Fraunhofer SIT

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@sit.fraunhofer.de

Arm Limited

UK Thomas.Fossati@arm.com

Huawei Technologies

william.panwei@huawei.com

Universität Bremen TZI

Bibliothekstr. 1 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org

Security RATS Working Group Internet-Draft Abstract Text About This Document Status information for this document may be found at . Discussion of this document takes place on the rats Working Group mailing list (), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction Systems that are required to interact via secure interactions often require a shared understanding of the freshness of conveyed information, especially in the domain of remote attestation procedures. Establishing a notion of freshness between various involved entities taking on roles that rely on information that is not outdated is not simple. In general, establishing a shared understanding of freshness in a secure manner is not simple. The RATS architecture dedicates an extensive appendix solely on the topic of freshness considerations and that fact alone should be considered a telltale sign on how necessary yet complex establishing a trusted and shared understanding of freshness between systems actually is. This document provides a prominent way to establish a notion of freshness between systems: Epoch Markers. Epoch Markers are messages that are like time ticks produced and conveyed by a system in a freshness domain: the Epoch Bell. Systems that receive Epoch Markers do not have to track freshness with their own local understanding of time (e.g., a local real time clock). Instead, each reception of a specific Epoch Marker rings in a new age of freshness 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. The layout of the freshness domain in which Epoch Markers are conveyed like the ticks of a clock, introduces a domain-specific latency -- and therefore a certain uncertainty about tick accuracy. While all Epoch Markers share the common characteristic of being like clock ticks in a freshness domain, there are various payload types that can make up the content of an Epoch Marker. These different types of Epoch Marker payloads address several specific use cases and are laid out in this document. While Epoch Markers are encoded in CBOR and many of the payload types are encoded in CBOR as well, a prominent payload is the Time Stamp Token content as defined by : a DER-encoded TSTInfo value. Time Stamp Tokens (TST) produced by Time Stamp Authorities (TSA) are conveyed by the Time Stamp Protocol (TSP). At the time of writing, TSAs are the most common world-wide implemented secure timestamp token systems. Reusing the essential TST payload structure as a payload type for CBOR encoded Epoch Markers makes sense with respect to migration paths and general interoperability. But there is more than one way to represent a signed timestamp and other kinds of freshness ticks that can be used for Epoch Markers. In this document, basic interaction models on how to convey Epoch Marchers are illustrated as they impact the message design of a generic Epoch Marker. Then, the structure of Epoch Markers is specified using CDDL and the corresponding payload types are introduced and elaborated on. To increase the level of trustworthiness in the Epoch Bell and the system that produces them, Epoch Markers also provide the option to include (concise) remote attestation evidence or corresponding remote attestation results.

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 when, and only when, they appear in all capitals, as shown here.

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. Epoch Markers are a solution that includes the lessons learned from TSAs and provides several means to identify a new freshness epoch as illustrated by the RATS architecture.

Interaction Models The interaction models illustrated in this section are derived from the RATS Reference Interaction Models. In general there are three of them:

• unsolicited distribution (e.g., via uni-directional methods, such as broad- or multicasting from Epoch Bells)
• solicited distribution (e.g., via a subscription to Epoch Bells)
• ad-hoc requests (e.g., via challenge-response requests addressed at Epoch Bells)

Epoch Marker CDDL epoch-marker = [ header, $payload, ] header = { ? challenge-response-nonce, ? remote-attestation-evidence, ; could be EAT or Concise Evidence ? remote-attestation-results, ; hopefully EAT with AR4SI Claims } challenge-response-nonce = (1: "PLEASE DEFINE") remote-attestation-evidence = (2: "PLEASE DEFINE") remote-attestation-results = (3: "PLEASE DEFINE") ;payload types independent on interaction model $payload /= native-rfc3161-TST-info $payload /= TST-info-based-on-CBOR-time-tag $payload /= CBOR-time-tag $payload /= multi-nonce $payload /= multi-nonce-list $payload /= strictly-monotonically-increasing-counter native-rfc3161-TST-info = bytes ; DER-encoded value of TSTInfo TST-info-based-on-CBOR-time-tag = "PLEASE DEFINE" ; ~~~ ; ~~~ verbatim translation of ASN.1 TSTInfo into CDDL ; ~~~ (GeneralName is TODO atm, due to its terrible callousness) ; ~~~ TSTInfo = { &(version : 0) => int .default 1 &(policy : 1) => oid &(messageImprint : 2) => MessageImprint &(serialNumber : 3) => int &(genTime : 4) => GeneralizedTime ? &(accuracy : 5) => Accuracy &(ordering : 6) => bool .default false ? &(nonce : 7) => int ? &(tsa : 8) => GeneralName * $$TSTInfoExtensions } MessageImprint = [ hashAlgorithm: AlgorithmIdentifier hashedMessage: bytes ] AlgorithmIdentifier = [ algorithm: oid ? parameters: any ] Accuracy = non-empty<{ ? &(seconds : 0) => int ? &(millis: 1) => 1..999 ? &(micros: 2) => 1..999 }> ; https://datatracker.ietf.org/doc/html/rfc5280#section-4.1.2.5.2 GeneralizedTime = tstr .regexp '[0-9]{14}(\.[0-9]+)?Z' GeneralName = "todo" ; stuff oid = #6.111(bstr) non-empty<M> = (M) .and ({ + any => any }) CBOR-time-tag = [ time-tag, ? nonce ] time-tag = "PLEASE DEFINE" nonce = "PLEASE DEFINE" multi-nonce = tstr / bstr / int multi-nonce-list = [+ multi-nonce] strictly-monotonically-increasing-counter = uint ; counter context? per issuer? per indicator?

RFC 3161 TSTInfo 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 }

References Normative References This document describes the format of a request sent to a Time Stamping Authority (TSA) and of the response that is returned. It also establishes several security-relevant requirements for TSA operation, with regards to processing requests to generate responses. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Fraunhofer SIT Microsoft Sandelman Software Works Intel Corporation Huawei Technologies In network protocol exchanges it is often useful for one end of a communication to know whether the other end is in an intended operating state. This document provides an architectural overview of the entities involved that make such tests possible through the process of generating, conveying, and evaluating evidentiary claims. An attempt is made to provide for a model that is neutral toward processor architectures, the content of claims, and protocols.

Acknowledgements TBD