RATS Working Group H. Birkholz
Internet-Draft M. Eckel
Intended status: Standards Track Fraunhofer SIT
Expires: July 10, 2020 January 07, 2020
Reference Interaction Models for Remote Attestation Procedures
draft-birkholz-rats-reference-interaction-model-02
Abstract
This document defines interaction models for basic remote attestation
procedures. Different methods of conveying attestation evidence
securely are defined and illustrated. Analogously, the required
information elements used by conveyance protocols are defined and
illustrated.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Disambiguation . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Normative Prerequisites . . . . . . . . . . . . . . . . . . . 3
5. Remote Attestation Interaction Model . . . . . . . . . . . . 4
5.1. Information Elements . . . . . . . . . . . . . . . . . . 4
5.2. Interaction Model . . . . . . . . . . . . . . . . . . . . 6
6. Further Context . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Confidentiality . . . . . . . . . . . . . . . . . . . . . 7
6.2. Mutual Authentication . . . . . . . . . . . . . . . . . . 8
6.3. Hardware-Enforcement/Support . . . . . . . . . . . . . . 8
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 8
7.1. Implementer . . . . . . . . . . . . . . . . . . . . . . . 8
7.2. Implementation Name . . . . . . . . . . . . . . . . . . . 9
7.3. Implementation URL . . . . . . . . . . . . . . . . . . . 9
7.4. Maturity . . . . . . . . . . . . . . . . . . . . . . . . 9
7.5. Coverage and Version Compatibility . . . . . . . . . . . 9
7.6. License . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.7. Implementation Dependencies . . . . . . . . . . . . . . . 9
7.8. Contact . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Security and Privacy Considerations . . . . . . . . . . . . . 9
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. CDDL Specification for a simple CoAP
Challenge/Response Interaction . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Remote ATtestation procedureS [I-D.ietf-rats-architecture] are
workflows composed of roles and interactions, in which a Verifier
creates assessments based on evidence about the trustworthiness of an
Attester's system component characteristics. The roles _Attester_
and _Verifier_, as well as the message _Evidence_ are terms defined
by the RATS Architecture. The goal of this document is to enable the
design and adoption of secure conveyance methods for attestation
evidence from an Attester to a Verifier.
This document defines three [note: pub/sub & time-based are still
missing] reference interaction models that describe the conveyance of
evidence between Attester and Verifier in order to provide the basis
for reliable and believable appraisal of evidence by a Verifier.
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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.
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 entity in the scope of
network topologies or the Internet. It rather refers to a decoupled
system or different Types of Environments
[I-D.ietf-rats-architecture], which also can be present locally as
separate system components of a composite device (in a single RATS
Entity). Examples include: a Trusted Execution Environment (TEE),
Baseboard Management Controllers (BMCs), as well as other physical or
logical protected/isolated/shielded Computing Environments.
3. Scope
This document focuses on generic interaction models between Verifiers
and Attesters. Complementary procedures, duties and functions that
are required for a complete semantic binding of RATS are not in
scope. Examples include: identity establishment, key distribution
and enrollment, as well as certificate revocation.
Furthermore, any processes and duties that go beyond carrying out
remote attestation procedures are out-of-scope. For instance, using
the results of a remote attestation that are created by the Verifier,
e.g., triggering remediation actions or recovery processes, as well
as the remediation actions and recovery processes themselves, is also
out-of-scope.
The definition of Reference Interaction Models for RATS uses the role
definitions of Attester and Verifier as defined in
[I-D.ietf-rats-architecture].
4. Normative Prerequisites
Attester Identity: The provenance of Attestation Evidence with
respect to a distinguishable Attesting Environment MUST be correct
and unambiguous.
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An Attester Identity MAY be a unique identity, or it MAY be
included in a zero-knowledge proof (ZKP), or it MAY be part of a
group signature.
Attestation Evidence: Attestation Evidence MUST be a set of well-
formatted and well-protected Claims that an Attester can create
and convey to a Verifier.
Attestation Evidence Authenticity: Attestation Evidence MUST be
correct and authentic.
Attestation Evidence, in order to provide proof of authenticity,
SHOULD be cryptographically associated with an identity document
(e.g. an X.509 certificate), or SHOULD include a correct and
unambiguous reference to an accessible identity document.
Authentication Secret: An Authentication Secret MUST be available
exclusively to an Attester's Attesting Environment. The Attester
MUST sign Claims with that Authentication Secret, thereby proving
the authenticity of the Claims included in the signed Attestation
Evidence. The Authentication Secret MUST be established before
RATS can take place. How it is established is out-of-scope for
this document.
5. 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.
5.1. 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
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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.
Claims ('claims'): _mandatory_
Claims are assertions that represent characteristics of an
Attester. Claims compose attestation evidence and are, for
example, used to appraise the integrity of an Attester. Examples
are Claims about sensor data, policies that are active on the
entity, versions of composite firmware of a platform, running
software, routing tables, or information about a local time
source.
Reference Claims ('refClaims') _mandatory_
Reference Claims are a specific subset of Appraisal Policies as
defined in [I-D.ietf-rats-architecture]. Reference Claims are
used to appraise the Claims received from an Attester via
appraisal by direct comparison. For example, Reference Claims MAY
be Reference Integrity Measurements (RIMs) or assertions that are
implicitly trusted because they are signed by a trusted authority
(see Endorsements in [I-D.ietf-rats-architecture]). RIMs
represent (trusted) Claim sets about an Attester's intended
platform operational state.
Claim Selection ('claimSelection'): _optional_
An Attester MAY provide a selection of Claims in order to reduce
or increase retrieved assertions to those that are relevant to the
appraisal policies. Usually, all available Claims that are
available to the Attester SHOULD be conveyed. The Claim Selection
MAY be composed as complementary signed Claim sets or MAY be
encapsulated Claims in the signed Attestation Evidence. An
Attester MAY decide whether or not to provide all requested Claims
or not. An example of a Claim 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
Claims, and the Verifier-provided Nonce. Attestation Evidence
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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 the output of
the appraisal of Attestation Evidence. The Attestation Result
represents Claims about integrity and other characteristics of the
corresponding Attester.
5.2. Interaction Model
The following sequence diagram illustrates the reference remote
attestation procedure defined by this document.
[Attester] [Verifier]
| |
measureClaims(attestedEnvironment) |
| => claims |
| |
| <---------- requestEvidence(nonce, authSecID, claimSelection) |
| |
collectClaims(claimSelection) |
| => claims |
| |
signAttestationEvidence(authSecID, claims, nonce) |
| => signedAttestationEvidence |
| |
| signedAttestationEvidence ----------------------------------> |
| |
| appraiseAttestationEvidence(signedAttestationEvidence, refClaims)
| 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 a Claim
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 Claim Selection
narrows down or increases the amount of received Claims, if required.
If the Claim Selection is empty, then by default all Claims that are
available on the Attester MUST be signed and returned as Attestation
Evidence. For example, a Verifier may only be requesting a
particular subset of information about the Attester, such as evidence
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about BIOS and firmware the Attester booted up with - and not include
information about all currently running software.
The Attester, after receiving the attestation request, collects the
corresponding Claims that have been measured beforehand to compose
the Attestation Evidence that the Verifier requested. In the case
that the Verifier did not provide a Claim Selection, the Attester
collects all information that can be used as complementary Claims in
the scope of the semantics of the remote attestation procedure.
Conclusively, the Attester creates Attestation Evidence by signing
the Attester Identity, the Claims, and the Nonce with the
Authentication Secret identified by the Authentication Secret ID.
The signed Attestation Evidence is transferred back to the Verifier.
It is crucial at this point that Claims, 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 section Section 8.
As soon as the Verifier receives the signed Attestation Evidence, it
verifies the signature, the Attester Identity, the Nonce, and
appraises the Claims. This procedure is application-specific and can
be carried out by comparing the Claims with corresponding Reference
Claims, e.g., Reference Integrity Measurements (RIMs), or using other
appraisal policies. The final output of the Verifier are Attestation
Results. Attestation Results constitute new Claims about an
Attester's properties and characteristics that enables relying
parties, for example, to assess an Attester's trustworthiness.
6. Further Context
Depending on the use cases covered, there can be additional
requirements. An exemplary subset is illustrated in this section.
6.1. 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.
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6.2. 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.
6.3. 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
roots of trusts, such as Hardware Security Modules (HSM), Physically
Unclonable Functions (PUFs), Shielded Secrets, or Trusted Executions
Environments (TEEs).
7. Implementation Status
Note to RFC Editor: Please remove this section as well as references
to [BCP205] before AUTH48.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [BCP205].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [BCP205], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
7.1. Implementer
The open-source implementation was initiated and is maintained by the
Fraunhofer Institute for Secure Information Technology - SIT.
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7.2. Implementation Name
The open-source implementation is named "CHAllenge-Response based
Remote Attestation" or in short: CHARRA.
7.3. Implementation URL
The open-source implementation project resource can be located via:
https://github.com/Fraunhofer-SIT/charra
7.4. Maturity
The code's level of maturity is considered to be "prototype".
7.5. Coverage and Version Compatibility
The current version (commit '847bcde') is aligned with the exemplary
specification of the CoAP FETCH bodies defined in section Appendix A
of this document.
7.6. License
The CHARRA project and all corresponding code and data maintained on
github are provided under the BSD 3-Clause "New" or "Revised"
license.
7.7. Implementation Dependencies
The implementation requires the use of the official Trusted Computing
Group (TCG) open-source Trusted Software Stack (TSS) for the Trusted
Platform Module (TPM) 2.0. The corresponding code and data is also
maintained on github and the project resources can be located via:
https://github.com/tpm2-software/tpm2-tss/
The implementation uses the Constrained Application Protocol
[RFC7252] (http://coap.technology/) and the Concise Binary Object
Representation [RFC7049] (https://cbor.io/).
7.8. Contact
Michael Eckel (michael.eckel@sit.fraunhofer.de)
8. Security and Privacy Considerations
In a remote attestation procedure 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).
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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
Olaf Bergmann and Ned Smith
10. Change Log
o Initial draft -00
o Changes from version 00 to version 01:
* Added details to the flow diagram
* Integrated comments from Ned Smith (Intel)
* Reorganized sections and
* Updated interaction model
* Replaced "claims" with "assertions"
* Added proof-of-concept CDDL for CBOR via CoAP based on a TPM
2.0 quote operation
o Changes from version 01 to version 02:
* Revised the relabeling of "claims" with "assertion" in
alignment with the RATS Architecture I-D.
* Added Implementation Status section
* Updated interaction model
* Text revisions based on changes in [I-D.ietf-rats-architecture]
and comments provided on rats@ietf.org.
11. References
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11.1. Normative References
[BCP205] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[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>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[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>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
11.2. Informative References
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., and N. Smith,
"Remote Attestation Procedures Architecture", draft-ietf-
rats-architecture-00 (work in progress), December 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.
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In this example, the root-of-trust for reporting primitive operation
"quote" is provided by a TPM 2.0.
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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