1.1. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

1.2. Terminology

A number of terms are reused from [RFC9334]. These include Appraisal Policy for Evidence, Attestation Result, Attester, Evidence, Reference Value, Relying Party, Verifier, and Verifier Owner.¶

Additionally, this document defines the following term:¶

Attestation:

The process of generating, conveying, and appraising claims, backed by evidence, about device trustworthiness characteristics, including supply chain trust, identity, device provenance, software configuration, device composition, compliance to test suites, functional and assurance evaluations, etc.¶

The goal of attestation is simply to assure an administrator or auditor that the device's configuration and software were authentic and unmodified when the device started. The determination of software authenticity is not prescribed in this document, but it's typically taken to mean a software image generated by an authority trusted by the administrator, such as the device manufacturer.¶

Within the context of the Trusted Computing Group (TCG), the scope of attestation is typically narrowed to describe the process by which an independent Verifier can obtain cryptographic proof as to the identity of the device in question, evidence of the integrity of the device's software that was loaded upon startup, and verification that the current configuration matches the intended configuration. For network equipment, a Verifier capability can be embedded in a Network Management Station, a posture collection server, or other network analytics tool (such as a software asset management solution, or a threat detection and mitigation tool, etc.). This document focuses on a specific subset of attestation tasks, defined here as Remote Integrity Verification (RIV), and informally referred to as attestation. RIV in this document takes a network-equipment-centric perspective that includes a set of protocols and procedures for determining whether a particular device was launched with authentic software, starting from Roots of Trust. While there are many ways to accomplish attestation, RIV sets out a specific set of protocols and tools that work in environments commonly found in network equipment. RIV does not cover other device characteristics that could be attested (e.g., geographic location or connectivity; see [RATS-USECASES]), although it does provide evidence of a secure infrastructure to increase the level of trust in other device characteristics attested by other means (e.g., by Entity Attestation Tokens [RATS-EAT]).¶

In line with definitions found in [RFC9334], this document uses the term Endorser to refer to the role that signs identity and attestation certificates used by the Attester, while Reference Values are signed by a Reference Value Provider. Typically, the manufacturer of a network device would be accepted as both the Endorser and Reference Value Provider, although the choice is ultimately up to the Verifier Owner.¶

1.3. Document Organization

The remainder of this document is organized into several sections:¶

• The remainder of this section covers goals and requirements, plus a top-level description of RIV.¶
• The Solution Overview section (Section 2) outlines how RIV works.¶
• The Standards Components section (Section 3) links components of RIV to normative standards.¶
• The Privacy and Security Considerations sections (Sections 4 and 5) shows how specific features of RIV contribute to the trustworthiness of the Attestation Result.¶
• Supporting material is in an appendix (Appendix A).¶

1.4. Goals

Network operators benefit from a trustworthy attestation mechanism that provides assurance that their network comprises authentic equipment and has loaded software free of known vulnerabilities and unauthorized tampering. In line with the overall goal of assuring integrity, attestation can be used to assist in asset management, vulnerability and compliance assessment, plus configuration management.¶

The RIV attestation workflow outlined in this document is intended to meet the following high-level goals:¶

• Provable Device Identity - This specification requires that an Attester (i.e., the attesting device) includes a cryptographic identifier unique to each device. Effectively, this means that the device's TPM must be provisioned with this during the manufacturing cycle.¶
• Software Inventory - Key goals are to identify the software release(s) installed on the Attester and to provide evidence that the software stored within hasn't been altered without authorization.¶
• Verifiability - Verification of the device's software and configuration shows that the software that the administrator authorized for use was actually launched.¶

In addition, RIV is designed to operate either in a centralized environment, such as with a central authority that manages and configures a number of network devices, or "peer-to-peer", where network devices independently verify one another to establish a trust relationship. (See Section 3.3.)¶

1.5. Description of Remote Integrity Verification (RIV)

Attestation requires two interlocking mechanisms between the Attester network device and the Verifier:¶

• Device Identity is the mechanism that provides trusted identity, which can reassure network managers that the specific devices they ordered from authorized manufacturers for attachment to their network are those that were installed and that they continue to be present in their network. As part of the mechanism for Device Identity, cryptographic proof of the manufacturer's identity is also provided.¶
• Software Measurement is the mechanism that reports the state of mutable software components on the device and that can assure administrators that they have known, authentic software configured to run in their network.¶

By using these two interlocking mechanisms, RIV, which is a component in a chain of procedures, can assure a network operator that the equipment in their network can be reliably identified and that authentic software of a known version is installed on each device. Equipment in the network includes devices that make up the network itself, such as routers, switches, and firewalls.¶

Software used to boot a device can be identified by a chain of measurements, anchored at the start by a Root of Trust for Measurement (RTM) (see Appendix A.2). An attestation function embedded in each stage, verified by the previous stage, measures the next stage and records the result in tamper-resistant storage. A measurement signifies the identity, integrity, and version of each software component registered with an Attester's TPM [TPM-1.2] [TPM-2.0] so that a subsequent verification stage can determine if the software installed is authentic, up-to-date, and free of tampering.¶

RIV includes several major processes, which are split between the Attester and Verifier:¶

1. Generation of Evidence is the process whereby an Attester generates cryptographic proof (Evidence) of claims about device properties. In particular, the device identity and its software configuration are both of critical importance.¶
2. Device Identification refers to the mechanism assuring the Relying Party (ultimately, a network administrator) of the identities of devices, and the identities of their manufacturers, that make up their network.¶
3. Conveyance of Evidence reliably transports the collected Evidence from the Attester to a Verifier to allow a management station to perform a meaningful appraisal in Step 4. The transport is typically carried out via a management network. Although not required for reliable attestation, an encrypted channel may be used to provide integrity, authenticity, or confidentiality once attestation is complete. It should be noted that critical attestation evidence from the TPM is signed by a key known only to TPM, and is not dependent on encryption carried out as part of a reliable transport.¶
4. Finally, appraisal of evidence occurs. This is the process of verifying the Evidence received by a Verifier from the Attester and using an Appraisal Policy to develop an Attestation Result, which is used to inform decision-making. In practice, this means comparing the Attester's measurements reported as Evidence with the device configuration expected by the Verifier. Subsequently, the Appraisal Policy for Evidence might match Evidence found against Reference Values (aka Golden Measurements), which represent the intended configured state of the connected device.¶

All implementations supporting this RIV specification require the support of the following three technologies:¶

1. Identity: Device identity in RIV is based on Device Identity (DevID) defined by IEEE Std 802.1AR [IEEE-802-1AR], coupled with careful supply-chain management by the manufacturer. The Initial DevID (IDevID) certificate contains a statement by the manufacturer that establishes the identity of the device as it left the factory. Some applications with a more complex post-manufacture supply chain (e.g., value added resellers), or with different privacy concerns, may want to use alternative mechanisms for platform authentication (for example, TCG Platform Certificates [PLATFORM-CERTS] or post-manufacture installation of Local DevID (LDevID)).¶
2. Platform attestation provides evidence of configuration of software elements present in the device. This form of attestation can be implemented with TPM Platform Configuration Registers (PCRs) and Quote and Log mechanisms, which provide cryptographically authenticated evidence to report what software was started on the device through the boot cycle. Successful attestation requires an unbroken chain from a boot-time Root of Trust through all layers of software needed to bring the device to an operational state, in which each stage computes the hash of components of the next stage, then updates the attestation log and the TPM. The TPM can then report the hashes of all the measured hashes as signed evidence called a Quote (see Appendix A.1 for an overview of TPM operation or [TPM-1.2] and [TPM-2.0] for many more details).¶
3. Signed Reference Values (aka reference integrity measurements) must be conveyed from the Reference Value Provider (the entity accepted as the software authority, often the manufacturer of the network device) to the Verifier.¶

1.6. Solution Requirements

RIV must address the "Lying Endpoint" problem, in which malicious software on an endpoint may subvert the intended function and also prevent the endpoint from reporting its compromised status. (See Section 5 for further Security Considerations.)¶

RIV attestation is designed to be simple to deploy at scale. RIV should work "out of the box" as far as possible, that is, with the fewest possible provisioning steps or configuration databases needed at the end user's site. Network equipment is often required to "self-configure", to reliably reach out without manual intervention to prove its identity and operating posture, then download its own configuration, a process which precludes pre-installation configuration. See [RFC8572] for an example of Secure Zero Touch Provisioning (SZTP).¶

1.7. Scope

The need for assurance of software integrity, addressed by Remote Attestation, is a very general problem that could apply to most network-connected computing devices. However, this document includes several assumptions that limit the scope to network equipment (e.g., routers, switches, and firewalls):¶

• This solution is for use in non-privacy-preserving applications (for example, networking or industrial Internet of Things (IoT) applications), which avoids the need for a Privacy Certification Authority (also called an Attestation CA) for Attestation Keys (AKs) [AIK-ENROLL] or TCG Platform Certificates [PLATFORM-CERTS].¶
• This document assumes network protocols that are common in network equipment such as YANG [RFC7950] and Network Configuration Protocol (NETCONF) [RFC6241], but not generally used in other applications.¶
• The approach outlined in this document mandates the use of a TPM [TPM-1.2] [TPM-2.0] or a compatible cryptoprocessor.¶

2.1. RIV Software Configuration Attestation Using TPM

RIV Attestation is a process that can be used to determine the identity of software running on a specifically identified device. The Remote Attestation steps of Section 1.5 are split into two phases as shown in Figure 1:¶

• During system startup, or Boot Phase, each distinct software object is "measured" by the Attester. The object's identity, hash (i.e., cryptographic digest), and version information are recorded in a log. Hashes are also extended into the TPM (see Appendix A.1 for more on extending hashes) in a way that can be used to validate the log entries. The measurement process generally follows the layered chain-of-trust model used in Measured Boot, where each stage of the system measures the next one, and extends its measurement into the TPM, before launching it. See Section 3.2 of [RFC9334], "Layered Attestation Environments", for an architectural definition of this model.¶
• Once the device is running and has operational network connectivity, verification can take place. A separate Verifier, running in its own trusted environment, will interrogate the network device to retrieve the logs and a copy of the digests collected by hashing each software object, signed by an attestation private key secured by, but never released by, the TPM. The YANG model described in [RFC9684] facilitates this operation.¶

The result is that the Verifier can verify the device's identity by checking the subject [RFC5280] and signature of the certificate containing the TPM's attestation public key. The Verifier can then verify the log's correctness by accumulating all the hashes in the log and comparing that to the signed digests from the TPM. From there, the Verifier can validate the launched software by comparing the digests in the log with Reference Values.¶

It should be noted that attestation and identity are inextricably linked; signed Evidence that a particular version of software was loaded is of little value without cryptographic proof of the identity of the Attester producing the Evidence.¶

    +-------------------------------------------------------+
    | +---------+    +--------+   +--------+    +---------+ |
    | |UEFI BIOS|--->| Loader |-->| Kernel |--->|Userland | |
    | +---------+    +--------+   +--------+    +---------+ |
    |     |            |           |                        |
    |     |            |           |                        |
    |     +------------+-----------+-+                      |
    |                    Boot Phase  |                      |
    |                                V                      |
    |                            +--------+                 |
    |                            |  TPM   |                 |
    |                            +--------+                 |
    |   Router                       |                      |
    +--------------------------------|----------------------+
                                     |
                                     |  Verification Phase
                                     |    +-----------+
                                     +--->| Verifier  |
                                          +-----------+

    Reset---------------flow-of-time-during-boot...--------->

In the Boot Phase, measurements are "extended", or hashed, into the TPM as processes start, which result in the TPM containing hashes of all the measured hashes. Later, once the system is operational, signed digests are retrieved from the TPM during the Verification Phase for off-box analysis.¶

2.2. RIV Keying

RIV attestation relies on two credentials:¶

• An identity key pair and matching certificate is required to certify the identity of the Attester itself. RIV specifies the use of an IEEE 802.1AR DevID [IEEE-802-1AR] that is signed by the device manufacturer and contains the device serial number. This requirement goes slightly beyond 802.1AR; see Section 2.4 for notes.¶
• An Attestation key pair and matching certificate is required to sign the Quote generated by the TPM to report evidence of software configuration.¶

In a TPM application, both the Attestation private key and the DevID private key MUST be protected by the TPM. Depending on other TPM configuration procedures, the two keys are likely to be different; some of the considerations are outlined in the "TPM 2.0 Keys for Device Identity and Attestation" document [PLATFORM-DEVID-TPM-2.0].¶

The "TPM 2.0 Keys for Device Identity and Attestation" document [PLATFORM-DEVID-TPM-2.0] specifies further conventions for these keys:¶

• When separate Identity and Attestation keys are used, the AK and its X.509 certificate should parallel the DevID, with the same unique device identification as the DevID certificate (that is, the same subject and subjectAltName (if present), even though the key pairs are different). By examining the corresponding AK certificate, the Verifier can directly link a device's quote, which was signed by an AK, to the device that provided it. If the subject in the AK certificate doesn't match the corresponding DevID certificate, or if they're signed by different authorities, the Verifier may signal the detection of an Asokan-style person-in-the-middle attack (see Section 5.2).¶
• Network devices that are expected to use SZTP as specified in [RFC8572] MUST be shipped by the manufacturer with pre-provisioned keys (Initial DevID and Initial AK, called IDevID and IAK, respectively). IDevID and IAK certificates MUST both be signed by the Endorser (typically the device manufacturer). Inclusion of an IDevID and IAK by a vendor does not preclude a mechanism whereby an administrator can define LDevID and Local Attestation Keys (LAK) if desired.¶

2.3. RIV Information Flow

RIV workflow for network equipment is organized around a simple use case where a network operator wishes to verify the integrity of software installed in specific, fielded devices. A normative taxonomy of terms is given in [RFC9334], but as a reminder, this use case implies several roles and objects:¶

Attester:

The device that the network operator wants to examine.¶

Verifier:

Which might be a Network Management Station and is somewhat separate from the Device that will retrieve the signed evidence and measurement logs, and analyze them to pass judgment on the security posture of the device.¶

Relying Party:

Can act on Attestation Results. Interaction between the Relying Party and the Verifier is considered out of scope for RIV.¶

Signed Reference Integrity Manifests (RIMs):

Contains Reference Values. RIMs can either be created by the device manufacturer and shipped along with the device as part of its software image, or alternatively, could be obtained several other ways (direct to the Verifier from the manufacturer, from a third party, from the owner's concept of a "known good system", etc.). Retrieving RIMs from the device itself allows attestation to be done in systems that may not have access to the public Internet, or by other devices that are not management stations per se (e.g., a peer device; see Section 3.1.3). If Reference Values are obtained from multiple sources, the Verifier may need to evaluate the relative level of trust to be placed in each source in case of a discrepancy.¶

These components are illustrated in Figure 2.¶

+----------------+        +-------------+        +---------+--------+
|Reference Value |        | Attester    | Step 1 | Verifier|        |
|Provider        |        | (Device     |<-------| (Network| Relying|
|(Device         |        | under       |------->| Mgmt    | Party  |
|Manufacturer    |        | attestation)| Step 2 | Station)|        |
|or other        |        |             |        |         |        |
|authority)      |        |             |        |         |        |
+----------------+        +-------------+        +---------+--------+
       |                                             /\
       |                  Step 0                      |
       -----------------------------------------------

Step 0:

The Reference Value Provider (the device manufacturer or other authority) makes one or more RIMs, which correspond to the software image expected to be found on the device and are signed by the Reference Value Provider, available to the Verifier. (See Section 3.1.3 for "in-band" and "out of band" ways to make this happen.)¶

Step 1:

On behalf of a Relying Party, the Verifier (Network Management Station) requests DevID, Measurement Values, and possibly RIMs from the Attester.¶

Step 2:

The Attester responds to the request by providing a DevID, quotes (measured values that are signed by the Attester), and optionally RIMs.¶

The use of the following standards components allows for interoperability:¶

1. TPM keys MUST be configured according to [PLATFORM-DEVID-TPM-2.0] or [PLATFORM-ID-TPM-1.2].¶
2. For devices using UEFI and Linux, measurements of firmware and bootable modules MUST be taken according to "TCG EFI Platform Specification" [PC-CLIENT-EFI-TPM-1.2] or "TCG PC Client Specific Platform Firmware Profile Specification" [PC-CLIENT-BIOS-TPM-2.0], and Linux IMA [IMA].¶
3. DevID MUST be managed as DevID certificates as specified in IEEE Std 802.1AR [IEEE-802-1AR], with keys protected by TPMs.¶
4. Attestation logs from Linux-based systems MUST be formatted according to the "Canonical Event Log Format" [CEL]. UEFI-based systems MUST use the TCG UEFI BIOS event log [PC-CLIENT-EFI-TPM-1.2] for TPM 1.2 systems and the "TCG PC Client Specific Platform Firmware Profile" [PC-CLIENT-BIOS-TPM-2.0] for TPM 2.0 systems.¶
5. Quotes MUST be retrieved from the TPM according to the TCG Trusted Attestation Protocol Information Model (TAP IM) [TAP] and the Challenge-Response-based Remote Attestation (CHARRA) YANG model [RFC9684]. While the TAP IM gives a protocol-independent description of the data elements involved, it's important to note that quotes from the TPM are signed inside the TPM and MUST be retrieved in a way that does not invalidate the signature, to preserve the trust model. The CHARRA YANG model [RFC9684] is used for this purpose. (See Section 5, Security Considerations).¶
6. Reference Values MUST be encoded as defined in the TCG RIM document [RIM], typically using Software Identification (SWID) tags [SWID] [NIST-IR-8060] or Concise SWID (CoSWID) tags [RFC9393].¶

2.4. RIV Simplifying Assumptions

This document makes the following simplifying assumptions to reduce complexity:¶

• The product to be attested MUST be shipped by the equipment vendor with both a DevID as specified by IEEE Std 802.1AR and an IAK, with certificates in place. The IAK certificate must contain the same identity information as the DevID (specifically, the same subject and subjectAltName (if used), signed by the manufacturer). The IAK is a type of key that can be used to sign a TPM Quote, but not other objects (i.e., it's marked as a TCG "Restricted" key; this convention is described in "TPM 2.0 Keys for Device Identity and Attestation" [PLATFORM-DEVID-TPM-2.0]). For network equipment, which is generally not privacy sensitive, shipping a device with both an IDevID and an IAK already provisioned substantially simplifies initial startup.¶

• IEEE Std 802.1AR does not require a product serial number as part of the subject, but RIV-compliant devices MUST include their serial numbers in the DevID/IAK certificates to simplify tracking logistics for network equipment users. All other optional 802.1AR fields remain optional in RIV.¶

  It should be noted that the use of X.509 certificate fields as specified by IEEE Std 802.1AR is not identical to that described in [RFC9525] for representation of application service identity.¶

• The product MUST be equipped with an RTM, a Root of Trust for Storage, and a Root of Trust for Reporting (as defined in [SP800-155]), which together are capable of conforming to the TCG TAP IM [TAP].¶
• The authorized software supplier MUST make available Reference Values in the form of signed SWID or CoSWID tags.¶

3.1. Prerequisites for RIV

The Reference Interaction Model for Challenge-Response-based Remote Attestation ([RATS-INTERACTION-MODELS]) is based on the standard roles defined in [RFC9334]. However, additional prerequisites have been established to allow for interoperable implementations of RIV use cases. These prerequisites are intended to provide sufficient context information so that the Verifier can acquire and evaluate measurements collected by the Attester.¶

3.2. Reference Model for Challenge-Response

Once the prerequisites for RIV are met, a Verifier is able to acquire Evidence from an Attester. Figure 3 illustrates a RIV information flow between a Verifier and an Attester, derived from Section 7.1 of [RATS-INTERACTION-MODELS]. In this diagram, each event with its input and output parameters is shown as "Event(input-params)=>(outputs)". The event times shown correspond to the time types described within Appendix A of [RFC9334]:¶

.----------.                               .-----------------------.
| Attester |                              | Relying Party/Verifier |
'----------'                              '------------------------'
  time(VG)                                                      |
generateClaims(attestingEnvironment)                            |
   | => claims, eventLogs                                       |
   |                                                            |
   |                                                        time(NS)
   | <-- requestAttestation(handle, authSecIDs, claimSelection) |
   |                                                            |
 time(EG)                                                       |
collectClaims(claims, claimSelection)                           |
   | => collectedClaims                                         |
   |                                                            |
generateEvidence(handle, authSecIDs, collectedClaims)           |
   | => evidence                                                |
   |                                                    time(RG,RA)
   | evidence, eventLogs -------------------------------------> |
   |                                                            |
   |               appraiseEvidence(evidence, eventLogs, refValues)
   |                                       attestationResult <= |
   |                                                            |
   ~                                                            ~
   |                                                       time(RX)

Step 1 (time(VG)):

One or more attesting network device PCRs are extended with measurements. RIV provides no direct link between the time at which the event takes place and the time that it's attested, although streaming attestation as described in [RATS-NET-DEV-SUB] could.¶

Step 2 (time(NS)):

The Verifier generates a unique random nonce ("number used once") and makes a request for one or more PCRs from an Attester. For interoperability, this must be accomplished as specified in "A YANG Data Model for Challenge-Response-Based Remote Attestation (CHARRA) Procedures Using Trusted Platform Modules (TPMs)" [RFC9684]. Both TPM 1.2 and TPM 2.0 allow nonces as large as the operative digest size (i.e., 20 or 32 bytes; see [TPM-1.2] Part 2, Section 5.5, and [TPM-2.0] Part 2, Section 10.4.4).¶

Step 3 (time(EG)):

On the Attester, measured values are retrieved from the Attester's TPM. This requested PCR evidence along with the Verifier's nonce is called a Quote and is signed by the AK associated with the DevID. Quotes are retrieved according to the CHARRA YANG model [RFC9684]. At the same time, the Attester collects log evidence showing the values have been extended into that PCR. Appendix A.1 gives more detail on how this works and includes references to the structure and contents of quotes in TPM documents.¶

Step 4:

The collected Evidence is passed from the Attester to the Verifier.¶

Step 5 (time(RG,RA)):

The Verifier reviews the Evidence and takes action as needed. As the interaction between Relying Party and Verifier is out of scope for RIV, this can be described as one step.¶

• If the signature covering TPM Evidence is not correct, the device SHOULD NOT be trusted.¶
• If the nonce in the response doesn't match the Verifier's nonce, the response may be a replay, and the device SHOULD NOT be trusted.¶
• If the signed PCR values do not match the set of log entries that have extended a particular PCR, the device SHOULD NOT be trusted.¶
• If the log entries that the Verifier considers important do not match known good values, the device SHOULD NOT be trusted. We note that the process of collecting and analyzing the log can be omitted if the value in the relevant PCR is already a known-good value.¶
• If the set of log entries are not seen as acceptable by the Appraisal Policy for Evidence, the device SHOULD NOT be trusted.¶
• If time(RG)-time(NS) is greater than the Appraisal Policy for Evidence's threshold for assessing freshness, the Evidence is considered stale and SHOULD NOT be trusted.¶

3.3. Centralized vs. Peer-to-Peer

Figure 3 assumes that the Verifier is trusted, while the Attester is not. In a peer-to-peer application such as two routers negotiating a trust relationship, the two peers can each ask the other to prove software integrity. In this application, the information flow is the same, but each side plays a role both as an Attester and a Verifier. Each device issues a challenge, and each device responds to the other's challenge, as shown in Figure 4. Peer-to-peer challenges, particularly if used to establish a trust relationship between routers, require devices to carry their own signed reference measurements (RIMs). Devices may also have to carry an appraisal policy for evidence for each possible peer device so that each device has everything needed for remote attestation, without having to resort to a central authority.¶

+---------------+                            +---------------+
| RefVal        |                            | RefVal        |
| Provider A    |                            | Provider B    |
| Firmware      |                            | Firmware      |
| Configuration |                            | Configuration |
| Authority     |                            | Authority     |
|               |                            |               |
+---------------+                            +---------------+
      |                                             |
      |                                             |Step 0B
      |       +------------+        +------------+  |
      |       |            | Step 1 |            |  |   \
      |       | Attester   |<------>| Verifier   |  |   |
      |       |            |<------>|            |  |   |  Router B
      +------>|            | Step 2 |            |  |   |- Challenges
       Step 0A|            |        |            |  |   |  Router A
              |            |------->|            |  |   |
              |- Router A -| Step 3 |- Router B -|  |   /
              |            |        |            |  |
              |            |        |            |  |
              |            | Step 1 |            |  |   \
              | Verifier   |<------>| Attester   |<-+   |  Router A
              |            |<------>|            |      |- Challenges
              |            | Step 2 |            |      |  Router B
              |            |        |            |      |
              |            |<-------|            |      |
              +------------+ Step 3 +------------+      /

In this application, each device may need to be equipped with signed RIMs to act as an Attester, and to allow each device to act as a Verifier, each may need to be equipped with an Appraisal Policy for Evidence and a selection of trusted X.509 root certificates also. An existing link layer protocol such as 802.1X [IEEE-802.1X] or 802.1AE [IEEE-802.1AE], with Evidence being enclosed over a variant of the Extensible Authentication Protocol (EAP) [RFC3748] or Link Layer Discovery Protocol (LLDP) [LLDP], are suitable methods for such an exchange. Details of peer-to-peer operation are out of scope for this document.¶

5.1. Keys Used in RIV

Trustworthiness of RIV attestation depends strongly on the validity of keys used for identity and attestation reports. RIV takes full advantage of TPM capabilities to ensure that evidence can be trusted.¶

Two sets of key pairs are relevant to RIV attestation:¶

• A DevID key pair is used to certify the identity of the device in which the TPM is installed.¶
• An AK key pair is used to certify attestation Evidence (i.e., quotes) and to provide evidence for integrity of the device software.¶

TPM practices usually require that these keys be different to ensure that a general-purpose signing key cannot be used to spoof an attestation quote.¶

In each case, the private half of the key is known only to the TPM and cannot be retrieved externally, even by a trusted party. To ensure that's the case, specification-compliant private/public key pairs are generated inside the TPM, where they are never exposed and cannot be extracted (see [PLATFORM-DEVID-TPM-2.0]).¶

Keeping keys safe is a critical enabler of trustworthiness, but it's just part of attestation security; knowing which keys are bound to the device in question is just as important in an environment where private keys are never exposed.¶

While there are many ways to manage keys in a TPM (see [PLATFORM-DEVID-TPM-2.0]), RIV includes support for "zero touch" provisioning (also known as zero touch onboarding) of fielded devices (e.g., SZTP [RFC8572]), where keys that have predictable trust properties are provisioned by the device vendor.¶

Device identity in RIV is based on DevID defined by IEEE Std 802.1AR. This specification provides several elements:¶

• A DevID requires a unique key pair for each device, accompanied by an X.509 certificate.¶
• The private portion of the DevID key is to be stored in the device, in a manner that provides confidentiality (Section 6.2.5 of [IEEE-802-1AR]).¶

The X.509 certificate contains several components:¶

• The public part of the unique DevID key assigned to that device allows a challenge of identity.¶
• An identifying string that's unique to the manufacturer of the device. This is normally the serial number of the unit, which might also be printed on a label on the device.¶
• The certificate must be signed by a key traceable to the manufacturer's root key.¶

With these elements, the device's manufacturer and serial number can be identified by analyzing the DevID certificate plus the chain of intermediate certificates leading back to the manufacturer's root certificate. As is conventional in TLS or SSH connections, a random nonce must be signed by the device in response to a challenge, proving possession of its DevID private key.¶

RIV uses the DevID to validate a TLS or SSH connection to the device as the attestation session begins. Security of this process derives from TLS or SSH security, with the DevID, which contains a device serial number, providing proof that the session terminates on the intended device. See [RFC8446] [RFC4253].¶

Evidence of software integrity is delivered in the form of a quote that is signed by the TPM itself and accompanied by an IAK certificate containing the same identity information as the DevID. Because the contents of the quote are signed inside the TPM, any external modification (including reformatting to a different data format) after measurements have been taken will be detected as tampering. An unbroken chain of trust is essential for ensuring that blocks of code that are taking measurements have been verified before execution (see Figure 1).¶

Requiring measurements of the operating software to be signed by a key known only to the TPM also removes the need to trust the device's operating software (beyond the first measurement in the RTM; see below). If malicious software makes any changes to a quote in the device itself, or in the path back to the Verifier, the signature on the quote will be invalidated.¶

A critical feature of the YANG model described in [RFC9684] is the ability to carry TPM data structures in their TCG-defined format, without requiring any changes to the structures as they were signed and delivered by the TPM. While alternate methods of conveying TPM quotes could reduce redundant information, or add another layer of signing using external keys, the implementation MUST preserve the TPM signing so that tampering anywhere in the path between the TPM itself and the Verifier can be detected.¶

5.2. Prevention of Spoofing and Person-in-the-Middle Attacks

Prevention of spoofing attacks against attestation systems is also important. There are several cases to consider:¶

• The entire device could be spoofed. If the Verifier goes to appraise a specific Attester, it might be redirected to a different Attester.¶
• A compromised device could have a valid DevID, but substitute a quote from a known-good device instead of returning its own, as described in [RFC6813].¶
• A device with a compromised OS could return a fabricated quote providing spoofed attestation Evidence.¶

Use of the 802.1AR DevID in the TPM provides protection against the case of a spoofed device by ensuring that the Verifier's TLS or SSH session is in fact terminating on the right device.¶

Protection against spoofed quotes from a device with valid identity is a bit more complex. An identity key must be available to sign any kind of nonce or hash offered by the Verifier, and consequently, could be used to sign a fabricated quote. To block a spoofed Attestation Result, the quote generated inside the TPM must be signed by a key, known as an AK, that's different from the DevID.¶

Given separate Attestation and DevID keys, the binding between the AK and the same device must also be proven to prevent a person-in-the-middle attack (e.g., the "Asokan Attack" [RFC6813]).¶

This is accomplished in RIV through use of an AK certificate with the same elements as the DevID (same manufacturer's serial number and signed by the same manufacturer's key), but containing the device's unique AK public key instead of the DevID public key. This binding between DevID and AK certificates is critical to reliable attestation.¶

The TCG document "TPM 2.0 Keys for Device Identity and Attestation" [PLATFORM-DEVID-TPM-2.0] specifies OIDs for Attestation Certificates that allow the CA to mark a key as specifically known to be an AK.¶

These two key pairs and certificates are used together:¶

• The DevID is used to validate a TLS connection terminating on the device with a known serial number.¶
• The AK is used to sign attestation quotes, which provides proof that the attestation evidence comes from the same device.¶

5.3. Replay Attacks

Replay attacks, where the results of a previous attestation are submitted in response to subsequent requests, are usually prevented by the inclusion of a random nonce in the request to the TPM for a quote. Each request from the Verifier includes a new random number (a nonce). The resulting quote signed by the TPM contains the same nonce, which allows the Verifier to determine freshness (i.e., that the resulting quote was generated in response to the Verifier's specific request). "Time-Based Uni-Directional Attestation" [RATS-TUDA] provides an alternate mechanism to verify freshness without requiring a request/response cycle.¶

5.4. Owner-Signed Keys

Although device manufacturers must pre-provision devices with easily verified DevID and AK certificates if SZTP such as described in [RFC8572] is to be supported, use of those credentials is not mandatory. IEEE Std 802.1AR incorporates the idea of an IDevID, which is provisioned by the manufacturer, and a LDevID, which is provisioned by the owner of the device. RIV and [PLATFORM-DEVID-TPM-2.0] extend that concept by defining an IAK and LAK with the same properties.¶

Device owners can use any method to provision the local credentials.¶

• The TCG document [PLATFORM-DEVID-TPM-2.0] shows how the IAKs can be used to certify LDevID and LAK keys. The use of the LDevID and LAK allows the device owner to use a uniform identity structure across device types from multiple manufacturers (in the same way that an "Asset Tag" is used by many enterprises to identify devices they own). The TCG document [PROV-TPM-2.0] also contains guidance on provisioning local identity keys in TPM 2.0. Owners should follow the same practice of binding LDevID and LAK as the manufacturer would for IDevID and IAK. See Section 2.2.¶
• Device owners, however, can use any other mechanism they want, including physical inspection and programming in a secure location, to assure themselves that local identity certificates are inserted into the intended device if they prefer to avoid placing trust in the manufacturer-provided keys.¶

Clearly, local keys can't be used for SZTP; installation of the local keys can only be done by some process that runs before the device is installed for network operation, or by using procedures such as those outlined in Bootstrapping Remote Secure Key Infrastructure (BRSKI) [RFC8995].¶

On the other end of the device lifecycle, provision should be made to wipe local keys when a device is decommissioned to indicate that the device is no longer owned by the enterprise. The manufacturer's initial identity keys must be preserved, as they contain no information that's not already printed on the device's serial number plate.¶

5.5. Other Factors for Trustworthy Operation

In addition to the trustworthy provisioning of keys, RIV depends on a number of other factors for trustworthy operation.¶

• Secure identity depends on mechanisms to prevent per-device secret keys from being compromised. The TPM provides this capability as a Root of Trust for Storage.¶
• Attestation depends on an unbroken chain of measurements, starting from the very first measurement. See Appendix A.1 for background on TPM practices.¶
• That first measurement is made by code called the RTM, typically done by trusted firmware stored in boot flash. Mechanisms for maintaining the trustworthiness of the RTM are out of scope for RIV, but could include immutable firmware, signed updates, or a vendor-specific hardware verification technique. See Appendix A.2 for background on Roots of Trust.¶
• The device owner SHOULD provide some level of physical defense for the device. If a TPM that has already been programmed with an authentic DevID is stolen and is inserted into a counterfeit device, attestation of that counterfeit device may become indistinguishable from an authentic device.¶

RIV also depends on reliable Reference Values, as expressed by the RIM [RIM]. The definition of trust procedures for RIMs is out of scope for RIV, and the device owner is free to use any policy to validate a set of reference measurements. It should also be noted that, while RIV can provide a reliable indication that a known software package is in use by the device and that the package has not been tampered with, it is the device owner's responsibility to determine that it's the correct package for the application.¶

RIMs may be conveyed either out-of-band or in-band as part of the attestation process (see Section 3.1.3). However, for network devices, where software is usually shipped as a self-contained package, RIMs signed by the manufacturer and delivered in-band may be more convenient for the device owner.¶

The validity of RIV attestation results is also influenced by procedures used to create Reference Values:¶

• While the RIM itself is signed, supply chains SHOULD be carefully scrutinized to ensure that the values are not subject to unexpected manipulation prior to signing. Insider attacks against code bases and build chains are particularly hard to spot.¶
• Designers SHOULD guard against hash collision attacks. RIMs often give hashes for large objects of indeterminate size. If one of the measured objects can be replaced with an implant engineered to produce the same hash, RIV will be unable to detect the substitution. TPM 1.2 only uses SHA-1 hashes, which have been shown to be susceptible to collision attack. TPM 2.0 will produce quotes with SHA-256, which so far has resisted such attacks. Consequently, RIV implementations SHOULD use TPM 2.0.¶

A.1. Using a TPM for Attestation

The TPM and surrounding ecosystem provide three interlocking capabilities to enable secure collection of evidence from a remote device: PCRs, a Quote mechanism, and a standardized Event Log.¶

Each TPM has at least eight and at most twenty-four PCRs (depending on the profile and vendor choices), each one large enough to hold one hash value (SHA-1, SHA-256, and other hash algorithms can be used, depending on TPM version). PCRs can't be accessed directly from outside the chip, but the TPM interface provides a way to "extend" a new security measurement hash into any PCR, a process by which the existing value in the PCR is hashed with the new security measurement hash, and the result placed back into the same PCR. The result is a composite fingerprint comprising the hash of all the security measurements extended into each PCR since the system was reset.¶

Every time a PCR is extended, an entry should be added to the corresponding Event Log. Logs contain the security measurement hash plus informative fields offering hints as to which event generated the security measurement. The Event Log itself is protected against accidental manipulation, but it is implicitly tamper-evident: Any verification process can read the security measurement hash from the log events, compute the composite value, and compare that to what is in the PCR. If there's no discrepancy, the logs do provide an accurate view of what was placed into the PCR.¶

Note that the composite hash-of-hashes recorded in PCRs is order-dependent, resulting in different PCR values for different ordering of the same set of events (e.g., Event A followed by Event B yields a different PCR value than B followed by A). For single-threaded code, where both the events and their order are fixed, a Verifier may validate a single PCR value, and use the log only to diagnose a mismatch from Reference Values. However, operating system code is usually nondeterministic, meaning that there may never be a single "known good" PCR value. In this case, the Verifier may have to verify that the log is correct, and then analyze each item in the log to determine if it represents an authorized event.¶

In a conventional TPM Attestation environment, the first measurement must be made and extended into the TPM by trusted device code (called the RTM). That first measurement should cover the segment of code that is run immediately after the RTM, which then measures the next code segment before running it, and so on, forming an unbroken chain of trust. See [TCG-RT] for more on Mutable vs. Immutable Roots of Trust.¶

The TPM provides another mechanism called a Quote that can read the current value of the PCRs and package them, along with the Verifier's nonce, into a TPM-specific data structure signed by an Attestation private key, known only to the TPM.¶

It's important to note that the Quote data structure is signed inside the TPM (see Section 5, Security Considerations). The trust model is preserved by retrieving the Quote in a way that does not invalidate the signature, as specified in [RFC9684]. The structure of the command and response for a quote, including its signature, as generated by the TPM, can be seen in Part 3, Section 16.5, of [TPM-1.2] and Section 18.4.2 of [TPM-2.0].¶

The Verifier uses the Quote and Log together. The Quote contains the composite hash of the complete sequence of security measurement hashes, signed by the TPM's private AK. The Log contains a record of each measurement extended into the TPM's PCRs. By computing the composite hash of all the measurements, the Verifier can verify the integrity of the Event Log, even though the Event Log itself is not signed. Each hash in the validated Event Log can then be compared to corresponding expected values in the set of Reference Values to validate overall system integrity.¶

A summary of information exchanged in obtaining quotes from TPM 1.2 and TPM 2.0 can be found in [TAP], Section 4. Detailed information about PCRs and Quote data structures can be found in [TPM-1.2], [TPM-2.0]. Recommended log formats include [PC-CLIENT-BIOS-TPM-2.0], and [CEL].¶

A.2. Root of Trust for Measurement (RTM)

The measurements needed for attestation require that the device being attested is equipped with an RTM, that is, some trustworthy mechanism that can compute the first measurement in the chain of trust required to attest that each stage of system startup is verified, a Root of Trust for Storage (i.e., the TPM PCRs) to record the results, and a Root of Trust for Reporting to report the results.¶

While there are many complex aspects of Roots of Trust ([TCG-RT] [SP800-155] [SP800-193]), two aspects that are important in the case of attestation are:¶

• The first measurement computed by the RTM and stored in the TPM's Root of Trust for Storage must be assumed to be correct.¶
• There must not be a way to reset the Root of Trust for Storage without re-entering the RTM code.¶

The first measurement must be computed by code that is implicitly trusted; if that first measurement can be subverted, none of the remaining measurements can be trusted. (See [SP800-155].)¶

It's important to note that the trustworthiness of the RTM code cannot be assured by the TPM or TPM supplier -- code or procedures external to the TPM must guarantee the security of the RTM.¶

A.3. Layering Model for Network Equipment Attester and Verifier

Retrieval of identity and attestation state uses one protocol stack, while retrieval of Reference Values uses a different set of protocols. Figure 5 shows the components involved.¶

+-----------------------+              +-------------------------+
|                       |              |                         |
|       Attester        |<-------------|        Verifier         |
|       (Device)        |------------->|   (Management Station)  |
|                       |      |       |                         |
+-----------------------+      |       +-------------------------+
                               |
           -------------------- --------------------
           |                                        |
-------------------------------    ---------------------------------
|      Reference Values       |    |          Attestation          |
-------------------------------    ---------------------------------

********************************************************************
*           IETF Remote Attestation Conceptual Data Flow           *
*                        RFC9334, Figure 1                         *
********************************************************************

    .........................          .........................
    .  Reference Integrity  .          .       TAP Info        .
    .       Manifest        .          .  Model and Canonical  .
    .                       .          .      Log Format       .
    .........................          .........................

    *************************          *************************
    *    YANG SWID Module   *          *    YANG Attestation   *
    *       RFC9393         *          *        Module         *
    *                       *          *        RFC9684        *
    *                       *          *                       *
    *************************          *************************

    *************************          *************************
    * XML, JSON, CBOR, etc. *          * XML, JSON, CBOR, etc. *
    *************************          *************************

    *************************          *************************
    *   RESTCONF/NETCONF    *          *   RESTCONF/NETCONF    *
    *************************          *************************

    *************************          *************************
    *       TLS, SSH        *          *       TLS, SSH        *
    *************************          *************************

IETF documents are captured in boxes surrounded by asterisks. TCG documents are shown in boxes surrounded by dots.¶

A.4. Implementation Notes

Table 2 summarizes many of the actions needed to complete an Attestation system, with links to relevant documents. While documents are controlled by several standards organizations, the implied actions required for implementation are all the responsibility of the manufacturer of the device, unless otherwise noted.¶

As noted, SWID tags can be generated many ways, but one possible tool is [SWID-GEN].¶