RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                                 E. Voit
Expires: 18 February 2022                                          Cisco
                                                                  W. Pan
                                                                  Huawei
                                                          17 August 2021


                 Attestation Event Stream Subscription
           draft-birkholz-rats-network-device-subscription-03

Abstract

   This memo defines how to subscribe to YANG Event Streams for Remote
   Attestation Procedures (RATS).  In RATS, Conceptional Messages, are
   defined.  Analogously, the YANG module defined in this memo augments
   the YANG module for TPM-based Challenge-Response based Remote
   Attestation (CHARRA) to allow for subscription to remote attestation
   Evidence.  Additionally, this memo provides the methods and means to
   define additional Event Streams for other Conceptual Message as
   illustrated in the RATS Architecture, e.g.  Attestation Results,
   Endorsements, or Event Logs.

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
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   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 18 February 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.






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   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 . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   5
   3.  Operational Model . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Sequence Diagram  . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Continuously Verifying Freshness  . . . . . . . . . . . .   7
       3.2.1.  TPM 1.2 Quote . . . . . . . . . . . . . . . . . . . .   8
       3.2.2.  TPM 2 Quote . . . . . . . . . . . . . . . . . . . . .   8
   4.  Remote Attestation Event Stream . . . . . . . . . . . . . . .   9
     4.1.  Subscription to the <attestation> Event Stream  . . . . .   9
     4.2.  Replaying a history of previous TPM extend operations . .  10
       4.2.1.  TPM2 Heartbeat  . . . . . . . . . . . . . . . . . . .  11
     4.3.  YANG notifications placed on the <attestation> Event
           Stream  . . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.3.1.  pcr-extend  . . . . . . . . . . . . . . . . . . . . .  11
       4.3.2.  tpm12-attestation . . . . . . . . . . . . . . . . . .  13
       4.3.3.  tpm20-attestation . . . . . . . . . . . . . . . . . .  13
     4.4.  Filtering Evidence at the Attester  . . . . . . . . . . .  14
     4.5.  Replaying previous PCR Extend events  . . . . . . . . . .  14
     4.6.  Configuring the <attestation> Event Stream  . . . . . . .  14
   5.  YANG Module . . . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Event Streams for Conceptual Messages . . . . . . . . . . . .  22
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  24
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24










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1.  Introduction

   [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 and other
   Conceptional Messages from a TPM-based network device.  However,
   there are limitations inherent in the challenge-response based remote
   attestation (CHARRA [I-D.ietf-rats-reference-interaction-models])
   upon which these documents are based.  One of these limitation is
   that it is a RATS role's duty to request Conceptional Messages, such
   as Evidence as provided by [I-D.ietf-rats-yang-tpm-charra], from
   another RATS entity.  The result is that the interval between the
   occurrence of a security-relevant change event, and the event's
   visibility within the interested RATS entity, such as a Verifier or a
   Relying Party, can be unacceptably long.  It is common to convey
   Conceptual Messages ad-hoc or periodically via requests.  As new
   technologies emerge, some of these solutions require Conceptual
   Messages to be conveyed from one RATS entity to another without the
   need of continuous polling.  Subscription to YANG Notifications
   [RFC8639] provides a set of standardized tools to facilitate these
   emerging requirements.  This memo specifies a YANG augment to
   subscribe to YANG modeled remote attestation Evidence as defined in
   [I-D.ietf-rats-yang-tpm-charra].  Additionally, this memo provides
   the means to define further Event Streams to convey Conceptional
   Messages other than Evidence, such as Attestation Results,
   Endorsements, or Event Logs.

   In essence, the limitation of poll-based interactions results in two
   adverse effects:

   1.  Conceptual Messages are not streamed to an interested consumer of
       information, e.g., Verifiers or Relying Parties, as soon as they
       are generated.

   2.  If they were to be streamed, Conceptual Messages are not
       appraisable for their freshness in every scenario.  This becomes
       more important with Conceptional Messages that have a strong
       dependency on freshness, such as Evidence and corresponding
       Attestation Results.












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   This specification addresses the first adverse effect by enabling a
   consumer of Conceptual Messages (the subscriber) to request a
   continuous stream of new or updated Conceptual Messages via an
   [RFC8639] subscription to an <attestation> Event Stream.  This new
   Event Stream is defined in this document and exists upon the producer
   of Conceptual Messages (the publisher).  In the case of a Verifier's
   subscription to an Attester's Evidence, the Attester will
   continuously stream a requested set of freshly generated Evidence to
   the subscribing Verifier.

   The second adverse effect results from the use of nonces in the
   challenge-response interaction model
   [I-D.ietf-rats-reference-interaction-models] realized in
   [I-D.ietf-rats-yang-tpm-charra].  In [I-D.ietf-rats-yang-tpm-charra],
   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 via the streaming attestation interaction model
   [I-D.ietf-rats-reference-interaction-models].  To convey a RATS
   Conceptual Message, an initial nonce is provided during the
   subscription to an Event Stream.

   There are several options to refresh a nonce provided by the initial
   subscription or its freshness characteristics.  All of these methods
   are out-of-band of an established subscription to YANG Notifications.
   Two complementary methods are taken into account by this memo:

   1.  a central provider supplies new fresh nonces, e.g. via a Handle
       Provider that distributes Epoch IDs to all entities in a domain
       as described in [I-D.ietf-rats-architecture] and as facilitated
       by the Uni-Directional Remote Attestation described in
       [I-D.ietf-rats-reference-interaction-models] or

   2.  the freshness characteristics of a received nonce are updated by
       -- potentially periodic or ad-hoc -- out-of-band TPM Quote
       requests as facilitated by [I-D.ietf-rats-yang-tpm-charra].

   Both approaches to update the freshness characteristics of the
   Conceptual Messages conveyed via subscription to YANG Notification
   that are taken into account by this memo assume that clock drift
   between involved entities can occur.  In consequence, in some usage
   scenarios the timing considerations for freshness
   [I-D.ietf-rats-architecture] might have to be updated in some regular
   interval.  Analogously, there are can be additional methods that are
   not describe by but nevertheless supported by this memo.






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   This memo enables to remove the two adverse effects described by
   using the YANG augment specified.  The YANG augment supports, for
   example, a RATS Verifier to maintain a continuous appraisal procedure
   of verifiably fresh Attester Evidence without relying on continuous
   polling.

2.  Terminology

   The following terms are imported from [I-D.ietf-rats-architecture]:
   Attester, Conceptual Message, 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 are 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

   [I-D.ietf-rats-tpm-based-network-device-attest] describes the
   conveyance of TPM-based Evidence from a Verifier to an Attester using
   the CHARRA interaction model
   [I-D.ietf-rats-reference-interaction-models].  The operational model
   and corresponding sequence diagram described in this section is based
   on [I-D.ietf-rats-yang-tpm-charra].  The basis for interoperability
   required for additional types of Event Streams is covered in
   Section 6.  The following sub-section focuses on subscription to YANG
   Notifications to the <attestation> Event Stream.

3.1.  Sequence Diagram

   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]
   TPM-specific challenge-response interaction model with a [RFC8639]
   Dynamic Subscription to an <attestation> Event Stream.  The contents
   of the <attestation> Event Stream are defined below within Section 4.








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   .----------.                            .--------------------------.
   | Attester |                            | Relying Party / Verifier |
   '----------'                            '--------------------------'
      time(VG)                                                    |
   generateClaims(targetEnvironment)                              |
        | => claims, eventLogs                                    |
        |                                                         |
        |<---------establish-subscription(<attestation>)------time(NS)
        |                                                         |
      time(EG)                                                    |
   generateEvidence(nonce, PcrSelection, collectedClaims)         |
        | => SignedPcrEvidence(nonce, PcrSelection)               |
        | => LogEvidence(collectedClaims)                         |
        |                                                         |
        |--filter(<pcr-extend>)---------------------------------->|
        |--<tpm12-attestation> or <tpm20-attestation>------------>|
        |                                                         |
        |                                                  time(RG,RA)
        |     appraiseEvidence(SignedPcrEvidence, eventLog, refClaims)
        |                                    attestationResult <= |
        |                                                         |
        ~                                                         ~
      time(VG')                                                   |
   generateClaimes(targetEnvironment)                             |
        | => claims                                               |
        |                                                         |
      time(EG')                                                   |
   generateEvidence(handle, PcrSelection, collectedClaims)        |
        | => SignedPcrEvidence(nonce, PcrSelection)               |
        | => LogEvidence(collectedClaims)                         |
        |                                                         |
        |--filter(<pcr-extend>)---------------------------------->|
        |--<tpm12-attestation> or <tpm20-attestation>------------>|
        |                                                         |
        |                                                 time(RG',RA')
        |    appraiseEvidence(SignedPcrEvidence, eventLog, refClaims)
        |                                    attestationResult <= |
        |                                                         |

          Figure 1: YANG Subscription Model for Remote Attestation

   *  time(VG,RG,RA) are identical to the corresponding time definitions
      from [I-D.ietf-rats-tpm-based-network-device-attest].

   *  time(VG',RG',RA') are subsequent instances of the corresponding
      times from Figure 5 in
      [I-D.ietf-rats-tpm-based-network-device-attest].




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   *  time(NS) - the subscriber generates a nonce and makes an [RFC8639]
      <establish-subscription> request based on a nonce.  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 wants to appraise, and

      -  an optional filter which can reduce the logged events on the
         <attestation> stream pushed to the Verifier.

   *  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 <pcr-extend>
         notification, and

      -  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).

   *  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,

      -  a signed TPM Quote showing the result the PCR extension, and

      -  and a handle (see Section 6. in
         [I-D.ietf-rats-reference-interaction-models], which is either
         the initially received nonce or a more recently received Epoch
         ID (see Section 10.3. in [I-D.ietf-rats-architecture] that
         contains a new nonce or equivalent qualified data.

   One way to acquire a new time synchronization that allows for the
   reuse of the initially received nonce as a fresh handle is elaborated
   on in the follow section Section 3.2.

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.



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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
   [I-D.ietf-rats-yang-tpm-charra].

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 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:

   *  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.

   *  If the <reset-counter>, <restart-counter> has incremented.  The
      existing subscription MUST be terminated, and a new <establish-
      subscription> SHOULD be generated.

   *  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.





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   .----------.                        .--------------------------.
   | Attester |                        | Relying Party / Verifier |
   '----------'                        '--------------------------'
      time(VG',EG')                                         |
        |-<tpm20-attestation>------------------------------>|
        |                                    :              |
        ~                           Heartbeat interval      ~
        |                                    :              |
      time(EG')                              :              |
        |-<tpm20-attestation>------------------------------>|
        |                                                   |

4.  Remote Attestation Event Stream

   The <attestation> Event Stream is an [RFC8639] compliant 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 to 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.

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.




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   2.  a series of <pcr-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 <pcr-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 <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.






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   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.

4.2.1.  TPM2 Heartbeat

   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.

4.3.  YANG notifications placed on the <attestation> Event Stream

4.3.1.  pcr-extend

   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.





















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       +---n pcr-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-event-log)
                      +--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>.



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4.3.2.  tpm12-attestation

   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       tpm:certificate-name-ref
          +--ro up-time?               uint32
          +--ro TPM_QUOTE2?            binary
          +--ro TPM12-hash-algo?       identityref
          +--ro unsigned-pcr-values* []
             +--ro pcr-index*   tpm:pcr
             +--ro pcr-value*   binary

   All YANG objects above are defined within
   [I-D.ietf-rats-yang-tpm-charra].  The <tpm12-attestation> is not
   replayable.

4.3.3.  tpm20-attestation

   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:

   *  every time at least one PCR has changed from a previous
      tpm20-attestation.  In this case, the notification SHOULD be
      emitted within 10 seconds of the corresponding <pcr-extend> being
      sent:

   *  after a locally configurable minimum heartbeat period since a
      previous tpm20-attestation was sent.











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       +---n tpm20-attestation {taa:TPM20}?
          +--ro certificate-name       tpm:certificate-name-ref
          +--ro TPMS_QUOTE_INFO        binary
          +--ro quote-signature?       binary
          +--ro up-time?               uint32
          +--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.

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 <pcr-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 <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.

4.6.  Configuring the <attestation> Event Stream

   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.





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   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.

     augment /tpm:rats-support-structures:
       +--rw marshalling-period?                  uint8
       +--rw tpm12-subscribed-signature-scheme?
       |   -> ../tpm:attester-supported-algos/tpm12-asymmetric-signing
       |      {taa:TPM12}?
       +--rw tpm20-subscribed-signature-scheme?
       |   -> ../tpm:attester-supported-algos/tpm20-asymmetric-signing
       |      {taa:TPM20}?
       +--rw tpm20-subscription-heartbeat?        uint16
     augment /tpm:rats-support-structures/tpm:tpms:
       +--rw subscription-aik?        tpm:certificate-name-ref
       +--rw (subscribable)?
          +--:(tpm12-stream) {taa:TPM12}?
          |  +--rw TPM12-hash-algo?   identityref
          |  +--rw tpm12-pcr-index*   tpm:pcr
          +--:(tpm20-stream) {taa:TPM20}?
             +--rw TPM20-hash-algo?   identityref
             +--rw tpm20-pcr-index*   tpm:pcr

           Figure 2: Configuring the \<attestation\> Event Stream

5.  YANG Module

   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@2020-12-15.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



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         "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) 2021 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 XXXX
        (https://www.rfc-editor.org/info/rfcXXXX); see the RFC
        itself for full legal notices.";

     revision 2021-05-11 {
       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.";
     }



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     /*
      * 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



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         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 {
               if-feature "tpm:bios";
               description
                 "BIOS/UEFI event log format";
               uses tpm:bios-event-log;
             }
             case ima-event-log {
               if-feature "tpm:ima";
               description
                 "IMA event log format";
               uses tpm:ima-event-log;
             }
             case netequip-boot-event-log {
               if-feature "tpm:netequip_boot";



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               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.";
       leaf certificate-name {
         type tpm:certificate-name-ref;
         mandatory true;
         description
           "Allows a TPM quote to be associated with a certificate.";
       }
       uses tpm:tpm12-attestation;
       uses tpm:tpm12-hash-algo;
       list unsigned-pcr-values {
         description
           "Allows notifications to be filtered by PCR number or
           PCR value based on via YANG related mechanisms such as PATH.
           This is done without requiring the filtering structure to be
           applied against TCG structured data.";
         leaf-list pcr-index {
           type tpm:pcr;
           min-elements 1;
           description
             "PCR index number.";
         }
         leaf-list pcr-value {
           type binary;
           description
             "PCR value in a sequence which matches to the 'pcr-index'.";
         }
       }
     }

     notification tpm20-attestation {
       if-feature "taa:tpm20";
       description
         "Contains an instance of TPM2 style signed cryptoprocessor
         measurements.  It is supplemented by unsigned Attester



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         information.";
       leaf certificate-name {
         type tpm:certificate-name-ref;
         mandatory true;
         description
           "Allows a TPM quote to be associated with a certificate.";
       }
       uses tpm:tpm20-attestation {
         description
           "Provides the attestation info.  Also ensures PCRs can be XPATH
           filtered by refining the unsigned data so that it appears.";
         refine unsigned-pcr-values {
           min-elements 1;
         }
         refine unsigned-pcr-values/pcr-values {
           min-elements 1;
         }
       }
     }


     /*
      * DATA NODES
      */

     augment "/tpm:rats-support-structures" {
       description
         "Defines platform wide 'attestation' stream subscription
         parameters.";
       leaf marshalling-period {
         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.";



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       }
       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 {
         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.";



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           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>

6.  Event Streams for Conceptual Messages

   Analogous to the [RFC8639] compliant <attestation> Event Stream for
   the conveyance of remote attestation Evidence as defined in
   Section Section 4, additional Event Streams can be defined for this
   YANG augment.  Additional Event Streams require separate YANG augment
   specifications that provide the Event Stream definition and
   optionally a content format definition either via subscriptions to
   YANG datastores or dedicated YANG Notifications.  It is possible to
   use either YANG subscription methods to other YANG modules for RATS
   Conceptual Messages or to define Event Streams for other none-YANG-
   modeled data.  In the context of RATS Conceptual Messages, both
   options MUST be a specified via YANG augments to this specification.

7.  Security Considerations

   To be written.

8.  IANA Considerations

   To be written.

9.  References

9.1.  Normative References

   [I-D.ietf-rats-architecture]
              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture", Work
              in Progress, Internet-Draft, draft-ietf-rats-architecture-
              12, 23 April 2021, <https://www.ietf.org/archive/id/draft-
              ietf-rats-architecture-12.txt>.

   [I-D.ietf-rats-reference-interaction-models]
              Birkholz, H., Eckel, M., Pan, W., and E. Voit, "Reference
              Interaction Models for Remote Attestation Procedures",



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              Work in Progress, Internet-Draft, draft-ietf-rats-
              reference-interaction-models-04, 26 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-rats-
              reference-interaction-models-04.txt>.

   [I-D.ietf-rats-tpm-based-network-device-attest]
              Fedorkow, G., Voit, E., and J. Fitzgerald-McKay, "TPM-
              based Network Device Remote Integrity Verification", Work
              in Progress, Internet-Draft, draft-ietf-rats-tpm-based-
              network-device-attest-08, 26 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-rats-tpm-
              based-network-device-attest-08.txt>.

   [I-D.ietf-rats-yang-tpm-charra]
              Birkholz, H., Eckel, M., Bhandari, S., Voit, E., Sulzen,
              B., (Frank), L. X., Laffey, T., and G. C. Fedorkow, "A
              YANG Data Model for Challenge-Response-based Remote
              Attestation Procedures using TPMs", Work in Progress,
              Internet-Draft, draft-ietf-rats-yang-tpm-charra-10, 12
              August 2021, <https://www.ietf.org/archive/id/draft-ietf-
              rats-yang-tpm-charra-10.txt>.

   [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/>.

9.2.  Informative References

   [I-D.birkholz-rats-tuda]
              Fuchs, A., Birkholz, H., McDonald, I. E., and C. Bormann,
              "Time-Based Uni-Directional Attestation", Work in
              Progress, Internet-Draft, draft-birkholz-rats-tuda-05, 12
              July 2021, <https://www.ietf.org/archive/id/draft-
              birkholz-rats-tuda-05.txt>.



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   [KGV]      TCG, "KGV", October 2003,
              <https://trustedcomputinggroup.org/wp-content/uploads/TCG-
              NetEq-Attestation-Workflow-Outline_v1r9b_pubrev.pdf>.

Appendix A.  Change Log

   v01-v02

   *  Match YANG changes/simplifications made to charra

   v00-v01

   *  rename notification: pcr-extended, which supports multiple PCRs

   *  netequip boot added

   *  YANG structure extension removed

   *  Matched to structural changes made within charra

Acknowledgements

   Thanks to ...

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   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




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   Email: william.panwei@huawei.com


















































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