Epoch Markers
draft-birkholz-rats-epoch-markers-02
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| Authors | Henk Birkholz , Thomas Fossati , Wei Pan , Carsten Bormann | ||
| Last updated | 2022-10-24 | ||
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draft-birkholz-rats-epoch-markers-02
RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track T. Fossati
Expires: 27 April 2023 Arm Limited
W. Pan
Huawei Technologies
C. Bormann
Universität Bremen TZI
24 October 2022
Epoch Markers
draft-birkholz-rats-epoch-markers-02
Abstract
This document defines Epoch Markers as a way to establish a notion of
freshness among actors in a distributed system. Epoch Markers are
similar to "time ticks" and are produced and distributed by a
dedicated system, the Epoch Bell. Systems that receive Epoch Markers
do not have to track freshness using their own understanding of time
(e.g., via a local real-time clock). Instead, the reception of a
certain Epoch Marker establishes a new epoch that is shared between
all recipients.
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-birkholz-rats-epoch-markers/.
Discussion of this document takes place on the rats Working Group
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Source for this draft and an issue tracker can be found at
https://github.com/ietf-rats/draft-birkholz-rats-epoch-marker.
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4
2. Epoch IDs . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Interaction Models . . . . . . . . . . . . . . . . . . . . . 4
4. Epoch Marker Structure . . . . . . . . . . . . . . . . . . . 5
4.1. Epoch Marker Payloads . . . . . . . . . . . . . . . . . . 5
4.1.1. CBOR Time Tag (etime) . . . . . . . . . . . . . . . . 5
4.1.2. Classical RFC 3161 TST Info . . . . . . . . . . . . . 6
4.1.3. CBOR-encoded RFC3161 TST Info . . . . . . . . . . . . 6
4.1.4. Multi-Nonce . . . . . . . . . . . . . . . . . . . . . 7
4.1.5. Multi-Nonce-List . . . . . . . . . . . . . . . . . . 8
4.1.6. Strictly Monotonically Increasing Counter . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 10
A.1. RFC 3161 TSTInfo . . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Systems that need to interact securely often require a shared
understanding of the freshness of conveyed information. This is
certainly the case in the domain of remote attestation procedures.
In general, securely establishing a shared notion of freshness of the
exchanged information among entities in a distributed system is not a
simple task.
The entire Appendix A of [I-D.ietf-rats-architecture] deals solely
with the topic of freshness, which is in itself an indication of how
relevant, and complex, it is to establish a trusted and shared
understanding of freshness in a RATS system.
This document defines Epoch Markers as a way to establish a notion of
freshness among actors in a distributed system. Epoch Markers are
similar to "time ticks" and are produced and distributed by a
dedicated system, the Epoch Bell. Systems that receive Epoch Markers
do not have to track freshness using their own understanding of time
(e.g., via a local real-time clock). Instead, the reception of a
certain Epoch Marker establishes a new epoch that is shared between
all recipients. In essence, the emissions and corresponding
receptions of Epoch Markers are like the ticks of a clock where the
ticks are conveyed by the Internet.
In general (barring highly symmetrical topologies), epoch ticking
incurs differential latency due to the non-uniform distribution of
receivers with respect to the Epoch Bell. This introduces skew that
needs to be taken into consideration when Epoch Markers are used.
While all Epoch Markers share the same core property of behaving like
clock ticks in a shared domain, various "epoch id" types are defined
to accommodate different use cases and diverse kinds of Epoch Bells.
While Epoch Markers are encoded in CBOR [STD94], and many of the
epoch id types are themselves encoded in CBOR, a prominent format in
this space is the Time-Stamp Token defined by [RFC3161], a DER-
encoded TSTInfo value wrapped in a CMS envelope [RFC5652]. Time-
Stamp Tokens (TST) are produced by Time-Stamp Authorities (TSA) and
exchanged via the Time-Stamp Protocol (TSP). At the time of writing,
TSAs are the most common providers of secure time-stamping services.
Therefore, reusing the core TSTInfo structure as an epoch id type for
Epoch Markers is instrumental for enabling smooth migration paths and
promote interoperability. There are, however, several other ways to
represent a signed timestamp, and therefore other kinds of payloads
that can be used to implement Epoch Markers.
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To inform the design, this document discusses a number of interaction
models in which Epoch Markers are expected to be exchanged. The top-
level structure of Epoch Markers and an initial set of epoch id types
are specified using CDDL [RFC8610]. To increase trustworthiness in
the Epoch Bell, Epoch Markers also provide the option to include a
"veracity proof" in the form of attestation evidence, attestation
results, or SCITT receipts [I-D.birkholz-scitt-receipts] associated
with the trust status of the Epoch Bell.
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.
In this document, CDDL [RFC8610] is used to describe the data
formats. The examples in Appendix A use CBOR diagnostic notation as
defined in Section 8 of [STD94] and Appendix G of [RFC8610].
2. Epoch IDs
The RATS architecture introduces the concept of Epoch IDs that mark
certain events during remote attestation procedures ranging from
simple handshakes to rather complex interactions including elaborate
freshness proofs. The Epoch Markers defined in this document are a
solution that includes the lessons learned from TSAs, the concept of
Epoch IDs defined in the RATS architecture, and provides several
means to identify a new freshness epoch. Some of these methods are
introduced and discussed in Section 10.3 of the RATS architecture
[I-D.ietf-rats-architecture].
3. Interaction Models
The interaction models illustrated in this section are derived from
the RATS Reference Interaction Models. In general, there are three
interaction models:
* ad-hoc requests (e.g., via challenge-response requests addressed
at Epoch Bells), corresponding to Section 7.1 in
[I-D.ietf-rats-reference-interaction-models]
* unsolicited distribution (e.g., via uni-directional methods, such
as broad- or multicasting from Epoch Bells), corresponding to
Section 7.2 in [I-D.ietf-rats-reference-interaction-models]
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* solicited distribution (e.g., via a subscription to Epoch Bells),
corresponding to Section 7.3 in
[I-D.ietf-rats-reference-interaction-models]
4. Epoch Marker Structure
At the top level, an Epoch Marker is a CBOR array with a header
carrying an optional veracity proof about the Epoch Bell and a
payload.
epoch-marker = [
$tagged-epoch-id
? bell-veracity-proof
]
; veracity of the bell
bell-veracity-proof = non-empty<{
? remote-attestation-evidence ; could be EAT or Concise Evidence
? remote-attestation-result ; hopefully EAT with AR4SI Claims
? scitt-receipt ; SCITT receipt
}>
remote-attestation-evidence = (1: "PLEASE DEFINE")
remote-attestation-result = (2: "PLEASE DEFINE")
scitt-receipt = (3: "PLEASE DEFINE")
; epoch-id types independent of interaction model
$tagged-epoch-id /= cbor-epoch-id
$tagged-epoch-id /= #6.26980(classical-rfc3161-TST-info)
$tagged-epoch-id /= #6.26981(TST-info-based-on-CBOR-time-tag)
$tagged-epoch-id /= #6.26982(multi-nonce)
$tagged-epoch-id /= #6.26983(multi-nonce-list)
$tagged-epoch-id /= #6.26984(strictly-monotonic-counter)
Figure 1: Epoch Marker definition
4.1. Epoch Marker Payloads
This memo comes with a set of predefined payloads.
4.1.1. CBOR Time Tag (etime)
CBOR extended time tag (1001) optionally bundled with a nonce.
See Section 3 of [I-D.ietf-cbor-time-tag] for the (many) details
about the CBOR extended time format.
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cbor-epoch-id = [
etime
? nonce
]
etime = #6.1001({* (int/tstr) => any})
nonce = tstr / bstr / int
4.1.2. Classical RFC 3161 TST Info
DER-encoded [X.690] TSTInfo [RFC3161]. See Appendix A.1 for the
layout.
classical-rfc3161-TST-info = bytes
4.1.3. CBOR-encoded RFC3161 TST Info
Semantically equivalent to classical RFC3161 TSTInfo rewritten using
the CBOR type system.
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TST-info-based-on-CBOR-time-tag = {
&(version : 0) => int .default 1 ; obsolete?
&(policy : 1) => oid
&(messageImprint : 2) => MessageImprint
&(serialNumber : 3) => int
&(eTime : 4) => profiled-etime
&(ordering : 5) => bool .default false
? &(nonce : 6) => int
? &(tsa : 7) => GeneralName
* $$TSTInfoExtensions
}
oid = #6.110(bstr) / #6.111(bstr) / #6.112(bstr)
; based on COSE_Hash_Find (draft-ietf-cose-hash-algs)
MessageImprint = [
hashAlg : int
hashValue : bstr
]
; timeMap profiles etime, see:
; https://datatracker.ietf.org/doc/html/draft-ietf-cbor-time-tag
profiled-etime = #6.1001(timeMap)
timeMap = {
1 => #6.1(int / float) ; TIME_T
-8 => profiled-duration ; guarantee aka RFC 3161 accuracy
* int => any
}
profiled-duration = #6.1002({* int => any})
; Section 11.8 of I-D.ietf-cose-cbor-encoded-cert
GeneralName = [ GeneralNameType : int, GeneralNameValue : any ]
4.1.4. Multi-Nonce
Typically, a nonce is a number only used once. In the context of
Epoch Markers, one Nonce can be distributed to multiple consumers,
each of them using that Nonce only once. Technically, that is not a
Nonce anymore. This type of Nonce is called Multi-Nonce in Epoch
Markers.
; Multi-Nonce
; a single nonce used by multiple entities
multi-nonce = tstr / bstr / int
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4.1.5. Multi-Nonce-List
A list of nonces send to multiple consumers. The consumers use each
Nonce in the list of Nonces sequentially. Technically, each
sequential Nonce in the distributed list is not used just once, but
by every Epoch Marker consumer involved. This renders each Nonce in
the list a Multi-Nonce
; Multi-Nonce-List
; a list of nonces potentially used by multiple entities
multi-nonce-list = [ + multi-nonce ]
4.1.6. Strictly Monotonically Increasing Counter
A strictly monotonically increasing counter.
The counter context is defined by the Epoch bell.
strictly-monotonic-counter = uint
5. Security Considerations
Epoch Markers are signed using [STD96] and inheritance of security
considerations will be addressed here.
6. IANA Considerations
TODO
7. References
7.1. Normative References
[I-D.ietf-cbor-time-tag]
Bormann, C., Gamari, B., and H. Birkholz, "Concise Binary
Object Representation (CBOR) Tags for Time, Duration, and
Period", Work in Progress, Internet-Draft, draft-ietf-
cbor-time-tag-02, 4 October 2022,
<https://www.ietf.org/archive/id/draft-ietf-cbor-time-tag-
02.txt>.
[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>.
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[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
2001, <https://www.rfc-editor.org/info/rfc3161>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
[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>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[STD94] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[STD96] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/info/rfc9052>.
[X.690] International Telecommunications Union, "Information
technology -- ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", ITU-T Recommendation
X.690, August 2015, <https://www.itu.int/rec/T-REC-X.690>.
7.2. Informative References
[I-D.birkholz-scitt-receipts]
Birkholz, H., Riechert, M., Delignat-Lavaud, A., and C.
Fournet, "Countersigning COSE Envelopes in Transparency
Services", Work in Progress, Internet-Draft, draft-
birkholz-scitt-receipts-01, 5 September 2022,
<https://www.ietf.org/archive/id/draft-birkholz-scitt-
receipts-01.txt>.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture", Work
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in Progress, Internet-Draft, draft-ietf-rats-architecture-
22, 28 September 2022, <https://www.ietf.org/archive/id/
draft-ietf-rats-architecture-22.txt>.
[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-06, 7 September 2022,
<https://www.ietf.org/archive/id/draft-ietf-rats-
reference-interaction-models-06.txt>.
Appendix A. Examples
The example in Figure 2 shows an epoch marker with a cbor-epoch-id
and no bell veracity proof.
[
/ cbor-epoch-id / [
/ 1996-12-19T16:39:57-08:00[America//Los_Angeles][u-ca=hebrew] /
/ etime / 1001({
1: 851042397,
-10: "America/Los_Angeles",
-11: { "u-ca": "hebrew" }
})
]
/ no bell veracity proof /
]
Figure 2: CBOR epoch id without bell veracity proof
A.1. RFC 3161 TSTInfo
As a reference for the definition of TST-info-based-on-CBOR-time-tag
the code block below depects the original layout of the TSTInfo
structure from [RFC3161].
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TSTInfo ::= SEQUENCE {
version INTEGER { v1(1) },
policy TSAPolicyId,
messageImprint MessageImprint,
-- MUST have the same value as the similar field in
-- TimeStampReq
serialNumber INTEGER,
-- Time-Stamping users MUST be ready to accommodate integers
-- up to 160 bits.
genTime GeneralizedTime,
accuracy Accuracy OPTIONAL,
ordering BOOLEAN DEFAULT FALSE,
nonce INTEGER OPTIONAL,
-- MUST be present if the similar field was present
-- in TimeStampReq. In that case it MUST have the same value.
tsa [0] GeneralName OPTIONAL,
extensions [1] IMPLICIT Extensions OPTIONAL }
Acknowledgements
TBD
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
Thomas Fossati
Arm Limited
United Kingdom
Email: Thomas.Fossati@arm.com
Wei Pan
Huawei Technologies
Email: william.panwei@huawei.com
Carsten Bormann
Universität Bremen TZI
Bibliothekstr. 1
D-28359 Bremen
Germany
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Phone: +49-421-218-63921
Email: cabo@tzi.org
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