| RATS Working Group | E. Voit |
| Internet-Draft | Cisco |
| Intended status: Standards Track | March 09, 2020 |
| Expires: September 10, 2020 |
Trusted Path Routing using Remote Attestation
draft-voit-rats-trusted-path-routing-01
There are end-users who believe encryption technologies like IPSec alone are insufficient to protect the confidentiality of their highly sensitive traffic flows. This specification describes two alternatives for protecting these sensitive flows as they transit a network. In both alternatives, protection is accomplished by forwarding sensitive flows across network devices currently appraised as trustworthy.
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 September 10, 2020.
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.
There are end-users who believe encryption technologies like IPSec alone are insufficient to protect the confidentiality of their highly sensitive traffic flows. These customers want their highly sensitive flows to be transported over only network devices recently verified as trustworthy.
With the inclusion of cryptoprocessor hardware into network devices, it is now possible for network providers to identify those network devices which have potentially exploitable or even exploited vulnerabilities. Using this knowledge, it then becomes possible to redirect sensitive flows around these potentially compromised devices.
This specification describes two architectural alternatives for exchanging traffic with end-user customer identified “sensitive subnets”. Traffic going to and from these subnets will transit a path where the IP layer and above are only interpretable by those network devices recently evaluated as trustworthy. These two architectural alternatives are:
Beyond the definition of these two architectural alternatives, incremental technology enhancements needed for each are also specified within this document. The specification works under the assumptions that cryptoprocessors capable of supporting [TPM1.2] or [TPM2.0] interface specifications are available on each network device, and the device supports the concepts of remote attestation laid out in [RATS-Device].
The following terms are imported from [I-D.ietf-rats-architecture]: Attester, Composite Evidence, Evidence, Passport, Relying Party, and Verifier.
The following terms at imported from [RFC8639]: Event Stream.
Newly defined terms for this document:
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.
With this architectural alternative, a controller-based Verifier ensures communications with Sensitive Subnets traverses a Trusted Topology within the controller’s IGP domain. To do this, the Verifier continuously acquires Evidence about each potentially transited device. This access is done via the context established within [RATS-Device]. The controller then appraises all available Evidence and decides on a Trustworthiness Level for each device. Using the set of all appraisals, the controller identifies end-to-end path(s) which avoid any devices with an insufficient Trustworthiness Level. Finally, the controller provisions Sensitive Subnets to use just these end-to-end paths.
Evidence passed to the Verifier which are used to establish a device’s Trustworthiness Level will include but is not limited to:
It is the consideration of all Evidence which allows the establishment and maintenance of a Trustworthiness Level. Note that it is outside the scope of this specification to include algorithms for determining a Trustworthiness Level.
The prerequisites for this solution are:
Figure 1 provides a network diagram of where these four sit within a network topology.
.------------------------------------------------.
| Verifier + Relying Party (3) |
'------------------------------------------------'
(4) ^ ^ ^ ^ ^ (4)
| | (2) | | | |
| | .-------. | | (2) V
V (2) |Hacked | (2) (2) .--------.
.--------. |Router | .-------. .-------. | Edge |
| Edge | |(Attest| |Router | |Switch | | Router |
| Router | | =Fail)| |(Attest| |(Attest| | (Attest|
| | '-------' | =OK) | | =OK) | | =OK) |
(1) path==================================> (1)--- Sensitive
| <==================================path | Subnet
'--------' '-------' '-------' '--------'
Figure 1: Centralized Trusted Path Routing
The feature functionality describing how to achieve (1), (3), and (4) are outside the scope of this specification. The reasoning is that each of these can be accomplished via existing standard-based or standards-track technologies. For example, in step (4), it is possible for a Verifier to provision each ingress device with the set of Sensitive Subnets for which traffic would be placed into a specific [I-D.ietf-idr-segment-routing-te-policy] tunnel.
The new requirements which need to be supported for this specification come from prerequisite (2). To accomplish prerequisite (2), it is necessary for each network device to stream changes in Evidence to a Verifier. This can be accomplished by the Verifier establishing an [RFC8639] subscription to the <attestation> Event Stream described in Section 5 below within this document.
With this new <attestation> Event Stream, a Verifier can consume and continuously determine the Trustworthiness Level of various network devices within the IGP domain. Maintaining this information allows the Controller to calculate an appropriate network path (3).
With this architectural alternative, Composite Evidence is assembled into a passport [I-D.ietf-rats-architecture] by the Attester network device. Upon receiving this passport as part of link layer authentication credentials, a peer Relying Party decides if this Attester is trustworthy enough to be an Attested Device. It also appraises its Trustworthiness Level. If found trustworthy, the relevant link is included into any Trusted Topologies capable of supporting that Trustworthiness Level.
When enough links have been included, a Trusted Topology will now exist for a specified Trustworthiness Level. And traffic exchanged with Sensitive Subnets can be forwarded into that Trusted Topology from all edges of an IGP domain.
.--------. .---------.
| Hacked | | Edge |
.---------. | Router | | Router |
| Router | | | | |
| | | trust>-------------<no_trust |
| no_trust>--<trust | .--------. | |----Sensitive
| | '--------' | trust>==<trust | Subnet
| trust>=============<trust | | |
'---------' | | '---------'
| Router |
'--------'
Figure 2: Distributed Trusted Path Topology Assembly
Critical to the establishment and maintenance of a Trusted Topology is the passport. Within the passport, Composite Evidence is continuously exchanged between peering network devices over a link layer protocol. This Section 4.2 provides a protocol independent process for passport generation and evaluation. Section 7 later in the document binds the passport to specific link layer protocols, YANG models, and authentication methods.
The composite nature of the passport exposes multiple dimensions of an attesting router’s security posture to a network peer. Specifically, using capabilities defined as part of either the TCG [TPM1.2] or [TPM2.0] specifications, the following can be established about the Attester:
With this information, the Relying Party peer can make nuanced decisions. For example, when the Attester’s legitimate hardware identity credentials can be verified, it might choose to accept link layer connections and forward generic Internet traffic. Additionally, if the Attester’s Trustworthiness Level is acceptable, and it hasn’t been too long since the Trustworthiness Level was examined by a Verifier, the Relying Party can include that link in a Trusted Topology.
As the process described above repeats across the set of links within the IGP domain, Trusted Topologies can be extended and maintained. Traffic to and from Sensitive Subnets is then identified at the edges of the IGP domain and passed into this Trusted Topology.
The prerequisites for this solution to work are:
.--------------.
| Verifier A |
'--------------'
^ (2)
| Verifier A signed Trustworthiness Level
Evidence |
(1) V
.-------------. .---------------.
| Attester | | Relying Party |
| (Router) |<------------------nonce(3)| / Verifier B |
| .-----. | | (Router) |
| | TPM | |(4)-Passport containing--->| |
| '-----' | Composite Evidence | (5) |
'-------------' '---------------'
Figure 3: Distributed Trusted Path Passport Generation and Delivery
In Figure 3 above, Evidence from a TPM1.2 or TPM2.0 is generated and signed by that TPM. This Evidence is appraised by Verifier A, and the Attester is given a Trustworthiness Level which is signed and returned as Attestation Results to the Attester. Later, when a request comes in from a Relying Party, the Attester assembles and returns three independently signed elements of Evidence. These three comprise the Composite Evidence which when taken together allow Verifier B to appraise the current Trustworthiness Level of the Attester.
More details on the mechanisms used in the construction and verification of the passport match to the numbered steps of Figure 3:
The <attestation> Event Stream is an [RFC8639] complaint Event Stream which is defined within this section and within the YANG Module of Section 6. The Event Stream contains YANG notifications which carry Evidence which assists a Verifier in appraising the Trustworthiness Level of an Attester. Data Nodes allow the configuration of this Event Stream’s contents on a particular Attester.
This <attestation> Event Stream may only be exposed on Attesters capable of signing cryptoprocessor PCRs using a private key unavailable elsewhere within the Attester. There is not a requirement that the underlying cryptoprocessor of the Attester has undergone TCG certification.
To establish the subscription in a way which results in provably fresh Evidence, randomness must be provided to the Attester. One way this can be done for an [RFC8639] dynamic subscriptions is via the augmentation of the <establish-subscription> RPC:
augment /sn:establish-subscription/sn:input:
+---w nonce-value? binary
As part of the response to the <establish-subscription>, a YANG notification defined in this document is retuned. This notification MUST incorporate the randomness provided by the <nonce-value>. By including this YANG notification in the response, critical measurements are delivered in a way which provides protection against replay attacks. Additionally, the Verifier has immediate access to current measurements.
augment /sn:establish-subscription/sn:output:
+--ro latest-attestation
+--(instance of <tpm12-attestation> or <tpm20-attestation> )
It is also possible to subscribe to the <attestation> Event Stream via an [RFC8639] configured subscription. In this case the Verifier needs some proof of Evidence freshness. Where a TPM2 exists, this may be accomplished via the creation and exposure of a Sync-Token as described in [I-D.birkholz-rats-tuda]. For any type of TPM, centrally created nonces could by signed, and broacast to both the Attester and Verifier.
This notification is generated every time a PCR is extended within a cryptoprocessor. The notification contains a list of the one or more strings which have extended a PCR.
+--n tpm-extend
+--ro tpm_name string
+--ro tpm-physical-index? int32 {ietfhw:entity-mib}?
+--ro pcr-index-changed uint8
+--ro extended-with* binary
All notifications since boot MUST be retained, and replayable.
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 every time at least one PCR identified within the <pcr-list> has changed from the previous <tpm12-attestation> notification:
+---n tpm12-attestation
+--ro tpm_name? string
+--ro up-time? uint32
+--ro node-name? 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-indices* uint8
| +--ro value-size? uint32
| +--ro tpm12-pcr-value* binary
+--:(tpm12-quote2)
+--ro tag? uint8
+--ro pcr-indices* uint8
+--ro locality-at-release? uint8
+--ro digest-at-release? binary
The vast majority of the YANG objects above are defined within [RATS-YANG]. As a result, these objects are not redefined in this draft. The objects which are new include:
Only the most recent <tpm12-attestation> is replayable. All others are discarded from the Event Stream history.
Note that this notification alone does not fully handle replay attack protection for Centralized Trusted Path Routing. As a result, a Verifier MUST periodically receive a nonce based TPM1.2 style quote response. This can be done in several ways including via the <tpm12-challenge-response-attestation> RPC specified in [RATS-YANG]. This periodic query allows a synching on the freshness of the results. Such a periodic synching is not required for the Distributed Trusted Path Routing architecture as the nonce based quote at time(y) proves the freshness of every passport.
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:
Only the most recent <tpm20-attestation> is replayable. All others are discarded from the Event Stream history.
Note that [RATS-YANG] does not yet include the full set of [TPM2.0] objects. As soon as [RATS-YANG] is updated with the necessary information, a new version of this draft will include a tree diagram which identifies those objects within this notification.
It is possible for a receive just those PCR changes of interest from an Attester. To accomplish this, a RFC8639 <establish-subscription> RPC is made against the <attestation> Event Stream. To limit the set of notifications, a <stream-filter> as per RFC8639, Section 2.2 can be set to select the following:
To verify the value of a PCR, a Verifier must either know that the value is a known good value [KGV] or be able to reconstruct the hash value by viewing all the PCR-Extends since the Attester rebooted. Wherever a hash reconstruction might be needed, the <attestation> Event Stream MUST support the RFC8639 <replay> feature. Through the <replay> feature, it is possible for a Verifier to retrieve and sequentially hash all of the PCR extending events since an Attester booted. And thus, the Verifier has access to all the evidence needed to verify a PCR’s current value.
Figure 4 is tree diagram which exposes the 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.
The majority of the YANG objects below are defined via reference from [RATS-YANG].
+--rw attestation-config!
+--rw tpm12-stream
| +--rw tpm12-stream-config* [tpm_name]
| | +--rw tpm_name string
| | +--rw pcr-indices* uint8
| | +--rw (key-identifier)?
| | +--:(public-key)
| | | +--rw pub-key-id? binary
| | +--:(TSS_UUID)
| | +--rw TSS_UUID-value
| | +--rw ulTimeLow? uint32
| | +--rw usTimeMid? uint16
| | +--rw usTimeHigh? uint16
| | +--rw bClockSeqHigh? uint8
| | +--rw bClockSeqLow? uint8
| | +--rw rgbNode* uint8
| +--rw TPM_SIG_SCHEME-value uint8
+--rw tpm20-stream
+--rw tpm20-stream-config* [tpm_name]
| +--rw tpm_name string
| +--rw pcr-list* [pcr-index]
| | +--rw pcr-index uint8
| | +--rw (algo-registry-type)
| | +--:(tcg)
| | | +--rw tcg-hash-algo-id? uint16
| | +--:(ietf)
| | +--rw ietf-ni-hash-algo-id? uint8
| +--rw (key-identifier)?
| +--:(public-key)
| | +--rw pub-key-id? binary
| +--:(uuid)
| +--rw uuid-value? binary
+--rw (signature-identifier-type)
| +--:(TPM_ALG_ID)
| | +--rw TPM_ALG_ID-value? uint16
| +--:(COSE_Algorithm)
| +--rw COSE_Algorithm-value? int32
+--rw tpm2-heartbeat? uint8
Figure 4: Configuring the Attestation Stream
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.
If there is no configuration of any <tpm-name> information within this model, all subscriptions should be rejected with an [RFC8639] reason of <stream-unavailable>.
This YANG module imports modules from [RATS-YANG] and [RFC8639]. It is also work-in-progress.
<CODE BEGINS> ietf-rats-attestation-stream@2020-03-06.yang
module ietf-rats-attestation-stream {
yang-version 1.1;
namespace
"urn:ietf:params:xml:ns:yang:ietf-rats-attestation-stream";
prefix ats;
import ietf-subscribed-notifications {
prefix sn;
reference
"RFC 8639: Subscription to YANG Notifications";
}
import ietf-tpm-remote-attestation {
prefix yang-brat;
reference
"draft-ietf-rats-yang-tpm-charra-00";
}
import ietf-yang-types {
prefix yang;
reference
"RFC 6991: Common YANG Data Types";
}
organization "IETF";
contact
"WG Web: <http://tools.ietf.org/wg/rats/>
WG List: <mailto:rats@ietf.org>
Editor: Eric Voit
<mailto:evoit@cisco.com>";
description
"This module contains conceptual YANG specifications for
subscribing to attestation streams being generated from TPM chips.
Copyright (c) 2020 IETF Trust and the persons identified as authors
of the code. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, is permitted pursuant to, and subject to the license
terms contained in, the Simplified BSD License set forth in Section
4.c of the IETF Trust's Legal Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC XXXX; see the RFC
itself for full legal notices.";
revision 2020-03-06 {
description
"Initial version.";
reference
"draft-voit-rats-trusted-path-routing";
}
/*
* FEATURES
*/
feature passport {
description
"This feature indicates that an Attester supports passports.";
}
/*
* IDENTITIES
*/
identity trustworthiness-level {
if-feature "passport";
description
"Base identity for a verifier assessed trustworthiness level.";
}
identity compromised {
base trustworthiness-level;
description
"A Verifier has appraised an Attester as compromised.";
}
identity unverified {
base trustworthiness-level;
description
"There is no recent Verifier appraisal of an Attester.";
}
identity boot-verified {
base trustworthiness-level;
description
"A Verifier has appraised an Attester as Boot Integrity Verified.";
}
/*
* Groupings
*/
grouping tpm-name {
description
"Name of a TPM.";
leaf tpm_name {
type string;
description
"Name of the TPM or All";
}
}
grouping tpm12-attestation {
description
"Contains an instance of TPM1.2 style signed cryptoprocessor
measurements. It is supplemented by unsigned Attester information.
The vast majority of the YANG objects in the YANG tree are defined
within [RATS-YANG].";
uses tpm-name;
uses yang-brat:node-uptime;
uses yang-brat:compute-node;
uses yang-brat:tpm12-quote-info-common;
choice tpm12-quote {
mandatory true;
description
"Either a tpm12-quote-info or tpm12-quote-info2, depending
on whether TPM_Quote or TPM_Quote2 was used
(cf. input field add-verson).";
case tpm12-quote1 {
description
"BIOS/UEFI event logs";
uses yang-brat:tpm12-quote-info;
uses yang-brat:tpm12-pcr-composite;
}
case tpm12-quote2 {
description
"BIOS/UEFI event logs";
uses yang-brat:tpm12-quote-info2;
}
}
}
/*
* RPCs
*/
augment "/sn:establish-subscription/sn:input" {
description
"This augmentation adds a nonce to as a subscription parameters
that apply specifically to datastore updates to RPC input.";
leaf nonce-value {
type binary;
description
"This nonce should be generated via a registered
cryptographic-strength algorithm. In consequence, the length
of the nonce depends on the hash algorithm used. The algorithm
used in this case is independent from the hash algorithm used to
create the hash-value in the response of the attestor.";
reference
"draft-ietf-rats-yang-tpm-charra";
}
}
augment "/sn:establish-subscription/sn:output" {
description
"This augmentation allows a subscriber/verifier to understand the
state of the Attester at time of subscription.";
container latest-attestation {
description
"provides the current PCR values of a TPM.";
uses tpm12-attestation;
/* Awaiting WG progress on draft-ietf-rats-yang-tpm-charra
before completing for TPM2.0 style. */
}
}
/*
* NOTIFICATIONS
*/
notification tpm-extend {
description
"This notification indicates that a PCR has extended within a TPM 1.x
or 2.0 cryptoprocessor. Within a small number of seconds, it should be
followed with a tpm12-attestation or tpm20-attestation.";
uses yang-brat:tpm-name;
leaf pcr-index-changed {
type uint8;
mandatory true;
description
"The number of the PCR extended.";
}
leaf-list extended-with {
type binary;
ordered-by user;
description
"Includes the one or more elements extending the PCR. The sequence of
elements represented in list must match the sequence entered into the
TPM.";
}
}
notification tpm12-attestation {
description
"Contains an instance of TPM1.2 style signed cryptoprocessor
measurements. It is supplemented by unsigned Attester information.
The vast majority of the YANG objects in the YANG tree are defined
within [RATS-YANG].";
uses tpm12-attestation;
}
/* Awaiting WG progress on draft-ietf-rats-yang-tpm-charra before completing
notification tpm20-attestation {
description
"We still need to define the majority of the YANG objects in
within [RATS-YANG]. Redefining them here would just result in lots of
unnecessary churn.";
}
*/
/*
* DATA NODES
*/
container attestation-config {
presence
"Indicates that the set of notifications which comprise the attestation
stream can be modified or tuned by a network adminsitrator.";
description
"This allows an Attester to determine which TPMs and PCRs are evaluated
and included within the Attestation Stream.";
container tpm12-stream {
description
"Configuration elements for a TPM1.2 event stream. This includes
all of the TPM1.2s which are available on an Attester.";
list tpm12-stream-config {
key tpm_name;
description
"Allows the stream to be configured for the inclusion of TPM1.2
quotes and evidence.";
uses tpm-name;
uses yang-brat:tpm12-pcr-selection;
uses yang-brat:tpm12-attestation-key-identifier;
}
uses yang-brat:tpm12-signature-scheme;
}
container tpm20-stream {
description
"Configuration elements for a TPM2.0 event stream. This includes
all of the TPM2.0s which are available on an Attester.";
list tpm20-stream-config {
key "tpm_name";
description
"Allows the stream to be configured for the inclusion of TPM2.0
quotes and evidence.";
uses tpm-name;
list pcr-list {
key "pcr-index";
description
"For each PCR in this list an individual list of banks
(hash-algo) can be requested.";
leaf pcr-index {
type uint8;
description
"The number of the PCR. At the moment this is limited 32";
}
uses yang-brat:hash-algo;
}
uses yang-brat:tpm20-attestation-key-identifier;
}
uses yang-brat:tpm20-signature-scheme;
leaf tpm2-heartbeat {
type uint8;
description
"Number of seconds before the Attestation stream should send a new
Notification which with a fresh quote. This allows confirmation
that the PCR values haven't changed since the last
tpm20-attestation.";
}
}
}
container attestation-results {
if-feature "passport";
presence
"An attestation Verifier has appraised the security posture of the
device, and returned the results within this container.";
description
"Containes the latest Verifier appraisal of an Attester.";
leaf-list trustworthiness-level {
type identityref {
base trustworthiness-level;
}
min-elements 1;
description
"One or more Trustworthiness Levels assigned.";
}
leaf timestamp {
type yang:date-and-time;
mandatory true;
description
"The timestamp of the Verifier's appraisal.";
}
leaf tpmt-signature {
type binary;
description
"Must match a recent tpmt-signature sent in a notification to
a Verifier. This allows correlation of the Attestation Results to
a recent PCR change.";
}
leaf verifier-signature {
type binary;
description
"Signature of the Verifier across all the current objects in the
attestation-results container.";
}
leaf verifier-signature-key-name {
type binary;
description
"Name of the key the Verifier used to sign the results.";
}
}
}
<CODE ENDS>
This section provides details of how Composite Evidence described in Section 4.2 interacts with link layer protocols like [MACSEC] or [IEEE-802.1X], YANG subscriptions [RFC8639], and [RFC3748] methods. Additional linkages to the YANG module defined in Section 6 are described.
.--------------.
| Verifier A |
'--------------'
^ (2)
| Verifier A signed Attestation Results @time(x) (
Evidence( | Trustworthiness Level,
TpmQuote | signature from TpmQuote@time(x) )
@time(x)) |
(1) V
.-------------. .---------------.
| Attester |<------nonce @time(y)---(3)| Relying Party |
| .-----. | | / Verifier B |
| | Tpm | |(4)-Composite Evidence ( ->| (Router) |
| '-----' | TpmQuote@time(y), | (5) |
'-------------' TpmQuote@time(x), '---------------'
Verifier A signed Attestation Results @time(x) )
Figure 5: Details of Passport Generation
Figure 5 above expands upon the previously described Figure 3. The numbering in both figures is the same.
Step (1)
Verifier A subscribes to an Attester’s <attestation> Event Stream on via [RFC8639]. Within the <establish-subscription> RPC, a nonce is delivered as per Section 5.1. This nonce then is included into TPM quotes requests driven for the Attester’s cryptoprocessor. The result of the TPM quote is appended to the <establish-subscription> response. Following this delivery of a provably current TPM state, all the historical evidence needed to validate specific PCRs within this quote are delivered on the <attestation> Event Stream via the <replay> feature. Any changes to PCRs results in new notifications as described in Section 5.2. These are continuously streamed to Verifier A.
Step (2)
Whenever a PCR changes, Verifier A evaluates the totality of the Evidence received. This Evidence may include information not provided on the <attestation> Event Stream. Verifier A then decides the Trustworthiness Level of the Attester. Subsequently it sends back a signed Attestation Result which includes the Trustworthiness Level and the signature sent as part of (1) from the Attester. It is this signature which allows the Trustworthiness Level to be later provably associated with a recent TPM Quote.
The delivery of Attestation Results back to the Attester can be done via a YANG operational datastore write of the following objects:
+--rw attestation-results! {passport}?
+--rw trustworthiness-level* identityref
+--rw timestamp yang:date-and-time
+--rw tpmt-signature? binary
+--rw verifier-signature? binary
+--rw verifier-signature-key-name? binary
Figure 6: Attestation Results Tree
Step (3)
At time(y) a Relying Party makes a Link Layer connection request to an Attester via a protocol such as [MACSEC] or [IEEE-802.1X]. This connection request must include [RFC3748] credentials. Specifics of the EAP credentials are TBD. If there is no central distribution of time via [I-D.birkholz-rats-tuda] a nonce must be included to ensure freshness of a response.
This step can repeat periodically independently of any subsequent iteration (1) and (2). This allows for periodic reauthentication of the link layer in a way not bound to the updating of Verifier A’s Attestation Results.
Step (4)
Upon receipt of (3), a passport is generated as per Section 4.2, and sent to the Relying Party.
Step (5)
Upon receipt of (4), the Relying Party verifies the Composite Evidence as per Section 4.2. Most often, the relevant PCR values at time(x) will be the same as the PCR values at time(y). In this case, the Relying Party can simply accept the Trustworthiness Level assigned by the Verifier A. When the PCR values are different, and not much time has passed between time(x) and time(y), the Relying Party can either accept the previous Trustworthiness Level, or attempt another EAP request in a few seconds as new Attestation Results are delivered by Step (2). When there is a large time gap between time(x) and time(y) and the PCR values are different, the Attester should be given an <unverified> Trustworthiness Level.
Based on the link’s Trustworthiness Level, the Relying Party may adjust the link affinity of the corresponding [I-D.ietf-lsr-flex-algo] topology.
Successful attacks on an IGP domain Verifier has the potential of affecting traffic on the Trusted Topology.
For Distributed Trusted Path Routing, links which are part of the FlexAlgo are visible across the entire IGP domain. Therefore a compromised device will know when it is being bypassed.
Access control for the objects in Figure 6 should be tightly controlled so that it becomes difficult for the passport to become a denial of service vector.
| [I-D.ietf-rats-architecture] | Birkholz, H., Thaler, D., Richardson, M., Smith, N. and W. Pan, "Remote Attestation Procedures Architecture", Internet-Draft draft-ietf-rats-architecture-02, March 2020. |
| [RATS-YANG] | "A YANG Data Model for Challenge-Response-based Remote Attestation Procedures using TPMs", January 2020. |
| [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. |
| [TPM1.2] | TCG, ., "TPM 1.2 Main Specification", October 2003. |
| [TPM2.0] | TCG, ., "TPM 2.0 Library Specification", October 2003. |
Shwetha Bhandari, Henk Birkholz, Chennakesava Reddy Gaddam, Sujal Sheth, Peter Psenak, Nancy Cam Winget, Siva Sivabalan, Ned Smith, Guy Fedorkow, Liang Xia.
[THIS SECTION TO BE REMOVED BY THE RFC EDITOR.]
v00-v01
Do we need functional requirements on how to handle traffic to/from Sensitive Subnets when no Trusted Topology exists between IGP edges? The network typically can make this unnecessary. For example it is possible to construct a local IPSec tunnel to make untrusted devices appear as Transparently-Transited Devices. This way Secure Subnets could be tunneled between FlexAlgo nodes where an end-to-end path doesn’t currently exist. However there still is a corner case where all IGP egress points are not considered sufficiently trustworthy.
Deep discussions on the Trustworthiness Levels which need standardization. Perhaps these could be mapped to the “Figure 2: Attested Objects” from [RATS-Device].
Should the “extended-with” object support a choice of structured data, or should it be binary only.
Should we have multiple attestation streams identified? E.g.: pcr-trust-evidence, bios-log-trust-evidence, and ima-log-trust-evidence? Should each stream have its own draft?
Should we include define how to acquires attestation-certificates. Perhaps through something like draft-ietf-netconf-keystore?