Direct Anonymous Attestation for the Remote Attestation Procedures Architecture
draft-ietf-rats-daa-05
| Document | Type | Active Internet-Draft (rats WG) | |
|---|---|---|---|
| Authors | Henk Birkholz , Christopher Newton , Liqun Chen , Dave Thaler | ||
| Last updated | 2024-03-04 | ||
| Replaces | draft-birkholz-rats-daa | ||
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draft-ietf-rats-daa-05
RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Informational C. Newton
Expires: 5 September 2024 L. Chen
University of Surrey
D. Thaler
Microsoft
4 March 2024
Direct Anonymous Attestation for the Remote Attestation Procedures
Architecture
draft-ietf-rats-daa-05
Abstract
This document maps the concept of Direct Anonymous Attestation (DAA)
to the Remote Attestation Procedures (RATS) Architecture. The
protocol entity DAA Issuer is introduced and its mapping with
existing RATS roles in DAA protocol steps is specified.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-rats-daa/.
Discussion of this document takes place on the Remote ATtestation
ProcedureS (rats) Working Group mailing list (mailto:rats@ietf.org),
which is archived at https://mailarchive.ietf.org/arch/browse/rats/.
Subscribe at https://www.ietf.org/mailman/listinfo/rats/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-rats-wg/draft-ietf-rats-daa.
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This Internet-Draft will expire on 5 September 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Direct Anonymous Attestation . . . . . . . . . . . . . . . . 3
4. DAA changes to the RATS Architecture . . . . . . . . . . . . 6
5. Additions to Remote Attestation principles . . . . . . . . . 7
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Implementation Considerations . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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
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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,
the protocol entity DAA Issuer as described in [DAA] is introduced
and its duties as well as its mappings with other RATS roles are
specified.
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, Endorser, Conceptual Message, Evidence, Attestation Result,
Attesting Environment.
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
[RFC5280].
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.
3. Direct Anonymous Attestation
Two protocols as described in [DAA] are illustrated: the Join
Protocol and the DAA-Signing Protocol. This section specifies the
mapping of the protocol entity DAA Issuer described in [DAA] as an
actor in the Join Protocol as well as an actor in the corresponding
DAA-Signing Protocol to roles specified in the RATS Architecture.
In the Join Protocol, the protocol entity DAA Issuer takes on the
RATS roles of Verifier and associated Relying Party. The mapping is
illustrated in Figure Figure 1.
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.--------. .---------. .--------. .-------------.
| Endorser | | Reference | | Verifier | | Relying Party |
'-+------' | Value | | Owner | | Owner |
| | Provider | '---+----' '----+--------'
| '-----+---' | |
| | | |
| Endorsements | Reference | Appraisal | Appraisal
| | Values | Policy for | Policy for
| | | Evidence | Attestation
'-----------. | | | Results
| | | |
.----|----|---------------|-----------------|------.
| | | | | |
| v v v | |
| .-------------------------. | |
.------->| Verifier +-----. | |
| | '-------------------------' | | |
| | | | |
| Evidence Attestation | | |
| | Results | | |
| | | | |
| | v v |
.-----+----. | .---------------. |
| Attester | | | Relying Party | |
'----------' | DAA Issuer '---------------' |
'--------------------------------------------------'
Figure 1: RATS Architecture for the Join Protocol
The Join Protocol is essentially an enrollment protocol that consumes
Evidence from the Attester (therefore the mapping to the Verifier
role). Corresponding Appraisal Policies for Evidence specific to the
Join Protocol are used to produce Attestation Results to decide
whether to issue a DAA credential to an Attester or not (therefore
the mapping to the Relying Party role).
In the DAA-Signing Protocol, the RATS role Endorser is then taken on
by the DAA Issuer protocol entity. The mapping is illustrated in
Figure Figure 2.
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.---------------.
| DAA Issuer |
| .--------. | .---------. .--------. .-------------.
| | Endorser | | | Reference | | Verifier | | Relying Party |
| '-+------' | | Value | | Owner | | Owner |
| | | | Provider | '---+----' '----+--------'
'-----|---------' '-----+---' | |
| | | |
| Endorsements | Reference | Appraisal | Appraisal
| | Values | Policy for | Policy for
| | | Evidence | Attestation
'-----------. | | | Results
| | | |
v v v |
.-------------------------. |
.------>| Verifier +-----. |
| '-------------------------' | |
| | |
| Evidence Attestation | |
| Results | |
| | |
| v v
.-----+----. .---------------.
| Attester | | Relying Party |
'----------' '---------------'
Figure 2: RATS Architecture for the DAA-Signing Protocol
The DAA Issuer acts as the Endorser for the Group Public Key that is
used by the Verifier for the appraisal of evidence of anonymized
Attesters that use the DAA credentials and associated key material to
produce Evidence.
In consequence, DAA provides 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 it 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
in this group. The DAA Issuer's group public key certificate
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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 it now
provides Endorsements to the DAA Issuer rather than directly to the
Verifier. These Endorsements 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, randomization ensures
that the Attester's identity cannot be revealed to anyone,
including the DAA 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 randomized DAA credential.
The following information elements define extensions for
corresponding information elements defined in
[I-D.ietf-rats-reference-interaction-models], which 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
randomized 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 randomized each time it used.
6. Privacy Considerations
As outlined above, for DAA to provide privacy for the Attester, the
DAA group must be large enough to stop the Verifier identifying the
Attester.
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
uniquely identify the Attester. If the Attestation Evidence is
unique to an Attester other cryptographic techniques can be used, for
example, property based attestation [PBA].
7. Security Considerations
The anonymity property of DAA makes revocation difficult. Well known
solutions include:
1. Rogue Attester revocation -- if 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 considerations for DAA over and
above those specified in the RATS architecture document
[I-D.ietf-rats-architecture].
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.
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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.
9. References
9.1. Normative References
[DAA] Brickell, E., Camenisch, J., and L. Chen, "Direct
anonymous attestation", DOI 10.1145/1030083.1030103,
Proceedings of the 11th ACM conference on Computer and
communications security, October 2004,
<https://doi.org/10.1145/1030083.1030103>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://doi.org/10.17487/RFC2119>.
[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,
<https://doi.org/10.17487/RFC5280>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://doi.org/10.17487/RFC8174>.
9.2. Informative References
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote ATtestation procedureS (RATS)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-rats-architecture-22, 28 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
architecture-22>.
[I-D.ietf-rats-reference-interaction-models]
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-08, 10 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
reference-interaction-models-08>.
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[PBA] Chen, L., Löhr, H., Manulis, M., and A. Sadeghi,
"Property-Based Attestation without a Trusted Third
Party", DOI 10.1007/978-3-540-85886-7_3, Lecture Notes in
Computer Science pp. 31-46, September 2008,
<https://doi.org/10.1007/978-3-540-85886-7_3>.
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@ietf.contact
Christopher Newton
University of Surrey
Email: cn0016@surrey.ac.uk
Liqun Chen
University of Surrey
Email: liqun.chen@surrey.ac.uk
Dave Thaler
Microsoft
United States of America
Email: dthaler@microsoft.com
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