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 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 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 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 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 Event Stream. The contents of the 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()--time(ns) | | time(eg) | |--filter()---------------------------->| |-- or ------>| | | | verify time(eg) Evidence @ time(rg,ra) | | ~ ~ time(vg',eg') | |--filter()---------------------------->| |-- or ------>| | | | 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] 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 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 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 RPC are included in a or ). 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 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 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 . These counters include the , , and 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 has incremented for no more than the same duration as both the 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 , has incremented. The existing subscription MUST be terminated, and a new 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') | |------------------------------->| | : | ~ Heartbeat interval ~ | : | time(eg') : | |------------------------------->| | | 4. Remote Attestation Event Stream The 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 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 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 into [RFC8639] the 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. and notifications which include the provided . These attestation notifications MUST at least include all the requested in the RPC. 2. a series of notifications which reference the requested PCRs on all TPM based cryptoprocessors on the Attester. 3. and notifications generated within a few seconds of the 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 RPC. This request is accomplished by placing a very old within the original RPC request. As the very old will pre-date the time of Attester boot, a will be returned in the RPC response, indicating when the Attester booted. Immediately following the response (and before the notifications above) one or more 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 can be adjusted so that known extensions are not forwarded. The end of this history replay will be indicated with the [RFC8639] notification. For more on this sequence, see Section 2.4.2.1 of [RFC8639]. After the notification is provided, a TPM Quote will be requested and the result passed to the Verifier via a and 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 and 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 heartbeat interval. This MAY be done with a single notification that includes all requested PCRs every heartbeat interval. Alternatively, this MAY be done with several notifications at different times during that heartbeat interval. 4.3. YANG notifications placed on the 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 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 or 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? Each MUST include one or more values being extended into the PCR. These are conveyed within the object. For each extension, details of the event MAY be provided within the object. The format of any included is identified by the . This document includes two YANG structures which may be inserted into the . These two structures are: and . Implementations wanting to provide additional documentation of a type of PCR extension may choose to define additional YANG structures which can be placed into . 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 . In this case, the notification SHOULD be emitted within a 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 . 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 which are not signed. The 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 . In this case, the notification SHOULD be emitted within a since a the first subscribed PCR changed. 3. After a minimum heartbeat interval since a previous was sent. Birkholz, et al. Expires December 26, 2020 [Page 10] Internet-Draft RATS Subscription June 2020 This notification MUST NOT include the returned the results from any PCR extensions not previously reportable by a . +---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 which are not signed. The 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 RPC is sent, a as per RFC8639, Section 2.2 can be set to discard a notification when the 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 Event Stream MUST support the RFC8639 feature. Through the 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 Event Stream Figure 2 is tree diagram which exposes the operator configurable elements of the 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. defines the maximum amount of time which should pass before a subscriber to the Event Stream should get a 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. 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: WG List: Editor: Eric Voit "; 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; } } 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., . [RATS-YANG] "A YANG Data Model for Challenge-Response-based Remote Attestation Procedures using TPMs", n.d., . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . [TPM2.0] TCG, "TPM 2.0 Library Specification", n.d., . 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, . 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]