Oblivious Credential State
draft-steele-spice-oblivious-credential-state-00
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Orie Steele | ||
| Last updated | 2024-01-13 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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draft-steele-spice-oblivious-credential-state-00
Secure Patterns for Internet CrEdentials O. Steele
Internet-Draft Transmute
Intended status: Informational 13 January 2024
Expires: 16 July 2024
Oblivious Credential State
draft-steele-spice-oblivious-credential-state-00
Abstract
Issuers of Digital Credentials enable dynamic state or status checks
through the use of dereferenceable identifiers, that resolve to
resources providing herd privacy. Privacy in such systems is
determined not just from the size of the herd, and the cryptographic
structure encoding it, but also from the observability of access to
shared state. This document describes a privacy preserving state
management system for digital credentials based on Oblivious HTTP
that addresses both data model and protocol risks associated with
digital credentials with dynamic state.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://OR13.github.io/draft-steele-spice-oblivious-credential-state/
draft-steele-spice-oblivious-credential-state.html. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-steele-spice-oblivious-
credential-state/.
Discussion of this document takes place on the Secure Patterns for
Internet CrEdentials Working Group mailing list
(mailto:spice@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/spice/. Subscribe at
https://www.ietf.org/mailman/listinfo/spice/.
Source for this draft and an issue tracker can be found at
https://github.com/OR13/draft-steele-spice-oblivious-credential-
state.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 16 July 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Credential State . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Identifier . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Issuer's Credential State Resource . . . . . . . . . 4
2.1.2. Mediator's Credential State Resource . . . . . . . . 5
2.2. Resources . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Processing Credential State Resources . . . . . . . . . . 5
2.4. Techniques . . . . . . . . . . . . . . . . . . . . . . . 6
2.4.1. CRL Distribution Points . . . . . . . . . . . . . . . 6
2.4.2. Online Certificate Status Protocol . . . . . . . . . 6
2.4.3. Bitmaps . . . . . . . . . . . . . . . . . . . . . . . 6
2.4.4. Cryptographic Accumulators . . . . . . . . . . . . . 7
2.4.5. Bloom Filters . . . . . . . . . . . . . . . . . . . . 7
2.4.6. Transparency Services . . . . . . . . . . . . . . . . 7
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 8
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Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Digitial Credentials often have a validity period, which indicates
the time at which the claims become active for the subject according
to the issuer, and the time at which the issuer specifies the claims
are no longer to be considered asserted by the issuer.
A typical example is a digital drivers license, which has an
activation date, and an expiration date.
When the activation date is in the past, and the expiration date is
in the future, we consider the license to be valid at the current
time.
A verifier might wonder if such licenses are suspended or revoked,
even if the validity period is acceptable.
A common solution is to the issuer of the credential to provide a
resource that reflects information about the state of the credential
over time.
Because issuer's track the presentation of digital credentials if a
verifier where to ask the issuer about the state of a specific
digital credential, it common to see credential states be merged into
blocks, or herds, where an issuer can deliver the block to the
verifier upon request, without learning which specific digital
credential the verifier is interested in.
Unfortunatly, the metadata associated with resolving credential state
can leak time and location information about the presentation of
credentials over time.
This document addresses this risk by introducing a mediator which is
trusted by the verifier.
2. Credential State
To simplify interpretation of resolution of credential state
resources, this document uses the following aliases for the terms
defined in [RFC9458].
The Verifier's Software is the Oblivious HTTP Client.
The Mediator's Credential State Resource is the Oblivious Relay
Resource.
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The Issuer's Gateway Resource is the Gateway Resource.
The Issuer's Credential State Resource is the Target Resource.
The critical privacy property is obtained by the verifier's client
relying on the Mediator's Credential State Resource instead of
Issuer's Credential State Resource.
In order to achieve this property, while preserving the property that
issuer's do not know which verifier's will be interested in dynamic
state information associated with their credentials, the issuer
includes the identifier for their Issuer's Credential State Resource
in their credential claim sets, however the verifier rewrites this
URL to be their Mediator's Credential State Resource before
resolution.
Editor note: is there a simpler solution here?
.--------. +----------+ .-------.
| Verifier +<------>+ Mediator +<------>+ Issuer |
'--------' +----------+ '-------'
2.1. Identifier
While many different protocol schemes can be used to identify
resources, to improve interoperability and reduce attack surface,
this document requires credential state resources to be identified
with https URLS, as described in [WHATWG.URL].
The following URI Templates, as described in [RFC6570] are required
to improve interoperability and reduce the chances of degrading the
privacy properties through the inclusion of extraneos information in
the identifiers embedded in credentials.
* issuer MUST support internationalizaton considerations, as
described in [WHATWG.URL], for example: 🏛️.example
* mediator MUST support internationalizaton considerations, as
described in [WHATWG.URL], for example: 🚛.example
* resource-name MUST be a URN as described in [RFC4122], for
example: urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6
2.1.1. Issuer's Credential State Resource
https://{issuer}\
/credential-states/{resource-name}
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https://🏛️.example\
/credential-states/urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6
Figure 1: Issuer Credential State Resources
2.1.2. Mediator's Credential State Resource
https://{issuer}.{mediator}\
/credential-states/{resource-name}
https://🏛️.example.🚛.example\
/credential-states/urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6
Figure 2: Mediator Credential State Resources
2.2. Resources
Credential state resources typically rely on a similar content type
as the credentials that require them.
Mixing different content types for credentials and their state,
increases implementation costs and harms interoperability.
The credential state resource MUST be secured with the same content
type that was used to secure the digital credential that has dynamic
state.
For example, a JSON Web Token, [RFC7519] based digital credential
must rely on a [RFC7519] based credential state resource.
There are many different content types that can be used to secure
digital credentials, this document does not require a specific
content type to be used.
The Accept header MUST be supported, and the application/cose content
type SHOULD be supported.
2.3. Processing Credential State Resources
Validation of the digital credential state MUST occur after
verification.
Validation of the digital credential validity period MUST occur
before credential state checks.
Implementers are cautioned that concepts like "suspended" or
"revoked" are interpretted differently and used differently by
issuers.
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All dynamic claims provided through credential state resources MUST
be considered issuer defined, and cannot be interpretted globally.
Interpretting the structure of the Issuer's Credential State Resource
is outside the scope of this document.
However, other documents describe this process in detail.
[W3C.VC-BITSTRING-STATUS-LIST] provides guidance on processing
resources that secure the content type application/vc+ld+json, such
as application/cose.
[I-D.draft-ietf-oauth-status-list] provides guidance on processing
resources of the content type application/cwt, and application/jwt.
2.4. Techniques
2.4.1. CRL Distribution Points
[RFC5755] described a mechanism for verifiers to check the revocation
status of attribute certificates.
2.4.2. Online Certificate Status Protocol
[RFC2560] described a protocol useful in determining the current
status of a digital certificate without requiring CRLs.
2.4.3. Bitmaps
In this approach, the size of the herd is the length of the bitmap,
and the state of a digital credential claim is the value of the bit
at a given index.
Scaling this approach can be difficult, as a seperate list is needed
for each dynamic claim in a digital credential.
This scalling challenge can be partially addressed by consuming
multiple bits at a given index, however, the resulting enumeration
needs to be consistently understood.
A common solution to consistent interpretation of enumerations is the
establishment of a registry, however this can become impractical
depending on the nature of the issuer's need to express dynamic
state.
Publishing a dictionary per issuer, or per sets of issuer's can help
address these challenges for some use cases.
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2.4.4. Cryptographic Accumulators
[ZKA] describes an approach to expressing proofs of set membership.
2.4.5. Bloom Filters
Appendix B.2.7 of [RFC8932] mentions an application of bloom filters,
that can be applied to communicating credential state assuming the
probabilistic nature of bloom filters is acceptable to the verifier.
2.4.6. Transparency Services
Tree structures, such as described in
[I-D.draft-mcmillion-keytrans-architecture] can be used to provide
advanced membership proofs, such as proving inclusion, consistency,
non inclusion, and freshness.
3. Terminology
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.
4. Security Considerations
TODO Security
5. IANA Considerations
This document has no IANA actions.
6. References
6.1. 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/rfc/rfc2119>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/rfc/rfc4122>.
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[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/rfc/rfc6570>.
[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/rfc/rfc8174>.
[RFC9458] Thomson, M. and C. A. Wood, "Oblivious HTTP", RFC 9458,
DOI 10.17487/RFC9458, January 2024,
<https://www.rfc-editor.org/rfc/rfc9458>.
[WHATWG.URL]
"URL - Living Standard", n.d.,
<https://url.spec.whatwg.org/>.
6.2. Informative References
[I-D.draft-ietf-oauth-status-list]
Looker, T., Bastian, P., and C. Bormann, "OAuth Status
List", Work in Progress, Internet-Draft, draft-ietf-oauth-
status-list-00, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
status-list-00>.
[I-D.draft-mcmillion-keytrans-architecture]
McMillion, B., "Key Transparency Architecture", Work in
Progress, Internet-Draft, draft-mcmillion-keytrans-
architecture-01, 4 December 2023,
<https://datatracker.ietf.org/doc/html/draft-mcmillion-
keytrans-architecture-01>.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560,
DOI 10.17487/RFC2560, June 1999,
<https://www.rfc-editor.org/rfc/rfc2560>.
[RFC5755] Farrell, S., Housley, R., and S. Turner, "An Internet
Attribute Certificate Profile for Authorization",
RFC 5755, DOI 10.17487/RFC5755, January 2010,
<https://www.rfc-editor.org/rfc/rfc5755>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/rfc/rfc7519>.
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[RFC8932] Dickinson, S., Overeinder, B., van Rijswijk-Deij, R., and
A. Mankin, "Recommendations for DNS Privacy Service
Operators", BCP 232, RFC 8932, DOI 10.17487/RFC8932,
October 2020, <https://www.rfc-editor.org/rfc/rfc8932>.
[W3C.VC-BITSTRING-STATUS-LIST]
"Bitstring Status List v1.0", n.d.,
<https://www.w3.org/TR/vc-bitstring-status-list/>.
[ZKA] "Zero-Knowledge Accumulators and Set Operations", n.d.,
<https://eprint.iacr.org/2015/404.pdf>.
Acknowledgments
TODO acknowledge.
Author's Address
Orie Steele
Transmute
Email: orie@transmute.industries
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