| Internet-Draft | Epoch Markers | March 2023 |
| Birkholz, et al. | Expires 14 September 2023 | [Page] |
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.¶
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/.¶
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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 [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.¶
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.¶
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].¶
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].¶
The interaction models illustrated in this section are derived from the RATS Reference Interaction Models. In general, there are three interaction models:¶
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)
$tagged-epoch-id /= #6.26985(stateless-nonce)
This memo comes with a set of predefined payloads.¶
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.¶
cbor-epoch-id = [
etime
? nonce
]
etime = #6.1001({* (int/tstr) => any})
nonce = tstr / bstr / int
¶
The following describes each member of the cbor-epoch-id map.¶
An extended time value as defined by [I-D.ietf-cbor-time-tag].¶
A never-repeating byte string used as extra data in challenge-response interaction models (see [I-D.ietf-rats-reference-interaction-models]).¶
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.¶
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}
GeneralNameAttr = ( GeneralNameType : int, GeneralNameValue : any )
GeneralName = [ + GeneralNameAttr ]
¶
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 [I-D.ietf-cbor-time-tag], indicated by CBOR tag 1001, profiled as follows:¶
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].¶
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¶
The following describes the multi-nonce type.¶
A list of nonces send to multiple consumer. 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 ]¶
The following describes the multi-nonce type.¶
A sequence of never-repeating byte strings used by RATS roles in 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 nonce in the list is used in a consecutive production of a conceptual messages. Asserting freshness of a conceptual message including a nonce from the multi-nonce-list requires some state on the receiver side to assess, if that nonce is the appropriate next unused nonce from the multi-nonce-list.¶
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 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.¶
In a highly available service (e.g., a cloud attestation verifier) having to keep per-session nonce state poses scalablity problems. An alternative is to use time-synchronised servers that share a symmetric key, which produce and consume nonces based on coarse-grained clock ticks that are signed using the shared secret. This way, a nonce minted by a server in the pool can be processed by any other server in pool, which avoids the need for session "stickiness."¶
A stateless-nonce supports the above use case by encoding a Posix time (i.e., the epoch identifier), alongside a minimal set of metadata, authenticated with a symmetric key in a self-contained and compact token.¶
stateless-nonce = [ TimeToken AuthTag: bstr .size 32 ] TimeToken = ( Version: bytes .size 1 KeyID: bytes .size 1 Timestamp: posix-time Pad: bytes ) posix-time = #6.1(int)¶
The following describes each member of the stateless-nonce array:¶
version of the TimeToken encoded as a single byte. The value MUST be 0x01.¶
opaque identifier shared across the server pool for the signing key used to compute AuthTag. It is semantically equivalent to the TID field defined in Section 3.1.3 of [RFC6896].¶
the timestamp associated with the current epoch encoded as CBOR tag for Posix time. It MUST use the int format.¶
zero or more pad bytes, used to make the stateless nonce the desired size.¶
HMAC [RFC2104] w/ SHA-256 computed over the CBOR serialisation of TimeToken encoded as a 32-bytes string.¶
RFC Editor: please replace RFCthis with the RFC number of this RFC and remove this note.¶
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 /
]
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].¶
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 }
¶