Fraunhofer SIT

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@sit.fraunhofer.de

Cisco Systems, Inc.

evoit@cisco.com

Huawei Technologies

101 Software Avenue, Yuhuatai District Nanjing, Jiangsu 210012 China william.panwei@huawei.com

Security RATS Working Group Internet-Draft This document defines how to subscribe to an Event Stream of attestation related Evidence on TPM-based network devices.

and 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: Evidence is not streamed to an interested Verifier as soon as it is generated. 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 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 . 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 : 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 : 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 when, and only when, they appear in all capitals, as shown here.

below is a sequence diagram which updates Figure 5 of . This sequence diagram replaces the challenge-response interaction model with an Dynamic Subscription to an <attestation> Event Stream. The contents of the <attestation> Event Stream are defined below within .

)--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') ]]>

time(VG,RG,RA) are identical to the corresponding times from Figure 5 of . time(RG’,RA’) are subsequent instances of the corresponding times from Figure 5 of . time(NS): The Verifier generates a nonce and makes an <establish-subscription> request. This request also includes the augmentations defined in this document’s YANG model. Key subscription RPC parameters include: the nonce a set of PCRs of interest which the Verifier wants to appraise 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. 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, and a signed TPM Quote showing the result the PCR extension.

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 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 .

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 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 , 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 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 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.

------------------------------>| | : | ~ Heartbeat interval ~ | : | time(EG') : | |------------------------------->| | | ]]>

The <attestation> Event Stream is an complaint Event Stream which is defined within this section and within the YANG Module of . This Event Stream contains YANG notifications which carry Evidence which assists a Verifier in appraising the Trustworthiness Level of an Attester. Data Nodes within allow the configuration of this Event Stream’s contents on an Attester. This <attestation> Event Stream may only be exposed on Attesters supporting . As with , 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 the <establish-subscription> RPC. Additionally, a Verifier must ask for PCRs of interest from a platform.

The result of the subscription will be that passing of the following information: <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. a series of <pcr-extend> notifications which reference the requested PCRs on all TPM based cryptoprocessors on the Attester. <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.

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 <replay-completed> notification. For more on this sequence, see Section 2.4.2.1 of . 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.

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>.

All YANG objects above are defined within . 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: 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.

All YANG objects above are defined within . 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 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.

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 . 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.

This YANG module imports modules from and . It is also work-in-progress.

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: 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. 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."; } } } } } ]]>

To be written.

To be written.

In network protocol exchanges, it is often the case that one entity (a Relying Party) requires evidence about a remote peer to assess the peer's trustworthiness, and a way to appraise such evidence. The evidence is typically a set of claims about its software and hardware platform. This document describes an architecture for such remote attestation procedures (RATS). This document describes a workflow for remote attestation of the integrity of firmware and software installed on network devices that contain Trusted Platform Modules [TPM1.2], [TPM2.0]. This document defines a YANG RPC and a minimal datastore required to retrieve attestation evidence about integrity measurements from a device following the operational context defined in [I-D.ietf-rats-tpm-based-network-device-attest]. Complementary measurement logs are also provided by the YANG RPC originating from one or more roots of trust of measurement. The module defined requires at least one TPM 1.2 or TPM 2.0 and corresponding Trusted Software Stack included in the device components of the composite device the YANG server is running on. This document defines a YANG data model and associated mechanisms enabling subscriber-specific subscriptions to a publisher's event streams. Applying these elements allows a subscriber to request and receive a continuous, customized feed of publisher-generated information. TCG In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This documents defines the method and bindings used to conduct Time- based Uni-Directional Attestation (TUDA) between two RATS (Remote ATtestation procedureS) entities over the Internet. TUDA does not require a challenge-response handshake and thereby does not rely on the conveyance of a nonce to prove freshness of remote attestation Evidence. Conversely, TUDA enables the creation of Secure Audit Logs that can constitute Evidence about current and past operational states of an Attester. Every RATS entity requires access to a trustable and synchronized time-source. A Handle Distributor takes on the corresponding role of a Time Stamp Authority (TSA) to provide Time Stamp Tokens (TST) to all RATS entities. TCG

v00-v01 rename notification: pcr-extended, which supports multiple PCRs netequip boot added YANG structure extension removed Matched to structural changes made within charra

Thanks to …