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rfc8366.txt
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Internet Engineering Task Force (IETF) K. Watsen
Request for Comments: 8366 Juniper Networks
Category: Standards Track M. Richardson
ISSN: 2070-1721 Sandelman Software
M. Pritikin
Cisco Systems
T. Eckert
Huawei
May 2018
A Voucher Artifact for Bootstrapping Protocols
Abstract
This document defines a strategy to securely assign a pledge to an
owner using an artifact signed, directly or indirectly, by the
pledge's manufacturer. This artifact is known as a "voucher".
This document defines an artifact format as a YANG-defined JSON
document that has been signed using a Cryptographic Message Syntax
(CMS) structure. Other YANG-derived formats are possible. The
voucher artifact is normally generated by the pledge's manufacturer
(i.e., the Manufacturer Authorized Signing Authority (MASA)).
This document only defines the voucher artifact, leaving it to other
documents to describe specialized protocols for accessing it.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8366.
Watsen, et al. Standards Track [Page 1]
RFC 8366 Voucher Profile May 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
4. Survey of Voucher Types . . . . . . . . . . . . . . . . . . . 5
5. Voucher Artifact . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Tree Diagram . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. YANG Module . . . . . . . . . . . . . . . . . . . . . . . 9
5.4. CMS Format Voucher Artifact . . . . . . . . . . . . . . . 15
6. Design Considerations . . . . . . . . . . . . . . . . . . . . 16
6.1. Renewals Instead of Revocations . . . . . . . . . . . . . 16
6.2. Voucher Per Pledge . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7.1. Clock Sensitivity . . . . . . . . . . . . . . . . . . . . 17
7.2. Protect Voucher PKI in HSM . . . . . . . . . . . . . . . 17
7.3. Test Domain Certificate Validity When Signing . . . . . . 17
7.4. YANG Module Security Considerations . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8.1. The IETF XML Registry . . . . . . . . . . . . . . . . . . 18
8.2. The YANG Module Names Registry . . . . . . . . . . . . . 19
8.3. The Media Types Registry . . . . . . . . . . . . . . . . 19
8.4. The SMI Security for S/MIME CMS Content Type Registry . . 20
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Normative References . . . . . . . . . . . . . . . . . . 20
9.2. Informative References . . . . . . . . . . . . . . . . . 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
Watsen, et al. Standards Track [Page 2]
RFC 8366 Voucher Profile May 2018
1. Introduction
This document defines a strategy to securely assign a candidate
device (pledge) to an owner using an artifact signed, directly or
indirectly, by the pledge's manufacturer, i.e., the Manufacturer
Authorized Signing Authority (MASA). This artifact is known as the
"voucher".
The voucher artifact is a JSON [RFC8259] document that conforms with
a data model described by YANG [RFC7950], is encoded using the rules
defined in [RFC8259], and is signed using (by default) a CMS
structure [RFC5652].
The primary purpose of a voucher is to securely convey a certificate,
the "pinned-domain-cert", that a pledge can use to authenticate
subsequent interactions. A voucher may be useful in several
contexts, but the driving motivation herein is to support secure
bootstrapping mechanisms. Assigning ownership is important to
bootstrapping mechanisms so that the pledge can authenticate the
network that is trying to take control of it.
The lifetimes of vouchers may vary. In some bootstrapping protocols,
the vouchers may include a nonce restricting them to a single use,
whereas the vouchers in other bootstrapping protocols may have an
indicated lifetime. In order to support long lifetimes, this
document recommends using short lifetimes with programmatic renewal,
see Section 6.1.
This document only defines the voucher artifact, leaving it to other
documents to describe specialized protocols for accessing it. Some
bootstrapping protocols using the voucher artifact defined in this
document include: [ZERO-TOUCH], [SECUREJOIN], and [KEYINFRA]).
2. Terminology
This document uses the following terms:
Artifact: Used throughout to represent the voucher as instantiated
in the form of a signed structure.
Domain: The set of entities or infrastructure under common
administrative control. The goal of the bootstrapping protocol is
to enable a pledge to discover and join a domain.
Watsen, et al. Standards Track [Page 3]
RFC 8366 Voucher Profile May 2018
Imprint: The process where a device obtains the cryptographic key
material to identify and trust future interactions with a network.
This term is taken from Konrad Lorenz's work in biology with new
ducklings: "during a critical period, the duckling would assume
that anything that looks like a mother duck is in fact their
mother" [Stajano99theresurrecting]. An equivalent for a device is
to obtain the fingerprint of the network's root certification
authority certificate. A device that imprints on an attacker
suffers a similar fate to a duckling that imprints on a hungry
wolf. Imprinting is a term from psychology and ethology, as
described in [imprinting].
Join Registrar (and Coordinator): A representative of the domain
that is configured, perhaps autonomically, to decide whether a new
device is allowed to join the domain. The administrator of the
domain interfaces with a join registrar (and Coordinator) to
control this process. Typically, a join registrar is "inside" its
domain. For simplicity, this document often refers to this as
just "registrar".
MASA (Manufacturer Authorized Signing Authority): The entity that,
for the purpose of this document, signs the vouchers for a
manufacturer's pledges. In some bootstrapping protocols, the MASA
may have an Internet presence and be integral to the bootstrapping
process, whereas in other protocols the MASA may be an offline
service that has no active role in the bootstrapping process.
Owner: The entity that controls the private key of the "pinned-
domain-cert" certificate conveyed by the voucher.
Pledge: The prospective device attempting to find and securely join
a domain. When shipped, it only trusts authorized representatives
of the manufacturer.
Registrar: See join registrar.
TOFU (Trust on First Use): Where a pledge device makes no security
decisions but rather simply trusts the first domain entity it is
contacted by. Used similarly to [RFC7435]. This is also known as
the "resurrecting duckling" model.
Voucher: A signed statement from the MASA service that indicates to
a pledge the cryptographic identity of the domain it should trust.
Watsen, et al. Standards Track [Page 4]
RFC 8366 Voucher Profile May 2018
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.
Watsen, et al. Standards Track [Page 5]
RFC 8366 Voucher Profile May 2018
A number of bootstrapping scenarios can be met using differing
combinations of this information. All scenarios address the primary
threat of a Man-in-The-Middle (MiTM) registrar gaining control over
the pledge device. The following combinations are "types" of
vouchers:
|Assertion |Registrar ID | Validity |
Voucher |Log-|Veri- |Trust |CN-ID or| RTC | Nonce |
Type | ged| fied |Anchor |DNS-ID | | |
---------------------------------------------------------|
Audit | X | | X | | | X |
-------------|----|-------|-------|--------|-----|-------|
Nonceless | X | | X | | X | |
Audit | | | | | | |
-------------|----|-------|-------|--------|-----|-------|
Owner Audit | X | X | X | | X | X |
-------------|----|-------|-------|--------|-----|-------|
Owner ID | | X | X | X | X | |
-------------|----|-------|----------------|-----|-------|
Bearer | X | | wildcard | optional |
out-of-scope | | | | |
-------------|----|-------|----------------|-------------|
NOTE: All voucher types include a 'pledge ID serial-number'
(not shown here for space reasons).
Audit Voucher: An Audit Voucher is named after the logging assertion
mechanisms that the registrar then "audits" to enforce local
policy. The registrar mitigates a MiTM registrar by auditing that
an unknown MiTM registrar does not appear in the log entries.
This does not directly prevent the MiTM but provides a response
mechanism that ensures the MiTM is unsuccessful. The advantage is
that actual ownership knowledge is not required on the MASA
service.
Nonceless Audit Voucher: An Audit Voucher without a validity period
statement. Fundamentally, it is the same as an Audit Voucher
except that it can be issued in advance to support network
partitions or to provide a permanent voucher for remote
deployments.
Ownership Audit Voucher: An Audit Voucher where the MASA service has
verified the registrar as the authorized owner. The MASA service
mitigates a MiTM registrar by refusing to generate Audit Vouchers
for unauthorized registrars. The registrar uses audit techniques
to supplement the MASA. This provides an ideal sharing of policy
decisions and enforcement between the vendor and the owner.
Watsen, et al. Standards Track [Page 6]
RFC 8366 Voucher Profile May 2018
Ownership ID Voucher: Named after inclusion of the pledge's CN-ID or
DNS-ID within the voucher. The MASA service mitigates a MiTM
registrar by identifying the specific registrar (via WebPKI)
authorized to own the pledge.
Bearer Voucher: A Bearer Voucher is named after the inclusion of a
registrar ID wildcard. Because the registrar identity is not
indicated, this voucher type must be treated as a secret and
protected from exposure as any 'bearer' of the voucher can claim
the pledge device. Publishing a nonceless bearer voucher
effectively turns the specified pledge into a "TOFU" device with
minimal mitigation against MiTM registrars. Bearer vouchers are
out of scope.
5. Voucher Artifact
The voucher's primary purpose is to securely assign a pledge to an
owner. The voucher informs the pledge which entity it should
consider to be its owner.
This document defines a voucher that is a JSON-encoded instance of
the YANG module defined in Section 5.3 that has been, by default, CMS
signed.
This format is described here as a practical basis for some uses
(such as in NETCONF), but more to clearly indicate what vouchers look
like in practice. This description also serves to validate the YANG
data model.
Future work is expected to define new mappings of the voucher to
Concise Binary Object Representation (CBOR) (from JSON) and to change
the signature container from CMS to JSON Object Signing and
Encryption (JOSE) or CBOR Object Signing and Encryption (COSE). XML
or ASN.1 formats are also conceivable.
This document defines a media type and a filename extension for the
CMS-encoded JSON type. Future documents on additional formats would
define additional media types. Signaling is in the form of a MIME
Content-Type, an HTTP Accept: header, or more mundane methods like
use of a filename extension when a voucher is transferred on a USB
key.
Watsen, et al. Standards Track [Page 7]
RFC 8366 Voucher Profile May 2018
5.1. Tree Diagram
The following tree diagram illustrates a high-level view of a voucher
document. The notation used in this diagram is described in
[RFC8340]. Each node in the diagram is fully described by the YANG
module in Section 5.3. Please review the YANG module for a detailed
description of the voucher format.
module: ietf-voucher
yang-data voucher-artifact:
+---- voucher
+---- created-on yang:date-and-time
+---- expires-on? yang:date-and-time
+---- assertion enumeration
+---- serial-number string
+---- idevid-issuer? binary
+---- pinned-domain-cert binary
+---- domain-cert-revocation-checks? boolean
+---- nonce? binary
+---- last-renewal-date? yang:date-and-time
5.2. Examples
This section provides voucher examples for illustration purposes.
These examples conform to the encoding rules defined in [RFC8259].
The following example illustrates an ephemeral voucher (uses a
nonce). The MASA generated this voucher using the 'logged' assertion
type, knowing that it would be suitable for the pledge making the
request.
{
"ietf-voucher:voucher": {
"created-on": "2016-10-07T19:31:42Z",
"assertion": "logged",
"serial-number": "JADA123456789",
"idevid-issuer": "base64encodedvalue==",
"pinned-domain-cert": "base64encodedvalue==",
"nonce": "base64encodedvalue=="
}
}
Watsen, et al. Standards Track [Page 8]
RFC 8366 Voucher Profile May 2018
The following example illustrates a non-ephemeral voucher (no nonce).
While the voucher itself expires after two weeks, it presumably can
be renewed for up to a year. The MASA generated this voucher using
the 'verified' assertion type, which should satisfy all pledges.
{
"ietf-voucher:voucher": {
"created-on": "2016-10-07T19:31:42Z",
"expires-on": "2016-10-21T19:31:42Z",
"assertion": "verified",
"serial-number": "JADA123456789",
"idevid-issuer": "base64encodedvalue==",
"pinned-domain-cert": "base64encodedvalue==",
"domain-cert-revocation-checks": "true",
"last-renewal-date": "2017-10-07T19:31:42Z"
}
}
5.3. YANG Module
Following is a YANG [RFC7950] module formally describing the
voucher's JSON document structure.
<CODE BEGINS> file "ietf-voucher@2018-05-09.yang"
module ietf-voucher {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-voucher";
prefix vch;
import ietf-yang-types {
prefix yang;
reference "RFC 6991: Common YANG Data Types";
}
import ietf-restconf {
prefix rc;
description
"This import statement is only present to access
the yang-data extension defined in RFC 8040.";
reference "RFC 8040: RESTCONF Protocol";
}
organization
"IETF ANIMA Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/anima/>
WG List: <mailto:anima@ietf.org>
Author: Kent Watsen
<mailto:kwatsen@juniper.net>
Watsen, et al. Standards Track [Page 9]
RFC 8366 Voucher Profile May 2018
Author: Max Pritikin
<mailto:pritikin@cisco.com>
Author: Michael Richardson
<mailto:mcr+ietf@sandelman.ca>
Author: Toerless Eckert
<mailto:tte+ietf@cs.fau.de>";
description
"This module defines the format for a voucher, which is produced by
a pledge's manufacturer or delegate (MASA) to securely assign a
pledge to an 'owner', so that the pledge may establish a secure
connection to the owner's network infrastructure.
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
'MAY', and 'OPTIONAL' in this document are to be interpreted as
described in BCP 14 (RFC 2119) (RFC 8174) when, and only when, they
appear in all capitals, as shown here.
Copyright (c) 2018 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, is permitted pursuant to, and subject to the license
terms contained in, the Simplified BSD License set forth in Section
4.c of the IETF Trust's Legal Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC 8366; see the RFC
itself for full legal notices.";
revision 2018-05-09 {
description
"Initial version";
reference "RFC 8366: Voucher Profile for Bootstrapping Protocols";
}
// Top-level statement
rc:yang-data voucher-artifact {
uses voucher-artifact-grouping;
}
// Grouping defined for future augmentations
grouping voucher-artifact-grouping {
description
"Grouping to allow reuse/extensions in future work.";
container voucher {
Watsen, et al. Standards Track [Page 10]
RFC 8366 Voucher Profile May 2018
description
"A voucher assigns a pledge to an owner (pinned-domain-cert).";
leaf created-on {
type yang:date-and-time;
mandatory true;
description
"A value indicating the date this voucher was created. This
node is primarily for human consumption and auditing. Future
work MAY create verification requirements based on this
node.";
}
leaf expires-on {
type yang:date-and-time;
must 'not(../nonce)';
description
"A value indicating when this voucher expires. The node is
optional as not all pledges support expirations, such as
pledges lacking a reliable clock.
If this field exists, then the pledges MUST ensure that
the expires-on time has not yet passed. A pledge without
an accurate clock cannot meet this requirement.
The expires-on value MUST NOT exceed the expiration date
of any of the listed 'pinned-domain-cert' certificates.";
}
leaf assertion {
type enumeration {
enum verified {
description
"Indicates that the ownership has been positively
verified by the MASA (e.g., through sales channel
integration).";
}
enum logged {
description
"Indicates that the voucher has been issued after
minimal verification of ownership or control. The
issuance has been logged for detection of
potential security issues (e.g., recipients of
vouchers might verify for themselves that unexpected
vouchers are not in the log). This is similar to
unsecured trust-on-first-use principles but with the
logging providing a basis for detecting unexpected
events.";
}
enum proximity {
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RFC 8366 Voucher Profile May 2018
description
"Indicates that the voucher has been issued after
the MASA verified a proximity proof provided by the
device and target domain. The issuance has been logged
for detection of potential security issues. This is
stronger than just logging, because it requires some
verification that the pledge and owner are
in communication but is still dependent on analysis of
the logs to detect unexpected events.";
}
}
mandatory true;
description
"The assertion is a statement from the MASA regarding how
the owner was verified. This statement enables pledges
to support more detailed policy checks. Pledges MUST
ensure that the assertion provided is acceptable, per
local policy, before processing the voucher.";
}
leaf serial-number {
type string;
mandatory true;
description
"The serial-number of the hardware. When processing a
voucher, a pledge MUST ensure that its serial-number
matches this value. If no match occurs, then the
pledge MUST NOT process this voucher.";
}
leaf idevid-issuer {
type binary;
description
"The Authority Key Identifier OCTET STRING (as defined in
Section 4.2.1.1 of RFC 5280) from the pledge's IDevID
certificate. Optional since some serial-numbers are
already unique within the scope of a MASA.
Inclusion of the statistically unique key identifier
ensures statistically unique identification of the hardware.
When processing a voucher, a pledge MUST ensure that its
IDevID Authority Key Identifier matches this value. If no
match occurs, then the pledge MUST NOT process this voucher.
When issuing a voucher, the MASA MUST ensure that this field
is populated for serial-numbers that are not otherwise unique
within the scope of the MASA.";
}
leaf pinned-domain-cert {
type binary;
mandatory true;
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RFC 8366 Voucher Profile May 2018
description
"An X.509 v3 certificate structure, as specified by RFC 5280,
using Distinguished Encoding Rules (DER) encoding, as defined
in ITU-T X.690.
This certificate is used by a pledge to trust a Public Key
Infrastructure in order to verify a domain certificate
supplied to the pledge separately by the bootstrapping
protocol. The domain certificate MUST have this certificate
somewhere in its chain of certificates. This certificate
MAY be an end-entity certificate, including a self-signed
entity.";
reference
"RFC 5280:
Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile.
ITU-T X.690:
Information technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER).";
}
leaf domain-cert-revocation-checks {
type boolean;
description
"A processing instruction to the pledge that it MUST (true)
or MUST NOT (false) verify the revocation status for the
pinned domain certificate. If this field is not set, then
normal PKIX behavior applies to validation of the domain
certificate.";
}
leaf nonce {
type binary {
length "8..32";
}
must 'not(../expires-on)';
description
"A value that can be used by a pledge in some bootstrapping
protocols to enable anti-replay protection. This node is
optional because it is not used by all bootstrapping
protocols.
When present, the pledge MUST compare the provided nonce
value with another value that the pledge randomly generated
and sent to a bootstrap server in an earlier bootstrapping
message. If the values do not match, then the pledge MUST
NOT process this voucher.";
}
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RFC 8366 Voucher Profile May 2018
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>
Watsen, et al. Standards Track [Page 14]
RFC 8366 Voucher Profile May 2018
5.4. CMS Format Voucher Artifact
The IETF evolution of PKCS#7 is CMS [RFC5652]. A CMS-signed voucher,
the default type, contains a ContentInfo structure with the voucher
content. An eContentType of 40 indicates that the content is a JSON-
encoded voucher.
The signing structure is a CMS SignedData structure, as specified by
Section 5.1 of [RFC5652], encoded using ASN.1 Distinguished Encoding
Rules (DER), as specified in ITU-T X.690 [ITU.X690.2015].
To facilitate interoperability, Section 8.3 in this document
registers the media type "application/voucher-cms+json" and the
filename extension ".vcj".
The CMS structure MUST contain a 'signerInfo' structure, as described
in Section 5.1 of [RFC5652], containing the signature generated over
the content using a private key trusted by the recipient. Normally,
the recipient is the pledge and the signer is the MASA. Another
possible use could be as a "signed voucher request" format
originating from the pledge or registrar toward the MASA. Within
this document, the signer is assumed to be the MASA.
Note that Section 5.1 of [RFC5652] includes a discussion about how to
validate a CMS object, which is really a PKCS7 object (cmsVersion=1).
Intermediate systems (such the Bootstrapping Remote Secure Key
Infrastructures (BRSKI) registrar) that might need to evaluate the
voucher in flight MUST be prepared for such an older format. No
signaling is necessary, as the manufacturer knows the capabilities of
the pledge and will use an appropriate format voucher for each
pledge.
The CMS structure SHOULD also contain all of the certificates leading
up to and including the signer's trust anchor certificate known to
the recipient. The inclusion of the trust anchor is unusual in many
applications, but third parties cannot accurately audit the
transaction without it.
The CMS structure MAY also contain revocation objects for any
intermediate certificate authorities (CAs) between the voucher issuer
and the trust anchor known to the recipient. However, the use of
CRLs and other validity mechanisms is discouraged, as the pledge is
unlikely to be able to perform online checks and is unlikely to have
a trusted clock source. As described below, the use of short-lived
vouchers and/or a pledge-provided nonce provides a freshness
guarantee.
Watsen, et al. Standards Track [Page 15]
RFC 8366 Voucher Profile May 2018
6. Design Considerations
6.1. Renewals Instead of Revocations
The lifetimes of vouchers may vary. In some bootstrapping protocols,
the vouchers may be created and consumed immediately, whereas in
other bootstrapping solutions, there may be a significant time delay
between when a voucher is created and when it is consumed. In cases
when there is a time delay, there is a need for the pledge to ensure
that the assertions made when the voucher was created are still
valid.
A revocation artifact is generally used to verify the continued
validity of an assertion such as a PKIX certificate, web token, or a
"voucher". With this approach, a potentially long-lived assertion is
paired with a reasonably fresh revocation status check to ensure that
the assertion is still valid. However, this approach increases
solution complexity, as it introduces the need for additional
protocols and code paths to distribute and process the revocations.
Addressing the shortcomings of revocations, this document recommends
instead the use of lightweight renewals of short-lived non-revocable
vouchers. That is, rather than issue a long-lived voucher, where the
'expires-on' leaf is set to some distant date, the expectation is for
the MASA to instead issue a short-lived voucher, where the 'expires-
on' leaf is set to a relatively near date, along with a promise
(reflected in the 'last-renewal-date' field) to reissue the voucher
again when needed. Importantly, while issuing the initial voucher
may incur heavyweight verification checks ("Are you who you say you
are?" "Does the pledge actually belong to you?"), reissuing the
voucher should be a lightweight process, as it ostensibly only
updates the voucher's validity period. With this approach, there is
only the one artifact, and only one code path is needed to process
it; there is no possibility of a pledge choosing to skip the
revocation status check because, for instance, the OCSP Responder is
not reachable.
While this document recommends issuing short-lived vouchers, the
voucher artifact does not restrict the ability to create long-lived
voucher, if required; however, no revocation method is described.
Note that a voucher may be signed by a chain of intermediate CAs
leading up to the trust anchor certificate known by the pledge. Even
though the voucher itself is not revocable, it may still be revoked,
per se, if one of the intermediate CA certificates is revoked.
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RFC 8366 Voucher Profile May 2018
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).
Watsen, et al. Standards Track [Page 17]
RFC 8366 Voucher Profile May 2018
Similarly,they are expected to contact the MASA to indicate that an
outstanding (presumably short lifetime) voucher should be blocked
from automated renewal. Protocols for voucher distribution are
RECOMMENDED to check for revocation of domain identity certificates
before the signing of vouchers.
7.4. YANG Module Security Considerations
The YANG module specified in this document defines the schema for
data that is subsequently encapsulated by a CMS signed-data content
type, as described in Section 5 of [RFC5652]. As such, all of the
YANG modeled data is protected from modification.
Implementations should be aware that the signed data is only
protected from external modification; the data is still visible.
This potential disclosure of information doesn't affect security so
much as privacy. In particular, adversaries can glean information
such as which devices belong to which organizations and which CRL
Distribution Point and/or OCSP Responder URLs are accessed to
validate the vouchers. When privacy is important, the CMS signed-
data content type SHOULD be encrypted, either by conveying it via a
mutually authenticated secure transport protocol (e.g., TLS
[RFC5246]) or by encapsulating the signed-data content type with an
enveloped-data content type (Section 6 of [RFC5652]), though details
for how to do this are outside the scope of this document.
The use of YANG to define data structures, via the 'yang-data'
statement, is relatively new and distinct from the traditional use of
YANG to define an API accessed by network management protocols such
as NETCONF [RFC6241] and RESTCONF [RFC8040]. For this reason, these
guidelines do not follow template described by Section 3.7 of
[YANG-GUIDE].
8. IANA Considerations
8.1. The IETF XML Registry
This document registers a URI in the "IETF XML Registry" [RFC3688].
IANA has registered the following:
URI: urn:ietf:params:xml:ns:yang:ietf-voucher
Registrant Contact: The ANIMA WG of the IETF.