A Voucher Artifact for Bootstrapping Protocols

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Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

1. Introduction

This document defines a strategy to securely assign a candidate device (pledge) to an owner using an artifact signed, directly or indirectly, by the pledge's manufacturer, i.e., the Manufacturer Authorized Signing Authority (MASA). This artifact is known as the "voucher".¶

The voucher artifact is a JSON [RFC8259] document that conforms with a data model described by YANG [RFC7950], is encoded using the rules defined in [RFC8259], and is signed using (by default) a CMS structure [RFC5652].¶

The primary purpose of a voucher is to securely convey a certificate, the "pinned-domain-cert", that a pledge can use to authenticate subsequent interactions. A voucher may be useful in several contexts, but the driving motivation herein is to support secure bootstrapping mechanisms. Assigning ownership is important to bootstrapping mechanisms so that the pledge can authenticate the network that is trying to take control of it.¶

The lifetimes of vouchers may vary. In some bootstrapping protocols, the vouchers may include a nonce restricting them to a single use, whereas the vouchers in other bootstrapping protocols may have an indicated lifetime. In order to support long lifetimes, this document recommends using short lifetimes with programmatic renewal, see Section 6.1.¶

This document only defines the voucher artifact, leaving it to other documents to describe specialized protocols for accessing it. Some bootstrapping protocols using the voucher artifact defined in this document include: [ZERO-TOUCH], [SECUREJOIN], and [BRSKI]).¶

2. Terminology

This document uses the following terms:¶

Artifact:

Used throughout to represent the voucher as instantiated in the form of a signed structure.¶

Domain:

The set of entities or infrastructure under common administrative control. The goal of the bootstrapping protocol is to enable a pledge to discover and join a domain.¶

Imprint:

The process where a device obtains the cryptographic key material to identify and trust future interactions with a network. This term is taken from Konrad Lorenz's work in biology with new ducklings: "during a critical period, the duckling would assume that anything that looks like a mother duck is in fact their mother" [Stajano99theresurrecting]. An equivalent for a device is to obtain the fingerprint of the network's root certification authority certificate. A device that imprints on an attacker suffers a similar fate to a duckling that imprints on a hungry wolf. Imprinting is a term from psychology and ethology, as described in [imprinting].¶

Join Registrar (and Coordinator):

A representative of the domain that is configured, perhaps autonomically, to decide whether a new device is allowed to join the domain. The administrator of the domain interfaces with a join registrar (and Coordinator) to control this process. Typically, a join registrar is "inside" its domain. For simplicity, this document often refers to this as just "registrar".¶

MASA (Manufacturer Authorized Signing Authority):

The entity that, for the purpose of this document, signs the vouchers for a manufacturer's pledges. In some bootstrapping protocols, the MASA may have an Internet presence and be integral to the bootstrapping process, whereas in other protocols the MASA may be an offline service that has no active role in the bootstrapping process.¶

Owner:

The entity that controls the private key of the "pinned-domain-cert" certificate conveyed by the voucher.¶

Pledge:

The prospective device attempting to find and securely join a domain. When shipped, it only trusts authorized representatives of the manufacturer.¶

Registrar:

See join registrar.¶

TOFU (Trust on First Use):

Where a pledge device makes no security decisions but rather simply trusts the first domain entity it is contacted by. Used similarly to [RFC7435]. This is also known as the "resurrecting duckling" model.¶

Voucher:

A signed statement from the MASA service that indicates to a pledge the cryptographic identity of the domain it should trust.¶

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. Survey of Voucher Types

A voucher is a cryptographically protected statement to the pledge device authorizing a zero-touch "imprint" on the join registrar of the domain. The specific information a voucher provides is influenced by the bootstrapping use case.¶

The voucher can impart the following information to the join registrar and pledge:¶

Assertion Basis:

Indicates the method that protects the imprint (this is distinct from the voucher signature that protects the voucher itself). This might include manufacturer-asserted ownership verification, assured logging operations, or reliance on pledge endpoint behavior such as secure root of trust of measurement. The join registrar might use this information. Only some methods are normatively defined in this document. Other methods are left for future work.¶

Authentication of Join Registrar:

Indicates how the pledge can authenticate the join registrar. This document defines a mechanism to pin the domain certificate. Pinning a symmetric key, a raw key, or "CN-ID" or "DNS-ID" information (as defined in [RFC6125]) is left for future work.¶

Anti-Replay Protections:

Time- or nonce-based information to constrain the voucher to time periods or bootstrap attempts.¶

A number of bootstrapping scenarios can be met using differing combinations of this information. All scenarios address the primary threat of a Man-in-The-Middle (MiTM) registrar gaining control over the pledge device. The following combinations are "types" of vouchers:¶

             |Assertion   |Registrar ID    | Validity    |
Voucher      |Log-|Veri-  |Trust  |CN-ID or| RTC | Nonce |
Type         | ged|  fied |Anchor |DNS-ID  |     |       |
---------------------------------------------------------|
Audit        |  X |       | X     |        |     | X     |
-------------|----|-------|-------|--------|-----|-------|
Nonceless    |  X |       | X     |        | X   |       |
Audit        |    |       |       |        |     |       |
-------------|----|-------|-------|--------|-----|-------|
Owner Audit  |  X |   X   | X     |        | X   | X     |
-------------|----|-------|-------|--------|-----|-------|
Owner ID     |    |   X   | X     |  X     | X   |       |
-------------|----|-------|----------------|-----|-------|
Bearer       |  X |       |   wildcard     | optional    |
out-of-scope |    |       |                |             |
-------------|----|-------|----------------|-------------|

NOTE: All voucher types include a 'pledge ID serial-number'
      (not shown here for space reasons).

Audit Voucher:

An Audit Voucher is named after the logging assertion mechanisms that the registrar then "audits" to enforce local policy. The registrar mitigates a MiTM registrar by auditing that an unknown MiTM registrar does not appear in the log entries. This does not directly prevent the MiTM but provides a response mechanism that ensures the MiTM is unsuccessful. The advantage is that actual ownership knowledge is not required on the MASA service.¶

Nonceless Audit Voucher:

An Audit Voucher without a validity period statement. Fundamentally, it is the same as an Audit Voucher except that it can be issued in advance to support network partitions or to provide a permanent voucher for remote deployments.¶

Ownership Audit Voucher:

An Audit Voucher where the MASA service has verified the registrar as the authorized owner. The MASA service mitigates a MiTM registrar by refusing to generate Audit Vouchers for unauthorized registrars. The registrar uses audit techniques to supplement the MASA. This provides an ideal sharing of policy decisions and enforcement between the vendor and the owner.¶

Ownership ID Voucher:

Named after inclusion of the pledge's CN-ID or DNS-ID within the voucher. The MASA service mitigates a MiTM registrar by identifying the specific registrar (via WebPKI) authorized to own the pledge.¶

Bearer Voucher:

A Bearer Voucher is named after the inclusion of a registrar ID wildcard. Because the registrar identity is not indicated, this voucher type must be treated as a secret and protected from exposure as any 'bearer' of the voucher can claim the pledge device. Publishing a nonceless bearer voucher effectively turns the specified pledge into a "TOFU" device with minimal mitigation against MiTM registrars. Bearer vouchers are out of scope.¶

5. Voucher Artifact

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

This document defines a voucher that is a JSON-encoded instance of the YANG module defined in Section 5.3 that has been, by default, CMS signed.¶

This format is described here as a practical basis for some uses (such as in NETCONF), but more to clearly indicate what vouchers look like in practice. This description also serves to validate the YANG data model.¶

Future work is expected to define new mappings of the voucher to Concise Binary Object Representation (CBOR) (from JSON) and to change the signature container from CMS to JSON Object Signing and Encryption (JOSE) or CBOR Object Signing and Encryption (COSE). XML or ASN.1 formats are also conceivable.¶

This document defines a media type and a filename extension for the CMS-encoded JSON type. Future documents on additional formats would define additional media types. Signaling is in the form of a MIME Content-Type, an HTTP Accept: header, or more mundane methods like use of a filename extension when a voucher is transferred on a USB key.¶

module: ietf-voucher

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

{
  "ietf-voucher:voucher": {
    "created-on": "2016-10-07T19:31:42Z",
    "assertion": "logged",
    "serial-number": "JADA123456789",
    "idevid-issuer": "base64encodedvalue==",
    "pinned-domain-cert": "base64encodedvalue==",
    "nonce": "base64encodedvalue=="
  }
}

{
  "ietf-voucher:voucher": {
    "created-on": "2016-10-07T19:31:42Z",
    "expires-on": "2016-10-21T19:31:42Z",
    "assertion": "verified",
    "serial-number": "JADA123456789",
    "idevid-issuer": "base64encodedvalue==",
    "pinned-domain-cert": "base64encodedvalue==",
    "domain-cert-revocation-checks": "true",
    "last-renewal-date": "2017-10-07T19:31:42Z"
  }
}

<CODE BEGINS>
 module iana-voucher-assertion-type {
  namespace "urn:ietf:params:xml:ns:yang:iana-voucher-assertion-type";
  prefix ianavat;

  organization
    "IANA";
  contact
    "Internet Assigned Numbers Authority
     Postal: ICANN
             12025 Waterfront Drive, Suite 300
             Los Angeles, CA 90094-2536
             United States of America
     Tel:    +1 310 301 5800
     <mailto:iana@iana.org>";
  description
    "This YANG module defines a YANG enumeration type for IANA
     -registered voucher assertion type. This YANG module is
     maintained by IANA and reflects the 'voucher assertion types'
     registry. The lastest revision of this YANG module can be
     obtained from the IANA web site.  Request for new enumerations
     should be made to IANA via email(iana@iana.org).

     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 (RFC 2119) (RFC 8174) when, and only when,
     they appear in all capitals, as shown here.

     Copyright (c) 2018 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject to
     the license terms contained in, the Simplified BSD License set
     forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC XXX; see the
     RFC itself for full legal notices.";

  revision 2021-07-04 {
    description
      "Initial version";
    reference "RFC XXXX: Voucher Artifact for Bootstrapping Protocols";
  }

  typedef voucher-assertion {
    type enumeration {
      enum verified {
        value 0;
        description
          "Indicates that the ownership has been positively verified
           by the MASA (e.g., through sales channel integration).";
      }
      enum logged {
        value 1;
        description
          "Indicates that the voucher has been issued after
           minimal verification of ownership or control.  The
           issuance has been logged for detection of
           potential security issues (e.g., recipients of
           vouchers might verify for themselves that unexpected
           vouchers are not in the log).  This is similar to
           unsecured trust-on-first-use principles but with the
           logging providing a basis for detecting unexpected
           events.";
      }
      enum proximity {
        value 2;
        description
          "Indicates that the voucher has been issued after
           the MASA verified a proximity proof provided by the
           device and target domain.  The issuance has been logged
           for detection of potential security issues.  This is
           stronger than just logging, because it requires some
           verification that the pledge and owner are
           in communication but is still dependent on analysis of
           the logs to detect unexpected events.";
      }
      enum agent-proximity {
        value 3;
        description
          "Indicates that the voucher has been issued after the
           MASA...support of asynchronous enrollment in BRSKI";
      }
    }
    description "Indicates what kind of ownership is being asserted by voucher";
  }
}

<CODE ENDS>

<CODE BEGINS>
module ietf-voucher {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-voucher";
  prefix vch;

  import ietf-yang-types {
    prefix yang;
    reference "RFC 6991: Common YANG Data Types";
  }
  import ietf-restconf {
    prefix rc;
    description
      "This import statement is only present to access
       the yang-data extension defined in RFC 8040.";
    reference "RFC 8040: RESTCONF Protocol";
  }

  import iana-voucher-assertion-type {
    prefix ianavat;
    reference "RFCZZZZ: Voucher Profile for Bootstrapping Protocols";
  }

  organization
    "IETF ANIMA Working Group";
  contact
    "WG Web:   <https://datatracker.ietf.org/wg/anima/>
     WG List:  <mailto:anima@ietf.org>
     Author:   Kent Watsen
               <mailto:kwatsen@juniper.net>
     Author:   Max Pritikin
               <mailto:pritikin@cisco.com>
     Author:   Michael Richardson
               <mailto:mcr+ietf@sandelman.ca>
     Author:   Toerless Eckert
               <mailto:tte+ietf@cs.fau.de>";
  description
    "This module defines the format for a voucher, which is produced by
     a pledge's manufacturer or delegate (MASA) to securely assign a
     pledge to an 'owner', so that the pledge may establish a secure
     connection to the owner's network infrastructure.

     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 (RFC 2119) (RFC 8174) when, and only when, they
     appear in all capitals, as shown here.

     Copyright (c) 2018 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or without
     modification, is permitted pursuant to, and subject to the license
     terms contained in, the Simplified BSD License set forth in Section
     4.c of the IETF Trust's Legal Provisions Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC 8366; see the RFC
     itself for full legal notices.";

  revision 2021-07-04 {
    description
      "updated to support new assertion enumerated type";
    reference "RFC ZZZZ Voucher Profile for Bootstrapping Protocols";
  }

  // Top-level statement
  rc:yang-data voucher-artifact {
    uses voucher-artifact-grouping;
  }

  // Grouping defined for future augmentations

  grouping voucher-artifact-grouping {
    description
      "Grouping to allow reuse/extensions in future work.";
    container voucher {
      description
        "A voucher assigns a pledge to an owner (pinned-domain-cert).";
      leaf created-on {
        type yang:date-and-time;
        mandatory true;
        description
          "A value indicating the date this voucher was created.  This
           node is primarily for human consumption and auditing. Future
           work MAY create verification requirements based on this
           node.";
      }
      leaf expires-on {
        type yang:date-and-time;
        must 'not(../nonce)';
        description
          "A value indicating when this voucher expires.  The node is
           optional as not all pledges support expirations, such as
           pledges lacking a reliable clock.

           If this field exists, then the pledges MUST ensure that
           the expires-on time has not yet passed. A pledge without
           an accurate clock cannot meet this requirement.

           The expires-on value MUST NOT exceed the expiration date
           of any of the listed 'pinned-domain-cert' certificates.";
      }
      leaf assertion {
        type ianavat:voucher-assertion;
        mandatory true;
        description
          "The assertion is a statement from the MASA regarding how
           the owner was verified.  This statement enables pledges
           to support more detailed policy checks.  Pledges MUST
           ensure that the assertion provided is acceptable, per
           local policy, before processing the voucher.";
      }
      leaf serial-number {
        type string;
        mandatory true;
        description
          "The serial-number of the hardware.  When processing a
           voucher, a pledge MUST ensure that its serial-number
           matches this value.  If no match occurs, then the
           pledge MUST NOT process this voucher.";
      }
      leaf idevid-issuer {
        type binary;
        description
          "The Authority Key Identifier OCTET STRING (as defined in
           Section 4.2.1.1 of RFC 5280) from the pledge's IDevID
           certificate.  Optional since some serial-numbers are
           already unique within the scope of a MASA.
           Inclusion of the statistically unique key identifier
           ensures statistically unique identification of the hardware.
           When processing a voucher, a pledge MUST ensure that its
           IDevID Authority Key Identifier matches this value.  If no
           match occurs, then the pledge MUST NOT process this voucher.

           When issuing a voucher, the MASA MUST ensure that this field
           is populated for serial-numbers that are not otherwise unique
           within the scope of the MASA.";
      }
      leaf pinned-domain-cert {
        type binary;
        mandatory true;
        description
          "An X.509 v3 certificate structure, as specified by RFC 5280,
           using Distinguished Encoding Rules (DER) encoding, as defined
           in ITU-T X.690.

           This certificate is used by a pledge to trust a Public Key
           Infrastructure in order to verify a domain certificate
           supplied to the pledge separately by the bootstrapping
           protocol.  The domain certificate MUST have this certificate
           somewhere in its chain of certificates.  This certificate
           MAY be an end-entity certificate, including a self-signed
           entity.";
        reference
          "RFC 5280:
             Internet X.509 Public Key Infrastructure Certificate
             and Certificate Revocation List (CRL) Profile.
           ITU-T X.690:
              Information technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER),
              Canonical Encoding Rules (CER) and Distinguished
              Encoding Rules (DER).";
      }
      leaf domain-cert-revocation-checks {
        type boolean;
        description
          "A processing instruction to the pledge that it MUST (true)
           or MUST NOT (false) verify the revocation status for the
           pinned domain certificate.  If this field is not set, then
           normal PKIX behavior applies to validation of the domain
           certificate.";
      }
      leaf nonce {
        type binary {
          length "8..32";
        }
        must 'not(../expires-on)';
        description
          "A value that can be used by a pledge in some bootstrapping
           protocols to enable anti-replay protection.  This node is
           optional because it is not used by all bootstrapping
           protocols.

           When present, the pledge MUST compare the provided nonce
           value with another value that the pledge randomly generated
           and sent to a bootstrap server in an earlier bootstrapping
           message.  If the values do not match, then the pledge MUST
           NOT process this voucher.";
      }
      leaf last-renewal-date {
        type yang:date-and-time;
        must '../expires-on';
        description
          "The date that the MASA projects to be the last date it
           will renew a voucher on. This field is merely informative; it
           is not processed by pledges.

           Circumstances may occur after a voucher is generated that
           may alter a voucher's validity period.  For instance, a
           vendor may associate validity periods with support contracts,
           which may be terminated or extended over time.";
      }
    } // end voucher
  } // end voucher-grouping
}

<CODE ENDS>

6. Design Considerations

7. Security Considerations

8. IANA Considerations

This section deals with actions and processes necessary for IANA to undertake to maintain the "iana-voucher-assertion-type" YANG module. The iana-voucher-assertion-type YANG module is intended to reflect the "voucher assertion types" registry in [TBD].¶

IANA is asked to create the "iana-voucher-assertion-type YANG module" registry.¶

Voucher assertion types must not be directly added to the iana-voucher-type YANG module. They must instead be added to the "voucher assertion types" registry.¶

Whenever a new enumerated type is added to the "voucher assertion types" registry, IANA must also update the "ietf-voucher-assertion-type" YANG module and add a new "enum" statement to the "voucher-assertion-type" type. The assigned name defined by the "enum" statement SHALL be the same as the mnemonic name of the new assertion type. The following substatements to the "enum" statement SHALL be defined:¶

• "value":

  Use the decimal value from the registry.¶

  "status":

  Include only if a class or type registration has been deprecated or obsoleted. IANA "deprecated" maps to YANG status "deprecated", and IANA "obsolete" maps to YANG status "obsolete".¶

  "description":

  Replicate the corresponding information from the registry, namely the full name of the new assertion type.¶

  "reference":

  Replicate the reference(s) from the registry.¶

Each time the "iana-voucher-assertion-type" YANG module is updated, a new "revision" statement SHALL be added before the existing "revision" statements. IANA has added this note to the "voucher assertion types" registries:¶

When this registry is modified, the YANG module "iana-voucher-assertion-type" must be updated as defined in [RFCXXXX]. The "Reference" text in the "voucher assertion types" registry has been updated as follows: OLD: | [RFC8366] NEW: | [RFC8366][RFCXXX]¶

Acknowledgements

The authors would like to thank for following for lively discussions on list and in the halls (ordered by last name): William Atwood, Toerless Eckert, and Sheng Jiang.¶

Russ Housley provided the upgrade from PKCS7 to CMS (RFC 5652) along with the detailed CMS structure diagram.¶

Authors' Addresses