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Versions: (draft-birkholz-reference-ra-interaction-model) 00

TBD                                                          H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Informational                                  M. Eckel
Expires: September 13, 2019                                       Huawei
                                                          March 12, 2019


    Reference Interaction Model for Challenge-Response-based Remote
                              Attestation
           draft-birkholz-rats-reference-interaction-model-00

Abstract

   This document defines an interaction model for a basic remote
   attestation procedure.  Additionally, the required information
   elements are illustrated.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 13, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements notation . . . . . . . . . . . . . . . . . .   2
   2.  Disambiguation  . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Component Roles . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Prerequisites . . . . . . . . . . . . . . . . . . . . . . . .   3
   6.  Remote Attestation Interaction Model  . . . . . . . . . . . .   4
     6.1.  Information Elements  . . . . . . . . . . . . . . . . . .   4
   7.  Interaction Model . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Further Context . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Confidentiality . . . . . . . . . . . . . . . . . . . . .   6
     8.2.  Mutual Authentication . . . . . . . . . . . . . . . . . .   6
     8.3.  Hardware-Enforcement/Support  . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   11. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     12.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Remote attestation procedures (RATS) are a combination of activities,
   in which a Verifier creates assertions about claims of integrity and
   about the characteristics of other system entities by the appraisal
   of corresponding signed claims (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 RFC
   2119, BCP 14 [RFC2119].

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



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   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

   Identity:  An Attester must have a unique Identity in such a way that
      a Verifier can uniquely identify an Attester.  This Identity MUST
      be part of the signed claims (attestation evidence) that the
      Attester conveys to the Verifier.

   Secret:  A Secret that is present on the Attester and that a Verifier
      can identify by its Secret ID, e.g. a public key.  This Secret
      MUST be established before a remote attestation procedure can take
      place.  How this Secret is established is out of scope for this
      reference model.





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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 scheme.

6.1.  Information Elements

   Nonce:  mandatory

      The Nonce (number used once) is a number intended to be unique and
      intended to be effectively infeasible to guess.  In this reference
      interaction model it MUST be provided by the Verifier and MUST be
      used as a proof of freshness, with respect to conveyed evidence
      ensuring that the result of an attestation activity was created
      recently (i. e. triggered by the challenge emitted by the
      Verifier).  As such, the Nonce MUST be part of the signed evidence
      sent by the Attester to the Verifier.

   Secret ID:  mandatory

      An identifier that MUST be associated with the Secret which is
      used to sign the evidence.

   Evidence:  mandatory

      The signed claims that are required to enable integrity proving of
      the corresponding characteristics of the Attester.  Examples of
      claims included in attestation evidence are claims about sensor
      data, policies that are active on the system entity, versions of
      composite firmware of a platform, running software, routing
      tables, or information about a local time source.  Attestation
      evidence must be cryptographically bound to the Verifier-provided
      Nonce, the Identity of the Attester, as well as to the Secret
      identified by the Secret ID.

   Claim Selection:  optional

      An Attester MAY provide a selection of claims that are relevant to
      the appraisal conducted by the Verifier in order to prove
      correctness of the (integrity) claims created by the Attester.
      Usually, all available claims that are available to the Attester
      SHOULD be conveyed.  This claim selection can be composed as
      complementary signed claims or can be encapsulated claims in the
      signed evidence that composes the evidence about integrity.  This
      information element is optional in order to enable a Verifier to
      narrow down or increase the amount of received evidence.  An



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      Attester MAY decide whether or not to provide the requested claims
      or not.  In either case, the claim selection MUST be
      cryptographically bound to the signed claim set.  An example for a
      claim selection is that a Verifier can request from an Attester
      (signed) Reference Integrity Measurements (RIMs), which represent
      a claim about the intended platform operational state of the
      Attester.

   Identity:  mandatory

      A statement about a distinguishable Attester made by an entity
      without accompanying evidence of its validity, used as proof of
      identity.

7.  Interaction Model

   The following sequence diagram illustrates the reference remote
   attestation procedure defined by this document.

   [Attester]                                                [Verifier]
       |                                                          |
       | <--- requestAttestation(nonce, secretID, claimSelection) |
       |                                                          |
    collectClaims(claimSelection)                                 |
       | ===> claims                                              |
       |                                                          |
    signEvidence(claims, secretID, nonce, identity)               |
       | ===> evidence, signature                                 |
       |                                                          |
       | evidence, signature, identity -------------------------> |
       |                                                          |
       |                 appraise(evidence, signature, identity, nonce)
       |                                     appraisalResult <=== |
       |                                                          |


   The remote attestation procedure is initiated by the Verifier,
   sending an attestation request to the Attester.  The attestation
   request consists of a Nonce, a Secret ID, and a Claim Selection.  The
   Nonce guarantees attestation freshness.  The Secret ID selects the
   secret the Attester is requested to sign the Evidence with.  The
   Claim Selection narrows down or increases the amount of received
   Evidence, if required.  If the Claim Selection is empty, then by
   default all claims that are available on the system of the Attester
   SHOULD be signed and returned as Evidence.  For example, the Verifier
   is only interested in particular information about the Attester, such
   as whether the device booted up in a known state, and not include
   information about all currently running software.



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   The Attester, after receiving the attestation request, collects the
   corresponding claims to compose the evidence the Verifier requested--
   or, in case 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.  After that, the Attester signs the evidence with the
   secret identified by the secret ID, including the nonce and the
   identity information.  Then the Attester sends the output back to the
   Verifier.  Important at this point is that the nonce as well as the
   identity information must be cryptographically bound to the
   signature, i. e. it is not required for them to be present in plain
   text.  For instance, those information can be part of the signature
   after a one-way function (e. g. a hash function) was applied to them.
   There is also a possibility to scramble the nonce or 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 as well as the Verifier.  This extra
   information can be used to scramble the Nonce in order to counter
   certain types of relay attacks.  As soon as the Verifier receives the
   evidence, it appraises it, including the verification of the
   signature, the identity, the nonce, and the claims included in the
   evidence.  This process is application-specific and can be done by e.
   g. comparing the claims to known (good), expected reference claims,
   such as Reference Integrity Measurements (RIMs), or evaluating it in
   other ways.  The final output, the appraisal result (also referred to
   as attestation result), is a new claim about properties of the
   Attester, i. e. whether or not it is compliant to policies, or even
   can be "trusted".

8.  Further Context

   Depending on the use cases to cover there may be additional
   requirements.

8.1.  Confidentiality

   Use confidential communication to exchange attestation information.
   This requirement usually is present when communication happens over
   insecure channels, such as the public Internet.  Speaking of a
   suitable communication protocol, TLS is a good candidate.  In private
   networks, such as carrier management networks, it must be evaluated
   whether or not the transport medium is considered confidential.

8.2.  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




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   Attester instead of only the Attester proving its identity to the
   Verifier.

8.3.  Hardware-Enforcement/Support

   In particular use cases hardware support can be desirable.  Depending
   on the requirements those can be secure storage of cryptographic
   keys, crypto accelerators, or protected or isolated execution
   environments.  Well-known technologies are Hardware Security Modules
   (HSM), Physical Unclonable Functions (PUFs), Shielded Secrets,
   Trusted Executions Environments (TEEs), etc.

9.  Security Considerations

   There are always some.

10.  Acknowledgements

   Very likely.

11.  Change Log

   Initial draft -00

   Changes from version 00 to version 01:

   Added details to the flow diagram

12.  References

12.1.  Normative References

   [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>.

12.2.  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-00 (work in progress), October 2018.







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Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   Darmstadt  64295
   Germany

   Email: henk.birkholz@sit.fraunhofer.de


   Michael Eckel
   Huawei Technologies
   Feldbergstrasse 78
   Darmstadt  64293
   Germany

   Email: michael.eckel@huawei.com

































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