Delegated Authority for Bootstrap Voucher Artifacts
draft-ietf-anima-voucher-delegation-00

Document Type Active Internet-Draft (anima WG)
Authors Michael Richardson  , Wei Pan 
Last updated 2021-04-28
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anima Working Group                                        M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                                  W. Pan
Expires: October 30, 2021                            Huawei Technologies
                                                          April 28, 2021

          Delegated Authority for Bootstrap Voucher Artifacts
                 draft-ietf-anima-voucher-delegation-00

Abstract

   This document describes an extension of the RFC8366 Voucher Artifact
   in order to support delegation of signing authority.  The initial
   voucher pins a public identity, and that public indentity can then
   issue additional vouchers.  This chain of authorization can support
   permission-less resale of devices, as well as guarding against
   business failure of the BRSKI [I-D.ietf-anima-bootstrapping-keyinfra]
   Manufacturer Authorized Signing Authority (MASA).

Status of This Memo

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

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

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

   This Internet-Draft will expire on October 30, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements for the Delegation . . . . . . . . . . . . .   3
       1.1.1.  Device Onboarding with Disconnected or Offline MASA .   3
       1.1.2.  Resale of Devices . . . . . . . . . . . . . . . . . .   3
       1.1.3.  Crypto-agility for Registrar  . . . . . . . . . . . .   3
       1.1.4.  Transparent Assemblers/Value-Added-Resellers  . . . .   4
     1.2.  Overview of Proposed Solution . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Delegation Voucher Artifact . . . . . . . . . . . . . . . . .   5
     3.1.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Bundling of The Vouchers  . . . . . . . . . . . . . . . .   9
     3.3.  Delegation of Multiple Devices  . . . . . . . . . . . . .   9
   4.  Enhanced Pledge Behavior  . . . . . . . . . . . . . . . . . .   9
   5.  Changes to Registrar Behavior . . . . . . . . . . . . . . . .  10
     5.1.  Discovering The Most Recent Delegated Authority to Use  .  10
   6.  Applying The Delegation Voucher to Requirements . . . . . . .  11
     6.1.  Case 1: Resale  . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Case 2: Assembly  . . . . . . . . . . . . . . . . . . . .  12
   7.  Constraints on Pinning The Delegated Authority  . . . . . . .  12
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     9.1.  YANG Module Security Considerations . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  The IETF XML Registry  . . . . . . . . . . . . . . . . .  13
     10.2.  YANG Module Names Registry . . . . . . . . . . . . . . .  13
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     13.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Extra references . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The [RFC8366] voucher artifact provides a proof from a manufacturer's
   authorizing signing authority (MASA) of the intended owner of a
   device.  This is used by an onboarding Pledge device in BRSKI
   ([I-D.ietf-anima-bootstrapping-keyinfra],
   [I-D.ietf-anima-constrained-voucher]), and SZTP ([RFC8572]).

   There are a number of criticisms of the MASA concept.  They include:

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   o  the MASA must be reachable to the Registar during the onboarding
      process.

   o  while the use of a nonceless voucher (see [RFC8366] section 4) can
      permit the MASA to be offline, it still requires the public key/
      certificate of the Registrar to be known at issuing time.  The
      device owner is always strongly dependent on the MASA service.

   o  the MASA must approve all transfers of ownership, impacting the
      rights of the supply chain distributors to transfer ownership as
      they see fit.

   o  if the Registrar has any nonceless vouchers, then it can not
      change it's public key, nor can it change which certification
      authority it uses.

   o  it is not possible for a MASA to pin ownership to a Registrar by
      Certification Authority plus DN.

   o  the creator of an assembly of parts/components can speak for the
      entire assembly of parts in a transparent way.

1.1.  Requirements for the Delegation

   This voucher artifact satisfies the following requirements:

1.1.1.  Device Onboarding with Disconnected or Offline MASA

   A Registrar wishes to onboard devices while it is not being connected
   to the Internet and MASA.

1.1.2.  Resale of Devices

   An owner of a device wishes to resale it which has previously been
   onboarded to a third party without specific authorization from the
   manufacturer.

1.1.3.  Crypto-agility for Registrar

   The owner/manager of a registrar wishes to be able to replace its
   domain registration key.  Replacing the registration key would
   invalidate any previously acquired (nonceless) vouchers.  Any devices
   which have not been onboarded, or which need to be factory reset,
   would not trust a replacement key.

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1.1.4.  Transparent Assemblers/Value-Added-Resellers

   An assembly may consist of a number of parts which are onboarded to a
   local controller during the manufacturing process.  Subsequent to
   this, the entire assembly will be shipped to a customer who wishes to
   onboard all the components.  The sub-components of the assembly needs
   to communicate with other sub-components, and so all the parts need
   to transparently onboarded.  (This is contrasted with an assembly
   where the controller acts as a security gateway.  Such a gateway
   might be a single point of failure)

   Assemblies may nest quite deeply.

1.2.  Overview of Proposed Solution

   The MASA will issue a voucher that delegates it's signing authority
   for one or more devices to a specific Registrar.  This is called a
   "delegation voucher".

   This Registrar can then operate as an authorized signing authority
   for the manufacturer, and can subsequently issue additional vouchers
   binding the pledge to new Registrars.

   This delegation can potentially be repeated multiple times to enable
   second, third, or n-th level of resale.

   The delegation voucher may be stored by the pledge for storage, to be
   included by the pledge in subsequent bootstrap operations.  The
   inclusion of the delegation voucher permits next Registrar with
   heuristics that permit it to find the delegated authorized signing
   authority (DASA).

   The delegation voucher pins the identity of the delegated authority
   using a variety of different mechanisms which are covered in
   Section 7.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Delegated Authorized Signing Authority :  the Delegated Authorized
      Signing Authority (DASA) is a service that can generate vouchers
      for one or more pledges to provide bootstrap authority, which is
      separated and delegated from the manufacturer.

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   Delegation Voucher:  a Delegation Voucher is an [RFC8366] format
      voucher that has additional fields to provide details of the
      entity to which authority has been delegated.

   Intermediate Voucher:  a voucher that is not the final voucher
      linking a pledge to its owner.

   End Voucher:  a voucher that is the final voucher linking a pledge to
      its owner.

3.  Delegation Voucher Artifact

   The following tree diagram shows the extensions to the [RFC8366]
   voucher.

   There are a few new fields:

   delegation-enable-flag:  A global enable flag to the pledge that it
      can be delegated (true) or not (false).  With default, this flag
      is false, which is consistent with the voucher artifact in
      RFC8366.

   pinned-delegation-cert-authority:  An subject-public-key-info for a
      public key of the new DASA

   pinned-delegation-cert-name:  A string for the rfc822Name
      SubjectAltName contents of the new DASA; (XXX- is it enough,
      should other DNs be considered?)

   delegation-voucher:  One or a series of Intermediate Vouchers that
      delegate authority to the DASA.  For the latter case, the series
      of Intermediate Vouchers constitute a nested structure, and the
      most inner voucher is from the MASA, which is called terminal
      voucher here

   intermediate-identities:  A set of voucher identities being
      consistent with the series of Intermediate Vouchers

   delegation-countdown:  Number of delegations still available.  If
      zero or omitted, then this is a terminal voucher and may not be
      further delegated.

   In addition, the serial-number field is no longer a plain leaf, but
   can also be an array (See Section 3.3).

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   module: ietf-delegated-voucher

     grouping voucher-delegated-grouping
       +-- voucher
          +-- created-on                          yang:date-and-time
          +-- expires-on?                         yang:date-and-time
          +-- assertion                           enumeration
          +-- serial-number                       string
          +-- idevid-issuer?                      binary
          +-- pinned-domain-cert?                 binary
          +-- domain-cert-revocation-checks?      boolean
          +-- nonce?                              binary
          +-- last-renewal-date?                  yang:date-and-time
          +-- delegation-enable-flag?             boolean
          +-- pinned-delegation-cert-authority?   binary
          +-- pinned-delegation-cert-name?        binary
          +-- delegation-voucher?                 binary
          +-- intermediate-identities?            binary
          +-- delegation-countdown?               int16

3.1.  YANG Module

   This module uses the grouping that was created in [RFC8366] to extend
   the definition.

<CODE BEGINS> file "ietf-delegated-voucher@2020-01-06.yang"
module ietf-delegated-voucher {
  yang-version 1.1;

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

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

  // maybe should import from constrained-voucher instead!
  import ietf-voucher {
    prefix "v";
  }

  organization
   "IETF ANIMA Working Group";

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  contact
   "WG Web:   <http://tools.ietf.org/wg/anima/>
    WG List:  <mailto:anima@ietf.org>
    Author:   Michael Richardson
              <mailto:mcr+ietf@sandelman.ca>";

  description
  "This module extends the RFC8366 voucher format to provide
   a mechanism by which the authority to issue additional vouchers
   may be delegated to another entity

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
   'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY',
   and 'OPTIONAL' in the module text are to be interpreted as
   described in BCP 14 RFC 2119, and RFC8174.";

  revision "2020-01-06" {
    description
     "Initial version";
    reference
     "RFC XXXX: Voucher Profile for Delegation Vouchers";
  }

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

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

    uses v:voucher-artifact-grouping {

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

      augment "voucher" {
        description "Base the delegated voucher
                     upon the regular one";

        leaf delegation-enable-flag {
          type boolean;
          description
            "A global enable flag to the pledge that it can be delegated
             (true) or not (false). With default, this flag is false,

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             which is consistent with the voucher artifact in RFC8366. ";
        }

        leaf pinned-delegation-cert-authority {
          type binary;
          description
            "An subject-public-key-info for a public key of the
             certificate authority that is to be trusted to issue
             a delegation voucher to the Registrar.
             This is not used by end-vouchers, and only valid
             when delegation-enable-flag is true.";
        }

        leaf pinned-delegation-cert-name {
          type binary;
          description
            "A string for the rfc822Name SubjectAltName contents
             which will be trusted to issue delegation vouchers.
             This is not used by end-vouchers, and only valid
             when delegation-enable-flag is true.";
        }

        leaf delegation-voucher {
          type binary;
          description
            "The intermediate voucher that delegates
             authority to the entity that signs this voucher
             is to be included here, and only valid
             when delegation-enable-flag is true.";
        }

        leaf intermediate-identities {
          type binary;
          description
            "A set of identities that will be needed to
             validate the chain of vouchers, and only valid
             when delegation-enable-flag is true. MAY BE REDUNDANT";
        }

        leaf delegation-countdown {
          type int16;
          description
          "Number of delegations still available, and only valid
             when delegation-enable-flag is true. If zero
           or omitted, then this is a terminal voucher and
           may not be further delegated";
        }
      }

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    }
  }
}
<CODE ENDS>

3.2.  Bundling of The Vouchers

   The [I-D.ietf-anima-bootstrapping-keyinfra] defines a mechanism to
   return a single voucher to the pledge.

   This protocol requires a number of additional items to be returned to
   the pledge for evaluation: the series of Intermediate Vouchers that
   leads to the DASA, and the public keys (often as certificates) of the
   Registrars on the Delegation Path that leads to each Authority.

3.3.  Delegation of Multiple Devices

   A MASA MAY delegate multiple devices to the same Registrar by putting
   an array of items in the "serial-number" attributes.  (XXX-how to
   describe this in the YANG, and the detailed mechanism, are TBD)

4.  Enhanced Pledge Behavior

   The use of a Delegation Voucher requires changes to how the pledge
   evaluates the voucher that is returned to by the Registrar.

   There are no significant changes to the voucher-request that is made.
   The pledge continues to pin the identity of the Registrar to which it
   is connected, providing a nonce to establish freshness.

   A pledge which has previously stored a Delegation Voucher and DASA ,
   SHOULD include it in its voucher request.  This will be in the form
   of a certificate provided by the "previous" owner.  This allows the
   Registrar to discover the previous authority for the pledge.  As the
   pledge has no idea if it connecting to an entity that it previously
   has connected to, it needs to include this certificate anyway.

   The pledge receives a voucher from the Registrar.  This voucher is
   called the zero voucher.  It will observe that the voucher is not
   signed with its built-in manufacturer trust anchor and it can not
   verify it.

   The pledge will examine the voucher to look for the "delegation-
   voucher" and the "intermediate-identities" attributes within the
   voucher.  A certificate from the set of intermediate-identities is
   expected to validate the signature on this zeroth end-entity voucher.
   (XXX- This attribute can be replaced by the CMS certificate chain)

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   The contained delegation-voucher object is to be interpreted as an
   (Intermediate) Voucher.  This first voucher is called the first
   voucher, or "voucher[1]".  Generically, for voucher[i], the voucher
   found in the delegation-voucher is called voucher[i+1].

   If voucher[i] can be validated by a built-in trust anchor, then the
   process is done.  If not, then voucher[i] is examined in a recursive
   process until there are no further embedded vouchers.  The last
   voucher[n] is expected to be validated by a built-in manufacturer
   trust anchor.

   Once the top (n-th) voucher is found, then the pinned-certificate-
   authority is added to the working set of trust anchors.  The "pinned-
   certificate-name" attribute is used along with the trust anchor to
   validate the certificate chain provided with the (n-1)th voucher.
   This is repeated (unwinding the recursive processing) until the
   zeroth voucher has been validated.

5.  Changes to Registrar Behavior

   The Registrar is the component that authenticates the pledge, makes
   authorization decisions, and distributes vouchers.  If the vouchers
   is delegated, then the registrar need to co-ordinate MASA and DASA.

5.1.  Discovering The Most Recent Delegated Authority to Use

   The pledge continues to use its manufacturer issued IDevID when
   performing BRSKI-style onboarding.  The IDevID contains an extension,
   the MASA URL (see [I-D.ietf-anima-bootstrapping-keyinfra] section
   2.3.2).  The IDevID certificate is not expected to be updated when
   the device is resold, nor may it be practical for an intermediate
   owner to be able to replace the IDevID with their own.  (Some devices
   may support having an intermediate owner replace the IDevID, in which
   case this section does not apply)

   The Registrar needs to be informed that it should not contact a MASA
   using the URL in the IDevID, but rather to contact the previous
   owner's DASA.

   This can be accomplished by local override, as described in
   [I-D.ietf-anima-bootstrapping-keyinfra] section 5.4:

   Registrars MAY include a mechanism to override
   the MASA URL on a manufacturer-by-manufacturer basis, and within that
   override it is appropriate to provide alternate anchors.  This will
   typically used by some vendors to establish explicit (or private)
   trust anchors for validating their MASA that is part of a sales
   channel integration.

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   The above override needs to be established on a per-device basis.  It
   requires per-device configuration which is very much non-autonomic.

   There are two other alternatives:

   1.  The Manufacturer could be aware of any Delegation Vouchers that
       it has issued for a particular device, and when contacted by the
       Registrar, it could redirect the Registrar to its DASA.  And the
       DASA may redirect the Registrar to its delegated DASA, this
       process is recursive to the final DASA.

   2.  The Pledge could provide a signed statement from the manufacturer
       providing the Registrar with a pointer to the DASA.

   Option 1 requires that the Registrar still contact the MASA,
   violating most of the goals from Section 1.1.

   Option 2 requires a signed artifact, and conveniently, the Delegation
   Voucher is exactly the item needed.  The most difficult problem is
   that the Pledge needs to (a) store one or more Delegation Vouchers in
   a non-volatile storage that survives factory reset operations, (b)
   attach these items to the pledge's voucher-request.

   The extension to the [I-D.ietf-anima-bootstrapping-keyinfra] voucher-
   request described below provides for a contained for these Delegation
   Vouchers.

6.  Applying The Delegation Voucher to Requirements

6.1.  Case 1: Resale

   This case has many scenarioes in application.

   For example, due to the willing of some devices' owner, or due to the
   creditor or bankruptcy, their devices need to resale to some third
   party, but they have previously been onboarded without specific
   authorization from the manufacturer.  Aother example is for some
   owner, which PKI system is on the cloud initially, but later, they
   wish to change its CA, and it is effectively a "resale".  Then, the
   registrar of third party must override MASA URL, contacting this
   owner's registrar for voucher.  Here, the owner's registrar is
   delegation authority.

   Furthurly, the pledges may be resaled many times, and when
   onboarding, they will receive all vouchers in order with the sale
   chain, firstly masa vouchour, then 1st intermidate, 2nd intermidate,
   till to the final dealer.  In this case, the pledge's authorization
   form a signed voucher chain.

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   In addition, for a pledge, resale can't be forever, so the delegation
   voucher need specify the limit number of resales with "delegation-
   countdown".

   The following illustrates a delegation voucher for a pledge: { "ietf-
   delegated-voucher:voucher": { "created-on": "2020-07-14T06:28:31Z",
   "expire-on": "2022-07-31T01:61:80Z", "assertion": "logged", "serial-
   number": "JADA123456789", "delegation-enable-flag": true, "pinned-
   delegation-cert-authority": "base64encodedvalue", "pinned-delegation-
   cert-name": "base64encodedvalue", "delegation-voucher":
   "base64encodedvalue", "intermediate-identities": "intermediateId1",
   "delegation-countdown": 1, } }

6.2.  Case 2: Assembly

   In some application, many pledges which come from multiple componet
   manufactures, need to be assemblied together in the first sale, In
   this time, the owner is assembly controller, so the pledge's voucher
   need to include these delegation options.

   In addition, there are also transparent assembly, for exmale rail
   wagon scenario.  Firstly, the assembly onboards normally to get all
   pledges' vouchers, then this assembly acts as intermidate registrar,
   who "sell" these pledges to every rail wagon registrar.

7.  Constraints on Pinning The Delegated Authority

   TBD

8.  Privacy Considerations

   YYY

9.  Security Considerations

9.1.  YANG Module Security Considerations

   As described in the Security Considerations section of [RFC8366]
   (section 7.4), the YANG module specified in this document defines the
   schema for data that is subsequently encapsulated by a CMS signed-
   data content type, as described in Section 5 of [RFC5652].  As such,
   all of the YANG modeled data is protected from modification.

   The use of YANG to define data structures, via the 'yang-data'
   statement, is relatively new and distinct from the traditional use of
   YANG to define an API accessed by network management protocols such
   as NETCONF [RFC6241] and RESTCONF [RFC8040].  For this reason, these

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   guidelines do not follow template described by Section 3.7 of
   [RFC8407].

10.  IANA Considerations

   This document requires the following IANA actions:

10.1.  The IETF XML Registry

   This document registers a URI in the "IETF XML Registry" [RFC3688].
   IANA is asked to register the following:

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

10.2.  YANG Module Names Registry

   This document registers a YANG module in the "YANG Module Names"
   registry [RFC6020].  IANA is asked to register the following:

        name:         ietf-delegated-voucher
        namespace:    urn:ietf:params:xml:ns:yang:ietf-delegated-voucher
        prefix:       NONE
        reference:    THIS DOCUMENT

11.  Acknowledgements

   Hello.

12.  Changelog

13.  References

13.1.  Normative References

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M. C., Eckert, T., Behringer, M.
              H., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-45 (work in progress), November 2020.

   [I-D.ietf-anima-constrained-voucher]
              Richardson, M., Stok, P. V. D., Kampanakis, P., and E.
              Dijk, "Constrained Voucher Artifacts for Bootstrapping
              Protocols", draft-ietf-anima-constrained-voucher-10 (work
              in progress), February 2021.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

13.2.  Informative References

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

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

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

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8407]  Bierman, A., "Guidelines for Authors and Reviewers of
              Documents Containing YANG Data Models", BCP 216, RFC 8407,
              DOI 10.17487/RFC8407, October 2018,
              <https://www.rfc-editor.org/info/rfc8407>.

   [RFC8572]  Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
              Touch Provisioning (SZTP)", RFC 8572,
              DOI 10.17487/RFC8572, April 2019,
              <https://www.rfc-editor.org/info/rfc8572>.

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Appendix A.  Extra references

   RFC Editor, please remove this section.  This section lists
   references in the YANG.  [RFC8174], [RFC8040].

Authors' Addresses

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

   Wei Pan
   Huawei Technologies

   Email: william.panwei@huawei.com

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