Voucher Profile for Bootstrapping Protocols
draft-ietf-anima-voucher-06

ANIMA Working Group                                            K. Watsen
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                           M. Richardson
Expires: April 28, 2018                               Sandelman Software
                                                             M. Pritikin
                                                           Cisco Systems
                                                               T. Eckert
                                                                  Huawei
                                                        October 25, 2017

              Voucher Profile for Bootstrapping Protocols
                      draft-ietf-anima-voucher-06

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 one artifact format to be a YANG-defined JSON
   document that has been signed using a CMS structure.  Other YANG-
   derived formats are possible.  The voucher artifact is normally
   generated by the pledge's manufacturer or delegate (i.e. the
   Manufacturer Authorized Signing Authority).

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

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 http://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 April 28, 2018.

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Copyright Notice

   Copyright (c) 2017 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
   (http://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   4.  Survey of Voucher Types . . . . . . . . . . . . . . . . . . .   4
   5.  Voucher artifact  . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   8
     5.4.  CMS format voucher artifact . . . . . . . . . . . . . . .  13
   6.  Design Considerations . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Renewals instead of Revocations . . . . . . . . . . . . .  14
     6.2.  Voucher Per Pledge  . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  Clock Sensitivity . . . . . . . . . . . . . . . . . . . .  15
     7.2.  Protect Voucher PKI in HSM  . . . . . . . . . . . . . . .  16
     7.3.  Test Domain Certificate Validity when Signing . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  The IETF XML Registry . . . . . . . . . . . . . . . . . .  16
     8.2.  The YANG Module Names Registry  . . . . . . . . . . . . .  17
     8.3.  The SMI Security for S/MIME CMS Content Type  Registry  .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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 or delegate, i.e. the

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   Manufacturer Authorized Signing Authority (MASA).  This artifact is
   known as the voucher.

   The voucher artifact is a JSON [RFC7159] document, conforming to a
   data model described by YANG [RFC7950], encoded using the rules
   defined in [RFC7159], and signed using (by default) a CMS structure
   [RFC5652].

   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's 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 in others the vouchers 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
   draft include: [I-D.ietf-netconf-zerotouch],
   [I-D.ietf-6tisch-dtsecurity-secure-join], and
   [I-D.ietf-anima-bootstrapping-keyinfra]).

2.  Terminology

   This document uses the following terms (sorted by name):

   Artifact:  The term "artifact" is used throughout to represent the
      voucher as instantiated in the form of a signed structure.

   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."  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.  Securely imprinting is a
      primary focus of this document [imprinting].  The analogy to
      Lorenz's work was first noted in [Stajano99theresurrecting].

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   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.

   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:  The Manufacturer Authorized Signing Authority (MASA) service
      that signs vouchers.  In some bootstrapping protocols, the MASA
      may have 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.  The
      MAS concept is explained in more detail in
      [I-D.ietf-anima-bootstrapping-keyinfra]

   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. This is 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.

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   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 might include an indication
      of the private PKIX (Public Key Infrastructure using X.509) trust
      anchor used by the Registrar, or an indication of a public PKIX
      trust anchor and additional CN-ID or DNS-ID information to
      complete authentication.  Symmetric key or other methods are 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 for space reasons)

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   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.  This 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 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 a ideal sharing of policy
      decisions and enforcement between the vendor and the owner.

   Ownership ID Voucher:  An Ownership ID Voucher is 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 make it clear what vouchers look

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   like in practice.  This description also serves to validate the YANG
   model.

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

   The method of signaling alternative signature methods is out-of-scope
   for this document.  Documents that leverage vouchers can provide this
   signaling.  The signaling could be 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 transfered on a USB key.

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
   [I-D.ietf-netmod-yang-tree-diagrams]).  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.
   That these examples conform to the encoding rules defined in
   [RFC7159].

   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.

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

   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 later.  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@2017-10-25.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";
  }

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  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:   <http://tools.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', 'MAY', and 'OPTIONAL' in
    the module text are to be interpreted as described in RFC 2119.

    Copyright (c) 2017 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
    (http://trustee.ietf.org/license-info).

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

  revision "2017-10-25" {
    description
     "Initial version";

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    reference
     "RFC XXXX: 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 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 {

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            description
              "Indicates that the ownership has been positively
               verified by the MASA (e.g., through sales channel
               integration).";
          }
          enum logged {
            description
              "Indicates that this ownership assignment has been
               logged into a database maintained by the MASA, after
               first verifying that there has not been a previous
               claim in the database for the same pledge (voucher
               transparency).";
          }
          enum proximity {
            description
              "Indicates that this assertion is made based on
               the proximity of the signer as determined by
               local network information. This is useful for
               a pledge device to indicate it 'sees' a specific
               registrar on a TLS connection, or for a registrar
               to indicate it 'sees' a pledge.";
          }
        }
        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 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 RFC5280 4.2.1.1 Authority Key Identifier OCTET STRING
           from the pledge's IDevID certificate.  Optional since some
           serial-numbers are already unique within the scope of a

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           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,
           Section 4 encoded using the ASN.1 distinguished encoding
           rules (DER), as specified 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;
        must "../expires-on";
        description
          "A processing instruction to the pledge that it MUST verify
           the revocation status for the domain certificate.  This
           instruction is only available for vouchers that expire. If
           this field is not set, then normal PKIX behaviour applies
           to validation of the domain certificate.";
      }

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      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>

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 TBD1 indicates 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.

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   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.  A possible
   other use could be as a "signed voucher request" format originating
   from 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 BRSKI Registrar) which 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 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 without it third parties can not accurately audit
   the transaction.

   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 pledge provided nonce provides a freshness guarantee.

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

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   solution complexity, as it introduces the need for additional
   protocols and code paths to distribute and process the revocations.

   Addressing the short-comings 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 re-issue 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?), re-issuing 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, without any possibility for a pledge to choose 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 a long-lived
   vouchers, 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.

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 understand of time.

   To defend against this there are three things: devices are required
   to verify that the expires-on field has not yet passed.  Devices

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   without access to time can use nonces to get ephemeral vouchers.
   Thirdly, 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 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

   A voucher is signed by a CA, that may itself be signed by a chain of
   CAs leading to a trust anchor known to a pledge.  Revocation checking
   of the intermediate certificates may be difficult in some scenarios.
   The voucher format supports the existing PKIX revocation information
   distribution within the limits of the current PKI technology (a PKCS7
   structure can contain revocation objects as well), but pledges MAY
   accept vouchers without checking X.509 certificate revocation (when
   'domain-cert-revocation-checks' is false).  Without revocation
   checking, a compromised MASA keychain could be used to issue vouchers
   ad infinitum without recourse.  For this reason, MASA implementations
   wanting to support such deployments SHOULD ensure that all the CA
   private keys used for signing the vouchers are protected by hardware
   security modules (HSMs).

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).

   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 any domain identity
   certificates before automated renewal of vouchers.

8.  IANA Considerations

8.1.  The IETF XML Registry

   This document registers a URIs in the IETF XML registry [RFC3688].
   Following the format in [RFC3688], the following registration is
   requested:

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      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.

8.2.  The YANG Module Names Registry

   This document registers a YANG module in the YANG Module Names
   registry [RFC6020].  Following the format defined in [RFC6020], the
   the following registration is requested:

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

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

   This document registers an OID in the "SMI Security for S/MIME CMS
   Content Type" registry (1.2.840.113549.1.9.16.1), with the value:

             Decimal  Description                             References
             -------  --------------------------------------  ----------
             TBD1      id-ct-animaJSONVoucher                 [ThisRFC]

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)",
              RFC 5652, September 2009.

   [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>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

   [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>.

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   [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>.

9.2.  Informative References

   [I-D.ietf-6tisch-dtsecurity-secure-join]
              Richardson, M., "6tisch Secure Join protocol", draft-ietf-
              6tisch-dtsecurity-secure-join-01 (work in progress),
              February 2017.

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-08 (work in progress), October 2017.

   [I-D.ietf-netconf-zerotouch]
              Watsen, K., Abrahamsson, M., and I. Farrer, "Zero Touch
              Provisioning for NETCONF or RESTCONF based Management",
              draft-ietf-netconf-zerotouch-19 (work in progress),
              October 2017.

   [I-D.ietf-netmod-yang-tree-diagrams]
              Bjorklund, M. and L. Berger, "YANG Tree Diagrams", draft-
              ietf-netmod-yang-tree-diagrams-02 (work in progress),
              October 2017.

   [imprinting]
              Wikipedia, , "Wikipedia article: Imprinting", July 2015,
              <https://en.wikipedia.org/wiki/Imprinting_(psychology)>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [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>.

   [Stajano99theresurrecting]
              Stajano, F. and R. Anderson, "The resurrecting duckling:
              security issues for ad-hoc wireless networks", 1999,
              <https://www.cl.cam.ac.uk/~fms27/papers/1999-StajanoAnd-
              duckling.pdf>.

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Appendix A.  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, Sheng Jiang.

   Russ Housley provided the upgrade from PKCS7 to CMS(RFC5652), 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
   Futurewei Technologies Inc.
   2330 Central Expy
   Santa Clara  95050
   USA

   EMail: tte+ietf@cs.fau.de

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