Delegated Authority for Bootstrap Voucher Artifacts
draft-richardson-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

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This Internet-Draft will expire on July 9, 2020.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Requirements for the delegation
• 1.1.1. Disconnected or Offline MASA
• 1.1.2. Resale of devices
• 1.1.3. Crypto-agility for Registrar
• 1.1.4. Transparent Assemblers/Value-Added-Resellers
• 1.2. Overview of proposed solution
• 2. Terminology
• 3. Delegated Voucher artifact
• 3.1. YANG module
• 3.2. Bundling of the vouchers
• 3.3. Delegation of multiple devices
• 4. Enhanced Pledge behaviour
• 5. Changes to Registrar behaviour
• 5.1. Discoverying the most recent Delegated Authority to use
• 6. Applying the delegated voucher to requirements
• 6.1. applicability one
• 6.2. applicability two
• 7. Constraints on pinning the Delegated Authority
• 8. Privacy Considerations
• 9. Security Considerations
• 9.1. YANG Module Security Considerations
• 10. IANA Considerations
• 10.1. The IETF XML Registry
• 10.2. YANG Module Names Registry
• 11. Acknowledgements
• 12. Changelog
• 13. References
• 13.1. Normative References
• 13.2. Informative References
• Appendix A. Extra references
• Author's Address

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:

• the MASA must be reachable to the Registar during the onboarding process.
• 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 MASA must also approve all transfers of ownership, impacting the rights of the initial seller to transfer ownership as they see fit.
• 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
• it is not possible for a MASA to pin ownership to a Registrar by Certification Authority plus DN
• 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. Disconnected or Offline MASA

A Registrar wishes to onboard devices while not being connected to the Internet.

1.1.2. Resale of devices

An owner of a device wishes to resale devices which have 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.

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 permits next Registrars with heuristics that permit it to find the delegated authorized signing service (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 separate from the manufacturer.

Delegation Voucher:

a Delegation Voucher is an [RFC8366] format voucher that has additional fields to provide detailed 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. Delegated Voucher artifact

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

There are a few new fields: pinned-delegation-certificate-authority, pinned-delegation-name, delegation-count. In addition, the serial-number field is no longer a plain leaf, but can also be an array.

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
       +-- pinned-certificate-authority?    binary
       +-- pinned-certificate-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";

  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 pinned-certificate-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 voucher to the Registrar.
             This is not used by end-vouchers.";
        }

        leaf pinned-certificate-name {
          type binary;
          description
            "A string for the rfc822Name SubjectAltName contents
             which will be trusted to issue vouchers.
             This is not used by end-vouchers.";
        }

        leaf delegation-voucher {
          type binary;
          description
            "The intermediate voucher that delegates
             authority to the entity that signs this voucher
             is to be included here.";
        }

        leaf intermediate-identities {
          type binary;
          description
            "A set of identities that will be needed to
             validate the chain of vouchers. MAY BE REDUNDANT";
        }

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

4. Enhanced Pledge behaviour

The use of a delegated 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 delegated authority, 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)

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 no 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-1th voucher. This is repeated (unwinding the recursive processing) until the zeroth voucher has been validated.

5. Changes to Registrar behaviour

TBD

5.1. Discoverying 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.

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 the 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 document 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 documents 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 delegated voucher to requirements

6.1. applicability one

TBD

6.2. applicability two

TBD

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 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

13.2. Informative References

Appendix A. Extra references

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

Author's Address