RATS Working Group H. Birkholz
Internet-Draft M. Eckel
Intended status: Informational Fraunhofer SIT
Expires: January 9, 2020 July 08, 2019
Reference Interaction Model for Challenge-Response-based Remote
Attestation
draft-birkholz-rats-reference-interaction-model-01
Abstract
This document defines an interaction model for a basic remote
attestation procedure. Additionally, the required information
elements are illustrated.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 2
2. Disambiguation . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Component Roles . . . . . . . . . . . . . . . . . . . . . . . 3
5. Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . 3
6. Remote Attestation Interaction Model . . . . . . . . . . . . 4
6.1. Information Elements . . . . . . . . . . . . . . . . . . 4
6.2. Interaction Model . . . . . . . . . . . . . . . . . . . . 6
7. Further Context . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Confidentiality . . . . . . . . . . . . . . . . . . . . . 7
7.2. Mutual Authentication . . . . . . . . . . . . . . . . . . 7
7.3. Hardware-Enforcement/Support . . . . . . . . . . . . . . 7
8. Security and Privacy Considerations . . . . . . . . . . . . . 8
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. CDDL Specification for a simple CoAP
Challenge/Response Interaction . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Remote attestation procedures (RATS) are a combination of activities,
in which a Verifier creates assertions about assertions of integrity
and about characteristics of other system entities by the appraisal
of corresponding signed assertions (evidence). In this document, a
reference interaction model for a generic challenge-response-based
remote attestation procedure is provided. The minimum set of
components, roles and information elements that have to be conveyed
between Verifier and Attester are defined as a standard reference to
derive more complex RATS from.
1.1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2. Disambiguation
The term "Remote Attestation" is a common expression and often
associated with certain properties. The term "Remote" in this
context does not necessarily refer to a remote system entity in the
scope of network topologies or the Internet. It rather refers to a
decoupled system or different computing context, which also could be
present locally as components of a composite device. Examples
include: a Trusted Execution Environment (TEE), Baseboard Management
Controllers (BMCs), as well as other physical or logical protected/
isolated execution environments.
3. Scope
This document focuses on a generic interaction model between
Verifiers and Attesters. Complementary processes, functions and
activities that are required for a complete semantic binding of RATS
are not in scope. Examples include: identity establishment, key
enrollment, and certificate revocation. Furthermore, any processes
and activities that go beyond carrying out the remote attestation
process are out of scope. For instance, using the result of a remote
attestation that is emitted by the Verifier, such as triggering
remediation actions and recovery processes, as well as the
remediation actions and recovery processes themselves, are out of
scope.
4. Component Roles
The Reference Interaction Model for Challenge-Response-based Remote
Attestation is based on the standard roles defined in
[I-D.birkholz-rats-architecture]:
Attester: The role that designates the subject of the remote
attestation. A system entity that is the provider of evidence
takes on the role of an Attester.
Verifier: The role that designates the system entity and that is the
appraiser of evidence provided by the Attester. A system entity
that is the consumer of evidence takes on the role of a Verifier.
5. Prerequisites
Attester Identity:
Attestation Authenticity: An Attestation MUST be authentic.
An attestation, in order to be authentic, MAY This Identity MUST
be part of the signed assertions (attestation evidence) that the
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Attester conveys to the Verifier. An Identity MAY be a unique
identity or it MAY be included in a zero-knowledge proof (ZKP) or
be part of a group signature.
Authentication Secret: An Authentication Secret MUST be present on
the Attester. The Attester MUST sign assertions with that
Authentication Secret, proving the authenticity of the assertions.
The Authentication Secret MUST be established before a remote
attestation procedure can take place. How it is established is
out of scope for this reference model.
6. Remote Attestation Interaction Model
This section defines the information elements that have to be
conveyed via a protocol, enabling the conveyance of Evidence between
Verifier and Attester, as well as the interaction model for a generic
challenge-response remote attestation scheme.
6.1. Information Elements
Attester Identity ('attesterIdentity'): _mandatory_
A statement about a distinguishable Attester made by an entity
without accompanying evidence of its validity, used as proof of
identity.
Authentication Secret ID ('authSecID'): _mandatory_
An identifier that MUST be associated with the Authentication
Secret which is used to sign evidence.
Nonce ('nonce'): _mandatory_
The Nonce (number used once) is intended to be unique and
practically infeasible to guess. In this reference interaction
model the Nonce MUST be provided by the Verifier and MUST be used
as proof of freshness. With respect to conveyed evidence, it
ensures the result of an attestation activity to be created
recently, e. g. sent or derived by the challenge from the
Verifier. As such, the Nonce MUST be part of the signed
Attestation Evidence that is sent from the Attester to the
Verifier.
Assertions ('assertions'): _mandatory_
Assertions represent characteristics of an Attester. They are
required for proving the integrity of an Attester. Examples are
assertions about sensor data, policies that are active on the
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system entity, versions of composite firmware of a platform,
running software, routing tables, or information about a local
time source.
Reference Assertions ('refAssertions') _mandatory_
Reference Assertions are used to verify the assertions received
from an Attester in an attestation verification process. For
example, Reference Assertions MAY be Reference Integrity
Measurements (RIMs) or assertions that are implicitly trusted
because they are signed by a trusted authority. RIMs represent
(trusted) assertions about the intended platform operational state
of the Attester.
Assertion Selection ('assertionSelection'): _optional_
An Attester MAY provide a selection of assertions in order to
reduce or increase retrieved assertions to those that are relevant
to the conducted appraisal. Usually, all available assertions
that are available to the Attester SHOULD be conveyed. The
Assertion Selection MAY be composed as complementary signed
assertions or MAY be encapsulated assertions in the signed
Attestation Evidence. An Attester MAY decide whether or not to
provide all requested assertions or not. An example for an
Assertion Selection is a Verifier requesting (signed) RIMs from an
Attester.
(Signed) Attestation Evidence ('signedAttestationEvidence'): _mandat
ory_
Attestation Evidence consists of the Authentication Secret ID that
identifies an Authentication Secret, the Attester Identity, the
Assertions, and the Verifier-provided Nonce. Attestation Evidence
MUST cryptographically bind all of those elements. The
Attestation Evidence MUST be signed by the Authentication Secret.
The Authentication Secret MUST be trusted by the Verifier as
authoritative.
Attestation Result ('attestationResult'): _mandatory_
An Attestation Result is produced by the Verifier as a result of a
Verification of Attestation Evidence. The Attestation Result
represents assertions about integrity and other characteristics of
the corresponding Attester.
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6.2. Interaction Model
The following sequence diagram illustrates the reference remote
attestation procedure defined by this document.
[Attester] [Verifier]
| |
| <--- requestAttestation(nonce, authSecID, assertionSelection) |
| |
collectAssertions(assertionSelection) |
| => assertions |
| |
signAttestationEvidence(authSecID, assertions, nonce) |
| => signedAttestationEvidence |
| |
| signedAttestationEvidence ----------------------------------> |
| |
| verifyAttestationEvidence(signedAttestationEvidence, refAssertions)
| attestationResult <= |
| |
The remote attestation procedure is initiated by the Verifier,
sending an attestation request to the Attester. The attestation
request consists of a Nonce, a Authentication Secret ID, and an
Assertion Selection. The Nonce guarantees attestation freshness.
The Authentication Secret ID selects the secret with which the
Attester is requested to sign the Attestation Evidence. The
Assertions Selection narrows down or increases the amount of received
Assertions, if required. If the Assertions Selection is empty, then
by default all assertions that are available on the system of the
Attester SHOULD be signed and returned as Attestation Evidence. For
example, a Verifier may only be interested in particular information
about the Attester, such as proof of with which BIOS and firmware it
booted up, and not include information about all currently running
software.
The Attester, after receiving the attestation request, collects the
corresponding Assertions to compose the Attestation Evidence that the
Verifier requested--or, in case the Verifier did not provide an
Assertions Selection, the Attester collects all information that can
be used as complementary Assertions in the scope of the semantics of
the remote attestation procedure. After that, the Attester produces
Attestation Evidence by signing the Attester Identity, the
Assertions, and the Nonce with the Authentication Secret identified
by the Authentication Secret ID. Then the Attester sends the signed
Attestation Evidence back to the Verifier.
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Important at this point is that Assertions, the Nonce as well as the
Attester Identity information MUST be cryptographically bound to the
signature of the Attestation Evidence. It is not required for them
to be present in plain text, though. Cryptographic blinding MAY be
used at this point. For further reference see Security and Privacy
Considerations (Section 8)
As soon as the Verifier receives the signed Attestation Evidence, it
verifies the signature, the Attester Identity, the Nonce, and the
Assertions. This process is application-specific and can be carried
out by, e. g., comparing the Assertions to known (good), expected
Reference Assertions, such as Reference Integrity Measurements
(RIMs), or evaluating it in other ways. The final output of the
Verifier is the Attestation Result. It constitutes an new assertion
about properties and characteristics of the Attester, i. e. whether
or not it is compliant to policies, or even can be "trusted".
7. Further Context
Depending on the use cases to cover, there may be additional
requirements. Some of them are mentioned in this section.
7.1. Confidentiality
Confidentiality of exchanged attestation information may be
desirable. This requirement usually is present when communication
takes place over insecure channels, such as the public Internet. In
such cases, TLS may be uses as a suitable communication protocol that
preserves confidentiality. In private networks, such as carrier
management networks, it must be evaluated whether or not the
transport medium is considered confidential.
7.2. Mutual Authentication
In particular use cases mutual authentication may be desirable in
such a way that a Verifier also needs to prove its identity to the
Attester, instead of only the Attester proving its identity to the
Verifier.
7.3. Hardware-Enforcement/Support
Depending on the requirements, hardware support for secure storage of
cryptographic keys, crypto accelerators, or protected or isolated
execution environments may be useful. Well-known technologies are
Hardware Security Modules (HSM), Physically Unclonable Functions
(PUFs), Shielded Secrets, and Trusted Executions Environments (TEEs).
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8. Security and Privacy Considerations
In a remote attestation process the Verifier or the Attester MAY want
to cryptographically blind several attributes. For instance,
information can be part of the signature after applying a one-way
function (e. g. a hash function).
There is also a possibility to scramble the Nonce or Attester
Identity with other information that is known to both the Verifier
and Attester. A prominent example is the IP address of the Attester
that usually is known by the Attester itself as well as the Verifier.
This extra information can be used to scramble the Nonce in order to
counter certain types of relay attacks.
9. Acknowledgments
Very likely.
10. Change Log
o Initial draft -00
o Changes from version 00 to version 01:
* Added details to the flow diagram
o Changes from version 01 to version 02:
* Integrated comments from Ned Smith (Intel)
* Reorganized sections and
* Updated interaction model
o Changes from version 02 to version 03:
* Replaced "claims" with "assertions"
* Added proof-of-concept CDDL for CBOR via CoAP based on a TPM
2.0 quote operation
11. References
11.1. Normative References
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[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>.
11.2. Informative References
[I-D.birkholz-rats-architecture]
Birkholz, H., Wiseman, M., Tschofenig, H., and N. Smith,
"Architecture and Reference Terminology for Remote
Attestation Procedures", draft-birkholz-rats-
architecture-01 (work in progress), March 2019.
Appendix A. CDDL Specification for a simple CoAP Challenge/Response
Interaction
The following CDDL specification is an examplary proof-of-concept to
illustrate a potential implementation of the Reference Interaction
Model. The transfer protocol used is CoAP using the FETCH operation.
The actual resource operated on can be empty. Both the Challenge
Message and the Response Message are exchanged via the FETCH Request
and FETCH Response body.
In this example, the root-of-trust for reporting primitive operation
"quote" is provided by a TPM 2.0.
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RAIM-Bodies = CoAP-FETCH-Body / CoAP-FETCH-Response-Body
CoAP-FETCH-Body = [ hello: bool, ; if true, the AK-Cert is conveyed
nonce: bytes,
pcr-selection: [ + [ tcg-hash-alg-id: uint .size 2, ; TPM2_ALG_ID
[ + pcr: uint .size 1 ],
]
],
]
CoAP-FETCH-Response-Body = [ attestation-evidence: TPMS_ATTEST-quote,
tpm-native-signature: bytes,
? ak-cert: bytes, ; attestation key certificate
]
TPMS_ATTEST-quote = [ qualifiediSigner: uint .size 2, ;TPM2B_NAME
TPMS_CLOCK_INFO,
firmwareVersion: uint .size 8
quote-responses: [ * [ pcr: uint .size 1,
+ [ pcr-value: bytes,
? hash-alg-id: uint .size 2,
],
],
? pcr-digest: bytes,
],
]
TPMS_CLOCK_INFO = [ clock: uint .size 8,
resetCounter: uint .size 4,
restartCounter: uint .size 4,
save: bool,
]
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
Darmstadt 64295
Germany
Email: henk.birkholz@sit.fraunhofer.de
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Michael Eckel
Fraunhofer SIT
Rheinstrasse 75
Darmstadt 64295
Germany
Email: michael.eckel@sit.fraunhofer.de
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