| Internet-Draft | RATS Subscription | October 2020 |
| Birkholz, et al. | Expires 9 April 2021 | [Page] |
This document defines how to subscribe to an Event Stream of attestation related Evidence on TPM-based network devices.¶
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 9 April 2021.¶
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.¶
[I-D.ietf-rats-tpm-based-network-device-attest] and [I-D.ietf-rats-yang-tpm-charra] define the operational prerequisites and a YANG Model for the acquisition of Evidence from a TPM-based network device. However there is a limitation inherant 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:¶
This specification addresses the first adverse effect by enabling a Verifier to subscribe via [RFC8639] to an <attestation> Event Stream which exists upon the Attester. When subscribed, the Attester will continuously stream a requested set of Evidence to the Verifier.¶
The second adverse effect results from the nonce based challenge-response of [I-D.ietf-rats-yang-tpm-charra]. In that document an Attester must wait for a new nonce from a Verifier before it generates a new TPM Quote. To address delays resulting from such a wait, this specification enables freshness to be asserted asynchronously.¶
By removing these two adverse effects, it becomes possible for a Verifier to continously maintain an appraisal of the Attested device without relying on continous polling.¶
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 Appendix A. The following terms at imported from [RFC8639]: Event Stream, Subscription, Event Stream Filter, Dynamic Subscription.¶
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.¶
Figure 1 below is a sequence diagram which updates Figure 5 of [I-D.ietf-rats-tpm-based-network-device-attest]. This sequence diagram replaces the [I-D.ietf-rats-tpm-based-network-device-attest] 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.¶
.----------. .--------------------------.
| Attester | | Relying Party / Verifier |
'----------' '--------------------------'
time(VG) |
|<---------establish-subscription(<attestation>)--time(NS)
| |
time(EG) |
|--filter(<pcr-extend>)---------------------------->|
|--<tpm12-attestation> or <tpm20-attestation>------>|
| |
| verify time(EG) Evidence @ time(RG,RA)
| |
~ ~
time(VG',EG') |
|--filter(<pcr-extend>)---------------------------->|
|--<tpm12-attestation> or <tpm20-attestation>------>|
| |
| verify time(EG') Evidence @ time(RG',RA')
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:¶
time(EG) - An initial response of Evidence is returned to the Verifier. This includes:¶
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:¶
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.¶
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 [I-D.ietf-rats-yang-tpm-charra].¶
When the Attester includes a TPM2 compliant cryptoprocessor, internal time-related counters are included within the signed TPM Quote. By including a 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. And 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 countersr.¶
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. The rules for appraising these objects are as follows:¶
.----------. .--------------------------.
| Attester | | Relying Party / Verifier |
'----------' '--------------------------'
time(VG',EG') |
|-<tpm20-attestation>------------------------------>|
| : |
~ Heartbeat interval ~
| : |
time(EG') : |
|-<tpm20-attestation>------------------------------>|
| |
¶
The <attestation> Event Stream is an [RFC8639] complaint Event Stream which is defined within this section and within the YANG Module of [I-D.ietf-rats-yang-tpm-charra]. This Event Stream contains YANG notifications which carry Evidence which assists a Verifier in appraising the Trustworthiness Level of an Attester. Data Nodes within Section 4.6 allow the configuration of this Event Stream's contents on an Attester.¶
This <attestation> Event Stream may only be exposed on Attesters supporting [I-D.ietf-rats-tpm-based-network-device-attest]. As with [I-D.ietf-rats-tpm-based-network-device-attest], it is up to the Verifier to understand which types of cryptoprocessors and keys are acceptable.¶
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:¶
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.¶
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 <pcr-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 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 appraise the reported extend operations for each PCR, as well compare the expected value of the PCR value against that signed by the TPM.¶
For TPM2, make sure that every requested PCR is sent within an <tpm20-attestation> no less frequently than once per heartbeat interval. This MAY be done with a single <tpm20-attestation> notification that includes all requested PCRs every heartbeat interval. This MAY be done with several <tpm20-attestation> notifications at different times during that heartbeat interval.¶
This notification documents when a subscribed PCR is extended within a single TPM cryptoprocessor. It SHOULD be emmitted no less than the <marshalling-period> after an 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, and these should be reflected in the same notification.) This notification MUST be emmitted prior to a <tpm12-attestation> or <tpm20-attestation> notification which has included and signed the results of any specific PCR extension. If pcr extending events occur during the generation of the <tpm12-attestation> or <tpm20-attestation> notification, the marshalling period MUST be extended so that a new <pcr-extend> is not sent until the corresponding notifications have been sent.¶
+---n tpm-extend
+--ro certificate-name? certificate-name-ref
+--ro pcr-index-changed* tpm:pcr
+--ro attested-event* []
+--ro attested-event
+--ro extended-with binary
+--ro (event-details)?
+--:(bios-event-log)
| +--ro bios-event-entry* [event-number]
| +--ro event-number uint32
| +--ro event-type? uint32
| +--ro pcr-index? pcr
| +--ro digest-list* []
| | +--ro hash-algo? identityref
| | +--ro digest* binary
| +--ro event-size? uint32
| +--ro event-data* uint8
+--:(ima-event-log)
| +--ro ima-event-entry* [event-number]
| +--ro event-number uint64
| +--ro ima-template? string
| +--ro filename-hint? string
| +--ro filedata-hash? binary
| +--ro filedata-hash-algorithm? string
| +--ro template-hash-algorithm? string
| +--ro template-hash? binary
| +--ro pcr-index? pcr
| +--ro signature? binary
+--:(netequip-boot)
+--ro boot-event-entry* [event-number]
+--ro event-number uint64
+--ro filename-hint? string
+--ro filedata-hash? binary
+--ro filedata-hash-algorithm? string
+--ro file-version? string
+--ro file-type? string
+--ro pcr-index? pcr
¶
Each <pcr-extend> MUST include one or more values being extended into the PCR. These are passed within the <extended-with> object. For each extension, details of the event SHOULD 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>.¶
This notification contains an instance of a TPM1.2 style signed cryptoprocessor measurement. It is supplemented by Attester information which is not signed. This notification is generated and emitted from an Attester when at least one PCR identified within the subscribed <pcr-indices> has changed from the previous <tpm12-attestation> notification. This notification MUST NOT include the results of any PCR extensions not previously reported by a <pcr-extend>. This notification SHOULD be emitted as soon as a TPM Quote can extract the latest PCR hashed values. This notification MUST be emitted prior to a subsequent <pcr-extend>.¶
+---n tpm12-attestation {taa:TPM12}?
+--ro certificate-name? certificate-name-ref
+--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 [I-D.ietf-rats-yang-tpm-charra]. The <tpm12-attestation> is not replayable.¶
This notification contains an instance of TPM2 style signed cryptoprocessor measurements. It is supplemented by Attester information which is not signed. This notification is generated at two points in time:¶
+---n tpm20-attestation {taa:TPM20}?
+--ro certificate-name? certificate-name-ref
+--ro TPMS_QUOTE_INFO binary
+--ro quote-signature? binary
+--ro up-time? uint32
+--ro node-id? string
+--ro node-physical-index? int32 {ietfhw:entity-mib}?
+--ro unsigned-pcr-values* []
+--ro TPM20-hash-algo? identityref
+--ro pcr-values* [pcr-index]
+--ro pcr-index pcr
+--ro pcr-value? binary
¶
All YANG objects above are defined within [I-D.ietf-rats-yang-tpm-charra]. The <tpm20-attestation> is not replayable.¶
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 <pcr-extend> notification when the <pcr-index-changed> is uninteresting to the verifier.¶
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 <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.¶
Figure 2 is tree diagram which exposes the operator configurable elements of the <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 [I-D.ietf-rats-yang-tpm-charra]. 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.¶
+--ro rats-support-structures
+--ro tpms* [tpm-name]
| +--ro tpms:leafref-to-keystore? string
| +--ro (tpms:subscribable)?
| +--:(tpms:tpm12-stream) {tpm:TPM12}?
| | +--ro tpms:pcr-index* pcr
| +--:(tpms:tpm20-stream) {tpm:TPM20}?
| +--ro tpms:pcr-list* []
| +--ro tpms:pcr
| +--ro tpms:pcr-index* pcr
| +--ro (tpms:algo-registry-type)
| +--:(tpms:tcg)
| | +--ro tpms:tcg-hash-algo-id? uint16
| +--:(tpms:ietf)
| +--ro tpms:ietf-ni-hash-algo-id? uint8
+--ro tpms:marshalling-period? uint8
+--ro tpms:TPM_SIG_SCHEME-value? enumeration {tpm:TPM12}?
+--ro (tpms:signature-identifier-type) {tpm:TPM20}?
| +--:(tpms:TPM_ALG_ID)
| | +--ro tpms:TPM_ALG_ID-value? enumeration
| +--:(tpms:COSE_Algorithm)
| +--ro tpms:COSE_Algorithm-value? int32
+--ro tpms:tpm20-heartbeat? uint8
This YANG module imports modules from [I-D.ietf-rats-yang-tpm-charra] and [RFC8639]. It is also work-in-progress.¶
<CODE BEGINS> ietf-rats-attestation-stream@2020-09-17.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;
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-tcg-algs {
prefix taa;
}
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.
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 2020-09-17 {
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
*/
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 pcr-extend {
description
"This notification indicates that one or more PCRs have been
extended within a TPM based cryptoprocessor. In less than the
'marshalling-period', it MUST be followed with either a
corresponding tpm12-attestation or tpm20-attestation notification
which exposes the result of the PCRs updated.";
uses tpm:certificate-name-ref;
leaf-list pcr-index-changed {
type tpm:pcr;
min-elements 1;
description
"The number of each PCR extended. This list MUST contain the
set of PCRs descibed within the event log details. This leaf
can be derived from the list of attested events, but exposing
it here allows for easy filtering of the notifications of
interest to a verifier.";
}
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's PCR.";
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.";
}
choice event-details {
description
"Contains the event happened the Attester thought
was worthy of recording in a PCR.
choices are of types defined by the identityref
base tpm:attested_event_log_type";
case bios-event-log {
description
"BIOS/UEFI event log format";
uses tpm:bios-event-log;
}
case ima-event-log {
description
"IMA event log format";
uses tpm:ima-event-log;
}
case netequip-boot-event-log {
description
"IMA event log format";
uses tpm:network-equipment-boot-event-log;
}
}
}
}
}
notification tpm12-attestation {
if-feature "taa:TPM12";
description
"Contains an instance of TPM1.2 style signed cryptoprocessor
measurements. It is supplemented by unsigned Attester
information.";
uses tpm:certificate-name-ref {
description
"Certificate associated with this tpm20-attestation.";
}
uses tpm:tpm12-attestation;
}
notification tpm20-attestation {
if-feature "taa:TPM20";
description
"Contains an instance of TPM2 style signed cryptoprocessor
measurements. It is supplemented by unsigned Attester
information.";
uses tpm:certificate-name-ref {
description
"Certificate associated with this tpm20-attestation.";
}
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
"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 "taa:TPM12";
type leafref {
path "../tpm:attester-supported-algos" +
"/tpm:tpm12-asymmetric-signing";
}
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 "taa:TPM20";
type leafref {
path "../tpm:attester-supported-algos" +
"/tpm:tpm20-asymmetric-signing";
}
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 "taa:TPM20";
}
}
augment "/tpm:rats-support-structures/tpm:tpms" {
description
"Allows the configuration 'attestation' stream parameters for a
TPM.";
leaf subscription-aik {
config true;
type tpm:certificate-name-ref;
description
"Identifies the certificate-name associated with the
notifications in the 'attestation' stream.";
}
choice subscribable {
config true;
description
"Indicates that the set of notifications which comprise the
'attestation' event stream can be modified or tuned by a
network administrator.";
case tpm12-stream {
if-feature "taa:TPM12";
description
"Configuration elements for a TPM1.2 event stream.";
uses tpm:TPM12-hash-algo;
leaf-list tpm12-pcr-index {
type tpm:pcr;
description
"The numbers/indexes of the PCRs which can be subscribed.";
}
}
case tpm20-stream {
if-feature "taa:TPM20";
description
"Configuration elements for a TPM2.0 event stream.";
uses tpm:TPM20-hash-algo;
leaf-list tpm20-pcr-index {
type tpm:pcr;
description
"The numbers/indexes of the PCRs which can be subscribed.";
}
}
}
}
}
<CODE ENDS>¶
To be written.¶
To be written.¶
v00-v01¶
Thanks to ...¶