| Internet-Draft | Epoch Markers | April 2025 |
| Birkholz, et al. | Expires 13 October 2025 | [Page] |
This document defines Epoch Markers as a means 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 known as the Epoch Bell. Systems receiving Epoch Markers do not need to track freshness using their own understanding of time (e.g., via a local real-time clock). Instead, the reception of a specific Epoch Marker establishes a new epoch that is shared among all recipients. This document defines Epoch Marker types, including CBOR time tags, RFC 3161 TimeStampToken, nonce-like structures, and a CWT Claim to embed Epoch Markers in RFC 8392 CBOR Web Tokens, which serve as vehicles for signed protocol messages.¶
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-epoch-markers/.¶
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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.¶
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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 [RFC9334] 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 distributed systems. Epoch Markers are similar to "time ticks" and are produced and distributed by a dedicated system, the Epoch Bell. Actors in a system 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, with these ticks being conveyed over 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" values are defined as Epoch Marker types in this document to accommodate different use cases and diverse kinds of Epoch Bells.¶
While most Epoch Markers types 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 TimeStampToken (TST) defined by [RFC3161], a DER-encoded TSTInfo value wrapped in a CMS envelope [RFC5652]. TSTs 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 or the start of a new freshness epoch, respectively, and therefore other Epoch Marker types.¶
To inform the design, this document discusses a number of interaction models in which Epoch Markers are expected to be exchanged.
The default top-level structure of Epoch Markers described in this document is CBOR Web Tokens (CWT) [RFC8392].
The present document specifies an extensible set of Epoch Marker types, along with the em CWT claim to include them in CWTs.
CWTs are signed using COSE [STD96] and benefit from wide tool support.
However, CWTs are not the only containers in which Epoch Markers can be embedded.
Epoch Markers can be included in any type of message that allows for the embedding of opaque bytes or CBOR data items.
Examples include the Collection CMW in [I-D.ietf-lamps-csr-attestation], Evidence formats such as [TCG-CoEvidence] or [I-D.ietf-rats-eat], [I-D.ietf-rats-ar4si], or the CWT Claims Header Parameter of [I-D.ietf-scitt-architecture].¶
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 the CBOR Extended Diagnostic Notation (EDN, [I-D.ietf-cbor-edn-literals]).¶
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 [RFC9334] (the RATS architecture).¶
The interaction models illustrated in this section are derived from the RATS Reference Interaction Models [I-D.ietf-rats-reference-interaction-models]. In general, there are three major interaction models used in remote attestation:¶
ad-hoc requests (e.g., via challenge-response requests addressed at Epoch Bells), corresponding to Section 7.1 of [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 of [I-D.ietf-rats-reference-interaction-models]¶
solicited distribution (e.g., via a subscription to Epoch Bells), corresponding to Section 7.3 of [I-D.ietf-rats-reference-interaction-models]¶
In all three interaction models, Epoch Markers can be used as content for the generic information element handle as introduced by [I-D.ietf-rats-reference-interaction-models].
Handles are used to establish freshness in ad-hoc, unsolicited, and solicited distribution mechanisms of an Epoch Bell.
For example, an Epoch Marker can be used as a nonce in challenge-response remote attestation (e.g., for limiting the number of ad-hoc requests by a Verifier).
If embedded in a CWT, an Epoch Marker can be used as a handle by extracting the value of the em Claim or by using the complete CWT including an em Claim (e.g., functioning as a signed time-stamp token).
Using an Epoch Marker requires the challenger to acquire an Epoch Marker beforehand, which may introduce a sensible overhead compared to using a simple nonce.¶
Epoch Markers are tagged CBOR data items.
As a default, Epoch Markers are transported via the em Claim in CWTs.
In cases of challenge-response interactions that employ a nonce to show recentness, the em Claim can be paired with a Nonce Claim to bind the nonce with the Epoch Marker as a response message in an ad-hoc request.
This in fact means that it is possible to request an Epoch Marker via a challenge-response interaction using a nonce to than use the received CWT or the Epoch Marker included as a different nonce in a separate RATS reference interaction model.¶
epoch-marker = $tagged-epoch-id ; epoch-id types independent of interaction model $tagged-epoch-id /= cbor-time $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(epoch-tick) $tagged-epoch-id /= #6.26983(epoch-tick-list) $tagged-epoch-id /= #6.26984(strictly-monotonic-counter)
$$Claims-Set-Claims //= (&(em: 2000) => epoch-marker)
This specification comes with a set of predefined Epoch Marker types.¶
DER-encoded [X.690] TSTInfo [RFC3161]. See Appendix A.1 for the layout.¶
classical-rfc3161-TST-info = bytes¶
The following describes the classical-rfc3161-TST-info type.¶
The DER-encoded TSTInfo generated by a [RFC3161] Time Stamping Authority.¶
The Epoch Bell MUST use the following value as MessageImprint in its request to the TSA:¶
SEQUENCE {
SEQUENCE {
OBJECT 2.16.840.1.101.3.4.2.1 (sha256)
NULL
}
OCTET STRING
BF4EE9143EF2329B1B778974AAD445064940B9CAE373C9E35A7B23361282698F
}
¶
This is the sha-256 hash of the string "EPOCH_BELL".¶
The TimeStampToken obtained by the TSA MUST be stripped of the TSA signature. Only the TSTInfo is to be kept the rest MUST be discarded. The Epoch Bell COSE signature will replace the TSA signature.¶
Issue tracked at: https://github.com/ietf-rats/draft-birkholz-rats-epoch-marker/issues/18¶
The TST-info-based-on-CBOR-time-tag is semantically equivalent to classical [RFC3161] TSTInfo, rewritten using the CBOR type system.¶
TST-info-based-on-CBOR-time-tag = {
&(version : 0) => v1
&(policy : 1) => oid
&(messageImprint : 2) => MessageImprint
&(serialNumber : 3) => integer
&(eTime : 4) => profiled-etime
? &(ordering : 5) => bool .default false
? &(nonce : 6) => integer
? &(tsa : 7) => GeneralName
* $$TSTInfoExtensions
}
v1 = 1
oid = #6.111(bstr) / #6.112(bstr)
MessageImprint = [
hashAlg : int
hashValue : bstr
]
profiled-etime = #6.1001(timeMap)
timeMap = {
1 => ~time
? -8 => profiled-duration
* int => any
}
profiled-duration = {* int => any}
GeneralName = [ GeneralNameType : int, GeneralNameValue : any ]
; See Section 4.2.1.6 of RFC 5280 for type/value
¶
The following describes each member of the TST-info-based-on-CBOR-time-tag map.¶
The integer value 1. Cf. version, Section 2.4.2 of [RFC3161].¶
A [RFC9090] object identifier tag (111 or 112) representing the TSA's policy under which the tst-info was produced. Cf. policy, Section 2.4.2 of [RFC3161].¶
A [RFC9054] COSE_Hash_Find array carrying the hash algorithm identifier and the hash value of the time-stamped datum. Cf. messageImprint, Section 2.4.2 of [RFC3161].¶
A unique integer value assigned by the TSA to each issued tst-info. Cf. serialNumber, Section 2.4.2 of [RFC3161].¶
The time at which the tst-info has been created by the TSA. Cf. genTime, Section 2.4.2 of [RFC3161]. Encoded as extended time [RFC9581], indicated by CBOR tag 1001, profiled as follows:¶
The "base time" is encoded using key 1, indicating Posix time as int or float.¶
The stated "accuracy" is encoded using key -8, which indicates the maximum allowed deviation from the value indicated by "base time". The duration map is profiled to disallow string keys. This is an optional field.¶
The map MAY also contain one or more integer keys, which may encode supplementary information Allowing unsigned integer (i.e., critical) keys goes counter interoperability.¶
boolean indicating whether tst-info issued by the TSA can be ordered solely based on the "base time". This is an optional field, whose default value is "false". Cf. ordering, Section 2.4.2 of [RFC3161].¶
int value echoing the nonce supplied by the requestor. Cf. nonce, Section 2.4.2 of [RFC3161].¶
a single-entry GeneralNames array Section 11.8 of [I-D.ietf-cose-cbor-encoded-cert] providing a hint in identifying the name of the TSA. Cf. tsa, Section 2.4.2 of [RFC3161].¶
A CDDL socket (Section 3.9 of [RFC8610]) to allow extensibility of the data format. Note that any extensions appearing here MUST match an extension in the corresponding request. Cf. extensions, Section 2.4.2 of [RFC3161].¶
An Epoch Tick is a single opaque blob sent to multiple consumers.¶
; Epoch-Tick epoch-tick = tstr / bstr / int¶
The following describes the epoch-tick type.¶
Either a string, a byte string, or an integer used by RATS roles within a trust domain as extra data (handle) included in conceptual messages [RFC9334] to associate them with a certain epoch, similar to a nonce.
Technically, an Epoch Tick is not used just once (like a nonce), but by every Epoch Marker consumer involved.¶
The emitter MUST follow the requirements in Section 4.3.¶
A list of Epoch Ticks send to multiple consumers. The consumers use each Epoch Tick in the list of sequentially, similar to a list of nonces. Technically, each sequential Epoch Tick in the distributed list is not used just once (like a nonce), but by every Epoch Marker consumer involved.¶
; Epoch-Tick-List epoch-tick-list = [ + epoch-tick ]¶
The following describes the Epoch Tick List type.¶
A sequence of byte strings used by RATS roles in trust domain as extra data (handle) in the generation of conceptual messages as specified by the RATS architecture [RFC9334] to associate them with a certain epoch.
Each Epoch Tick in the list is used in a consecutive generation of a conceptual message.
Asserting freshness of a conceptual message including an Epoch Tick from the epoch-tick-list requires some state on the receiver side to assess if that Epoch Tick is the appropriate next unused Epoch Tick from the epoch-tick-list.¶
The emitter MUST follow the requirements in Section 4.3.¶
A strictly monotonically increasing counter.¶
The counter context is defined by the Epoch bell.¶
strictly-monotonic-counter = uint¶
The following describes the strictly-monotonic-counter type.¶
An unsigned integer used by RATS roles in a trust domain as extra data in the production of conceptual messages as specified by the RATS architecture [RFC9334] to associate them with a certain epoch. Each new strictly-monotonic-counter value must be higher than the last one.¶
Time MUST be sourced from a trusted clock.¶
A nonce value used in a protocol or message to retrieve an Epoch Marker MUST be freshly generated. The generated value MUST have at least 64 bits of entropy (before encoding). The generated value MUST be generated via a cryptographically secure random number generator.¶
A maximum nonce size of 512 bits is set to limit the memory requirements. All receivers MUST be able to accommodate the maximum size.¶
RFC Editor: please replace RFCthis with the RFC number of this RFC and remove this note.¶
This specification adds the following value to the "CBOR Web Token Claims" registry [IANA.cwt].¶
The example in Figure 3 shows an Epoch Marker with an etime as the Epoch Marker type.¶
/ epoch-marker for
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" }
})
The encoded data item in CBOR pretty-printed form (hex with comments) is shown in Figure 4.¶
d9 03e9 # tag(1001)
a3 # map(3)
01 # unsigned(1)
1a 32b9e05d # unsigned(851042397)
29 # negative(9)
73 # text(19)
416d65726963612f4c6f735f416e67656c6573 # "America/Los_Angeles"
2a # negative(10)
a1 # map(1)
64 # text(4)
752d6361 # "u-ca"
66 # text(6)
686562726577 # "hebrew"
As a reference for the definition of TST-info-based-on-CBOR-time-tag the code block below depicts the original layout of the TSTInfo structure from [RFC3161].¶
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 }
¶