A Voucher Artifact for Bootstrapping Protocols
RFC 8366

Internet Engineering Task Force (IETF)                         K. Watsen
Request for Comments: 8366                              Juniper Networks
Category: Standards Track                                  M. Richardson
ISSN: 2070-1721                                       Sandelman Software
                                                             M. Pritikin
                                                           Cisco Systems
                                                               T. Eckert
                                                                  Huawei
                                                                May 2018

             A Voucher Artifact for Bootstrapping Protocols

Abstract

   This document defines a strategy to securely assign a pledge to an
   owner using an artifact signed, directly or indirectly, by the
   pledge's manufacturer.  This artifact is known as a "voucher".

   This document defines an artifact format as a YANG-defined JSON
   document that has been signed using a Cryptographic Message Syntax
   (CMS) structure.  Other YANG-derived formats are possible.  The
   voucher artifact is normally generated by the pledge's manufacturer
   (i.e., the Manufacturer Authorized Signing Authority (MASA)).

   This document only defines the voucher artifact, leaving it to other
   documents to describe specialized protocols for accessing it.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8366.

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Copyright Notice

   Copyright (c) 2018 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 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   4.  Survey of Voucher Types . . . . . . . . . . . . . . . . . . .   5
   5.  Voucher Artifact  . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   9
     5.4.  CMS Format Voucher Artifact . . . . . . . . . . . . . . .  15
   6.  Design Considerations . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Renewals Instead of Revocations . . . . . . . . . . . . .  16
     6.2.  Voucher Per Pledge  . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
     7.1.  Clock Sensitivity . . . . . . . . . . . . . . . . . . . .  17
     7.2.  Protect Voucher PKI in HSM  . . . . . . . . . . . . . . .  17
     7.3.  Test Domain Certificate Validity When Signing . . . . . .  17
     7.4.  YANG Module Security Considerations . . . . . . . . . . .  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  The IETF XML Registry . . . . . . . . . . . . . . . . . .  18
     8.2.  The YANG Module Names Registry  . . . . . . . . . . . . .  19
     8.3.  The Media Types Registry  . . . . . . . . . . . . . . . .  19
     8.4.  The SMI Security for S/MIME CMS Content Type Registry . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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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 [KEYINFRA]).

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.

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   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.

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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.  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.

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   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.

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   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.

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5.1.  Tree Diagram

   The following tree diagram illustrates a high-level view of a voucher
   document.  The notation used in this diagram is described in
   [RFC8340].  Each node in the diagram is fully described by the YANG
   module in Section 5.3.  Please review the YANG module for a detailed
   description of the voucher format.

   module: ietf-voucher

     yang-data voucher-artifact:
         +---- 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

5.2.  Examples

   This section provides voucher examples for illustration purposes.
   These examples conform to the encoding rules defined in [RFC8259].

   The following example illustrates an ephemeral voucher (uses a
   nonce).  The MASA generated this voucher using the 'logged' assertion
   type, knowing that it would be suitable for the pledge making the
   request.

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

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   The following example illustrates a non-ephemeral voucher (no nonce).
   While the voucher itself expires after two weeks, it presumably can
   be renewed for up to a year.  The MASA generated this voucher using
   the 'verified' assertion type, which should satisfy all pledges.

   {
     "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"
     }
   }

5.3.  YANG Module

   Following is a YANG [RFC7950] module formally describing the
   voucher's JSON document structure.

<CODE BEGINS> file "ietf-voucher@2018-05-09.yang"
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";
  }

  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>

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     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 2018-05-09 {
    description
      "Initial version";
    reference "RFC 8366: 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 {

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      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 enumeration {
          enum verified {
            description
              "Indicates that the ownership has been positively
               verified by the MASA (e.g., through sales channel
               integration).";
          }
          enum logged {
            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 {

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            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.";
          }
        }
        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;

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        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.";
      }

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      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>

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5.4.  CMS Format Voucher Artifact

   The IETF evolution of PKCS#7 is CMS [RFC5652].  A CMS-signed voucher,
   the default type, contains a ContentInfo structure with the voucher
   content.  An eContentType of 40 indicates that the content is a JSON-
   encoded voucher.

   The signing structure is a CMS SignedData structure, as specified by
   Section 5.1 of [RFC5652], encoded using ASN.1 Distinguished Encoding
   Rules (DER), as specified in ITU-T X.690 [ITU.X690.2015].

   To facilitate interoperability, Section 8.3 in this document
   registers the media type "application/voucher-cms+json" and the
   filename extension ".vcj".

   The CMS structure MUST contain a 'signerInfo' structure, as described
   in Section 5.1 of [RFC5652], containing the signature generated over
   the content using a private key trusted by the recipient.  Normally,
   the recipient is the pledge and the signer is the MASA.  Another
   possible use could be as a "signed voucher request" format
   originating from the pledge or registrar toward the MASA.  Within
   this document, the signer is assumed to be the MASA.

   Note that Section 5.1 of [RFC5652] includes a discussion about how to
   validate a CMS object, which is really a PKCS7 object (cmsVersion=1).
   Intermediate systems (such the Bootstrapping Remote Secure Key
   Infrastructures (BRSKI) registrar) that might need to evaluate the
   voucher in flight MUST be prepared for such an older format.  No
   signaling is necessary, as the manufacturer knows the capabilities of
   the pledge and will use an appropriate format voucher for each
   pledge.

   The CMS structure SHOULD also contain all of the certificates leading
   up to and including the signer's trust anchor certificate known to
   the recipient.  The inclusion of the trust anchor is unusual in many
   applications, but third parties cannot accurately audit the
   transaction without it.

   The CMS structure MAY also contain revocation objects for any
   intermediate certificate authorities (CAs) between the voucher issuer
   and the trust anchor known to the recipient.  However, the use of
   CRLs and other validity mechanisms is discouraged, as the pledge is
   unlikely to be able to perform online checks and is unlikely to have
   a trusted clock source.  As described below, the use of short-lived
   vouchers and/or a pledge-provided nonce provides a freshness
   guarantee.

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6.  Design Considerations

6.1.  Renewals Instead of Revocations

   The lifetimes of vouchers may vary.  In some bootstrapping protocols,
   the vouchers may be created and consumed immediately, whereas in
   other bootstrapping solutions, there may be a significant time delay
   between when a voucher is created and when it is consumed.  In cases
   when there is a time delay, there is a need for the pledge to ensure
   that the assertions made when the voucher was created are still
   valid.

   A revocation artifact is generally used to verify the continued
   validity of an assertion such as a PKIX certificate, web token, or a
   "voucher".  With this approach, a potentially long-lived assertion is
   paired with a reasonably fresh revocation status check to ensure that
   the assertion is still valid.  However, this approach increases
   solution complexity, as it introduces the need for additional
   protocols and code paths to distribute and process the revocations.

   Addressing the shortcomings of revocations, this document recommends
   instead the use of lightweight renewals of short-lived non-revocable
   vouchers.  That is, rather than issue a long-lived voucher, where the
   'expires-on' leaf is set to some distant date, the expectation is for
   the MASA to instead issue a short-lived voucher, where the 'expires-
   on' leaf is set to a relatively near date, along with a promise
   (reflected in the 'last-renewal-date' field) to reissue the voucher
   again when needed.  Importantly, while issuing the initial voucher
   may incur heavyweight verification checks ("Are you who you say you
   are?"  "Does the pledge actually belong to you?"), reissuing the
   voucher should be a lightweight process, as it ostensibly only
   updates the voucher's validity period.  With this approach, there is
   only the one artifact, and only one code path is needed to process
   it; there is no possibility of a pledge choosing to skip the
   revocation status check because, for instance, the OCSP Responder is
   not reachable.

   While this document recommends issuing short-lived vouchers, the
   voucher artifact does not restrict the ability to create long-lived
   voucher, if required; however, no revocation method is described.

   Note that a voucher may be signed by a chain of intermediate CAs
   leading up to the trust anchor certificate known by the pledge.  Even
   though the voucher itself is not revocable, it may still be revoked,
   per se, if one of the intermediate CA certificates is revoked.

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6.2.  Voucher Per Pledge

   The solution described herein originally enabled a single voucher to
   apply to many pledges, using lists of regular expressions to
   represent ranges of serial-numbers.  However, it was determined that
   blocking the renewal of a voucher that applied to many devices would
   be excessive when only the ownership for a single pledge needed to be
   blocked.  Thus, the voucher format now only supports a single serial-
   number to be listed.

7.  Security Considerations

7.1.  Clock Sensitivity

   An attacker could use an expired voucher to gain control over a
   device that has no understanding of time.  The device cannot trust
   NTP as a time reference, as an attacker could control the NTP stream.

   There are three things to defend against this: 1) devices are
   required to verify that the expires-on field has not yet passed, 2)
   devices without access to time can use nonces to get ephemeral
   vouchers, and 3) vouchers without expiration times may be used, which
   will appear in the audit log, informing the security decision.

   This document defines a voucher format that contains time values for
   expirations, which require an accurate clock in order to be processed
   correctly.  Vendors planning on issuing vouchers with expiration
   values must ensure that devices have an accurate clock when shipped
   from manufacturing facilities and take steps to prevent clock
   tampering.  If it is not possible to ensure clock accuracy, then
   vouchers with expirations should not be issued.

7.2.  Protect Voucher PKI in HSM

   Pursuant the recommendation made in Section 6.1 for the MASA to be
   deployed as an online voucher signing service, it is RECOMMENDED that
   the MASA's private key used for signing vouchers is protected by a
   hardware security module (HSM).

7.3.  Test Domain Certificate Validity When Signing

   If a domain certificate is compromised, then any outstanding vouchers
   for that domain could be used by the attacker.  The domain
   administrator is clearly expected to initiate revocation of any
   domain identity certificates (as is normal in PKI solutions).

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   Similarly,they are expected to contact the MASA to indicate that an
   outstanding (presumably short lifetime) voucher should be blocked
   from automated renewal.  Protocols for voucher distribution are
   RECOMMENDED to check for revocation of domain identity certificates
   before the signing of vouchers.

7.4.  YANG Module Security Considerations

   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.

   Implementations should be aware that the signed data is only
   protected from external modification; the data is still visible.
   This potential disclosure of information doesn't affect security so
   much as privacy.  In particular, adversaries can glean information
   such as which devices belong to which organizations and which CRL
   Distribution Point and/or OCSP Responder URLs are accessed to
   validate the vouchers.  When privacy is important, the CMS signed-
   data content type SHOULD be encrypted, either by conveying it via a
   mutually authenticated secure transport protocol (e.g., TLS
   [RFC5246]) or by encapsulating the signed-data content type with an
   enveloped-data content type (Section 6 of [RFC5652]), though details
   for how to do this are outside the scope of this document.

   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
   [YANG-GUIDE].

8.  IANA Considerations

8.1.  The IETF XML Registry

   This document registers a URI in the "IETF XML Registry" [RFC3688].
   IANA has registered the following:

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

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8.2.  The YANG Module Names Registry

   This document registers a YANG module in the "YANG Module Names"
   registry [RFC6020].  IANA has registered the following:

      name:         ietf-voucher
      namespace:    urn:ietf:params:xml:ns:yang:ietf-voucher
      prefix:       vch
      reference:    RFC 8366

8.3.  The Media Types Registry

   This document registers a new media type in the "Media Types"
   registry [RFC6838].  IANA has registered the following:

   Type name:  application

   Subtype name:  voucher-cms+json

   Required parameters:  none

   Optional parameters:  none

   Encoding considerations:  CMS-signed JSON vouchers are ASN.1/DER
      encoded.

   Security considerations:  See Section 7

   Interoperability considerations:  The format is designed to be
      broadly interoperable.

   Published specification:  RFC 8366

   Applications that use this media type:  ANIMA, 6tisch, and NETCONF
      zero-touch imprinting systems.

   Fragment identifier considerations:  none

   Additional information:

      Deprecated alias names for this type:  none

      Magic number(s):  None

      File extension(s):  .vcj

      Macintosh file type code(s):  none

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   Person and email address to contact for further information:
      IETF ANIMA WG

   Intended usage:  LIMITED

   Restrictions on usage:  NONE

   Author:  ANIMA WG

   Change controller:  IETF

   Provisional registration? (standards tree only):  NO

8.4.  The SMI Security for S/MIME CMS Content Type Registry

   IANA has registered the following OID in the "SMI Security for S/MIME
   CMS Content Type (1.2.840.113549.1.9.16.1)" registry:

             Decimal  Description                             References
             -------  --------------------------------------  ----------
             40       id-ct-animaJSONVoucher                  RFC 8366

9.  References

9.1.  Normative References

   [ITU.X690.2015]
                International Telecommunication Union, "Information
                Technology - ASN.1 encoding rules: Specification of
                Basic Encoding Rules (BER), Canonical Encoding Rules
                (CER) and Distinguished Encoding Rules (DER)", ITU-T
                Recommendation X.690, ISO/IEC 8825-1, August 2015,
                <https://www.itu.int/rec/T-REC-X.690/>.

   [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>.

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

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

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

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

   [RFC8259]    Bray, T., Ed., "The JavaScript Object Notation (JSON)
                Data Interchange Format", STD 90, RFC 8259,
                DOI 10.17487/RFC8259, December 2017,
                <https://www.rfc-editor.org/info/rfc8259>.

9.2.  Informative References

   [imprinting] Wikipedia, "Wikipedia article: Imprinting", February
                2018, <https://en.wikipedia.org/w/index.php?title=
                Imprinting_(psychology)&oldid=825757556>.

   [KEYINFRA]   Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
                S., and K. Watsen, "Bootstrapping Remote Secure Key
                Infrastructures (BRSKI)", Work in Progress,
                draft-ietf-anima-bootstrapping-keyinfra-12, March 2018.

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

   [RFC5246]    Dierks, T. and E. Rescorla, "The Transport Layer
                Security (TLS) Protocol Version 1.2", RFC 5246,
                DOI 10.17487/RFC5246, August 2008,
                <https://www.rfc-editor.org/info/rfc5246>.

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

   [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>.

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   [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/info/rfc6838>.

   [RFC7435]    Dukhovni, V., "Opportunistic Security: Some Protection
                Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
                December 2014,
                <https://www.rfc-editor.org/info/rfc7435>.

   [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>.

   [RFC8340]    Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
                BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
                <https://www.rfc-editor.org/info/rfc8340>.

   [SECUREJOIN] Richardson, M., "6tisch Secure Join protocol", Work in
                Progress, draft-ietf-6tisch-dtsecurity-secure-join-01,
                February 2017.

   [Stajano99theresurrecting]
                Stajano, F. and R. Anderson, "The Resurrecting Duckling:
                Security Issues for Ad-Hoc Wireless Networks", 1999,
                <https://www.cl.cam.ac.uk/research/dtg/www/files/
                publications/public/files/tr.1999.2.pdf>.

   [YANG-GUIDE] Bierman, A., "Guidelines for Authors and Reviewers of
                YANG Data Model Documents", Work in Progress,
                draft-ietf-netmod-rfc6087bis-20, March 2018.

   [ZERO-TOUCH] Watsen, K., Abrahamsson, M., and I. Farrer, "Zero Touch
                Provisioning for Networking Devices", Work in Progress,
                draft-ietf-netconf-zerotouch-21, March 2018.

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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

   Kent Watsen
   Juniper Networks

   Email: kwatsen@juniper.net

   Michael C. Richardson
   Sandelman Software

   Email: mcr+ietf@sandelman.ca
   URI:   http://www.sandelman.ca/

   Max Pritikin
   Cisco Systems

   Email: pritikin@cisco.com

   Toerless Eckert
   Huawei USA - Futurewei Technologies Inc.
   2330 Central Expy
   Santa Clara  95050
   United States of America

   Email: tte+ietf@cs.fau.de, toerless.eckert@huawei.com

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