In particular, a Pledge may use the longer (well-known)
   and shorter URLs in any combination.

   A Registrar may also be implemented without support for the
   (optional) CoAP discovery.  In that case, it may for example return a
   4.04 Not Found as shown below.  In such case, the Pledge cannot
   discover any onboarding/enrollment options and so it has to rely on
   the default cBRSKI options and has to use the /.well-known/brski and
   /.well-known/est resources.

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

     RES: 4.04 Not Found

E.2.  Pledge Discovery Query for the Root cBRSKI Resource

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

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

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






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   The Pledge can now start using any of the cBRSKI resources /b/<short-
   name>.  In above example, the well-known resource present under
   /.well-known/brski is not returned because this is assumed to be
   well-known to the Pledge and would not require discovery anyway.

   As a follow-up example, the Pledge can now start the onboarding by
   sending its PVR:

     REQ: POST /b/rv
     Content-Format: 836
     Accept: 836
     Payload: (binary COSE-signed PVR)

E.3.  Usage of ct Attribute

   The return of multiple content-types in the "ct" attribute by the
   Registrar allows the Pledge to choose the most appropriate one for a
   particular operation, and allows extension with new voucher formats.
   Note that only Content-Format 836 ("application/voucher+cose") is
   defined in this document for the voucher request resource (/rv), both
   as request payload and as response payload.  If the "ct" attribute is
   not indicated for the /rv resource in the CoRE link format
   description, this implies that at least format 836 is supported and
   maybe more.

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

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

   In the below example, a Pledge queries specifically for the brski.rv
   resource type to learn what voucher formats are supported:

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

     RES: 2.05 Content
     Content-Format: 40
     Payload:
     </b/rv>;rt=brski.rv;ct="836 65123 65124"







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   The Registrar returns 3 supported voucher formats: 836, 65123, and
   65124.  The first is the mandatory "application/voucher+cose".  The
   other two are numbers from the Experimental Use number range of the
   CoAP Content-Formats sub-registry, which are used as mere examples.
   The Pledge can now make a selection between the supported formats.

   Note that if the Registrar only supports the default Content-Formats
   for each cBRSKI resource as specified by this document, it may also
   omit the ct attributes in the discovery query response.  For example
   as in the following interaction:

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

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

E.4.  EST-coaps Resource Discovery

   The Pledge can also use CoAP discovery to identify enrollment
   options, for example enrollment using EST-coaps or other methods.
   The below example shows a Pledge that wants to identify EST-coaps
   enrollment options by sending a discovery query:

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

     RES: 2.05 Content
     Content-Format: 40
     Payload:
     </e/crts>;rt=ace.est.crts;ct="62 281 287",
     </e/sen>;rt=ace.est.sen;ct="281 287",
     </e/sren>;rt=ace.est.sren;ct="281 287",
     </e/att>;rt=ace.est.att,
     </e/skg>;rt=ace.est.skg,
     </e/skc>;rt=ace.est.skc

   The response indicates that EST-coaps enrollment (/sen) and re-
   enrollment (/sren) is supported, with a choice of two Content-Formats
   for the return payload: either a PKCS#7 container with a single
   LDevID certificate ("application/pkcs7-mime;smime-type=certs-only",
   content-format 281) or just a single LDevID certificate
   ("application/pkix-cert", content-format 287).





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   For the EST cacerts resources (/crts) there are three Content-Formats
   supported: a multipart-core container (62) per Section 6.5.5, a
   PKCS#7 container with all CA certificates (287), or a single (most
   relevant) CA certificate.

   The Pledge can now send a CoAP request to one or more of the
   discovered resources, with the Accept Option to indicate which return
   payload format the Pledge wants to receive.

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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