ANIMA Working Group                                            K. Watsen
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                           M. Richardson
Expires: September 16, December 9, 2017                             Sandelman Software
                                                             M. Pritikin
                                                           Cisco Systems
                                                               T. Eckert
                                                                  Huawei
                                                          March 15,
                                                            June 7, 2017

              Voucher Profile for Bootstrapping Protocols
                      draft-ietf-anima-voucher-02
                      draft-ietf-anima-voucher-03

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".

   The voucher artifact is a YANG-defined JSON document that has been
   signed using a PKCS#7 structure.  The voucher artifact is generated
   by the pledge's manufacture or delegate (i.e. the 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 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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 16, December 9, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   4.  Survey of Voucher Types . . . . . . . . . . . . . . . . . . .   4
   5.  Voucher . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Design Considerations . . . . . . . . . . . . . . . . . . . .  14  13
     6.1.  Renewals instead of Revocations . . . . . . . . . . . . .  14  13
     6.2.  Voucher Per Pledge  . . . . . . . . . . . . . . . . . . .  16  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16  14
     7.1.  Clock Sensitivity . . . . . . . . . . . . . . . . . . . .  16  14
     7.2.  Protect Voucher PKI in HSM  . . . . . . . . . . . . . . .  16  14
     7.3.  Test Domain Certificate Validity when Signing . . . . . .  16  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17  15
     8.1.  The IETF XML Registry . . . . . . . . . . . . . . . . . .  17  15
     8.2.  The YANG Module Names Registry  . . . . . . . . . . . . .  17  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  18  16
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  19  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19  18

1.  Introduction

   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 or delegate (i.e. the MASA).  This artifact is
   known as the voucher.

   The voucher artifact is a JSON document, conforming to a data model
   described by YANG [RFC7950], that has been signed using a PKCS#7
   structure.

   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 be ephemeral, whereas in others the vouchers may be
   potentially long-lived.  In order to support the second category of
   vouchers, this document recommends using short-life vouchers with
   programatic renewal, enabling the MASA to communicate the ongoing
   validity of vouchers.

   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

   The following terms are defined for clarity:

   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 "during a critical period, the duckling would assume
      that anything that looks like a mother duck is in fact their
      mother.
      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].

   Pledge:  The prospective device attempting to find and join a secure
      remote key infrastructure.  When shipped it only trusts authorized
      representatives of the manufacturer.

   Voucher:  A signed statement from the MASA service that indicates to
      a Pledge the cryptographic identity of the Registrar it should
      trust.  There are different types of vouchers depending on how
      that trust asserted.  This document describes vouchers in detail.

   Domain:  The set of entities that trust a common key or infrastructure
      trust anchor.  This includes the Proxy, Registrar, Domain
      Certificate Authority, Management components and any existing
      entity that is already a member under common
      administrative control.  The goal of the domain.

   Domain CA:  The domain Certification Authority (CA) provides
      certification functionalities bootstrapping protocol is
      to the domain.  At enable a minimum it
      provides certification functionalities Pledge to a Registrar discover and stores
      the trust anchor that defines the domain.  Optionally, it
      certifies all elements. 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. i Typically a Join Registrar is "inside" its
      domain.  For simplicity this document often refers to this as just
      "Registrar".

   MASA:  The term JRC is used in common with other bootstrap
      mechanisms.

   MASA Service:  A third-party Manufacturer Authorized Signing Authority (MASA) service on the global Internet.  The MASA
      that signs vouchers.  It also provides  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.

   Pledge:  The prospective device attempting to find and securely join
      a repository for audit log information domain.  When shipped it only trusts authorized representatives
      of privacy protected bootstrapping events.  It does not track
      ownership.  It is trusted by the Pledge. manufacturer.

   Registrar  See Join Registrar

   TOFU:  Trust on First Use. Used similarly to [RFC7435]. This is where a Pledge device makes no
      security decisions but rather simply trusts the first Registrar 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", "MAY", and "OPTIONAL" in the
   sections below are to be interpreted as described in RFC 2119
   [RFC2119].

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 might include an indication
      of the private PKIX 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 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 |
   Name         | 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)

   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 direct 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 Voucher's
      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:  An Ownership ID Voucher is named after
      inclusion of the Pledge's CN-ID or DNS-ID within the voucher.  An
      example Ownership Voucher is defined in
      [I-D.ietf-netconf-zerotouch].  The
      MASA service mitigates a MiTM Registrar by identifying the
      specific Registrar (via WebPKI) authorized to own the Pledge.  [DISCUSS: still needed?]

   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

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

   The voucher is signed a PKCS#7 SignedData structure, as specified by
   Section 9.1 of [RFC2315], encoded using ASN.1 distinguished encoding
   rules (DER), as specified in ITU-T X.690.

   The PKCS#7 structure MUST contain JSON-encoded content conforming to
   the YANG module specified in Section 5.3.

   The PKCS#7 structure MUST also contain a 'signerInfo' structure, as
   described in Section 9.1 of [RFC2315], containing the signature
   generated over the content using the MASA's private key.

   The PKCS#7 structure SHOULD also contain all of the certificates
   leading up to and including the MASA's trust anchor certificate known
   to the pledges.

   The PKCS#7 structure MAY also contain revocation objects for any
   intermediate CAs between the voucher-issuer and the trust anchor
   known to the pledge.

5.1.  Tree Diagram

   The following tree diagram [I-D.bjorklund-netmod-yang-tree-diagrams]
   illustrates a high-level view of a voucher document.  Each field in
   the voucher is fully described by the YANG module provided in
   Section 5.3.  Please review this YANG module for a detailed
   description of the voucher format.

   module: ietf-voucher
     +--ro voucher
        +--ro authority-key-identifier?         binary
        +--ro created-on                       yang:date-and-time
        +--ro expires-on?                      yang:date-and-time
        +--ro assertion                        enumeration
        +--ro device-identifier serial-number                    string
        +--ro trusted-ca-certificate idevid-issuer?                   binary
        +--ro domain-certificate-identifier
        |  +--ro subject? pinned-domain-cert*              binary
        |  +--ro cn-id?     string
        |  +--ro dns-id?    string
        +--ro assert-certificate-revocations? domain-cert-revocation-checks?   boolean
        +--ro nonce?                           binary
        +--ro last-renewal-date?               yang:date-and-time

5.2.  Examples

   This section provides a couple Voucher examples for illustration
   purposes.

   The following example illustrates an ephemeral voucher (uses a nonce)
   encoded in JSON.  As is expected with a dynamically-generated
   voucher, only a single pledge (device-identifier) is specified.  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",
       "trusted-ca-certificate":
       "serial-number": "JADA123456789",
       "serial-number-issuer": "some binary identifier",
       "domain-cert-trusted-ca": "base64-encoded X.509 DER",
       "device-identifier": "JADA123456789",
       "created-on": "2016-10-07T19:31:42Z",
       "domain-cert-identifier": {
         "subject": "base64-encoded Subject DER"
       },
       "nonce": "base64-encoded octet string"
     }
   }

   The following illustrates a long-lived voucher (no nonce), encoded in
   XML.  This particular voucher applies to more than one pledge
   (unique-id), which might relate to, for instance, they were all
   issued as part of the same purchase order.  This voucher includes
   both a trust anchor certificate (trusted-ca-certificate) as well as
   some additional information (cn-id and dns-id) that can be used to
   identify a specific domain certificate issued, perhaps indirectly, by
   the trust anchor CA.

   {
     "ietf-voucher:voucher": {
       "created-on": "2016-10-07T19:31:42Z",
       "expires-on": "2016-10-21T19:31:42Z",
       "assertion": "verified",
       "trusted-ca-certificate":
       "serial-number": "JADA123456789",
       "serial-number-issuer": "some binary identifier",
       "domain-cert-trusted-ca": "base64-encoded X.509 DER",
       "domain-certificate-identifier":
       "domain-cert-identifier": {
         "subject": "base64-encoded Subject DER"
       },
       "device-identifier": "JADA123456789",
       "created-on": "2016-10-07T19:31:42Z"
       "assert-revocations-on-PKIX-certs": "false",
       "last-renewal-date": "2017-10-07T19:31:42Z"
     }
   }

5.3.  YANG Module

<CODE BEGINS> file "ietf-voucher@2017-03-15.yang" "ietf-voucher@2017-06-07.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.  The yang-data extension doesn't
        itself have anything to do with RESTCONF, but was placed in the
        that RFC for convenience.  This extension is being tracked to
        be moved to the next version of the YANG modeling language.
        Regardless where or how this extension statement is defined,
        there should not be any impact to a voucher's encoding."; 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>";

  description
   "This module defines the format for a voucher, which is produced by
    a pledge's manufacturer or delegate (MASA) to securely assign one
    or more pledges to an 'owner', so that the pledges may establish a
    secure connection to the owner's network infrastructure."; 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.";

  revision "2017-03-15" "2017-06-07" {
    description
     "Initial version";
    reference
     "RFC XXXX: Voucher Profile for Bootstrapping Protocols";
  }

  rc:yang-data voucher-artifact {
    uses voucher-grouping;
  }

  grouping voucher-grouping {
    description
      "Grouping only exists for pyang tree output...";

    container voucher {
      config false;
      description
        "A voucher that can be used to assign one or more
         pledges to an owner.";

      leaf authority-key-identifier {
         type binary;
         description
          "The Subject Key Identifier of the MASA's leaf certificate.
           Enables the pledge a definitively identify the voucher's
           issuer's certificate.  This field is optional as not all
           vouchers will be signed by a private key associated with
           an X.509 certificate.";
       }

       leaf created-on {
        type yang:date-and-time;
        mandatory true;
        description
          "A value indicating the date this voucher was created.  This
           node is optional because its primary purpose is for human
           consumption.  However, when present, pledges that have
           reliable clocks SHOULD ensure that this created-on value
           is not greater than the current time.";
      }

      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 the pledge supports expirations and the expires-on value
            is less this field exists, then the current time, then the pledge pledges MUST ensure that
           the expires-on time has not
            process yet passed. A pledge without
           an accurate clock cannot meet this voucher."; 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 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).";
          }
        }
        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 device-identifier serial-number {
        type string;
        mandatory true;
        description
           "A unique identifier (e.g.,
          "The serial number) within the scope number of the MASA. hardware.  When processing a vouchers, pledges
           voucher, a pledge MUST ensure that their
            unique identifier its serial number
           matches at least one regular expression in
            the list. this value.  If no matching regular expression is found, match occurs, then the
           pledge MUST NOT process this voucher.";
      }

      leaf trusted-ca-certificate idevid-issuer {
        type binary;
         mandatory true;
        description
           "An X.509 v3 certificate structure as specified by RFC 5280,
            Section 4 encoded using
          "The RFC5280 4.2.1.1 Authority Key Identifier OCTET STRING
           from the ASN.1 distinguished encoding
            rules (DER), as specified in ITU-T X.690.

            This 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-list pinned-domain-cert {
        type binary;
        min-elements 1;
        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 field is optional because it may not be needed by all
            bootstrapping protocols.

            Note: the expiration date of this certificate effectively
                  imposes
           MAY be an upper limit on the voucher's expiration."; 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).";
      }

       // DISCUSS: do we need this anymore, if short-lived vouchers
       // are expected, shouldn't the

      leaf certificate be pinned, or
       // perhaps just the immediate issuer CA?
       container domain-certificate-identifier domain-cert-revocation-checks {
        type boolean;
        must "../trusted-ca-certificate" { "../expires-on";
        description
          "A trusted-ca-certificate must be present whenever
              this node is present";
         }
         description
           "This container identifies specific values that a domain
            certificate, provided processing instruction to the pledge separately by the
            bootstrapping protocol, that it MUST contain.  This is useful
            when, for instance, the trust anchor is a long-lived
            public CA certificate, while the domain certificate is
            reissued periodically.

            When provided, verify
           the pledge MUST perform RFC 6125 style
            validation of revocation status for the domain certificate against all of
            the provided values. certificate.  This container is optional because it
           instruction is unneeded when, only available for instance, the trusted CA certificate vouchers that expire. If
           this field is owned by not set, then normal PKIX behaviour applies
           to validation of the domain (i.e.  a private PKI), and hence the trust model
            can be more relaxed."; certificate.";
      }

      leaf subject {
           type binary;
           description
             "The certificate's entire subject field MUST match
              this value.  This value is the Subject structure, as
              specified by RFC ???? Section ???, encoded using the
              ASN.1 distinguished encoding rules (DER), as specified
              in ITU-T X.690.";
         }
         leaf cn-id {
           type string;
           description
             "The certificate's subject field's 'common name' value
              MUST match this value.";
         }
         leaf dns-id {
           type string;
           description
             "A subjectAltName entry of type dNSName in the
              certificate MUST match this value.";
         }
       }

       // DISCUSS: does the transition to 'pinning' model mean we can
       // drop this leaf for now? future proofing allows it to be added
       // if needed but its a edge condition?
       //
       // DISCUSS: there must be such future proofing. not clear where
       // to add it in the voucher document. This is probably the most
       // important point of these discusses
       leaf assert-certificate-revocations {
         type boolean;
         must "../expires-on";
         default true;
         description
           "A processing instruction to the device that it should
            verify revocation information for the PKIX certificates
            involved in bootstrapping. This is available only if
            the pledge has a real-time-clock. This is in addition
            to any revocation checks performed by the MASA.";

           // DISCUSS: should this be a boolean or an enum indicating
           // "fail open" vs "fail closed" to make the meaning clearer.
       }

       leaf nonce 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 last date that the MASA projects to be the last date it
           will renew a voucher on (assuming the same validity duration
            used in this voucher. on. This field is merely infomrative, informative, it
           is not processed by pledges.

            Circustances

           Circumstances may occur after when a voucher was is generated that can
           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

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 delay
   between when a voucher is created and when it is consumed.  In cases
   when there is a delay, there is a need for the pledge to ensure that
   the assertions made when the voucher was created are still valid when
   it is consumed.

   A revocation artifact is generally used to verify the continued
   validity of an assertion such as a PKIX certificate, web token, or a
   "voucher".  Conceptually  With this approach, a potentially long-lived assertion is
   paired with a reasonably fresh revocation allows for issuance of assertions
   using long lifetimes and thereby avoiding ongoing protocol operations status check to renew the assertion.  In practice the use of revocation artifacts
   increases the solution complexity.  Rather than a single protocol, or
   operation, to obtain or renew ensure that
   the assertion the resulting is still valid.  However, this approach increases
   solution
   instead has two or more protocols: one for assertion maintanence and complexity, as it introduces the other(s) need for revocation verification.

   The PKIX use of CRLs additional
   protocols and OCSP responses provides an illustrative
   example.  Relying parties that verify revocation information must
   obtain code paths to distribute and parse the CRL or OCSP information.  Each revocation method
   has its own validity period that effectively shortens the certificate
   validity period (since without valid revocation checks process the
   certificate would be rejected).  In addition to having multiple
   revocation protocol options revocations.

   Addressing the resulting space is further
   complicated by inline distribution short-comings of revocations, this document recommends
   instead the revocation information.
   The TLS extension "Certificate Status Request" [RFC6066] for when
   "constrained clients may wish to use a certificate-status protocol"
   is an example of this.  Including revocation information into
   Cryptographic Message Syntax [RFC5652] is another example.

   If vouchers included revocation similar complexities would propagate
   to all related voucher distribution and assertion protocols.  Instead
   vouchers do not support revocation.  Instead lightweight renewals of short-lived non-revocable
   vouchers.  That is, rather than issue a long-lived voucher, the asserting party,
   or relying party, obtaining and distributing revocation information
   the asserting party MUST obtain an up-to-date valid voucher.  The
   protocol and operations infrastructures
   expectation is for this are expected to be
   the same as the initial methods used MASA to obtain instead issue a short-lived voucher in the first
   place,
   along with one important clarification: a promise (reflected in the MASA services MUST issue
   updated validity period vouchers 'last-renewal-date' field) to
   re-issue the same Registrar ID with
   minimal friction.  This is similar to how an OCSP revocation system
   is always willing to confirm that a certificate is not revoked.
   There is no requirement implied that vouchers be contiguously
   renewed.  For example if a two-week lifetime voucher is not used
   before it expires there is no requirement that it be still valid when
   renewed.  The domain MAY renew an expired voucher at any time.  The
   MASA always has authoritative control and MAY reject such renewals
   (such as again when requested by domain owner's to "block" renewals or if needed.  Importantly, while issuing
   the device has been successfully claimed by an alternate domain).
   Allowing non-contiguous lifetimes significantly reduces initial voucher may incur heavyweight verification checks (are
   you who you say you are?  does the
   operational load on pledge actually belong to you?),
   re-issuing the domain voucher should be a lightweight process, as it is not required to maintain
   valid vouchers;
   ostensibly only to ensure a valid voucher updates the voucher's validatity period.  With this
   approach, there is available during only the time window in which it needs one artifact, and only one code path is
   needed to be used.

   [[EDNOTE: It might be worth including an indication of maximum
   lifetime process it, without any possibility for which such automated renewal is available.  If so the
   language we'd use would be similar a pledge to the RFC5280 statement that
   certificate validity period is "the time interval during which the CA
   warrants that it will maintain information about the status of the
   certificate" only here used choose
   to inform the Registrar of "the time
   interval during which the MASA warrants that it will maintain
   information about skip the revocation status of the ownership claim".  Such a field
   would be independent of check because, for instance, the actual validity period of OCSP
   Responder is not reachable.

   While this document recommends issuing short-lived vouchers, the
   voucher and
   is artifact does not intended for consumption by restrict the Pledge.  A suggested name for
   this field would be "last-renewal-date".]]

   The communications ability to the MASA service regarding claiming and
   blocking of devices is out of scope of this specification.  Similarly create a long-lived
   vouchers, if required, however no revocation methods had been described, the method of reporting is described.

   Note that a
   revocation would have been out-of-scope.

   The lifetimes of vouchers may vary.  In some bootstrapping protocols
   the vouchers voucher may be ephemeral, whereas in others signed by a chain of intermediate CAs
   leading up to the vouchers trust anchor certificate known by the pledge.  Even
   though the voucher itself is not revocable, it may still be
   potentially long-lived.  For bootstrapping protocols that support
   ephemeral vouchers, there is no need to support renewal.  For
   bootstrapping protocols that support long-lived vouchers, final
   protocol complexity is reduced when short lifetime vouchers are
   easily renewed rather than layering on additional revocation methods.
   Manufacturers MAY issue long-lived vouchers to customers revoked,
   per se, if required
   but no revocation method one of the intermediate CA certificates is described. 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, 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
   without access to time can use nonces to get ephermal vouchers.
   Thirdly, vouchers without expiration times may be used, which will
   appear in the audit log, informing the security decision.

   This document defines artifacts containing time values for voucher
   expirations, which require an accurate clock in order to be processed
   correctly.  Vendors planning on issuing vouchers with expiration
   values MUST 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 should not be issued.

7.2.  Protect Voucher PKI in HSM

   This document favors using voucher-renewals over needing to support
   voucher-revocations (Section 6.1).  However, a

   A voucher is signed by a CA, that may itself be signed by a chain of intermediate
   CAs leading to the a trust anchor known to a pledge.  Revocation checking
   of these the intermediate certificates is similarly
   difficult. may be difficult in some scenarios.
   The current voucher format supports the existing PKIX revocation information
   distribution within the limits of the current PKI technology; technology (a PKCS7
   structure can contain revocation objects as well), but pledges MAY
   accept vouchers without checking X.509 certificate revocation. revocation (when
   'domain-cert-revocation-checks' is false).  Without revocation
   checking, a compromized 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 in 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:

      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

9.  References

9.1.  Normative References

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

   [RFC2315]  Kaliski, B., "PKCS #7: Cryptographic Message Syntax
              Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998,
              <http://www.rfc-editor.org/info/rfc2315>.

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

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

9.2.  Informative References

   [I-D.bjorklund-netmod-yang-tree-diagrams]
              Bjorklund, M. and L. Berger, "YANG Tree Diagrams", 2017.

   [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-04
              keyinfra-06 (work in progress), October 2016. May 2017.

   [I-D.ietf-netconf-zerotouch]
              Watsen, K. and M. Abrahamsson, "Zero Touch Provisioning
              for NETCONF or RESTCONF based Management", draft-ietf-
              netconf-zerotouch-12
              netconf-zerotouch-13 (work in progress), January March 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,
              DOI 10.17487/RFC3688, January 2004,
              <http://www.rfc-editor.org/info/rfc3688>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <http://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>.

Appendix A.  Acknowledgements

   The authors would like to thank for following for lively discussions
   on list and in the halls (ordered by last name):

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