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
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 10, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Requirements Notation . . . . . . . . . . . . . . . . . . 4
3. Centralized Trusted Path Routing . . . . . . . . . . . . . . 4
4. Distributed Trusted Path Routing . . . . . . . . . . . . . . 6
4.1. Trusted Topology . . . . . . . . . . . . . . . . . . . . 6
4.2. Passport with Composite Evidence . . . . . . . . . . . . 6
5. Attestation Event Stream . . . . . . . . . . . . . . . . . . 10
5.1. Subscribing to the stream . . . . . . . . . . . . . . . . 10
5.2. YANG notifications placed on the Event Stream . . . . . . 11
5.3. Pre-filtering the Event Stream . . . . . . . . . . . . . 13
5.4. Replaying previous PCR Extend events. . . . . . . . . . . 14
5.5. Configuring the Attestation Event Stream . . . . . . . . 14
6. YANG Module . . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Passport Protocol Bindings . . . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.1. Normative References . . . . . . . . . . . . . . . . . . 25
9.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 27
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 27
Appendix C. Open Questions . . . . . . . . . . . . . . . . . . . 27
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
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
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path where the IP layer and above are only interpretable by those
network devices recently evaluated as trustworthy. These two
architectural alternatives are:
1. Centralized Trusted Path Routing - For sensitive subnets, trusted
end-to-end paths are pre-assigned through a network provider
domain. Along these paths, attestation evidence of potentially
transited components has been assessed. Each path is guaranteed
to only include devices meeting the needs of a formally defined
trustworthiness level.
2. Distributed Trusted Path Routing - Through the exchange of
attestation evidence between peering network devices, a trusted
topology is established and maintained. Only devices meeting the
needs of a formally defined trustworthiness level are included as
members of this topology. Traffic exchanged with sensitive
subnets is forwarded into this topology.
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].
2. Terminology
2.1. Terms
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:
Attested Device - a device where a Verifier's most recent appraisal
of attestation evidence has successfully met the criteria for a
specific Trustworthiness Level. Attested Devices cannot be
appraised as unverified or compromised.
Sensitive Subnet - an IP address range where IP packets to or from
that range must only have their IP headers and encapsulated
payloads accessible/visible only by Attested Devices.
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Transparently-Transited Device - a network device within an IGP
domain where any packets passed into that IGP domain are
completely opaque at Layer 3 and above.
Trusted Topology - A topology which includes only Attested Devices
and Transparently-Transited Devices.
Trustworthiness Level - a specific grade of trust earned by a
device. The grade for a device is assigned by a Verifier during
the appraisal process and can be returned within Attestation
Results. Example levels include boot-verified, unverified and
compromised. (Note: significant discussion will be needed to
agree on definitions of these levels.)
2.2. 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. Centralized Trusted Path Routing
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:
o An Attester's security measurements being extended into [TPM1.2]
or [TPM2.0] compliant Platform Configuration Registers (PCR).
o An Attester's current PCR measurements.
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.
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The prerequisites for this solution are:
1. Customer designated Sensitive Subnet ranges and their demanded
Trustworthiness Levels have been identified and associated with
external interfaces to/from the edge of an IGP domain.
2. A Verifier which can continuously acquire Evidence and appraise
the Trustworthiness Levels of all network devices within the IGP
domain.
3. A Verifier which continuously optimizes a set of network paths/
tunnels. These paths must traverse only Attested Devices or
Transparently-Transited Devices while on their way to an egress
interface for an IGP Domain.
4. A Verifier which can provision and maintain the set of Sensitive
Subnets associated with specific network paths/tunnels.
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
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(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).
4. Distributed Trusted Path Routing
4.1. Trusted Topology
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
4.2. Passport with Composite Evidence
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
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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:
o its hardware-based identity,
o the Trustworthiness level according to its most recent Verifier
appraisal,
o the amount of time which has passed since the Attester has been at
a Trustworthiness Level, and
o if the PCRs haven't changed, the Attester's current
Trustworthiness Level
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:
o Customer designated Sensitive Subnets and their requested
Trustworthiness Levels have been identified and associated with
external interfaces to/from the edge of an IGP domain.
o A Trusted Topology such as one established by
[I-D.ietf-lsr-flex-algo] exists in an IGP domain for the
forwarding of Sensitive Subnet traffic. This Topology will carry
traffic of a Trustworthiness Level.
o Verifiers A and B are able to verify [TPM1.2] or [TPM2.0]
signatures of an Attester.
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o Verifier A can establish the Trustworthiness Level of an Attester
and return a signed result to that Attester.
o An Attester can assemble a passport of Composite Evidence for
Verifier B.
o Verifier B trusts the Attestation Results and can verify
signatures made by Verifier A.
o Within an IGP domain, a Relying Party is able to use affinity to
include/exclude links as part of the Trusted Topology based on
this appraisal.
o Traffic to a Sensitive Subnet can be passed into the Trusted
Topology.
.--------------.
| 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:
1. An Attester sends a signed TPM Quote which includes PCR
measurements to Verifier A at time(x).
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2. Verifier A appraises (1), then sends the following items back to
that Attester as Attestation Results:
1. the appraised Trustworthiness Level of an Attester,
2. the signature from the TPM Quote of (1),
3. a Verifier signature across (2.1) and (2.2).
3. A nonce known to the Relying Party is received by the Attester at
time(y).
4. The Attester generates and sends a passport. The encapsulated
Composite Evidence includes:
1. (1)
2. (2)
3. New signed, verifiably fresh PCR measurements at time(y),
which incorporates the nonce from (3).
5. On receipt of (4), the Relying Party makes its determination of
how the Composite Evidence will impact adjacencies within a
Trusted Topology. The decision process is:
1. Verify that (4.3) includes the nonce from (3).
2. Verify the TPM signature within (4.2) matches the signature
of (4.1).
3. Validate the signatures of (4.1), (4.2), (4.3).
4. Failure of (5.1), (5.2), or (5.3) means the link should be
assigned a <compromised> Trustworthiness Level, and
additionally jump to step (5.8).
5. If selected PCR values of (1) match (4.3), then Relying Party
can accept (2.1) as the link's Trustworthiness Level.
6. When the PCR values are different, and not much time has
passed between time(x) and time(y), the Relying Party can
either accept any previous Trustworthiness Level, or attempt
to acquire a new passport. In many cases, it should only be
a few seconds before a new Attestation Results should be
delivered to an Attester via (2).
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7. When the PCR values are different, but there is a large time
gap between time(x) and time(y), the link should be assigned
an <unverified> Trustworthiness Level.
8. Based on the link's Trustworthiness Level, add or remove it
from the appropriate Trusted Topology.
5. Attestation Event Stream
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.
5.1. Subscribing to the stream
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
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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.
5.2. YANG notifications placed on the Event Stream
5.2.1. tpm-extend
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.
5.2.2. 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 every time at least one PCR identified
within the <pcr-list> has changed from the previous
<tpm12-attestation> notification:
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+---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:
o pcr-index-changed* - this is a list of a PCRs which have new
values since the last <tpm12-attestation> notification.
o pcr-index-attested* - this is a list of all the PCRs contained in
the <tpms-attest-result>.
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
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Path Routing architecture as the nonce based quote at time(y) proves
the freshness of every passport.
5.2.3. 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:
o every time at least one PCR has changed from a previous
tpm20-attestation.
o after a locally configurable minimum heartbeat period since a
previous tpm20-attestation was sent. This heartbeat is
identifiable as the <pcr-index-changed> will be missing from the
notification. As a result, there is no need to match it to one or
more <tpm-extend> notifications.
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.
5.3. Pre-filtering the Event Stream
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:
o each <tpm-extend> containing a <pcr-index-changed> of a desired
PCR
o each <tpm12-attestation> containing a <pcr-index-changed> of a
desired PCR
o each <tpm20-attestation> containing a <pcr-index-changed> of a
desired PCR
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5.4. 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.
5.5. Configuring the Attestation Event Stream
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].
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+--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.
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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>.
6. YANG Module
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
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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;
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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;
}
}
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}
/*
* 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.";
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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
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"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.";
}
}
}
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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>
7. Passport Protocol Bindings
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.
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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
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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.
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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.
8. Security Considerations
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.
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",
draft-ietf-rats-architecture-02 (work in progress), March
2020.
[RATS-YANG]
"A YANG Data Model for Challenge-Response-based Remote
Attestation Procedures using TPMs", January 2020,
<https://tools.ietf.org/html/draft-ietf-rats-yang-tpm-
charra-00>.
[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>.
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[TPM1.2] TCG, ., "TPM 1.2 Main Specification", October 2003,
<https://trustedcomputinggroup.org/resource/tpm-main-
specification/>.
[TPM2.0] TCG, ., "TPM 2.0 Library Specification", October 2003,
<https://trustedcomputinggroup.org/resource/tpm-library-
specification/>.
9.2. Informative References
[I-D.birkholz-rats-tuda]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", draft-birkholz-
rats-tuda-01 (work in progress), September 2019.
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
Rosen, E., Jain, D., and S. Lin, "Advertising Segment
Routing Policies in BGP", draft-ietf-idr-segment-routing-
te-policy-08 (work in progress), November 2019.
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
algo-06 (work in progress), February 2020.
[IEEE-802.1X]
Parsons, G., "802.1AE: MAC Security (MACsec)", January
2020,
<https://standards.ieee.org/standard/802_1X-2010.html>.
[KGV] TCG, ., "KGV", October 2003,
<https://trustedcomputinggroup.org/wp-content/uploads/TCG-
NetEq-Attestation-Workflow-Outline_v1r9b_pubrev.pdf>.
[MACSEC] Seaman, M., "802.1AE: MAC Security (MACsec)", January
2006, <https://1.ieee802.org/security/802-1ae/>.
[RATS-Device]
Fedorkow, G. and J. Fitzgerald-McKay, "Network Device
Remote Integrity Verification", n.d.,
<https://tools.ietf.org/html/draft-fedorkow-rats-network-
device-attestation-02>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<https://www.rfc-editor.org/info/rfc3748>.
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Appendix A. Acknowledgements
Shwetha Bhandari, Henk Birkholz, Chennakesava Reddy Gaddam, Sujal
Sheth, Peter Psenak, Nancy Cam Winget, Siva Sivabalan, Ned Smith, Guy
Fedorkow, Liang Xia.
Appendix B. Change Log
[THIS SECTION TO BE REMOVED BY THE RFC EDITOR.]
v00-v01
o Move all FlexAlgo terminology to Section 7. This allows
Section 4.2 to be more generic.
o Edited Figure 1 so that (4) points to the egress router.
o Added text freshness mechanisms, and articulated configured
subscription support.
o Minor YANG model clarifications.
o Added a few open questions which Frank thinks interesting to work.
Appendix C. Open Questions
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?
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Author's Address
Eric Voit
Cisco Systems, Inc.
Email: evoit@cisco.com
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