Attestation Event Stream Subscription
draft-birkholz-rats-network-device-subscription-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Authors | Henk Birkholz , Eric Voit , Wei Pan | ||
| Last updated | 2020-06-24 | ||
| Replaced by | draft-ietf-rats-network-device-subscription | ||
| RFC stream | (None) | ||
| Formats | |||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-birkholz-rats-network-device-subscription-00
RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track E. Voit
Expires: December 26, 2020 Cisco
W. Pan
Huawei
June 24, 2020
Attestation Event Stream Subscription
draft-birkholz-rats-network-device-subscription-00
Abstract
This document defines how to subscribe to a stream of attestation
related Evidence on TPM-based network devices.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at 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 December 26, 2020.
Copyright Notice
Copyright (c) 2020 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.
Birkholz, et al. Expires December 26, 2020 [Page 1]
Internet-Draft RATS Subscription June 2020
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
3. Operational Model . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Sequence Diagram . . . . . . . . . . . . . . . . . . . . 3
3.2. Continuously Verifying Freshness . . . . . . . . . . . . 5
3.2.1. TPM 1.2 Quote . . . . . . . . . . . . . . . . . . . . 5
3.2.2. TPM 2 Quote . . . . . . . . . . . . . . . . . . . . . 5
4. Remote Attestation Event Stream . . . . . . . . . . . . . . . 6
4.1. Subscription to the <attestation> Event Stream . . . . . 7
4.2. Replaying a history of previous TPM extend operations . . 7
4.2.1. TPM2 Heartbeat . . . . . . . . . . . . . . . . . . . 8
4.3. YANG notifications placed on the <attestation> Event
Stream . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.1. tpm-extend . . . . . . . . . . . . . . . . . . . . . 8
4.3.2. tpm12-attestation . . . . . . . . . . . . . . . . . . 9
4.3.3. tpm20-attestation . . . . . . . . . . . . . . . . . . 10
4.4. Filtering Evidence at the Attester . . . . . . . . . . . 11
4.5. Replaying previous PCR Extend events . . . . . . . . . . 11
4.6. Configuring the <attestation> Event Stream . . . . . . . 12
5. YANG Module . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 20
A.1. Open Questions . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
[RATS-Device] and [RATS-YANG] define the operational prerequisites
and a YANG Model for the acquisition of Evidence from a TPM-based
network device. However, there is a limitation inherent in the
challenge-response interaction models upon which these documents are
based. This limitation is that it is up to the Verifier to request
Evidence. The result is that the interval between the occurrence of
a security event, and the event's visibility within the Relying Party
can be unacceptably long.
This limitation results in two adverse effects:
1. Evidence is not streamed to an interested Verifier as soon as it
is generated.
Birkholz, et al. Expires December 26, 2020 [Page 2]
Internet-Draft RATS Subscription June 2020
2. If it were to be streamed, the Evidence is not appraisable for
freshness.
This specification addresses the first adverse effect by enabling a
Verifier to subscribe via [RFC8639] to a YANG <attestation> Event
Stream which exists upon the Attester. When subscribed, the Attester
will continuously stream a subscribed set of Evidence to the
Verifier.
The second adverse effect results from the nonce based challenge-
response of [RATS-YANG]. In that document, an Attester must wait for
a new nonce from a Verifier before it generates a new TPM Quote. In
this case, the nonce acts as an implicit timestamp that a windows of
freshness is tied to. To address delays resulting from such a
synchronous wait for nonce based Evidence generation, this
specification enables freshness to be asserted in an asynchronous
manner.
By removing these two adverse effects, it becomes possible for a
Verifier to continuously maintain an appraisal of the Attested device
without relying on continuous polling.
2. Terminology
The following terms are imported from [I-D.ietf-rats-architecture]:
Attester, Evidence, Relying Party, and Verifier. Also imported are
the time definitions time(vg), time(ns), time(eg), time(rg), and
time(ra) from that document's appendices. The following terms at
imported from [RFC8639]: Event Stream, Subscription, Event Stream
Filter, Dynamic Subscription.
2.1. Requirements Notation
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.
3. Operational Model
3.1. Sequence Diagram
Figure 1 below is a sequence diagram which updates Figure 5 of
[RATS-Device]. This sequence diagram replaces the [RATS-Device]
challenge-response interaction model with an [RFC8639] Dynamic
Subscription to an <attestation> Event Stream. The contents of the
<attestation> Event Stream are defined below within Section 4.
Birkholz, et al. Expires December 26, 2020 [Page 3]
Internet-Draft RATS Subscription June 2020
.----------. .--------------------------.
| Attester | | Relying Party / Verifier |
'----------' '--------------------------'
time(vg) |
|<---------establish-subscription(<attestation>)--time(ns)
| |
time(eg) |
|--filter(<tpm-extend>)---------------------------->|
|--<tpm12-attestation> or <tpm20-attestation>------>|
| |
| verify time(eg) Evidence @ time(rg,ra)
| |
~ ~
time(vg',eg') |
|--filter(<tpm-extend>)---------------------------->|
|--<tpm12-attestation> or <tpm20-attestation>------>|
| |
| verify time(eg') Evidence @ time(rg',ra')
Figure 1: YANG Subscription Model for Remote Attestation
o time(vg,rg,ra) are identical to the corresponding times from
Figure 5 of [RATS-Device].
o time(rg',ra') are subsequent instances of the corresponding times
from Figure 5 of [RATS-Device].
o time(ns): The Verifier generates a nonce and makes an [RFC8639]
<establish-subscription> request. This request also includes the
augmentations defined in this document's YANG model. Key
subscription RPC parameters include:
* the nonce,
* a set of PCRs of interest which the Verifier wants to appraise,
and
* an optional filter which can reduce the logged events on the
<attestation> stream pushed to the Verifier.
o time(eg) - An initial response of Evidence is returned to the
Verifier. This includes:
* a replay of filtered log entries which have extended into a PCR
of interest since boot are sent in the <tpm-extend>
notification, and a
Birkholz, et al. Expires December 26, 2020 [Page 4]
Internet-Draft RATS Subscription June 2020
* a signed TPM quote that contains at least the PCRs from the
<establish-subscription> RPC are included in a
<tpm12-attestation> or <tpm20-attestation>). This quote must
have included the nonce provided at time(ns).
o time(vg',eg') - This occurs when a PCR is extended subsequent to
time(eg). Immediately after the extension, the following
information needs to be pushed to the Verifier:
* Any values extended into a PCR of interest, and
* a signed TPM Quote showing the result the PCR extension.
3.2. Continuously Verifying Freshness
As there is no new Verifier nonce provided at time(eg'), it is
important to validate the freshness of TPM Quotes which are delivered
at that time. The method of doing this verification will vary based
on the capabilities of the TPM cryptoprocessor used.
3.2.1. TPM 1.2 Quote
The [RFC8639] notification format includes the <eventTime> object.
This can be used to determine the amount of time subsequent to the
initial subscription each notification was sent. However, this time
is not part of the signed results which are returned from the Quote,
and therefore is not trustworthy as objects returned in the Quote.
Therefore, a Verifier MUST periodically issue a new nonce, and
receive this nonce within a TPM quote response in order to ensure the
freshness of the results. This can be done using the <tpm12-
challenge-response-attestation> RPC from [RATS-YANG].
3.2.2. TPM 2 Quote
When the Attester includes a TPM2 compliant cryptoprocessor, internal
time-related counters are included within the signed TPM Quote. By
including an initial nonce in the [RFC8639] subscription request,
fresh values for these counters are pushed as part of the first TPM
Quote returned to the Verifier. Then, as shown by
[I-D.birkholz-rats-tuda], subsequent TPM Quotes delivered to the
Verifier can the be appraised for freshness based on the predictable
incrementing of these time-related counters.
The relevant internal time-related counters defined within [TPM2.0]
can be seen within <tpms-clock-info>. These counters include the
<clock>, <reset-counter>, and <restart-counter> objects. Normative
rules for appraising these objects are as follows:
Birkholz, et al. Expires December 26, 2020 [Page 5]
Internet-Draft RATS Subscription June 2020
o If the <clock> has incremented for no more than the same duration
as both the <eventTime> and the Verifier's internal time since the
initial time(eg) and any previous time(eg'), then the TPM Quote
MAY be considered fresh. Note that [TPM2.0] allows for +/- 15%
clock drift. However, many chips significantly improve on this
maximum drift. If available, chip specific maximum drifts SHOULD
be considered during the appraisal process.
o If the <reset-counter>, <restart-counter> has incremented. The
existing subscription MUST be terminated, and a new <establish-
subscription> SHOULD be generated.
o If a TPM Quote on any subscribed PCR has not been pushed to the
Verifier for a duration of an Attester defined heartbeat interval,
then a new TPM Quote notification SHOULD be sent to the Verifier.
This may often be the case, as certain PCRs might be infrequently
updated.
.----------. .--------------------------.
| Attester | | Relying Party / Verifier |
'----------' '--------------------------'
time(vg',eg') |
|-<tpm20-attestation>------------------------------>|
| : |
~ Heartbeat interval ~
| : |
time(eg') : |
|-<tpm20-attestation>------------------------------>|
| |
4. Remote Attestation Event Stream
The <remote-attestation> Event Stream is an [RFC8639] complaint Event
Stream which is defined within this section and within the YANG
Module of [RATS-YANG]. This Event Stream contains YANG notifications
which carry Evidence assisting a Verifier in the appraisal of an
Attester. Data Nodes within Section 4.6 allow the configuration of
this Event Stream's contents on an Attester.
This <remote-attestation> Event Stream may only be exposed on
Attesters supporting [RATS-Device]. As with [RATS-Device], it is up
to the Verifier to understand which types of cryptoprocessors and
keys are acceptable.
Birkholz, et al. Expires December 26, 2020 [Page 6]
Internet-Draft RATS Subscription June 2020
4.1. Subscription to the <attestation> Event Stream
To establish a subscription to an Attester in a way which provides
provably fresh Evidence, initial randomness must be provided to the
Attester. This is done via the augmentation of a <nonce-value> into
[RFC8639] the <establish-subscription> RPC. Additionally, a Verifier
must ask for PCRs of interest from a platform.
augment /sn:establish-subscription/sn:input:
+---w nonce-value binary
+---w pcr-index* tpm:pcr
The result of the subscription will be that passing of the following
information:
1. <tpm12-attestation> and <tpm20-attestation> notifications which
include the provided <nonce-value>. These attestation
notifications MUST at least include all the <pcr-indicies>
requested in the RPC.
2. a series of <tpm-extend> notifications which reference the
requested PCRs on all TPM based cryptoprocessors on the Attester.
3. <tpm12-attestation> and <tpm20-attestation> notifications
generated within a few seconds of the <tpm-extend> notifications.
These attestation notifications MUST at least include any PCRs
extended.
If the Verifier does not want to see the logged extend operations for
all PCRs available from an Attester, an Event Stream Filter should be
applied. This filter will remove Evidence from any PCRs which are
not interesting to the Verifier.
4.2. Replaying a history of previous TPM extend operations
Unless it is relying on Known Good Values, a Verifier will need to
acquire a history of PCR extensions since the Attester has been
booted. This history may be requested from the Attester as part of
the <establish-subscription> RPC. This request is accomplished by
placing a very old <replay-start-time> within the original RPC
request. As the very old <replay-start-time> will pre-date the time
of Attester boot, a <replay-start-time-revision> will be returned in
the <establish-subscription> RPC response, indicating when the
Attester booted. Immediately following the response (and before the
notifications above) one or more <tpm-extend> notifications which
document all extend operations which have occurred for the requested
PCRs since boot will be sent. Many extend operations to a single PCR
Birkholz, et al. Expires December 26, 2020 [Page 7]
Internet-Draft RATS Subscription June 2020
index on a single TPM SHOULD be included within a single
notification.
Note that if a Verifier has a partial history of extensions, the
<replay-start-time> can be adjusted so that known extensions are not
forwarded.
The end of this history replay will be indicated with the [RFC8639]
<replay-completed> notification. For more on this sequence, see
Section 2.4.2.1 of [RFC8639].
After the <replay-complete> notification is provided, a TPM Quote
will be requested and the result passed to the Verifier via a
<tpm12-attestation> and <tpm20-attestation> notification. If there
have been any additional extend operations which have changed a
subscribed PCR value in this quote, these MUST be pushed to the
Verifier before the <tpm12-attestation> and <tpm20-attestation>
notification.
At this point, the Verifier has sufficient Evidence to appraise the
reported extend operations for each PCR, as well as compare the
expected value of the PCR value against that signed by the TPM.
4.2.1. TPM2 Heartbeat
For TPM2, every requested PCR MUST at least be sent once within a
<tpm20-attestation> heartbeat interval. This MAY be done with a
single <tpm20-attestation> notification that includes all requested
PCRs every heartbeat interval. Alternatively, this MAY be done with
several <tpm20-attestation> notifications at different times during
that heartbeat interval.
4.3. YANG notifications placed on the <attestation> Event Stream
4.3.1. tpm-extend
This notification documents when a single subscribed PCR is extended
within a single TPM cryptoprocessor. Corresponding notifications
SHOULD be emitted no less than a <marshalling-period> after the PCR
is first extended (the reason for the marshalling is that it is quite
possible that multiple extensions to the same PCR have been made in
quick succession). A notification MUST be emitted prior to a
<tpm12-attestation> or <tpm20-attestation> notification which has
included and signed the results of any specific PCR extension.
Birkholz, et al. Expires December 26, 2020 [Page 8]
Internet-Draft RATS Subscription June 2020
+---n tpm-extend
+--ro certificate-name? string
+--ro pcr-index-changed tpm:pcr
+--ro attested-event* []
+--ro attested-event
+--ro extended-with binary
+--ro event-type? identityref
+--ro event-details? <anydata>
Each <tpm-extend> MUST include one or more values being extended into
the PCR. These are conveyed within the <extended-with> object. For
each extension, details of the event MAY be provided within the
<event-details> object.
The format of any included <event-details> is identified by the
<event-type>. This document includes two YANG structures which may
be inserted into the <event-details>. These two structures are:
<ima-event-log> and <bios-event-log>. Implementations wanting to
provide additional documentation of a type of PCR extension may
choose to define additional YANG structures which can be placed into
<event-details>.
4.3.2. tpm12-attestation
This notification type contains an instance of a TPM1.2 style signed
cryptoprocessor measurement. This notification is generated at two
points in time:
1. Upon initial subscription
2. Every time at least one subscribed PCR has changed from the
directly previous <tpm12-attestation>. In this case, the
notification SHOULD be emitted within a <marshalling-period>
since a the first subscribed PCR changed.
This notification MUST NOT include the returned quote digest the
results from any PCR extensions not previously reportable by a <tpm-
extend>.
Birkholz, et al. Expires December 26, 2020 [Page 9]
Internet-Draft RATS Subscription June 2020
+---n tpm12-attestation {tpm:TPM12}?
+--ro certificate-name? string
+--ro up-time? uint32
+--ro node-id? string
+--ro node-physical-index? int32 {ietfhw:entity-mib}?
+--ro fixed? binary
+--ro external-data? binary
+--ro signature-size? uint32
+--ro signature? binary
+--ro (tpm12-quote)
+--:(tpm12-quote1)
| +--ro version* []
| | +--ro major? uint8
| | +--ro minor? uint8
| | +--ro revMajor? uint8
| | +--ro revMinor? uint8
| +--ro digest-value? binary
| +--ro TPM_PCR_COMPOSITE* []
| +--ro pcr-index* pcr
| +--ro value-size? uint32
| +--ro tpm12-pcr-value* binary
+--:(tpm12-quote2)
+--ro tag? uint8
+--ro pcr-index* pcr
+--ro locality-at-release? uint8
+--ro digest-at-release? binary
All YANG objects above are defined within [RATS-YANG]. The objects
MAY include Attester information such as <tpm12-pcr-value> which are
not signed. The <tpm12-attestation> is not replayable.
4.3.3. tpm20-attestation
This notification contains an instance of TPM2 style signed
cryptoprocessor measurements. This notification is generated at
three points in time:
1. Upon initial subscription
2. Every time at least one subscribed PCR has changed from the
directly previous <tpm20-attestation>. In this case, the
notification SHOULD be emitted within a <marshalling-period>
since a the first subscribed PCR changed.
3. After a minimum heartbeat interval since a previous
<tpm20-attestation> was sent.
Birkholz, et al. Expires December 26, 2020 [Page 10]
Internet-Draft RATS Subscription June 2020
This notification MUST NOT include the returned <quote> the results
from any PCR extensions not previously reportable by a <tpm-extend>.
+---n tpm20-attestation {tpm:TPM20}?
+--ro certificate-name? string
+--ro up-time? uint32
+--ro node-id? string
+--ro node-physical-index? int32 {ietfhw:entity-mib}?
+--ro quote? binary
+--ro quote-signature? binary
+--ro pcr-bank-values* []
| +--ro TPM2_Algo? identityref
| +--ro pcr-values* [pcr-index]
| +--ro pcr-index pcr
| +--ro pcr-value? binary
+--ro pcr-digest-algo-in-quote
+--ro TPM2_Algo? identityref
All YANG objects above are defined within [RATS-YANG]. The objects
MAY include Attester information such as <pcr-bank-values> which are
not signed. The <tpm20-attestation> is not replayable.
4.4. Filtering Evidence at the Attester
It can be useful NOT to receive all Evidence related to a PCR. An
example of this is would be a when a Verifier maintains Known Good
Values of a PCR. In this case, it is not necessary to send each
extend operation.
To accomplish this reduction, when an RFC8639 <establish-
subscription> RPC is sent, a <stream-filter> as per RFC8639,
Section 2.2 can be set to discard a <tpm-extend> notification when
the <pcr-index-changed> is uninteresting to the verifier.
4.5. Replaying previous PCR Extend events
To verify the value of a PCR, a Verifier must either know that the
value is a Known Good Value [KGV] or be able to reconstruct the hash
value by viewing all the PCR-Extends since the Attester rebooted.
Wherever a hash reconstruction might be needed, the <remote-
attestation> Event Stream MUST support the RFC8639 <replay> feature.
Through the <replay> feature, it is possible for a Verifier to
retrieve and sequentially hash all of the PCR extending events since
an Attester booted. And thus, the Verifier has access to all the
evidence needed to verify a PCR's current value.
Birkholz, et al. Expires December 26, 2020 [Page 11]
Internet-Draft RATS Subscription June 2020
4.6. Configuring the <attestation> Event Stream
Figure 2 is tree diagram which exposes the operator configurable
elements of the <remote-attestation> Event Stream. This allows an
Attester to select what information should be available on the
stream. A fetch operation also allows an external device such as a
Verifier to understand the current configuration of stream.
Almost all YANG objects below are defined via reference from
[RATS-YANG]. There is one object which is new with this model
however. <tpm2-heartbeat> defines the maximum amount of time which
should pass before a subscriber to the Event Stream should get a
<tpm20-attestation> notification from devices which contain a TPM2.
+--rw rats-support-structures
+--rw rats-support-structures
+--rw supported-algos* identityref
+--rw tpms* [tpm-name]
| +--rw tpm-name string
| +--rw tras:leafref-to-keystore? string
| +--rw (tras:subscribable)?
| +--:(tras:tpm12-stream) {tpm:TPM12}?
| | +--rw tras:tpm12-pcr-index* tpm:pcr
| +--:(tras:tpm20-stream) {tpm:TPM20}?
| +--rw tras:tpm20-pcr-index* tpm:pcr
+--rw tras:marshalling-period? uint8
+--rw tras:tpm12-subscribed-signature-scheme?
| -> ../tpm:supported-algos {tpm:TPM12}?
+--rw tras:tpm20-subscribed-signature-scheme?
| -> ../tpm:supported-algos {tpm:TPM20}?
+--rw tras:tpm20-subscription-heartbeat? uint16
{tpm:TPM20}?
Figure 2: Configuring the Attestation Stream
5. YANG Module
This YANG module imports modules from [RATS-YANG] and [RFC8639]. It
is also work-in-progress.
<CODE BEGINS> ietf-tpm-remote-attestation-stream@2020-06-10.yang
module ietf-tpm-remote-attestation-stream {
yang-version 1.1;
namespace
"urn:ietf:params:xml:ns:yang:ietf-tpm-remote-attestation-stream";
prefix tras;
Birkholz, et al. Expires December 26, 2020 [Page 12]
Internet-Draft RATS Subscription June 2020
import ietf-subscribed-notifications {
prefix sn;
reference
"RFC 8639: Subscription to YANG Notifications";
}
import ietf-tpm-remote-attestation {
prefix tpm;
reference
"draft-ietf-rats-yang-tpm-charra";
}
import ietf-yang-structure-ext {
prefix sx;
reference
"draft-ietf-netmod-yang-data-ext";
}
organization "IETF";
contact
"WG Web: <http://tools.ietf.org/wg/rats/>
WG List: <mailto:rats@ietf.org>
Editor: Eric Voit
<mailto:evoit@cisco.com>";
description
"This module contains conceptual YANG specifications for
subscribing to attestation streams being generated from TPM chips.
Copyright (c) 2020 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).
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 XXXX
(https://www.rfc-editor.org/info/rfcXXXX); see the RFC
itself for full legal notices.
Birkholz, et al. Expires December 26, 2020 [Page 13]
Internet-Draft RATS Subscription June 2020
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.";
revision 2020-06-10 {
description
"Initial version.";
reference
"draft-birkholz-rats-network-device-subscription";
}
/*
* IDENTITIES
*/
identity pcr-unsubscribable {
base sn:establish-subscription-error;
description
"Requested PCR is subscribable by the Attester.";
}
/*
* Groupings
*/
grouping heartbeat {
description
"Allows an Attester to push verifiable, current TPM PCR values even
when there have been no recent changes to PCRs.";
leaf tpm20-subscription-heartbeat {
type uint16;
description
"Number of seconds before the Attestation stream should send a new
notification with a fresh quote. This allows confirmation
that the PCR values haven't changed since the last
tpm20-attestation.";
}
}
/*
* RPCs
*/
Birkholz, et al. Expires December 26, 2020 [Page 14]
Internet-Draft RATS Subscription June 2020
augment "/sn:establish-subscription/sn:input" {
when 'derived-from-or-self(sn:stream, "attestation")';
description
"This augmentation adds a nonce to as a subscription parameters
that apply specifically to datastore updates to RPC input.";
uses tpm:nonce;
leaf-list pcr-index {
type tpm:pcr;
min-elements 1;
description
"The numbers/indexes of the PCRs. This will act as a filter for the
subscription so that 'tpm-extend' notifications related to
non-requested PCRs will not be sent to a subscriber.";
}
}
/*
* NOTIFICATIONS
*/
notification tpm-extend {
description
"This notification indicates that a PCR has extended within a TPM based
cryptoprocessor. In less that 10 seconds, it should be followed with
either a tpm12-attestation or tpm20-attestation notification.";
uses tpm:certificate-name;
leaf pcr-index-changed {
type tpm:pcr;
mandatory true;
description
"The number of the PCR extended.";
}
list attested-event {
description
"A set of events which extended an Attester PCR. The sequence of
elements represented in list must match the sequence of events
placed into the TPM.";
container attested-event {
description
"An instance of an event which extended an Attester PCR";
leaf extended-with {
type binary;
mandatory true;
description
"Information extending the PCR.";
}
leaf event-type {
type identityref {
Birkholz, et al. Expires December 26, 2020 [Page 15]
Internet-Draft RATS Subscription June 2020
base tpm:attested-event-log-type;
}
description
"Indicates what kind of event happened the Attester thought was
worthy of recording in a PCR.";
}
anydata event-details {
description
"Any structure reference 'event-type' contains supporting
information which allows an Attester to evaluate the trust
implications.
Event details may be populated with YANG log structures defined
at the bottom of this module.";
}
}
}
}
notification tpm12-attestation {
if-feature "tpm:TPM12";
description
"Contains an instance of TPM1.2 style signed cryptoprocessor
measurements. It is supplemented by unsigned Attester information.";
uses tpm:tpm12-attestation;
}
notification tpm20-attestation {
if-feature "tpm:TPM20";
description
"Contains an instance of TPM2 style signed cryptoprocessor
measurements. It is supplemented by unsigned Attester information.";
uses tpm:tpm20-attestation;
}
/*
* DATA NODES
*/
augment "/tpm:rats-support-structures" {
description
"Defines platform wide 'attestation' stream subscription parameters.";
leaf marshalling-period {
config true;
type uint8;
default 5;
description
Birkholz, et al. Expires December 26, 2020 [Page 16]
Internet-Draft RATS Subscription June 2020
"The maximum number of seconds between the time an event extends a PCR,
and the 'tpm-extend' notification which reports it to a subscribed
Verifier. This period allows multiple extend operations bundled
together and handled as a group.";
}
leaf tpm12-subscribed-signature-scheme {
if-feature "tpm:TPM12";
type leafref {
path "../tpm:supported-algos";
/* a specific algorithm, need to check syntax */
}
description
"A single signature-scheme which will be used to sign the evidence
from a TPM 1.2. which is then placed onto the 'attestation' event
stream.";
}
leaf tpm20-subscribed-signature-scheme {
if-feature "tpm:TPM20";
type leafref {
path "../tpm:supported-algos";
/* a specific algorithm, need to check syntax */
}
description
"A single signature-scheme which will be used to sign the evidence
from a TPM 2.0. which is then placed onto the 'attestation' event
stream.";
}
uses heartbeat{
if-feature "tpm:TPM20";
}
}
augment "/tpm:rats-support-structures/tpm:tpms" {
description
"Allows the configuration 'attestation' stream parameters for a TPM.";
leaf leafref-to-keystore {
config true;
type string;
description
"needs to be replaced with Reference to keystore draft.";
}
choice subscribable {
config true;
description
"Indicates that the set of notifications which comprise the attestation
stream can be modified or tuned by a network administrator.";
case tpm12-stream {
if-feature "tpm:TPM12";
Birkholz, et al. Expires December 26, 2020 [Page 17]
Internet-Draft RATS Subscription June 2020
description
"Configuration elements for a TPM1.2 event stream.";
leaf-list tpm12-pcr-index {
type tpm:pcr;
description
"The numbers/indexes of the PCRs which can be subscribed.";
}
}
case tpm20-stream {
if-feature "tpm:TPM20";
description
"Configuration elements for a TPM2.0 event stream.";
leaf-list tpm20-pcr-index {
type tpm:pcr;
description
"The numbers/indexes of the PCRs which can be subscribed.";
}
/* We need to decide if more than one hash-algo is subscribable */
}
}
}
/*
* STRUCTURES - these contain the schema of reportable event types
*/
sx:structure bios-event-log {
when 'derived-from(../event-type, "bios-event-log")';
description
"BIOS/UEFI event log format";
uses tpm:bios-event-log;
}
sx:structure ima-event-log {
when 'derived-from(../event-type, "ima-event-log")';
description
"IMA event log format";
uses tpm:ima-event-log;
}
}
<CODE ENDS>
6. Security Considerations
To be written.
Birkholz, et al. Expires December 26, 2020 [Page 18]
Internet-Draft RATS Subscription June 2020
7. IANA Considerations
To be written.
8. References
8.1. Normative References
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture",
draft-ietf-rats-architecture-04 (work in progress), May
2020.
[RATS-Device]
"Network Device Remote Integrity Verification", n.d.,
<https://tools.ietf.org/html/draft-ietf-rats-tpm-based-
network-device-attest-00>.
[RATS-YANG]
"A YANG Data Model for Challenge-Response-based Remote
Attestation Procedures using TPMs", n.d.,
<https://datatracker.ietf.org/doc/draft-ietf-rats-yang-
tpm-charra/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8639] Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
E., and A. Tripathy, "Subscription to YANG Notifications",
RFC 8639, DOI 10.17487/RFC8639, September 2019,
<https://www.rfc-editor.org/info/rfc8639>.
[TPM2.0] TCG, "TPM 2.0 Library Specification", n.d.,
<https://trustedcomputinggroup.org/resource/tpm-library-
specification/>.
8.2. Informative References
Birkholz, et al. Expires December 26, 2020 [Page 19]
Internet-Draft RATS Subscription June 2020
[I-D.birkholz-rats-tuda]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", draft-birkholz-
rats-tuda-02 (work in progress), March 2020.
[KGV] TCG, "KGV", October 2003,
<https://trustedcomputinggroup.org/wp-content/uploads/TCG-
NetEq-Attestation-Workflow-Outline_v1r9b_pubrev.pdf>.
Appendix A. Acknowledgements
Tim Jenkins, Paul Merlo, Wayne Mills
A.1. Open Questions
o Do we need a notification correlator object to easily allow
correlation on which extensions have been embodied within a
specific attestation?
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
Darmstadt 64295
Germany
Email: henk.birkholz@sit.fraunhofer.de
Eric Voit
Cisco Systems, Inc.
Email: evoit@cisco.com
Wei Pan
Huawei Technologies
101 Software Avenue, Yuhuatai District
Nanjing, Jiangsu 210012
China
Email: william.panwei@huawei.com
Birkholz, et al. Expires December 26, 2020 [Page 20]