]> Fraunhofer Institute for Secure Information Technology

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Fraunhofer Institute for Secure Information Technology

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Security RATS Working Group Internet-Draft This document defines the method and bindings used to convey Evidence via Time-based Uni-Directional Attestation (TUDA) in Remote ATtestation procedureS (RATS). TUDA does not require a challenge-response handshake and thereby does not rely on the conveyance of a nonce to prove freshness of remote attestation Evidence. TUDA enables the creation of Secure Audit Logs that can constitute believable Evidence about both current and past operational states of an Attester. In TUDA, RATS entities require access to a Handle Distributor to which a trustable and synchronized time-source is available. The Handle Distributor takes on the role of a Time Stamp Authority (TSA) to distribute Handles incorporating Time Stamp Tokens (TST) to the RATS entities. RATS require an Attesting Environment that generates believable Evidence. While a TPM is used as the corresponding root of trust in this specification, any other type of root of trust can be used with TUDA. About This Document Status information for this document may be found at . Discussion of this document takes place on the Remote ATtestation ProcedureS (rats) Working Group mailing list (), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction Remote ATtestation procedureS (RATS) describe the attempt to determine and appraise system properties, such as integrity and trustworthiness, of a remote peer -- the Attester -- by the use of a Verifier in support of Relying Parties that intend to interact with the Attester. The Verifier carries the burden of appraisal of detailed Evidence about an Attester's trustworthiness. Evidence is generated by the Attester and consumed by the Verifier. To support security decisions, the Verifier generates digestable Attestation Results that can be easily consumed by Relying Parties. The RATS architecture specifies the corresponding concepts and terms . TUDA uses the architectural constituents of the RATS Architecture, such as the roles Attester and Verifier, and defines a method to convey Conceptual Messages between them. TUDA uses the Uni-Directional Remote Attestation interaction model described in . While the Conceptual Message focused on in this document is RATS Evidence, any type of Conceptual Message content that requires a believable indication about the message's content freshness can be conveyed with TUDA (e.g. Attestation Results). The conveyance of Evidence in RATS must ensure that Evidence always remains integrity protected, tamper-evident, originates from a trustable entity (or group of entities), and is accompanied by a proof of its freshness. In contrast to bi-directional interactions as described by Challenge/Response Remote Attestation in , TUDA enables uni-directional conveyance in the interactions between Attester and Verifier. TUDA allows a Verifier to receive Evidence from an Attester without solicitation. Conversely, it allows a Verifier to retrieve Evidence from an Attester without it being generated ad-hoc. Exemplary applications of TUDA are the creation of beacons in vehicular environments or authentication mechanisms based on EAP . The generation of Evidence in RATS requires an Attesting Environment. In this specification, the root of trust acting as an Attesting Environment is a Trusted Platform Module (TPM, see and ). The Protected Capabilities provided by a TPM support various activities in RATS, e.g., Claims collection and Evidence generation. A trusted coupling of Evidence generation with a global timescale is enabled via a Handle Distributor. Handles generated by a Handle Distributor can include nonces, signed timestamps, or other structured or opaque content used as qualifying data in Evidence generation. In TUDA, all RATS entities, such as the entities taking on the roles of Attester and Verifier, can receive signed timestamps from the Handle Distributor. These trusted timestamps replace nonces in Evidence generation and Evidence appraisal .

Forward Authenticity Nonces enable an implicit time-keeping in which the freshness of Evidence is inferred by recentness. Recentness is estimated via the time interval between sending a nonce as part of a challenge for Evidence and the reception of Evidence based on that nonce (as outlined in the interaction model depicted in section 8.1 in ). Conversely, the omission of nonces in TUDA allows for explicit time-keeping where freshness is not inferred from recentness. Instead, a cryptographic binding of a trusted synchronization to a global timescale in the Evidence itself allows for Evidence that can prove past operational states of an Attester. To capture and support this concept, this document introduces the term Forward Authenticity.

Forward Authenticity:

A property of secure communication protocols, in which later compromise of the long-term keys of a data origin does not compromise past authentication of data from that origin. Forward Authenticity is achieved by timely recording of authenticity Claims from Target Environments (via "audit logs" during "audit sessions") that are authorized for this purpose and trustworthy (via endorsed roots of trusts, for example), in a time-frame much shorter than that expected for the compromise of the long-term keys.

Forward Authenticity enables new levels of assurance and can be included in basically every protocol, such as ssh, YANG Push, router advertisements, link layer neighbor discovery, or even ICMP echo requests.

TUDA Objectives Time-Based Uni-directional Attestation is designed to:

• increase the confidence in authentication and authorization procedures,
• address the requirements of constrained-node networks,
• support interaction models that do not maintain connection-state over time, such as REST architectures ,
• be able to leverage existing management interfaces, such as SNMP (RFC 3411, ). RESTCONF or CoMI --- and corresponding bindings,
• support broadcast and multicast schemes (e.g. ),
• be able to cope with temporary loss of connectivity, and to
• provide trustworthy audit logs of past endpoint states.

Terminology This document uses the terms defined in the RATS Architecture and by the RATS Reference Interaction Models . 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.

Remote Attestation Principles Based on the RATS Architecture, the processing of TPM generated Evidence can be separated in three activities.

Evidence Generation:

The retrieval of signed digests from an RTR based on a sequence of collected Claims about software component integrity (measurements).

Evidence Conveyance:

The transfer of Evidence from the Attester to the Verifier via the Internet.

Evidence Appraisal:

The validation of Evidence signatures as well as the assessment of Claim values in Evidence by comparing them with Reference Values.

TUDA is specified in support of these RATS activities that align with the definitions presented in and .

Authenticity of Evidence Remote attestation Evidence is composed of a set of Claims (assertions about the trustworthiness of an Attester's Target Environments) that is accompanied by a proof of its veracity -- typically a signature based on shielded, private, and potentially use-restricted key material used as an Authentication Secret as specified in section 6 of (or a secure channel as illustrated in ). As key material alone is typically not self-descriptive with respect to its intended use (its semantics), the Evidence created via TUDA MUST be accompanied by two kinds of certificates that are cryptographically associated with a trust anchor (TA) via certification paths:

• an Attestation Key (AK) Certificate (AK-Cert) that represents the attestation provenance of the Attesting Environment (see section 4.2. in ) that generates Evidence, and
• an Endorsement Key (EK) Certificate (EK-Cert) that represents the Protection Capabilities of an Attesting Environment the AK is stored in.

If a Verifier decides to trust the TA of both an AK-Cert and an EK-Cert presented by an Attester -- and thereby the included Claims about the trustworthiness of an Attester's Target Environments -- the Evidence generated by the Attester can be considered trustable and believable. Ultimately, all trustable and believable Evidence MUST be appraised by a Verifier in order to assess the trustworthiness of the corresponding Attester. Assertions represented via Claims MUST NOT be considered believable by themselves. In this document, Evidence is generated via TPMs that come with an AK-Cert and a EK-Cert as a basis for believable Evidence generation.

Generating Evidence about Software Component Integrity Evidence generated by a TPM for TUDA is based on measured hash values of all software components deployed in Target Environments (see section 4.2. in ) before they are executed ("measure then execute"). The underlying concept of "Attestation Logs" is elaborated on in Section 2.4.2. of . This concept is implemented, for example, in the Linux kernel where it is called the Linux Integrity Measurement Architecture (IMA) and used to generates such a sequence of hash values. A representation for conveyance of corresponding event logs is described in the Canonical Event Log specification. Open source solutions, for example, based on use an IMA log to enable remote attestation . An Attester MUST generate such an event/measurement log.

Measurements and Digests Generated by an Attester A hash value of a software component is created before it is executed by Attesters. These hash values are typically represented as event log entries referred to as measurements, which often occur in large quantities. Capabilities such as Linux IMA can be used to generate these measurements on an Attester. Measurements are chained by Attesters using a rolling hash function. A TPM acts as a root of trust for storage (RTS) by providing an Extend (, ) operation to feed hash values in a rolling hash function. Each measurement added to the sequence of all measurements results in a new current digest hash value. A TPM acts as a root of trust for reporting (RTR) by providing Quote (, ) operations to generate a digest of all currently extended hash values as Evidence. TUDA requirements on TPM primitive operations and the information elements processed by them are illustrated using pseudocode in Appendix C and D.

Attesting Environments and Roots of Trust The primitive operations used to generate an initial set of measurements at the beginning of an Attester's boot sequence MUST be provided by a Root of Trust for Measurement (RTM) that is a system component of the Attester. An RTM MUST be trusted via trust-relationships to TAs enabled by appropriate Endorsements (e.g.,EK-Certs). If a Verifier cannot trust an RTM, measurements based on values generated by the RTM MUST be considered invalid. At least one RTM MUST be accessible to the first Attesting Environment in Attester conducting Layered Attestation (see section 4.3. in ). An RTM MAY aggregate and retain measurements until the first RTS becomes available in a Layered Attestation procedure -- instead of feeding measurements into an RTS, instantly. The Protection Capabilities of an RTM to also act as a temporary RTS MUST be trusted via trust-relationships to TAs enabled by appropriate Endorsements. System components supporting the use of a TPM typically include such an appropriate RTM. In general, feeding measurements from an initial RTM into a TPM is automated and separated from Protected Capabilities that provide Claims collection from Target Environments that are regular execution environments. A TPM providing the Protection Capabilities for an isolated and shielded location to feed measurements into (integrity and confidentiality) is an appropriate RTS for TUDA. The primitive operations used to store and chain measurements via a rolling hash function MUST be provided by an appropriate root of trust for storage (RTS) that is a system component of the Attester. An RTS MUST be trusted via trust-relationships to TAs enabled by appropriate Endorsements (e.g.,EK-Certs). If a Verifier cannot trust an RTS, Evidence generated based on digest values acquired from the RTS MUST be considered invalid. An RTS MUST be accessible to all Attesting Environments that are chained in a Layered Attestation procedure. A TPM providing the primitive operation for Extend is an appropriate RTM for TUDA. The primitive operations used to generate Evidence based on digests MUST be provided by roots of trust for reporting (RTR) that are system components of the Attester. An RTR MUST be be trusted via trust-relationships to TAs enabled by appropriate Endorsements (e.g.,EK-Certs). If a Verifier cannot trust an RTR, Evidence generated by the RTR MUST be considered invalid. A TPM providing the primitive operations for Quote is an appropriate RTR for TUDA. In a Composite Device (see Section 3.5. in conducting a Layered Attestation procedure, Attesting Environments MAY not be TPMs. At least one Attesting Environment MUST be a TPM. At least one TPM MUST act as an RTR. Attesting Environments that are not TPMs MUST NOT act as an RTR. A concise definition of the terms RTM, RTS, and RTR can be found in the Trusted Computing Group (TCG) Glossary . An RTS and an RTR are often tightly coupled. In TUDA, a Trusted Platform Module (TPM, see and ) takes on the roles of an RTS and an RTR. The specification in this document requires the use of a TPM as a component of the Attester. The protocol part of this specification can also be used with other RTS and RTR as long as essential functional requirements are satisfied (e.g., a trusted relative source of time, such as a tick-counter). A sequence of Layered Attestation using at least an RTM, RTS, and RTR enables an authenticated boot sequence typically referred to as Secure Boot.

Indeterministic Measurements The sequence of measurements that is extended into the RTS provided by a TPM may not be deterministic due to race conditions that are side-effects of parallelization. Parallelization occurs, for example, between different isolated execution environments or separate software components started in a execution environment. In order to enable the appraisal of Evidence in cases where sequence of measurement varies, a corresponding event log that records all measurements in sequence, such as the IMA log, has to be conveyed next to the Evidence as depicted in section 8.2. in . In contrast to Evidence, event logs do not necessarily have to be integrity protected or tamper-evident. Event logs are conveyed to a Verifier in order to compute the reference values required for comparison with digest values (output of TPM Quote operations). While digest values MUST constitute Evidence, measurements in event logs MAY be part of Evidence, but do not have to be MAY be conveyed separately. If the values in event logs or their sequence are tampered with before or during conveyance from an Attester to a Verifier, the corresponding Evidence Appraisal fails. While this dependency reflects the intended behavior of RATS, integrity protected or tamper-evident can be beneficial or convenient in some usage scenarios. Additionally, event logs my allow insights into the composition of an Attester and typically come with confidentiality requirements. In order to compute reference values to compare digest Claims in Evidence with, a Verifier MUST be able to replay the rolling hash function of the Extend operation provided by a TPM (see Section 2.4.2. in ). A Verifier has to replay the event log using its own extend operation with an identical rolling hash function in order to generate reference values as outlined in section 2.4.1. of . During reply, the validity of each event log record MUST be appraised individually by the Verifier in order to infer if each started software component satisfies integrity requirements. These appraisal procedures require Reference Integrity Measurements/Manifests (RIM) as are provided via or . Each RIM includes Reference Values that are nominal reference hash values for sets of software components. The Reference Values can be compared with hash values about executed software components included in an event log. A Verifier requires an appropriate set of RIMs to compare every record in an event log successfully. RIMs or other sets Reverence Value are supplied by Reference Value Providers as defined in the RATS Architecture . Corresponding procedures that enable a Verifier to acquire Reference Values are out-of-scope of this document.

TUDA Principles and Requirements Traditional remote attestation protocols typically use bi-directional challenge/response interaction models. Examples include the Platform Trust Service protocol or CAVES , where one entity sends a challenge that is included inside the response to prove the freshness of Evidence via recentness. The corresponding interaction model depicted in Section 8.1. of tightly couples the three RATS activities of generating, conveying and appraising Evidence. Time-Based Uni-directional Attestation can decouple these three activities. As a result, TUDA provides additional capabilities, such as:

• remote attestation for Attesters that might not always be able to reach the Internet by enabling the appraisal of past states,
• secure audit logs by combining the Evidence generated with integrity measurement logs (e.g. IMA logs) that represent a detailed record of corresponding past states,
• the use of the uni-directional interaction model that can traverse "diode-like" network security functions (NSF) or can be leveraged RESTful telemetry as enabled by the CoAP Observe option ).

Attesting Environment Requirements An Attesting Environment that generates Evidence in TUDA MUST support three specific Protected Capabilities:

• Platform Configuration Registers (PCR) that can extend measurements consecutively and represent the sequence of measurements as a single digest,
• Restricted Signing Keys (RSK) that can only be accessed, if a specific signature about a set of measurements can be provided as authentication, and
• a dedicated source of (relative) time, e.g. a tick counter (a tick being a specific time interval, for example 10 ms).

A TPM is capable of providing these Protected Capabilities for TUDA.

Handle Distributor Requirements: Time Stamp Authority Both Evidence generation and Evidence appraisal require a Handle Distributor that can take on the role of a trusted Time Stamp Authority (TSA) as an additional third party. Time Stamp Tokens (TST) included in Handles MUST be generated by Time Stamp Authority based on that acts as the Handle Distributor. The combination of a local source of time provided by a TPM (on the Attester) and the TST provided by the Handle Distributor (to both the Attester and the Verifier) enable an appropriate proof of freshness.

Information Elements and Conveyance TUDA defines a set of information elements (IE) that represent a set of Claims, are generated and stored on the Attester, and are intended to be transferred to the Verifier in order to enable the appraisal of Evidence. Each TUDA IE:

• MUST be encoded in the Concise Binary Object Representation (CBOR ) to minimize the volume of data in motion. In this document, the composition of the CBOR data items that represent IE is described using the Concise Data Definition Language, CDDL .
• that requires a certain freshness SHOULD only be re-generated when out-dated (not fresh, but stale), which reduces the overall resources required from the Attester, including the usage of a TPM's resources (re-generation of IE is determined by their age or by specific state changes on the Attester, e.g., due to a reboot-cycle)
• SHOULD only be transferred when required, which reduces the amount of data in motion necessary to conduct remote attestation significantly (only IE that have changed since their last conveyance have to be transferred)
• that requires a certain freshness SHOULD be reused for multiple remote attestation procedures in the limits of its corresponding freshness-window, further reducing the load imposed on the Attester and corresponding TPMs.

TUDA Core Concept Traditional Challenge/Response Remote Attestation includes sending a nonce in the challenge to be used in ad-hoc Evidence generation. Using the TPM 1.2 as an example, a corresponding nonce-challenge would be included within the signature created by the TPM_Quote command in order to prove the freshness of a response containing evidence, see e.g. . In contrast, the TUDA protocol uses the combined output of TPM_CertifyInfo and TPM_TickStampBlob. The former provides a proof about the Attester's state by creating Evidence that a certain key is bound to that state. The latter provides proof that the Attester was in the specified state by using the bound key in a time operation. This combination enables a time-based attestation scheme. The approach is based on the concepts introduced in and . Each TUDA IE has an individual time-frame, in which it is considered to be fresh (and therefore valid and trustworthy). In consequence, each TUDA IE that composes data in motion is based on different methods of creation. As highlighted above, the freshness properties of a challenge-response based protocol enable implicit time-keeping via a time window between:

• the time of transmission of the nonce, and
• the reception of the corresponding response.

Given the time-based attestation scheme, the freshness property of TUDA is equivalent to that of bi-directional challenge response attestation, if the point-in-time of attestation lies between:

• the transmission of a TUDA time-synchronization token, and
• the typical round-trip time between the Verifier and the Attester.

The accuracy of this time-frame is defined by two factors:

• the time-synchronization between the Attester and the Handle Distributor. The time between the two tickstamps acquired via the RoT define the scope of the maximum drift (time "left" and time "right" in respect to the timeline) to the handle including the signed timestamp, and
• the drift of clocks included in the RoT.

Since the conveyance of TUDA Evidence does not rely upon a Verifier provided value (i.e. the nonce), the security guarantees of the protocol only incorporate the Handle Distributor and the RoT used. In consequence, TUDA Evidence can even serve as proof of integrity in audit logs with precise point-in-time guarantees. contains guidance on how to utilize a REST architecture. contains guidance on how to create an SNMP binding and a corresponding TUDA-MIB. contains a corresponding YANG module that supports both RESTCONF and CoREDONF. contains a realization of TUDA using TPM 1.2 primitives. contains a realization of TUDA using TPM 2.0 primitives.

TPM Specific Terms

PCR:

A Platform Configuration Register that is part of the TPM and is used to securely store and report measurements about security posture

PCR-Hash:

A hash value of the security posture measurements stored in a TPM PCR (e.g. regarding running software instances) represented as a byte-string

Certificates

HD-CA:

The Certificate Authority that provides the certificate for the TSA role of a Handle Distributor (HD)

AIK-CA:

The Certificate Authority that provides the certificate for the AK of the TPM. This is the client platform credential for this protocol. It is a placeholder for a specific CA and AK-Cert is a placeholder for the corresponding certificate, depending on what protocol was used. The specific protocols are out of scope for this document, see also and .

The TUDA Protocol Family Time-Based Uni-Directional Attestation consists of the following seven information elements:

Handle Distributor Certificate:

The certificate of the Handle Distributor that takes on the role of TSA. The Handle Distributor certificate is used in a subsequent synchronization protocol tokens. This certificate is signed by the HD-CA.

AK Certificate:

A certificate about the Attestation Key (AIK) used. An AK-Cert may be an IDevID or LDevID, depending on their setting of the corresponding identity property (, ; see ).

Synchronization Token:

The reference frame for Evidence is provided by the relative timestamps generated by the TPM. In order to put Evidence into relation with a Real Time Clock (RTC), it is necessary to provide a cryptographic synchronization between these trusted relative timestamps and the regular RTC that is a hardware component of the Attester. To do so, trustable timestamps are acquired from a Handle Distributor.

Restriction Info:

Evidence Generation relies on the capability of the Rot to operate on restricted keys. Whenever the PCR values of an Attesting Environment change, a new restricted key is created that can only be operated as long as the PCRs remain in their current state.

In order to prove to the Verifier that this restricted temporary key actually has these properties and also to provide the PCR value that it is restricted, the corresponding signing capabilities of the RoT are used. The TPM creates a signed certificate using the AK about the newly created restricted key.

Measurement Log:

A Verifier requires the means to derive the PCRs' values in order to appraise the trustworthiness of an Attester. As such, a list of those elements that were extended into the PCRs is reported. For certain environments, this step may be optional if a list of valid PCR configurations (in the form of RIM available to the Verifier) exists and no measurement log is required.

Implicit Evidence:

The actual Evidence is then based on a signed timestamp provided by the RoT using the restricted temporary key that was certified in the steps above. The signed timestamp generated provides the trustable assertion that at this point in time (with respect to the relative time of the TPM s tick counter) a certain configuration existed (namely the PCR values associated with the restricted key). In combination with the synchronization token this timestamp represented in relative time can then be related to the real-time clock.

Concise SWID tags:

As an option to better assess the trustworthiness of an Attester, a Verifier can request the reference hashes (RIM, sometimes called golden measurements, known-good-values, or nominal values) of all started software components to compare them with the entries in a measurement log. References hashes regarding installed (and therefore running) software can be provided by the manufacturer via SWID tags. SWID tags are provided by the Attester using the Concise SWID representation and bundled into a collection (a RIM Manifest ).

These information elements can be sent en bloc, but it is recommended to retrieve them separately to save bandwidth, since these elements have different update cycles. In most cases, retransmitting all seven information elements would result in unnecessary redundancy. Furthermore, in some scenarios it might be feasible not to store all elements on the Attester, but instead they could be retrieved from another location or be pre-deployed to the Verifier. It is also feasible to only store public keys on the Verifier and skip certificate provisioning completely in order to save bandwidth and computation time for certificate verification.

TUDA Information Elements Update Cycles An Attester can be in various states during its uptime cycles. For TUDA, a subset of these states (which imply associated information) are important to the Evidence Generation. The specific states defined are:

• persistent, even after a hard reboot: includes certificates that are associated with the endpoint itself or with services it relies on.
• volatile to a degree: may change at the beginning of each boot cycle. This includes the capability of a TPM to provide relative time which provides the basis for the synchronization token and implicit attestation -- and which can reset after an Attester is powered off.
• very volatile: can change during any time of an uptime cycle (periods of time an Attester is powered on, starting with its boot sequence). This includes the content of PCRs of a hardware RoT and thereby also the PCR-restricted signing keys used for attestation.

Depending on this "lifetime of state", data has to be transported over the wire, or not. E.g. information that does not change due to a reboot typically has to be transported only once between the Attester and the Verifier. There are three kinds of events that require fresh Evidence to be generated:

• The Attester completes a boot-cycle
• A relevant PCR changes
• Too much time has passed since the Evidence Generation

The third event listed above is variable per application use case and also depends on the precision of the clock included in the RoT. For usage scenarios, in which the Attester would periodically push information to be used in an audit-log, a time-frame of approximately one update per minute should be sufficient. For those usage scenarios, where Verifiers request (pull) fresh Evidence, an implementation could potentially use a TPM continuously to always present the most freshly created Evidence. This kind of utilization can result in a bottle-neck with respect to other purposes: if unavoidable, a periodic interval of once per ten seconds is recommended, which typically leaves about 80% of available TPM resource for other applications. The following diagram is based on the reference interaction model found in section 8.1. of and is enriched with the IE update cycles defined in this section.

Example sequence of events claims | | .--------------------. | | <----------handle | | | | | Handle Distributor | | | | | handle----------> | | '--------------------' | syncTokenGeneration | | => syncToken | | | restrictedKeyGeneration | restrictedKeyCertify | | | evidenceGeneration(handle, authSecIDs, collectedClaims) | | => evidence | | | | pushAKCert ----------------------------------------------> | | pushSyncToken -------------------------------------------> | | pushCertifyInfo -----------------------------------------> | | pushEventLog --------------------------------------------> | | pushEvidenceon ------------------------------------------> | | | | evidenceAppraisal(evidence, eventLog, refClaims) | attestationResult <= | ~ ~ pcr-change | | | restrictedKeyGeneration | restrictedKeyCertify | | | evidenceGeneration(handle, authSecIDs, collectedClaims) | | => evidence | | | | pushCertifyInfo -----------------------------------------> | | pushEventLog --------------------------------------------> | | pushEvidenceon ------------------------------------------> | | | | evidenceAppraisal(evidence, eventLog, refClaims) | attestationResult <= | | | ]]>

Sync Base Protocol The uni-directional approach of TUDA requires evidence on how the TPM time represented in ticks (relative time since boot of the TPM) relates to the standard time provided by the TSA. The Sync Base Protocol (SBP) creates evidence that binds the TPM tick time to the TSA timestamp. The binding information is used by and conveyed via the Sync Token (TUDA IE). There are three actions required to create the content of a Sync Token:

• At a given point in time (called "left"), a signed tickstamp counter value is acquired from the hardware RoT. The hash of counter and signature is used as a nonce in the request directed at the TSA.
• The corresponding response includes a data-structure incorporating the trusted timestamp token and its signature created by the TSA.
• At the point-in-time the response arrives (called "right"), a signed tickstamp counter value is acquired from the hardware RoT again, using a hash of the signed TSA timestamp as a nonce.

The three time-related values --- the relative timestamps provided by the hardware RoT ("left" and "right") and the TSA timestamp --- and their corresponding signatures are aggregated in order to create a corresponding Sync Token to be used as a TUDA Information Element that can be conveyed as evidence to a Verifier. The drift of a clock incorporated in the hardware RoT that drives the increments of the tick counter constitutes one of the triggers that can initiate a TUDA Information Element Update Cycle in respect to the freshness of the available Sync Token.

IANA Considerations This memo includes requests to IANA, including registrations for media type definitions. TBD

Security Considerations There are Security Considerations. TBD

Contributors TBD

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] The Extensible Authentication Protocol (EAP), defined in RFC 3748, enables extensible network access authentication. This document specifies the EAP key hierarchy and provides a framework for the transport and usage of keying material and parameters generated by EAP authentication algorithms, known as "methods". It also provides a detailed system-level security analysis, describing the conditions under which the key management guidelines described in RFC 4962 can be satisfied. [STANDARDS-TRACK] This specification defines Web Linking using a link format for use by constrained web servers to describe hosted resources, their attributes, and other relationships between links. Based on the HTTP Link Header field defined in RFC 5988, the Constrained RESTful Environments (CoRE) Link Format is carried as a payload and is assigned an Internet media type. "RESTful" refers to the Representational State Transfer (REST) architecture. A well-known URI is defined as a default entry point for requesting the links hosted by a server. [STANDARDS-TRACK] The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations. The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. Section 1.1.1 of RFC 3986 defines URI syntax as "a federated and extensible naming system wherein each scheme's specification may further restrict the syntax and semantics of identifiers using that scheme." In other words, the structure of a URI is defined by its scheme. While it is common for schemes to further delegate their substructure to the URI's owner, publishing independent standards that mandate particular forms of substructure in URIs is often problematic. This document provides guidance on the specification of URI substructure in standards. This document obsoletes RFC 7320 and updates RFC 3986. This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients. This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged. This document describes an HTTP-based protocol that provides a programmatic interface for accessing data defined in YANG, using the datastore concepts defined in the Network Configuration Protocol (NETCONF). This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. 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 This memo defines the second version of the Management Information Base (MIB-II) for use with network management protocols in TCP/IP-based internets. [STANDARDS-TRACK] This memo obsoletes RFC 1514, the "Host Resources MIB". This memo extends that specification by clarifying changes based on implementation and deployment experience and documenting the Host Resources MIB in SMIv2 format while remaining semantically identical to the existing SMIv1-based MIB. [STANDARDS-TRACK] This Glossary provides definitions, abbreviations, and explanations of terminology for information system security. The 334 pages of entries offer recommendations to improve the comprehensibility of written material that is generated in the Internet Standards Process (RFC 2026). The recommendations follow the principles that such writing should (a) use the same term or definition whenever the same concept is mentioned; (b) use terms in their plainest, dictionary sense; (c) use terms that are already well-established in open publications; and (d) avoid terms that either favor a particular vendor or favor a particular technology or mechanism over other, competing techniques that already exist or could be developed. This memo provides information for the Internet community. This document defines the problem statement, scope, and protocol requirements between the components of the NEA (Network Endpoint Assessment) reference model. NEA provides owners of networks (e.g., an enterprise offering remote access) a mechanism to evaluate the posture of a system. This may take place during the request for network access and/or subsequently at any time while connected to the network. The learned posture information can then be applied to a variety of compliance-oriented decisions. The posture information is frequently useful for detecting systems that are lacking or have out-of-date security protection mechanisms such as: anti-virus and host-based firewall software. In order to provide context for the requirements, a reference model and terminology are introduced. This memo provides information for the Internet community. This memo defines a portion of the Management Information Base (MIB) for use with network management protocols in the Internet community. In particular, it describes managed objects used for managing multiple logical and physical entities managed by a single Simple Network Management Protocol (SNMP) agent. This document specifies version 4 of the Entity MIB. This memo obsoletes version 3 of the Entity MIB module published as RFC 4133. This document describes an architecture for describing Simple Network Management Protocol (SNMP) Management Frameworks. The architecture is designed to be modular to allow the evolution of the SNMP protocol standards over time. The major portions of the architecture are an SNMP engine containing a Message Processing Subsystem, a Security Subsystem and an Access Control Subsystem, and possibly multiple SNMP applications which provide specific functional processing of management data. This document obsoletes RFC 2571. [STANDARDS-TRACK] This document describes the Message Processing and Dispatching for Simple Network Management Protocol (SNMP) messages within the SNMP architecture. It defines the procedures for dispatching potentially multiple versions of SNMP messages to the proper SNMP Message Processing Models, and for dispatching PDUs to SNMP applications. This document also describes one Message Processing Model - the SNMPv3 Message Processing Model. This document obsoletes RFC 2572. [STANDARDS-TRACK] This document describes five types of Simple Network Management Protocol (SNMP) applications which make use of an SNMP engine as described in STD 62, RFC 3411. The types of application described are Command Generators, Command Responders, Notification Originators, Notification Receivers, and Proxy Forwarders. This document also defines Management Information Base (MIB) modules for specifying targets of management operations, for notification filtering, and for proxy forwarding. This document obsoletes RFC 2573. [STANDARDS-TRACK] This document describes the User-based Security Model (USM) for Simple Network Management Protocol (SNMP) version 3 for use in the SNMP architecture. It defines the Elements of Procedure for providing SNMP message level security. This document also includes a Management Information Base (MIB) for remotely monitoring/managing the configuration parameters for this Security Model. This document obsoletes RFC 2574. [STANDARDS-TRACK] This document describes the View-based Access Control Model (VACM) for use in the Simple Network Management Protocol (SNMP) architecture. It defines the Elements of Procedure for controlling access to management information. This document also includes a Management Information Base (MIB) for remotely managing the configuration parameters for the View- based Access Control Model. This document obsoletes RFC 2575. [STANDARDS-TRACK] This document defines version 2 of the protocol operations for the Simple Network Management Protocol (SNMP). It defines the syntax and elements of procedure for sending, receiving, and processing SNMP PDUs. This document obsoletes RFC 1905. [STANDARDS-TRACK] This document defines the transport of Simple Network Management Protocol (SNMP) messages over various protocols. This document obsoletes RFC 1906. [STANDARDS-TRACK] This document defines managed objects which describe the behavior of a Simple Network Management Protocol (SNMP) entity. This document obsoletes RFC 1907, Management Information Base for Version 2 of the Simple Network Management Protocol (SNMPv2). [STANDARDS-TRACK] Trilliant Networks Inc. consultant Acklio YumaWorks Acklio This document describes a network management interface for constrained devices and networks, called CoAP Management Interface (CORECONF). The Constrained Application Protocol (CoAP) is used to access datastore and data node resources specified in YANG, or SMIv2 converted to YANG. CORECONF uses the YANG to CBOR mapping and converts YANG identifier strings to numeric identifiers for payload size reduction. CORECONF extends the set of YANG based protocols, NETCONF and RESTCONF, with the capability to manage constrained devices and networks. Fraunhofer SIT National Security Agency The MITRE Corporation National Institute of Standards and Technology ISO/IEC 19770-2:2015 Software Identification (SWID) tags provide an extensible XML-based structure to identify and describe individual software components, patches, and installation bundles. SWID tag representations can be too large for devices with network and storage constraints. This document defines a concise representation of SWID tags: Concise SWID (CoSWID) tags. CoSWID supports a similar set of semantics and features as SWID tags, as well as new semantics that allow CoSWIDs to describe additional types of information, all in a more memory efficient format. Fraunhofer SIT Fraunhofer SIT Huawei Technologies Cisco Systems This document describes interaction models for remote attestation procedures (RATS). Three conveying mechanisms -- Challenge/Response, Uni-Directional, and Streaming Remote Attestation -- are illustrated and defined. Analogously, a general overview about the information elements typically used by corresponding conveyance protocols are highlighted. 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. Juniper Networks, Inc. Cisco Systems, Inc. National Security Agency This document describes a workflow for remote attestation of integrity of network devices. Fraunhofer SIT Red Hat National Institute of Standards and Technology Cisco Systems, Inc. Department of Defense This document specifies the CDDL and usage description for Reference Integrity Measurements (RIM) in Remote Attestation Procedures (RATS). The specification is based on Concise Software Identification (CoSWID) and TCG Reference Integrity Manifest Information Model - based on Host Integrity at Runtime and Start-up (HIRS). Extension points defined in CoSWID used to augment CoSWID tags with new attributes that can express the TCG Reference Integrity Manifest extensions. Fraunhofer SIT Qualcomm Technologies Inc. Cisco Systems Universitaet Bremen TZI CBOR Web Token (CWT, RFC 8392) Claims Sets sometimes do not need the protection afforded by wrapping them into COSE, as is required for a true CWT. This specification defines a CBOR tag for such unprotected CWT Claims Sets (UCCS) and discusses conditions for its proper use. TCG TNC Working Group Trusted Computing Group TCG TNC Working Group TCG TCG TCG Infrastructure Working Group TCG Infrastructure Working Group University of California, Irvine IEEE Computer Society IEEE Computer Society

REST Realization Each of the seven data items is defined as a media type (). Representations of resources for each of these media types can be retrieved from URIs that are defined by the respective servers . As can be derived from the URI, the actual retrieval is via one of the HTTPs (, ) or CoAP . How a client obtains these URIs is dependent on the application; e.g., CoRE Web links can be used to obtain the relevant URIs from the self-description of a server, or they could be prescribed by a RESTCONF data model .

SNMP Realization SNMPv3 (RFC 3411, ) is widely available on computers and also constrained devices. To transport the TUDA information elements, an SNMP MIB is defined below which encodes each of the seven TUDA information elements into a table. Each row in a table contains a single read-only columnar SNMP object of datatype OCTET-STRING. The values of a set of rows in each table can be concatenated to reconstitute a CBOR-encoded TUDA information element. The Verifier can retrieve the values for each CBOR fragment by using SNMP GetNext requests to "walk" each table and can decode each of the CBOR-encoded data items based on the corresponding CDDL definition. Design Principles:

1. Over time, TUDA attestation values age and should no longer be used. Every table in the TUDA MIB has a primary index with the value of a separate scalar cycle counter object that disambiguates the transition from one attestation cycle to the next.
2. Over time, the measurement log information (for example) may grow large. Therefore, read-only cycle counter scalar objects in all TUDA MIB object groups facilitate more efficient access with SNMP GetNext requests.
3. Notifications are supported by an SNMP trap definition with all of the cycle counters as bindings, to alert a Verifier that a new attestation cycle has occurred (e.g., synchronization data, measurement log, etc. have been updated by adding new rows and possibly deleting old rows).

Structure of TUDA MIB The following table summarizes the object groups, tables and their indexes, and conformance requirements for the TUDA MIB:

Group/Table	Cycle	Instance	Fragment	Required
General				x
AIKCert	x	x	x
TSACert	x	x	x
SyncToken	x		x	x
Restrict	x			x
Measure	x	x
VerifyToken	x			x
SWIDTag	x	x	x

Cycle Index A tudaV1<Group>CycleIndex is the:

1. first index of a row (element instance or element fragment) in the tudaV1<Group>Table;
2. identifier of an update cycle on the table, when rows were added and/or deleted from the table (bounded by tudaV1<Group>Cycles); and
3. binding in the tudaV1TrapV2Cycles notification for directed polling.

Instance Index A tudaV1<Group>InstanceIndex is the:

1. second index of a row (element instance or element fragment) in the tudaV1<Group>Table; except for
2. a row in the tudaV1SyncTokenTable (that has only one instance per cycle).

Fragment Index A tudaV1<Group>FragmentIndex is the:

1. last index of a row (always an element fragment) in the tudaV1<Group>Table; and
2. accomodation for SNMP transport mapping restrictions for large string elements that require fragmentation.

Relationship to Host Resources MIB The General group in the TUDA MIB is analogous to the System group in the Host Resources MIB and provides context information for the TUDA attestation process. The Verify Token group in the TUDA MIB is analogous to the Device group in the Host MIB and represents the verifiable state of a TPM device and its associated system. The SWID Tag group (containing a Concise SWID reference hash profile ) in the TUDA MIB is analogous to the Software Installed and Software Running groups in the Host Resources MIB .

Relationship to Entity MIB The General group in the TUDA MIB is analogous to the Entity General group in the Entity MIB v4 and provides context information for the TUDA attestation process. The SWID Tag group in the TUDA MIB is analogous to the Entity Logical group in the Entity MIB v4 .

Relationship to Other MIBs The General group in the TUDA MIB is analogous to the System group in MIB-II and the System group in the SNMPv2 MIB (RFC 3418, ) and provides context information for the TUDA attestation process.

Definition of TUDA MIB

YANG Realization

Realization with TPM functions

TPM Functions The following TPM structures, resources and functions are used within this approach. They are based upon the TPM specifications and .

Tick-Session and Tick-Stamp On every boot, the TPM initializes a new Tick-Session. Such a tick-session consists of a nonce that is randomly created upon each boot to identify the current boot-cycle -- the phase between boot-time of the device and shutdown or power-off -- and prevent replaying of old tick-session values. The TPM uses its internal entropy source that guarantees virtually no collisions of the nonce values between two of such boot cycles. It further includes an internal timer that is being initialize to Zero on each reboot. From this point on, the TPM increments this timer continuously based upon its internal secure clocking information until the device is powered down or set to sleep. By its hardware design, the TPM will detect attacks on any of those properties. The TPM offers the function TPM_TickStampBlob, which allows the TPM to create a signature over the current tick-session and two externally provided input values. These input values are designed to serve as a nonce and as payload data to be included in a TickStampBlob: TickstampBlob := sig(TPM-key, currentTicks || nonce || externalData). As a result, one is able to proof that at a certain point in time (relative to the tick-session) after the provisioning of a certain nonce, some certain externalData was known and provided to the TPM. If an approach however requires no input values or only one input value (such as the use in this document) the input values can be set to well-known value. The convention used within TCG specifications and within this document is to use twenty bytes of zero h'0000000000000000000000000000000000000000' as well-known value.

Platform Configuration Registers (PCRs) The TPM is a secure cryptoprocessor that provides the ability to store measurements and metrics about an endpoint's configuration and state in a secure, tamper-proof environment. Each of these security relevant metrics can be stored in a volatile Platform Configuration Register (PCR) inside the TPM. These measurements can be conducted at any point in time, ranging from an initial BIOS boot-up sequence to measurements taken after hundreds of hours of uptime. The initial measurement is triggered by the Platforms so-called pre-BIOS or ROM-code. It will conduct a measurement of the first loadable pieces of code; i.e.\ the BIOS. The BIOS will in turn measure its Option ROMs and the BootLoader, which measures the OS-Kernel, which in turn measures its applications. This describes a so-called measurement chain. This typically gets recorded in a so-called measurement log, such that the values of the PCRs can be reconstructed from the individual measurements for validation. Via its PCRs, a TPM provides a Root of Trust that can, for example, support secure boot or remote attestation. The attestation of an endpoint's identity or security posture is based on the content of an TPM's PCRs (platform integrity measurements).

PCR restricted Keys Every key inside the TPM can be restricted in such a way that it can only be used if a certain set of PCRs are in a predetermined state. For key creation the desired state for PCRs are defined via the PCRInfo field inside the keyInfo parameter. Whenever an operation using this key is performed, the TPM first checks whether the PCRs are in the correct state. Otherwise the operation is denied by the TPM.

CertifyInfo The TPM offers a command to certify the properties of a key by means of a signature using another key. This includes especially the keyInfo which in turn includes the PCRInfo information used during key creation. This way, a third party can be assured about the fact that a key is only usable if the PCRs are in a certain state.

IE Generation Procedures for TPM 1.2

AIK and AIK Certificate Attestations are based upon a cryptographic signature performed by the TPM using a so-called Attestation Identity Key (AIK). An AIK has the properties that it cannot be exported from a TPM and is used for attestations. Trust in the AIK is established by an X.509 Certificate emitted by a Certificate Authority. The AIK certificate is either provided directly or via a so-called PrivacyCA . This element consists of the AIK certificate that includes the AIK's public key used during verification as well as the certificate chain up to the Root CA for validation of the AIK certificate itself.

TUDA-Cert element in CDDL

The TSA-Cert is a standard certificate of the TSA. The AIK-Cert may be provisioned in a secure environment using standard means or it may follow the PrivacyCA protocols. gives a rough sketch of this protocol. See for more information. The X.509 Certificate is built from the AIK public key and the corresponding PKCS #7 certificate chain, as shown in . Required TPM functions:

Creating the TUDA-Cert element

Synchronization Token The reference for Attestations are the Tick-Sessions of the TPM. In order to put Attestations into relation with a Real Time Clock (RTC), it is necessary to provide a cryptographic synchronization between the tick session and the RTC. To do so, a synchronization protocol is run with a Time Stamp Authority (TSA) that consists of three steps:

• The TPM creates a TickStampBlob using the AIK
• This TickStampBlob is used as nonce to the Timestamp of the TSA
• Another TickStampBlob with the AIK is created using the TSA's Timestamp a nonce

The first TickStampBlob is called "left" and the second "right" in a reference to their position on a time-axis. These three elements, with the TSA's certificate factored out, form the synchronization token

TUDA-Sync element in CDDL

Required TPM functions:

Creating the Sync-Token element

RestrictionInfo The attestation relies on the capability of the TPM to operate on restricted keys. Whenever the PCR values for the machine to be attested change, a new restricted key is created that can only be operated as long as the PCRs remain in their current state. In order to prove to the Verifier that this restricted temporary key actually has these properties and also to provide the PCR value that it is restricted, the TPM command TPM_CertifyInfo is used. It creates a signed certificate using the AIK about the newly created restricted key. This token is formed from the list of:

• PCR list,
• the newly created restricted public key, and
• the certificate.

TUDA-Key element in CDDL we represent this case as null ] ]]>

Required TPM functions:

Creating the pubkey

Measurement Log Similarly to regular attestations, the Verifier needs a way to reconstruct the PCRs' values in order to estimate the trustworthiness of the device. As such, a list of those elements that were extended into the PCRs is reported. Note though that for certain environments, this step may be optional if a list of valid PCR configurations exists and no measurement log is required.

Implicit Attestation The actual attestation is then based upon a TickStampBlob using the restricted temporary key that was certified in the steps above. The TPM-Tickstamp is executed and thereby provides evidence that at this point in time (with respect to the TPM internal tick-session) a certain configuration existed (namely the PCR values associated with the restricted key). Together with the synchronization token this tick-related timing can then be related to the real-time clock. This element consists only of the TPM_TickStampBlock with no nonce.

TUDA-Verify element in CDDL

Required TPM functions:

Creating the Verify Token

Attestation Verification Approach The seven TUDA information elements transport the essential content that is required to enable verification of the attestation statement at the Verifier. The following listings illustrate the verification algorithm to be used at the Verifier in pseudocode. The pseudocode provided covers the entire verification task. If only a subset of TUDA elements changed (see ), only the corresponding code listings need to be re-executed.

Verification of Certificates

Verification of Synchronization Token

Verification of Restriction Info

Verification of Measurement Log

Verification of Attestation Token

IE Generation Procedures for TPM 2.0 The pseudocode below includes general operations that are conducted as specific TPM commands:

• hash() : description TBD
• sig() : description TBD
• X.509-Certificate() : description TBD

These represent the output structure of that command in the form of a byte string value.

AIK and AIK Certificate Attestations are based upon a cryptographic signature performed by the TPM using a so-called Attestation Identity Key (AIK). An AIK has the properties that it cannot be exported from a TPM and is used for attestations. Trust in the AIK is established by an X.509 Certificate emitted by a Certificate Authority. The AIK certificate is either provided directly or via a so-called PrivacyCA . This element consists of the AIK certificate that includes the AIK's public key used during verification as well as the certificate chain up to the Root CA for validation of the AIK certificate itself.

TUDA-Cert element for TPM 2.0

Synchronization Token The synchronization token uses a different TPM command, TPM2 GetTime() instead of TPM TickStampBlob(). The TPM2 GetTime() command contains the clock and time information of the TPM. The clock information is the equivalent of TUDA v1's tickSession information.

TUDA-Sync element for TPM 2.0

Measurement Log The creation procedure is identical to .

TUDA-Log element for TPM 2.0

Explicit time-based Attestation The TUDA attestation token consists of the result of TPM2_Quote() or a set of TPM2_PCR_READ followed by a TPM2_GetSessionAuditDigest. It proves that --- at a certain point-in-time with respect to the TPM's internal clock --- a certain configuration of PCRs was present, as denoted in the keys restriction information.

TUDA-Attest element for TPM 2.0

Sync Proof In order to proof to the Verifier that the TPM's clock was not 'fast-forwarded' the result of a TPM2_GetTime() is sent after the TUDA-AttestationToken.

TUDA-Proof element for TPM 2.0

Acknowledgements