RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Informational                                 C. Newton
Expires: 5 June 2022                                             L. Chen
                                                    University of Surrey
                                                               D. Thaler
                                                         2 December 2021

   Direct Anonymous Attestation for the Remote Attestation Procedures


   This document maps the concept of Direct Anonymous Attestation (DAA)
   to the Remote Attestation Procedures (RATS) Architecture.  The role
   DAA Issuer is introduced and its interactions with existing RATS
   roles is specified.

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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Direct Anonymous Attestation  . . . . . . . . . . . . . . . .   3
   4.  DAA changes to the RATS Architecture  . . . . . . . . . . . .   5
   5.  Additions to Remote Attestation principles  . . . . . . . . .   6
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Implementation Considerations . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Remote ATtestation procedureS (RATS, [I-D.ietf-rats-architecture])
   describe interactions between well-defined architectural constituents
   in support of Relying Parties that require an understanding about the
   trustworthiness of a remote peer.  The identity of an Attester and
   its corresponding Attesting Environments play a vital role in RATS.
   A common way to refer to such an identity is the Authentication
   Secret ID as defined in the Reference Interaction Models for RATS
   [I-D.ietf-rats-reference-interaction-models].  The fact that every
   Attesting Environment can be uniquely identified in the context of
   the RATS architecture is not suitable for every application of remote
   attestation.  Additional issues may arise when Personally
   identifiable information (PII) -- whether obfuscated or in clear text
   -- are included in attestation Evidence or even corresponding
   Attestation Results.  This document illustrates how Direct Anonymous
   Attestation (DAA) can mitigate the issue of uniquely
   (re-)identifiable Attesting Environments.  To accomplish that goal, a
   new RATS role -- the DAA Issuer -- is introduced and its duties as
   well as its interactions with other RATS roles are specified.

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2.  Terminology

   This document uses the following set of terms, roles, and concepts as
   defined in [I-D.ietf-rats-architecture]: Attester, Verifier, Relying
   Party, Conceptual Message, Evidence, Attestation Result, Attesting
   Environment.  The role of Endorser, also defined in
   [I-D.ietf-rats-architecture], needs to be adapted and details are
   given below.

   Additionally, this document uses and adapts, as necessary, the
   following concepts and information elements as defined in
   [I-D.ietf-rats-reference-interaction-models]: Attester Identity,
   Authentication Secret, Authentication Secret ID

   A PKIX Certificate is an X.509v3 format certificate as specified by

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  Direct Anonymous Attestation

   Figure 1 shows the data flows between the different RATS roles
   involved in DAA.

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     ************       *************   ************   *****************
     * Endorser *       * Reference *   * Verifier *   * Relying Party *
     ************       * Value     *   *  Owner   *   *  Owner        *
          |             * Provider  *   ************   *****************
          |             *************          |            |
          |                       |            |            |
          |Endorsements           |Reference   |Appraisal   |Appraisal
          |                       |Values      |Policy      |Policy for
          |                       |            |for         |Attestation
          |                       |            |Evidence    |Results
          V                       |            |            |
   .-----------------.            |            |            |
   |   DAA Issuer    |---------.  |            |            |
   '-----------------'         |  |            |            |
     ^          |         Group|  |            |            |
     |          |        Public|  |            |            |
     |Credential|           Key|  |            |            |
     |Request   |              v  v            v            |
     |          |             .----------------------.      |
     |          |          .->|      Verifier        |--.   |
     |          |          |  '----------------------'  |   |
     |          |          |                            |   |
     |          |          |Evidence         Attestation|   |
     |          |          |                     Results|   |
     |          |          |                            |   |
     |          |Credential|                            |   |
     |          |          |                            |   |
     |          v          |                            v   v
     |        .-------------.                     .---------------.
     '--------|  Attester   |                     | Relying Party |
              '-------------'                     '---------------'

                          Figure 1: DAA data flows

   DAA [DAA] is a signature scheme that allows the privacy of users that
   are associated with an Attester (e.g. its owner) to be maintained.
   Essentially, DAA can be seen as a group signature scheme with the
   feature that given a DAA signature no-one can find out who the signer
   is, i.e., the anonymity is not revocable.  To be able to sign
   anonymously, an Attester has to obtain a credential from a DAA
   Issuer.  The DAA Issuer uses a private/public key pair to generate
   credentials for a group of Attesters and makes the public key (in the
   form of a public key certificate) available to the verifier in order
   to enable them to validate the Evidence received.

   In order to support these DAA signatures, the DAA Issuer MUST
   associate a single key pair with a group of Attesters and use the
   same key pair when creating the credentials for all of the Attesters

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   in this group.  The DAA Issuer's group public key certificate
   replaces the individual Attester Identity documents during
   authenticity validation as a part of the appraisal of Evidence
   conducted by a verifier.  This is in contrast to intuition that there
   has to be a unique Attester Identity per device.

   For DAA, the role of the Endorser is essentially the same, but they
   now provide Attester endorsement documents to the DAA Issuer rather
   than directly to the verifier.  These Attester endorsement documents
   enable the Attester to obtain a credential from the DAA Issuer.

4.  DAA changes to the RATS Architecture

   In order to enable the use of DAA, a new conceptual message, the
   Credential Request, is defined and a new role, the DAA Issuer role,
   is added to the roles defined in the RATS Architecture.

   Credential Request:  An Attester sends a Credential Request to the
      DAA Issuer to obtain a credential.  This request contains
      information about the DAA key that the Attester will use to create
      evidence and together with Attester endorsement information that
      is provided by the Endorser to confirm that the request came from
      a valid Attester.

   DAA Issuer:  A RATS role that offers zero-knowledge proofs based on
      public-key certificates used for a group of Attesters (Group
      Public Keys) [DAA].  How this group of Attesters is defined is not
      specified here, but the group must be large enough for the
      necessary anonymity to be assured.

   Effectively, these certificates share the semantics of Endorsements,
   with the following exceptions:

   *  Upon receiving a Credential Request from an Attester, the
      associated group private key is used by the DAA Issuer to provide
      the Attester with a credential that it can use to convince the
      Verifier that its Evidence is valid.  To keep their anonymity the
      Attester randomizes this credential each time that it is used.
      Although the DAA Issuer knows the Attester Identity and can
      associate this with the credential issued, randomisation ensures
      that the Attester's identity cannot be revealed to anyone,
      including the Issuer.

   *  The Verifier can use the DAA Issuer's group public key
      certificate, together with the randomized credential from the
      Attester, to confirm that the Evidence comes from a valid Attester
      without revealing the Attester's identity.

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   *  A credential is conveyed from a DAA Issuer to an Attester in
      combination with the conveyance of the group public key
      certificate from DAA Issuer to Verifier.

5.  Additions to Remote Attestation principles

   In order to ensure an appropriate conveyance of Evidence via
   interaction models in general, the following prerequisite considering
   Attester Identity MUST be in place to support the implementation of
   interaction models.

   Attestation Evidence Authenticity:  Attestation Evidence MUST be
      correct and authentic.

      In order to provide proofs of authenticity, Attestation Evidence
      SHOULD be cryptographically associated with an identity document
      that is a randomised DAA credential.

   The following information elements define extensions for
   corresponding information elements defined in
   [I-D.ietf-rats-reference-interaction-models] and that are vital to
   all types of reference interaction models.  Varying from solution to
   solution, generic information elements can be either included in the
   scope of protocol messages (instantiating Conceptual Messages defined
   by the RATS architecture) or can be included in additional protocol
   parameters of protocols that facilitate the conveyance of RATS
   Conceptual Messages.  Ultimately, the following information elements
   are required by any kind of scalable remote attestation procedure
   using DAA with one of RATS's reference interaction models.

   Attester Identity ('attesterIdentity'):  _mandatory_

      In DAA, the Attester's identity is not revealed to the verifier.
      The Attester is issued with a credential by the DAA Issuer that is
      randomised and then used to anonymously confirm the validity of
      their evidence.  The evidence is verified using the DAA Issuer's
      group public key.

   Authentication Secret IDs ('authSecID'):  _mandatory_

      In DAA, Authentication Secret IDs are represented by the DAA
      Issuer's group public key that MUST be used to create DAA
      credentials for the corresponding Authentication Secrets used to
      protect Evidence.

      In DAA, an Authentication Secret ID does not identify a unique

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      Attesting Environment but is associated with a group of Attesting
      Environments.  This is because an Attesting Environment should not
      be distinguishable and the DAA credential which represents the
      Attesting Environment is randomised each time it used.

6.  Privacy Considerations

   As outlined about for DAA to provide privacy for the Attester the DAA
   group must be large enough to stop the Verifier identifying the

   Randomization of the DAA credential by the Attester means that
   collusion between the DAA Issuer and Verifier, will not give them any
   advantage when trying to identify the Attester.

   For DAA, the Attestation Evidence conveyed to the Verifier MUST not
   uniqely identify the Attester.  If the Attestation Evidence is unique
   to an Attester other cryptographic techniques can be used, for
   example, property based attestation.  (Henk -- reference follows)

   Chen L., Loehr H., Manulis M., Sadeghi AR. (2008) Property-Based
   Attestation without a Trusted Third Party.  Information Security.
   ISC 2008.  Lecture Notes in Computer Science, vol 5222.  Springer.

7.  Security Considerations

   The anonymity property of DAA makes revocation difficult.  Well known
   solutions include: 1.  Rogue attester revocation -- if the an
   Attester's private key is compromised and known by the Verifier then
   any DAA signature from that Attester can be revoked. 2.  EPID -
   Intel's Enhanced Privacy ID -- this requires the Attester to prove
   (as part of their Attestation) that their credential was not used to
   generate any signature in a signature revocation list.

   There are no other special security conderations for DAA over and
   above those specifed in the RATS architecture document

8.  Implementation Considerations

   The new DAA Issuer role can be implemented in a number of ways, for
   example: 1.  As a stand-alone service like a Certificate Authority, a
   Privacy CA. 2.  As a part of the Attester's manufacture.  The
   Endorser and the DAA Issuer could be the same entity and the
   manufacturer would then provide a certificate for the group public
   key to the Verifier.

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9.  IANA Considerations

   We don't yet.

10.  References

10.1.  Normative References

   [DAA]      Brickell, E., Camenisch, J., and L. Chen, "Direct
              Anonymous Attestation", page 132-145, ACM Proceedings of
              the 11rd ACM conference on Computer and Communications
              Security, 2004.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [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>.

10.2.  Informative References

              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture", Work
              in Progress, Internet-Draft, draft-ietf-rats-architecture-
              13, 8 November 2021, <https://www.ietf.org/archive/id/

              Birkholz, H., Eckel, M., Pan, W., and E. Voit, "Reference
              Interaction Models for Remote Attestation Procedures",
              Work in Progress, Internet-Draft, draft-ietf-rats-
              reference-interaction-models-04, 26 July 2021,

Authors' Addresses

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   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt

   Email: henk.birkholz@sit.fraunhofer.de

   Christopher Newton
   University of Surrey

   Email: cn0016@surrey.ac.uk

   Liqun Chen
   University of Surrey

   Email: liqun.chen@surrey.ac.uk

   Dave Thaler
   United States of America

   Email: dthaler@microsoft.com

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