richardson-anima-rfc8366bis-04.txt

Internet DRAFT - draft-richardson-anima-rfc8366bis

draft-richardson-anima-rfc8366bis

Last Version:	draft-richardson-anima-rfc8366bis-04.txt	Tracker Entry
Date:	`02-Dec-2021`
Disposition:	.draft-ietf-anima-rfc8366bis
Previous Versions:	draft-richardson-anima-rfc8366bis-00.txt (diff) - 05-Jul-2021

ANIMA Working Group K. Watsen
Internet-Draft Juniper Networks
Intended status: Standards Track M. Richardson
Expires: 4 June 2022 Sandelman Software
M. Pritikin
Cisco Systems
T. Eckert
Huawei
December 2021

A Voucher Artifact for Bootstrapping Protocols
draft-richardson-anima-rfc8366bis-04

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.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the Autonomic Networking
Integrated Model and Approach Working Group mailing list
(anima@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/anima/.

Source for this draft and an issue tracker can be found at
https://github.com/anima-wg/voucher.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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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 https://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 4 June 2022.

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

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
4. Survey of Voucher Types . . . . . . . . . . . . . . . . . . . 5
5. Voucher Artifact . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Tree Diagram . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. YANG Module . . . . . . . . . . . . . . . . . . . . . . . 9
5.3.1. "iana-voucher-assertion-type" Module . . . . . . . . 9
5.3.2. "ietf-voucher" Module . . . . . . . . . . . . . . . . 11
5.4. CMS Format Voucher Artifact . . . . . . . . . . . . . . . 15
6. Design Considerations . . . . . . . . . . . . . . . . . . . . 16
6.1. Renewals Instead of Revocations . . . . . . . . . . . . . 17
6.2. Voucher Per Pledge . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7.1. Clock Sensitivity . . . . . . . . . . . . . . . . . . . . 18
7.2. Protect Voucher PKI in HSM . . . . . . . . . . . . . . . 18
7.3. Test Domain Certificate Validity When Signing . . . . . . 18
7.4. YANG Module Security Considerations . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19

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8.1. The IETF XML Registry . . . . . . . . . . . . . . . . . . 20
8.2. The YANG Module Names Registry . . . . . . . . . . . . . 20
8.3. The Media Types Registry . . . . . . . . . . . . . . . . 21
8.4. The SMI Security for S/MIME CMS Content Type Registry . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . 23
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25

1. Introduction

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

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

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

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

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

2. Terminology

This document uses the following terms:

Artifact: Used throughout to represent the voucher as instantiated

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in the form of a signed structure.

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

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

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

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

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

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

Registrar: See join registrar.

TOFU (Trust on First Use): Where a pledge device makes no security
decisions but rather simply trusts the first domain entity it is
contacted by. Used similarly to [RFC7435]. This is also known as
the "resurrecting duckling" model.

Voucher: A signed statement from the MASA service that indicates to

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a pledge the cryptographic identity of the domain it should trust.

3. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

4. Survey of Voucher Types

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

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

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

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

Anti-Replay Protections: Time- or nonce-based information to
constrain the voucher to time periods or bootstrap attempts.

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

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

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

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

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

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

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

Bearer Voucher: A Bearer Voucher is named after the inclusion of a

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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.

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.

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module: ietf-voucher

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

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=="
}
}

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.

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{
"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

5.3.1. "iana-voucher-assertion-type" Module

Following is a YANG [RFC7950] module formally describing the
voucher's assertion type.

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

organization
"IETF ANIMA Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/anima/>
WG List: <mailto:anima@ietf.org>
Author: Kent Watsen
<mailto:kwatsen@juniper.net>
Author: Max Pritikin
<mailto:pritikin@cisco.com>
Author: Michael Richardson
<mailto:mcr+ietf@sandelman.ca>
Author: Toerless Eckert
<mailto:tte+ietf@cs.fau.de>";
description
"This YANG module defines a YANG enumeration type for IANA
-registered voucher assertion type. This YANG module is
maintained by IANA and reflects the 'voucher assertion types'
registry. The lastest revision of this YANG module can be
obtained from the IANA web site. Request for new enumerations
should be made to IANA via email(iana@iana.org).

The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',

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'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.

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

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

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

typedef voucher-assertion {
type enumeration {
enum verified {
value 0;
description
"Indicates that the ownership has been positively verified
by the MASA (e.g., through sales channel integration).";
}
enum logged {
value 1;
description
"Indicates that the voucher has been issued after
minimal verification of ownership or control. The
issuance has been logged for detection of
potential security issues (e.g., recipients of
vouchers might verify for themselves that unexpected
vouchers are not in the log). This is similar to
unsecured trust-on-first-use principles but with the
logging providing a basis for detecting unexpected
events.";
}
enum proximity {
value 2;
description
"Indicates that the voucher has been issued after

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the MASA verified a proximity proof provided by the
device and target domain. The issuance has been logged
for detection of potential security issues. This is
stronger than just logging, because it requires some
verification that the pledge and owner are
in communication but is still dependent on analysis of
the logs to detect unexpected events.";
}
enum agent-proximity {
value 3;
description
"Indicates that the voucher has been issued after the
MASA...support of asynchronous enrollment in BRSKI";
}
}
description "Indicates what kind of ownership is being asserted by voucher";
}
}
<CODE ENDS>

5.3.2. "ietf-voucher" Module

The revised ietf-voucher YANG module imports the typedef defined in
"iana-voucher-assertion-type" YANG module specified in this document.

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

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

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

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

The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
'MAY', and 'OPTIONAL' in this document are to be interpreted as
described in BCP 14 (RFC 2119) (RFC 8174) when, and only when, they
appear in all capitals, as shown here.

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

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

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

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

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// Grouping defined for future augmentations

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

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

The expires-on value MUST NOT exceed the expiration date
of any of the listed 'pinned-domain-cert' certificates.";
}
leaf assertion {
type ianavat:voucher-assertion;
mandatory true;
description
"The assertion is a statement from the MASA regarding how
the owner was verified. This statement enables pledges
to support more detailed policy checks. Pledges MUST
ensure that the assertion provided is acceptable, per
local policy, before processing the voucher.";
}
leaf serial-number {
type string;
mandatory true;
description
"The serial-number of the hardware. When processing a
voucher, a pledge MUST ensure that its serial-number

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matches this value. If no match occurs, then the
pledge MUST NOT process this voucher.";
}
leaf idevid-issuer {
type binary;
description
"The Authority Key Identifier OCTET STRING (as defined in
Section 4.2.1.1 of RFC 5280) from the pledge's IDevID
certificate. Optional since some serial-numbers are
already unique within the scope of a MASA.
Inclusion of the statistically unique key identifier
ensures statistically unique identification of the hardware.
When processing a voucher, a pledge MUST ensure that its
IDevID Authority Key Identifier matches this value. If no
match occurs, then the pledge MUST NOT process this voucher.

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

This certificate is used by a pledge to trust a Public Key
Infrastructure in order to verify a domain certificate
supplied to the pledge separately by the bootstrapping
protocol. The domain certificate MUST have this certificate
somewhere in its chain of certificates. This certificate
MAY be an end-entity certificate, including a self-signed
entity.";
reference
"RFC 5280:
Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile.
ITU-T X.690:
Information technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER).";
}
leaf domain-cert-revocation-checks {
type boolean;
description

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"A processing instruction to the pledge that it MUST (true)
or MUST NOT (false) verify the revocation status for the
pinned domain certificate. If this field is not set, then
normal PKIX behavior applies to validation of the domain
certificate.";
}
leaf nonce {
type binary {
length "8..32";
}
must 'not(../expires-on)';
description
"A value that can be used by a pledge in some bootstrapping
protocols to enable anti-replay protection. This node is
optional because it is not used by all bootstrapping
protocols.

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

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

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.

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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-T.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.

6. Design Considerations

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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

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

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

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

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

"value": Use the decimal value from the registry.

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

"description": Replicate the corresponding information from the registry, namely the full name of the new assertion type.

"reference": Replicate the reference(s) from the registry.

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

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

8.1. The IETF XML Registry

This document registers two URIs 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.

IANA is asked to register a second URI as follows:

URI: urn:ietf:params:xml:ns:yang:iana-voucher-assertion-type
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 two YANG module in the "YANG Module Names"
registry [RFC6020].

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IANA has registered the following:

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

IANA is asked to register a second YANG module as follows:

name: iana-voucher-assertion-type
namespace: urn:ietf:params:xml:ns:yang:iana-voucher-assertion-type
prefix: ianavat
reference: RFC XXXX

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

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Macintosh file type code(s): none

Person and email address to contact for further information: IETF AN
IMA 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-T.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/rfc/rfc2119>.

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

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[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/rfc/rfc6020>.

[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/rfc/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/rfc/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/rfc/rfc8259>.

9.2. Informative References

[BRSKI] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/rfc/rfc8995>.

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

[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/rfc/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/rfc/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/rfc/rfc6125>.

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[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/rfc/rfc6241>.

[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/rfc/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/rfc/rfc7435>.

[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/rfc/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/rfc/rfc8340>.

[SECUREJOIN]
Richardson, M., "6tisch Secure Join protocol", Work in
Progress, Internet-Draft, draft-ietf-6tisch-dtsecurity-
secure-join-01, 25 February 2017,
<https://datatracker.ietf.org/doc/html/draft-ietf-6tisch-
dtsecurity-secure-join-01>.

[Stajano99theresurrecting]
Stajano, F. and R. Anderson, "The Resurrecting Duckling:
Security Issues for Ad-Hoc Wireless Networks", 1999, <http
s://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
Documents Containing YANG Data Models", BCP 216, RFC 8407,
DOI 10.17487/RFC8407, October 2018,
<https://www.rfc-editor.org/rfc/rfc8407>.

[ZERO-TOUCH]
Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
Touch Provisioning (SZTP)", RFC 8572,
DOI 10.17487/RFC8572, April 2019,
<https://www.rfc-editor.org/rfc/rfc8572>.

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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
Futurewei Technologies Inc.
2330 Central Expy
Santa Clara, 95050
United States of America

Email: tte+ietf@cs.fau.de

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