Time-Based Uni-Directional AttestationFraunhofer Institute for Secure Information TechnologyRheinstrasse 75Darmstadt64295Germanyandreas.fuchs@sit.fraunhofer.deFraunhofer Institute for Secure Information TechnologyRheinstrasse 75Darmstadt64295Germanyhenk.birkholz@sit.fraunhofer.deHigh North IncPO Box 221Grand Marais49839USblueroofmusic@gmail.comUniversitaet Bremen TZIBibliothekstr. 1BremenD-28359Germany+49-421-218-63921cabo@tzi.org
Security
RATS Working GroupInternet-DraftThis documents defines the method and bindings used to conduct Time-based Uni-Directional Attestation (TUDA) between two RATS (Remote ATtestation procedureS) Principals over the Internet.
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.
Conversely, TUDA enables the creation of Secure Audit Logs that can constitute Evidence about current and past operational states of an Attester.
As a prerequisite for TUDA, every RATS Principal requires access to a trusted and synchronized time-source.
Per default, in TUDA this is a Time Stamp Authority (TSA) issuing signed Time Stamp Tokens (TST).Remote ATtestation procedureS (RATS) describe the attempt to determine and appraise properties, such as integrity and trustworthiness, of a communication partner – the Attester – over the Internet to another communication parter – the Verifier – without direct access.
TUDA uses the architectural constituents of the RATS Architecture that defines the Roles Attester and Verifier in detail.
The RATS Architecture also defines Role Messages.
TUDA creates and conveys a specific type of Role Message called Evidence, a composition of trustwrthiness Claims provided by an Attester and consumed by a Verifier (potentially relayed by another RATS Role that is a Relying Party).
TUDA – in contrast to traditional bi-directional challenge-response protocols – enables a uni-directional conveyance of attestation Evidence that allows for providing attestation information without solicitation (e.g. as beacons or push data via YANG Push , , ).As a result, this document introduces the term 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.
FA is achieved by timely recording of assessments of the authenticity from system components (via “audit logs” during “audit sessions”) that are authorized for this purpose and trustworthy (e.g via endorsed roots of trust), 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.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 Evidence is basically a set of trustworthiness claims (assertions about the Attester and its system characteristics including security posture and protection characteristics) that are accompanied by a proof of their veracity – typically a signature based on shielded, private and potentially restricted key material.
As key material alone is typically not self-descriptive with respect to its intended use (its semantics), the remote attestation Evidence created via TUDA is accompanied by two kinds of certificates that are cryptographically associated with a Trust Anchor (TA) via a certification path:an Attestation Key (AK) Certificate (AK-Cert) that represents the attestation provenance of the created Evidence, andan Endorsement Key (EK) Certificate (EK-Cert) that represents the protection characteristics of the system components the AK is stored in.If a Verifier decides to trust both the TA of an AK-Cert and an EK-Cert presented by an Attester – and the included assertions about the system characteristics describing the Attester, the attestation Evidence created via TUDA by the Attester is considered believable.
Ultimately, believable Evidence is appraised by a Verifier in order to assess the trustworthiness of the corresponding Attester.The TUDA protocol mechanism uses hash values of all started software components as a basis to provide and create Evidence about the integrity of the software components of an Attester.
This section defines the processed data items, the required system components, and corresponding operations to enable the creation of Evidence about software component integrity for TUDA.The hash value of a software component created before it is executed is referred to as a “measurement” in the remainder of this document.
Measurements are chained using a rolling hash function.
Each measurement added to the sequence of all measurements results in a new current hash value that is referred to as a “digest” in the remainder of this document.The function to store these measurements via a rolling hash function is provided by a root of trust for storage – a system component that MUST be a component of the attester.With respect to the boot sequence of an Attester, the very first measurements of software components (e.g. the BIOS, or a sometimes a bootloader) have to be conducted by a root of trust for measurement that is implemented in hardware and MUST be a system component of the Attester.All measurements retained in the root of trust for measurements are handed over to the root of trust for storage when it becomes available during the boot procedure of the Attester.
During that hand-over the sequence of measurements retained in the root of trust for measurement are processed by the rolling hash function of the root of trust for storage.The function of retrieving the current output value of the rolling hash function, including a signature to provide a proof of veracity, is provided by a root of trust for reporting and MUST be a system component of the Attester.Typically, a root of trust for storage and a root of trust for reporting are tightly coupled.
Analogously, a root of trust for measurement is typically independent from the root of trust for storage, but has to be able to interact with root of trust for storage at some point of the boot sequence of the Attester to hand over the retained measurements.The operation of processing a measurement and adding it to the sequence of measurements via the rolling hash function is called “extend” and is provided by the root of trust for storage.The operation of retrieving the current available hash value that is the result of the rolling hash function including a signature based on an Attestation Key is called “quote” and is provided by the corresponding root of trust for reporting.In essence, RATS are composed of three base activities. The following definitions are derived from the definitions presented in and , and are a simplified summary of the RATS Architecture relevant for TUDA. The complete RATS Architecture and every corresponding constituent, message and interaction is defined in .
The creation of one ore more claims about the trustworthiness properties of an Attester, such that the claims can be used as Evidence.
The transfer of Evidence from the Attester to the Verifier via an interconnect.
The appraisal of Evidence by evaluating it against known-good-values (a type of declarative guidance).With TUDA, the claims that compose the evidence are signatures over trustworthy integrity measurements created by leveraging roots of trust. The evidence is appraised via corresponding signatures over reference integrity measurements (RIM, represented, for example via ).Protocols that facilitate Trust-Anchor based signatures in order to provide
RATS are usually bi-directional challenge/response protocols, such as the Platform Trust Service protocol or CAVES , where one entity sends a challenge that is included inside the response to prove the recentness – the freshness (see fresh in ) – of the attestation information. The corresponding interaction model tightly couples the three activities of creating, transferring and appraising evidence.The Time-Based Uni-directional Attestation family of protocols – TUDA – described in this document can decouple the three activities RATS are composed of. 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 verification of past states,secure audit logs by combining the evidence created via TUDA with integrity measurement logs that represent a detailed record of corresponding past states,an uni-directional interaction model that can traverse “diode-like” network security functions (NSF) or can be leveraged in RESTful architectures (e.g. CoAP ), analogously.TUDA is a family of protocols that bundles results from specific attestation activities. The attestation activities of TUDA are based on a hardware roots of trust that provides the following 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, anda dedicated source of (relative) time, e.g. a tick counter (a tick being a specific time interval, for example 10 ms).To appraise the evidence created by an Attester, the Verifier requires corresponding Reference Integrity Measurements (RIM). Typical set of RIMs are required to assess the integrity of an Attester. These sets are called RIM Bundles. The scope of a RIM Bundle encompasses, e.g., a platform, a device, a computing context, or a virtualised function. In order to be comparable, the hashing algorithms used by the Attester to create the integrity measurements have to match the hashing algorithms used to create the corresponding RIM that are used by the Verifier to appraise the attestation Evidence about software component integrity.Depending on the platform (i.e. one or more computing contexts including a dedicated hardware RoT), a generic RA activity results in platform-specific actions that have to be conducted. In consequence, there are multiple specific operations and data models (defining the input and output of operations). Hence, specific actions are are not covered by this document. Instead, the requirements on operations and the information elements that are the input and output to these operations are illustrated using pseudo code in Appendix C and D.Both the attestation and the verification activity of TUDA also require a trusted Time Stamp Authority (TSA) as an additional third party next to the Attester and the Verifier.
The protocol uses a Time Stamp Authority based on . The combination of the local source of time provided by the hardware RoT (located on the Attester) and the Time Stamp Tokens provided by the TSA (to both the Attester and the Verifier) enable the attestation and verification of an appropriate freshness of the evidence conveyed by the Attester — without requiring a challenge/response interaction model that uses a nonce to ensure the freshness.Typically, the verification activity requires declarative guidance (representing desired or compliant endpoint characteristics in the form of RIM, see above) to appraise the individual integrity measurements the conveyed evidence is composed on. The acquisition or representation (data models) of declarative guidance as well as the corresponding evaluation methods are out of the scope of this document.TUDA defines a set of information elements (IE) that are created and stored on the Attester and are intended to be transferred to the Verifier in order to enable appraisal. Each TUDA IE:is 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 is only created/updated when out-dated, which reduces the overall resources required from the Attester, including the utilization of the hardware root of trust. The IE that have to be created are determined by their age or by specific state changes on the Attester (e.g. state changes due to a reboot-cycle)is only 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 transferredthat requires a certain freshness can be reused for multiple remote attestation procedures in the limits of its corresponding freshness-window, further reducing the load imposed on the Attester and its corresponding hardware RoT.The Time-Based Uni-directional Attestation family of protocols 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 . RESTCONF or CoMI — and corresponding bindings,support broadcast and multicast schemes (e.g. ),be able to cope with temporary loss of connectivity, and toprovide trustworthy audit logs of past endpoint states.The binding of the attestation scheme used by TUDA to generate the TUDA IE is specific to the methods provided by the hardware RoT used (see above). In this document,expositional text and pseudo-code that is provided as a reference to instantiate the TUDA IE is based on TPM 1.2 and TPM 2.0 operations. The corresponding TPM commands are specified in and . The references to TPM commands and corresponding pseudo-code only serve as guidance to enable a better understanding of the attestation scheme and is intended to encourage the use of any appropriate hardware RoT or equivalent set of functions available to a CPU or Trusted Execution Environment .There are significant differences between conventional bi-directional attestation and TUDA regarding both the information elements conveyed between Attester and Verifier and the time-frame, in which an attestation can be considered to be fresh (and therefore trustworthy).In general, remote attestation using a bi-directional communication scheme includes sending a nonce-challenge within a signed attestation token. 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 the attestation response, 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 platform’s state by creating evidence that a certain key is bound to that state. The latter provides proof that the platform 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 trustworthy). In consequence, each TUDA IE that composes data in motion is based on different methods of creation.The freshness properties of a challenge-response based protocol define the point-of-time of attestation between:the time of transmission of the nonce, andthe 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, andthe 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 TSA. The time between the two tickstamps acquired via the hardware RoT define the scope of the maximum drift (“left” and “right” in respect to the timeline) to the TSA timestamp, andthe drift of clocks included in the hardware 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 TSA and the hardware RoT. In consequence, TUDA evidence can even serve as proof of integrity in audit logs with precise point-in-time guarantees, in contrast to classical attestations. 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 CoMI. contains a realization of TUDA using TPM 1.2 primitives. contains a realization of TUDA using TPM 2.0 primitives.This document introduces roles, information elements and types required to conduct TUDA and uses terminology (e.g. specific certificate names) typically seen in the context of attestation or hardware security modules.
a special purpose signature (therefore asymmetric) key that supports identity related operations. The private portion of the key pair is maintained confidential to the entity via appropriate measures (that have an impact on the scope of confidence). The public portion of the key pair may be included in AIK credentials that provide a claim about the entity.
A piece of information asserted about a subject . A claim is represented as a name/value pair consisting of a Claim Name and a Claim Value .In the context of SACM, a claim is also specialized as an attribute/value pair that is intended to be related to a statement .
the creation of evidence on the Attester that provides proof of a set of the endpoints’s integrity measurements. This is done by digitally signing a set of PCRs using an AIK shielded by the hardware RoT.
the context, composition, configuration, state, and behavior of an endpoint.
a trustworthy set of claims about an endpoint’s characteristics.
a set of claims that is intended to be related to an entity.
Metrics of endpoint characteristics (i.e. composition, configuration and state) that
affect the confidence in the trustworthiness of an endpoint. Digests of integrity measurements
can be stored in shielded locations (i.e. PCR of a TPM).
Signed measurements about the characteristics of an endpoint’s characteristics that are provided by a vendor and are intended to be used as declarative guidance (e.g. a signed CoSWID).
the qualities of an endpoint that guarantee a specific behavior and/or endpoint characteristics defined by declarative guidance.
Analogously, trustworthiness is the quality of being trustworthy with respect to declarative guidance.
Trustworthiness is not an absolute property but defined with respect to an entity, corresponding declarative guidance, and has a scope of confidence.Trustworthy Endpoint: an endpoint that guarantees trustworthy behavior and/or composition (with respect to certain declarative guidance and a scope of confidence).Trustworthy Statement: evidence that is trustworthy conveyed by an endpoint that is not necessarily trustworthy.
the endpoint that is the subject of the attestation to another endpoint.
the endpoint that consumes the attestation of another endpoint to conduct a verification.
a Time Stamp Authority
the now customary synonym for octet
an X.509 certificate represented as a byte-string
a Platform Configuration Register that is part of a hardware root of trust and is used to securely store and report measurements about security posture
a hash value of the security posture measurements stored in a TPM PCR (e.g. regarding running software instances) represented as a byte-string
the Certificate Authority that provides the certificate for the TSA represented as a Cert
the Certificate Authority that provides the certificate for the attestation identity key of the TPM. This is the client platform credential for this protocol. It is a placeholder for a specific CA and AIK-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 .A Time-Based Uni-Directional Attestation (TUDA) consists of the
following seven information elements. They are used to gain assurance of the Attester’s
platform configuration at a certain point in time:
The certificate of the Time Stamp Authority that is used in a subsequent synchronization
protocol token. This certificate is signed by the TSA-CA.
A certificate about the Attestation Identity Key (AIK) used. This may or may not
also be an IDevID or LDevID, depending on their setting of the corresponding identity property.
(, ; see .)
The reference for attestations are the relative timestanps provided by the hardware RoT. In
order to put attestations 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, a synchronization
protocol is run with a Time Stamp Authority (TSA).
The attestation relies on the capability of the hardware RoT 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 corresponding
signing capabilities of the hardware RoT are used. It creates a signed certificate using the AIK about
the newly created restricted key.
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
(in the form of RIM available to the Verifier) exists and no measurement log is required.
The actual attestation is then based upon a signed timestamp provided by the hardware RoT using the restricted
temporary key that was certified in the steps above. The signed timestamp provides evidence that at this point in time (with respect to the relative time of the hardware RoT)
a certain configuration existed (namely the PCR values associated
with the restricted key). Together with the synchronization token this timestamp represented in relative time
can then be related to the real-time clock.
As an option to better assess the trustworthiness of an Attester, a Verifier can request the
reference hashes (RIM, which are often referred to as golden measurements) of all started software components
to compare them with the entries in the 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 CBOR array (a RIM Manifest).
Ideally, the reference hashes include a signature created by the manufacturer of the software to prove their integrity.These information elements could 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 endpoint, 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 the whole
certificate provisioning completely in order to save bandwidth and computation
time for certificate verification.An endpoint can be in various states and have various information associated
with it during its life cycle. For TUDA, a subset of the states
(which can include associated information) that an endpoint and its hardware root of trust can be in, is
important to the attestation process. States can be:persistent, even after a hard reboot. This includes certificates
that are associated with the endpoint itself or with services it relies on.volatile to a degree, because they change at the beginning of each boot cycle.
This includes the capability of a hardware RoT to provide relative time which provides the basis for the
synchronization token and implicit attestation—and which can reset after an endpoint is powered off.very volatile, because they change during an uptime cycle
(the period of time an endpoint is powered on, starting with its boot).
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 a renewed attestation:The Attester completes a boot-cycleA relevant PCR changesToo much time has passed since the last attestation statementThe third event listed above is variable per application use case and also depends on the precision of the clock included in the hardware RoT.
For usage scenarios, in which the device would periodically
push information to be used in an audit-log, a time-frame of approximately one update
per minute should be sufficient in most cases. For those usage scenarios, where
Verifiers request (pull) a fresh attestation statement, an implementation could use the hardware RoT
continuously to always present the most freshly created results. To save some
utilization of the hardware RoT for other purposes, however, a time-frame of once per ten
seconds is recommended, which would typically leave about 80% of utilization for other applications.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.content TBDThis memo includes requests to IANA, including registrations for media
type definitions.TBDThere are Security Considerations. TBDChanges from version 04 to I2NSF related document version 00:
* Refactored main document to be more technology agnostic
* Added first draft of procedures for TPM 2.0
* Improved content consistency and structure of all sectionsChanges from version 03 to version 04:Refactoring of Introduction, intend, scope and audienceAdded first draft of Sync Base Prootoll section illustrated background for interaction with TSAAdded YANG moduleAdded missing changelog entryChanges from version 02 to version 03:Moved base concept out of IntroductionFirst refactoring of Introduction and ConceptFirst restructuring of Appendices and improved referencesChanges from version 01 to version 02:Restructuring of Introduction, highlighting conceptual prerequisitesRestructuring of Concept to better illustrate differences to hand-shake based attestation and deciding factors regarding freshness propertiesSubsection structure added to TerminologyClarification of descriptions of approach (these were the FIXMEs)Correction of RestrictionInfo structure: Added missing signature memberChanges from version 00 to version 01:Major update to the SNMP MIB and added a table for the Concise SWID profile Reference Hashes that provides additional information to be compared with the measurement logs.TBDKey words for use in RFCs to Indicate Requirement LevelsIn 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.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 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.Subscription to YANG Notifications for Datastore UpdatesThis document describes a mechanism that allows subscriber applications to request a continuous and customized stream of updates from a YANG datastore. Providing such visibility into updates enables new capabilities based on the remote mirroring and monitoring of configuration and operational state.Dynamic Subscription to YANG Events and Datastores over NETCONFThis document provides a Network Configuration Protocol (NETCONF) binding to the dynamic subscription capability of both subscribed notifications and YANG-Push.Subscription to YANG NotificationsThis document defines a YANG data model and associated mechanisms enabling subscriber-specific subscriptions to a publisher's event streams. Applying these elements allows a subscriber to request and receive a continuous, customized feed of publisher-generated information.Architecture and Reference Terminology for Remote Attestation ProceduresRemote ATtestation ProcedureS (RATS) architecture facilitates the attestation of device characteristics that, in general, are based on specific trustworthiness qualities intrinsic to a device or service. It includes trusted computing functionality provided by device hardware and software that allows trustworthiness qualities to be asserted and verified as part of, or pre-requisite to, the device's normal operation. The RATS architecture maps corresponding attestation functions and capabilities to specific RATS Roles. The goal is to enable an appropriate conveyance of evidence about device trustworthiness via network protocols. RATS Roles provide the endpoint context for understanding the various interaction semantics of the attestation lifecycle. The RATS architecture provides the building block concepts, semantics, syntax and framework for interoperable attestation while remaining hardware-agnostic. This flexibility is intended to address a significant variety of use-cases and scenarios involving interoperable attestation. Example usages include, but are not limited to: financial transactions, voting machines, critical safety systems, network equipment health, or trustworthy end-user device management. Existing industry attestation efforts may be helpful toward informing RATS architecture. Such as: Remote Integrity VERification (RIVER), the creation of Entity Attestation Tokens (EAT), software integrity Measurement And ATtestation (MAAT)Internet Security Glossary, Version 2This 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.Host Resources MIBThis 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]Entity MIB (Version 4)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.Management Information Base for Network Management of TCP/IP-based internets: MIB-IIThis 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]Management Information Base (MIB) for the Simple Network Management Protocol (SNMP)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]Concise Binary Object Representation (CBOR)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.Internet Standard 62Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data StructuresThis 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.Security Automation and Continuous Monitoring (SACM) TerminologyThis memo documents terminology used in the documents produced by SACM (Security Automation and Continuous Monitoring).CoAP Management InterfaceThis document describes a network management interface for constrained devices and networks, called CoAP Management Interface (CoMI). The Constrained Application Protocol (CoAP) is used to access datastore and data node resources specified in YANG, or SMIv2 converted to YANG. CoMI uses the YANG to CBOR mapping and converts YANG identifier strings to numeric identifiers for payload size reduction. The complete solution composed of CoMI, [I-D.ietf-core-yang-cbor] and [I-D.ietf-core-sid] is called CORECONF. CORECONF extends the set of YANG based protocols, NETCONF and RESTCONF, with the capability to manage constrained devices and networks.Concise Software Identification TagsISO/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 the same features as SWID tags, as well as additional semantics that allow CoSWIDs to describe additional types of information, all in a more memory efficient format.Reference Interaction Model for Challenge-Response-based Remote AttestationThis document defines an interaction model for a basic remote attestation procedure. Additionally, the required information elements are illustrated.Improving Scalability for Remote AttestationPrinciples of Remote AttestationImproving the scalability of platform attestationInformation technology -- Trusted Platform Module -- Part 1: OverviewTrusted Platform Module Library Specification, Family 2.0, Level 00, Revision 01.16 ed., Trusted Computing GroupTEE System Architecture v1.1, GPD_SPE_009Global PlatformTCG Attestation PTS Protocol Binding to TNC IF-MTCG TNC Working GroupTCG GlossaryTCGA CMC Profile for AIK Certificate EnrollmentTCG Infrastructure Working GroupTCG Credential ProfileTCG Infrastructure Working GroupArchitectural Styles and the Design of Network-based Software ArchitecturesUniversity of California, IrvineInternet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)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]An Architecture for Describing Simple Network Management Protocol (SNMP) Management FrameworksThis 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]URI Design and OwnershipSection 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 URI substructure is inappropriate, because that essentially usurps ownership. This document further describes this problematic practice and provides some acceptable alternatives for use in standards.JSON Web Token (JWT)JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted.Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingThe 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)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.Hypertext Transfer Protocol Version 2 (HTTP/2)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.Constrained RESTful Environments (CoRE) Link FormatThis 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]RESTCONF ProtocolThis 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).802.1AR-2009 - IEEE Standard for Local and metropolitan area networks - Secure Device IdentityIEEE Computer Society1609.4-2016 - IEEE Standard for Wireless Access in Vehicular Environments (WAVE) -- Multi-Channel OperationIEEE Computer SocietyEach 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 .SNMPv3 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: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.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.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).The following table summarizes the object groups, tables and their indexes, and conformance requirements for the TUDA MIB:A tudaV1<Group>CycleIndex is the:first index of a row (element instance or element fragment) in the
tudaV1<Group>Table;identifier of an update cycle on the table, when rows were added and/or
deleted from the table (bounded by tudaV1<Group>Cycles); andbinding in the tudaV1TrapV2Cycles notification for directed polling.A tudaV1<Group>InstanceIndex is the:second index of a row (element instance or element fragment) in the
tudaV1<Group>Table; except fora row in the tudaV1SyncTokenTable (that has only one instance per cycle).A tudaV1<Group>FragmentIndex is the:last index of a row (always an element fragment) in the
tudaV1<Group>Table; andaccomodation for SNMP transport mapping restrictions for large string
elements that require fragmentation.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 .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 .The General group in the TUDA MIB is analogous to the System group in MIB-II
and the System group in the SNMPv2 MIB and provides
context information for the TUDA attestation process.The following TPM structures, resources and functions are used within this approach.
They are based upon the TPM specifications and .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.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).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.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.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.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: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 AIKThis TickstampBlob is used as nonce to the Timestamp of the TSAAnother TickStampBlob with the AIK is created using the TSA’s Timestamp a nonceThe 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 tokenRequired TPM functions: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, andthe certificate.Required TPM functions: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.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.Required TPM functions: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.The pseudo code below includes general operations that are conducted as specific TPM commands:hash() : description TBDsig() : description TBDX.509-Certificate() : description TBDThese represent the output structure of that command in the form of a byte string value.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.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.The creation procedure is identical to .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.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.