Internet DRAFT - draft-birkholz-rats-reference-interaction-model

draft-birkholz-rats-reference-interaction-model







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.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 9, 2020.

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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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