Internet-Draft Epoch Markers March 2023
Birkholz, et al. Expires 15 September 2023 [Page]
RATS Working Group
Intended Status:
Standards Track
H. Birkholz
Fraunhofer SIT
T. Fossati
Arm Limited
W. Pan
Huawei Technologies
C. Bormann
Universität Bremen TZI

Epoch Markers


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.

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Table of Contents

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.

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:

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 = [
  ? 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)
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.

cbor-epoch-id = [
   ? 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]).

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

The following describes the classical-rfc3161-TST-info type.


The response message of a Time Stamp Authority including a [RFC3161] Time Stamp Token Info structure.

4.1.3. CBOR-encoded RFC3161 TST Info

Issue tracked at:

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 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 [I-D.ietf-cbor-time-tag], 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].

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

The following describes the multi-nonce type.


A never-repeating byte string used by RATS roles in a trust domain as extra data included in the production of conceptual messages as specified by the RATS architecture [RFC9334] to associate them with a certain epoch.

4.1.5. Multi-Nonce-List

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.

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

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.

4.1.7. Stateless Nonce

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 = [
  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.

5. Security Considerations


6. IANA Considerations

RFC Editor: please replace RFCthis with the RFC number of this RFC and remove this note.

6.1. New CBOR Tags

IANA is requested to allocate the following tags in the "CBOR Tags" registry [IANA.cbor-tags], preferably with the specific CBOR tag value requested:

Table 1: New CBOR Tags
Tag Data Item Semantics Reference
26980 bytes DER-encoded RFC3161 TSTInfo Section 4.1.2 of RFCthis
26981 map CBOR-encoding of RFC3161 TSTInfo semantics Section 4.1.3 of RFCthis
26982 tstr / bstr / int a nonce that is shared among many participants but that can only be used once by each participant Section 4.1.4 of RFCthis
26983 array a list of multi-nonce Section 4.1.5 of RFCthis
26984 uint strictly monotonically increasing counter Section 4.1.6 of RFCthis
26985 array stateless nonce Section 4.1.7 of RFCthis

7. References

7.1. Normative References

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-05, , <>.
Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and M. Furuhed, "CBOR Encoded X.509 Certificates (C509 Certificates)", Work in Progress, Internet-Draft, draft-ietf-cose-cbor-encoded-cert-05, , <>.
IANA, "Concise Binary Object Representation (CBOR) Tags", <>.
Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
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, , <>.
Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
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, , <>.
Schaad, J., "CBOR Object Signing and Encryption (COSE): Hash Algorithms", RFC 9054, DOI 10.17487/RFC9054, , <>.
Bormann, C., "Concise Binary Object Representation (CBOR) Tags for Object Identifiers", RFC 9090, DOI 10.17487/RFC9090, , <>.
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and W. Pan, "Remote ATtestation procedureS (RATS) Architecture", RFC 9334, DOI 10.17487/RFC9334, , <>.
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <>.
Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <>.
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, , <>.

7.2. Informative References

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-02, , <>.
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, , <>.
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-07, , <>.
Barbato, S., Dorigotti, S., and T. Fossati, Ed., "SCS: KoanLogic's Secure Cookie Sessions for HTTP", RFC 6896, DOI 10.17487/RFC6896, , <>.

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].

   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  }



Authors' Addresses

Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Thomas Fossati
Arm Limited
United Kingdom
Wei Pan
Huawei Technologies
Carsten Bormann
Universität Bremen TZI
Bibliothekstr. 1
D-28359 Bremen