]> Arm Limited

brendan.moran.ietf@gmail.com

hannes.tschofenig@gmx.net

Security SUIT Internet-Draft The Manufacturer Usage Description (MUD) specification describes the access and network functionality required for a device to properly function. This description has to reflect the software running on the device and its configuration. Because of this, the most appropriate entity for describing device network access requirements is the same as the entity developing the software and its configuration. A network presented with a MUD file by a device allows detection of misbehavior by the device software and configuration of access control. This document defines a way to link the Software Updates for Internet of Things (SUIT) manifest to a MUD file offering a stronger binding between the two.

Introduction A Manufacturer Usage Description (MUD) file describes what sort of network communication behavior a device is designed to have. For example, a manufacturer may use a MUD file to describe that a device uses HTTP, DNS and NTP communication but no other protocols. The communication patterns are described in a JSON-based format in the MUD file. The MUD files do, however, need to be presented by the device to a MUD manager in the operational network where the device is deployed. Under , devices report a MUD URL to a MUD manager in the operational network. The MUD URL is a URL that can be used by the MUD manager to receive the MUD file from a MUD file server to ultimately obtain the MUD file. shows the MUD architecture, as defined in RFC 8520.

get URL-->| MUD | . | Manager | .(https) | File Server | . End system network |____________|<-MUD file<-<|_____________| . . . . . . . ________ _________ . .| | | router | . .| Device |--->MUD URL-->| or | . .|________| | switch | . . |_________| . ....................................... ]]>

RFC 8520 envisions different approaches for conveying the MUD URL from the device to the operational network. Section 4 of provides additional description of the MUD URLs sources, which include: DHCP, IEEE 802.1AB Link Layer Discovery Protocol (LLDP), and Device Certificates, such as IEEE 802.1X, whereby the URL to the MUD File would be contained in the certificate. The MUD manager must trust the MUD file server from which the MUD file is fetched to return the most up-to-date MUD file. It must also trust the device to report the correct MUD URL. In case of DHCP and LLDP the URL is unprotected and not bound to the device itself. When the MUD URL is included in a certificate then it is authenticated and integrity protected. However, the certificate only proves possession of a private key and endorsements by the certificate issuer. This does not prove what software is in use, nor does it prove that the MUD file is the correct file for the deployed software: instead, the responsibility falls on the certificate issuer to identify the MUD URL correctly and to supply a MUD Signer correctly. There is a need to bind the entity that creates the software and configuration to the MUD file. The developer is in the best position to describe the communication requirements of the software it developed and configured for a device. This specification defines an extension to the Software Updates for Internet of Things (SUIT) manifest to include a MUD URL. A SUIT manifest is a bundle of metadata about code/data for an IoT device, where to find the code/data, the devices to which it applies, and cryptographic information protecting the manifest. When combining a MUD URL with a manifest used for software/firmware updates then a network operator has more confidence in the description of the communication requirements for a device to properly function.

Terminology 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. This document re-uses the terms defined in related to remote attestation. Readers of this document are assumed to be familiar with the following terms: Evidence, Claim, Attester, Verifier, and Relying Party (RP). This document also uses terms defined in , such as MUD, MUD file, MUD manager, MUD URL, etc.

Workflow shows the architectural extensions introduced by combining SUIT and MUD. The key elements are that the developer, who produces the firmware is also generating a manifest and the MUD file. Information about the MUD file is embedded into the SUIT manifest and provided to the device via firmware update mechanism. Once this information is available on the device it can be presented during device onboarding, during network access authentication, or as part of other interactions that involve the conveyance of Evidence to the operational network. After retrieving the manifest, the MUD file can be obtained as well.

get URL-->| MUD | . | Manager | .(https) | File Server | . End system network |____________|<-MUD file<-<| | . ^ +Signature |_____________| . . . . . . . . . . ________ _____________ . .| | Attestation | NAS, AAA or | . .| Device |-->Evidence-->| Onboarding | . .|________| (+ Manifest | Server | . . ^ Claim) |_____________| . ......*.................................... * //-\\ * \-/ * SUIT Manifest | +************************(+ MUD URL) ----*----- Firmware / \ / \ Developer ]]>

The intended workflow is as follows, and assumes an attestation mechanism between the device and the MUD Manager: At the time of onboarding, devices report their manifest in use to the MUD Manager via some form of attestation Evidence and a conveyance protocol. The device thereby acts as an Attester. The normative specification of these mechanisms is out of scope for this document. An example of an Evidence format is the Entity Attestation Token (EAT) , which offers a rich set of claims. This specification assumes that Evidence includes a link to the SUIT manifest via the "manifests" claim (see Section 4.2.15 of ) or that the manifest itself is embedded in the Evidence. This Evidence is conveyed to the operational network via some protocol, such as network access authentication protocol (for example using the EAP-TLS 1.3 method utilizing the attestation extensions ) or an onboarding protocol like FIDO Device Onboard (FDO) or Bootstrapping Remote Secure Key Infrastructure (BRSKI) . The MUD Manager, acting as a Relying Party, relays the Evidence to the Verifier and receives an Attestation Result in response. This allows the MUD Manager to check that the device is operating with the expected version of software and configuration. Since a URL to the manifest is contained in the Evidence, the MUD Manager can look up the corresponding manifest. The MUD Manager acquires the MUD file from the MUD URL found in the SUIT manifest. The SUIT manifest contains the MUD URL and not the MUD file primarily to due the size of the MUD file. This also allows the MUD file to be updated rapidly in response to evolving threats. The MUD Manager verifies the MUD file signature using the Subject Key Identifier (SKI) provided in the SUIT manifest. Then, the MUD Manager can apply any appropriate policy as described by the MUD file. Each time a device is updated, rebooted, or otherwise substantially changed, it will execute the remote attestation procedures again.

Operational Considerations This specification assumes that the software/firmware author provides a MUD file that describes the behavior of the software running on a device.

Pros The approach described in this document has several advantages over the RFC 8520 MUD URL reporting mechanisms: The MUD URL is tightly coupled to device software/firmware version. The device does not report the MUD URL, so the device cannot tamper with the MUD URL. The author explicitly authorizes a key to sign MUD files, providing a tight coupling between the party that knows device behavior best (the author of the software/firmware) and the party that declares device behavior (MUD file signer). Network operators do not need to know, a priori, which MUD URL to use for each device; this can be harvested from the device's manifest and only replaced if necessary. A network operator can still replace a MUD URL in a SUIT manifest: By providing a SUIT manifest that overrides the MUD URL. By replacing the MUD URL in their network infrastructure. Devices can be quarantined if the Attestation Result indicates that an out-dated or compromised software/firmware version has been used. Devices cannot lie about which MUD URL to use.

Cons This mechanism relies on the use of SUIT manifests to encode the MUD URL. Conceptually, the MUD file is similar to a Software Bill of Material (SBOM) but focuses on the external visible communication behavior, which is essential for network operators, rather than describing the software libraries contained within the device itself. MUD Manager must be aware of the Status Tracker or vice versa so that the MUD Manager can obtain MUD URLs and MUD Signer SKIs from the Status Tracker. This implies a new API in the MUD manager or Status Tracker. The MUD manager requires a failover mechanism to trigger the status tracker to obtain a copy of the SUIT Manifest in order to extract the MUD URL if it is not already aware of a device. This could be done, for example, as a part of an onboarding flow. Attestation Evidence may convey the SUIT Manifest, in which case the Status Tracker becomes a Relying Party since it depends on Attestation Evidence. This workflow is expected, however. This approach explicitly moves the decisions about device behaviour away from the Network Operator and towards the Manifest Author. While this is appropriate when the Manifest Author is trusted, not all IoT devices are fully trusted, and MUD files enable a Network Operator to restrict their capabilities. For a Network Operator to override a device's manufacturer-provided MUD URL will require the MUD manager to have a mechanism to enable this override, which adds complexity

Extensions to SUIT To enable strong assertions about the network access requirements that a device should have for a particular software/configuration pair a MUD URL is added to the SUIT manifest along with a subject key identifier (ski). Note that the subject key identifier refers to a more generic version of SubjectPublicKeyInfo defined in , which refers to an X.509-based ski. The subject key identifier MUST be generated according to the process defined in and the SUIT_Digest structure MUST be populated with the selected hash algorithm and obtained fingerprint. The subject key identifier corresponds to the key used in the MUD signature file described in Section 13.2 of . Note: A key need not be in COSE Key format to create a COSE Key Thumbprint of it. The following Concise Data Definition Language (CDDL) describes the extension to the SUIT_Manifest structure: The extension to the SUIT_Manifest is described here:

bstr .cbor SUIT_MUD_container ) ]]>

The SUIT_MUD_container structure is defined as follows:

#6.32(tstr), suit-mud-ski => SUIT_Digest, } ]]>

Security Considerations This specification links MUD files to SUIT manifests for improving security protection and ease of use. By including MUD URLs in SUIT manifests an extra layer of protection has been created and synchronization risks can be minimized. Used in this way, the MUD manager presents an additional layer of security on networks where they are enabled. The MUD manager configures the L2/L3 infrastructure of a Local Area Network to apply restrictive policies to certain devices. The MUD manager only has the ability to elevate or restrict the network privileges of a device. Therefore, attacks on the MUD Manager cannot compromise devices, they can only enable a compromised device to access more of the network. Further security considerations related to the MUD Manager are covered in . If the MUD file and the software/firmware loaded onto the device gets out-of-sync a device may be firewalled and, with firewalling by networks in place, the device may stop functioning. This is, however, not a concern specific to this specification but rather to the use of MUD in general. Below are two mitigations: A manufacturer must update the MUD file in advance of network service or product changes so that the new services can be supported. Because the MUD file is accessed by a URL means that it can be subsequently updated. This requires a MUD file being retrieved again. This handles the case when the device is already deployed and in use. There is a possibility that an IoT device has remained on-shelf inventory for an extended period, resulting in its MUD file being inaccessible at its previous location. This necessitates a decision on how to implement a fail-safe tailored to the particular environment.

IANA Considerations IANA is requested to add a new value to the SUIT manifest elements registry created with : Label: TBD1 [[Value allocated from the standards action address range]] Name: Manufacturer Usage Description (MUD) Reference: [[TBD: This document]]

This memo specifies a component-based architecture for Manufacturer Usage Descriptions (MUDs). The goal of MUD is to provide a means for end devices to signal to the network what sort of access and network functionality they require to properly function. The initial focus is on access control. Later work can delve into other aspects. This memo specifies two YANG modules, IPv4 and IPv6 DHCP options, a Link Layer Discovery Protocol (LLDP) TLV, a URL, an X.509 certificate extension, and a means to sign and verify the descriptions. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Security Theory LLC Mediatek USA Qualcomm Technologies Inc. Red Hound Software, Inc. An Entity Attestation Token (EAT) provides an attested claims set that describes state and characteristics of an entity, a device like a smartphone, IoT device, network equipment or such. This claims set is used by a relying party, server or service to determine the type and degree of trust placed in the entity. An EAT is either a CBOR Web Token (CWT) or JSON Web Token (JWT) with attestation-oriented claims. Arm Limited Fraunhofer SIT Inria Nordic Semiconductor This specification describes the format of a manifest. A manifest is a bundle of metadata about code/data obtained by a recipient (chiefly the firmware for an Internet of Things (IoT) device), where to find the code/data, the devices to which it applies, and cryptographic information protecting the manifest. Software updates and Trusted Invocation both tend to use sequences of common operations, so the manifest encodes those sequences of operations, rather than declaring the metadata. SECOM CO., LTD. University of Applied Sciences Bonn-Rhein-Sieg Transmute This specification defines a method for computing a hash value over a CBOR Object Signing and Encryption (COSE) Key. It specifies which fields within the COSE Key structure are included in the cryptographic hash computation, the process for creating a canonical representation of these fields, and how to hash the resulting byte sequence. The resulting hash value, referred to as a "thumbprint," can be used to identify or select the corresponding key. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. In network protocol exchanges, it is often useful for one end of a communication to know whether the other end is in an intended operating state. This document provides an architectural overview of the entities involved that make such tests possible through the process of generating, conveying, and evaluating evidentiary Claims. It provides a model that is neutral toward processor architectures, the content of Claims, and protocols. The Extensible Authentication Protocol (EAP), defined in RFC 3748, provides a standard mechanism for support of multiple authentication methods. This document specifies the use of EAP-TLS with TLS 1.3 while remaining backwards compatible with existing implementations of EAP-TLS. TLS 1.3 provides significantly improved security and privacy, and reduced latency when compared to earlier versions of TLS. EAP-TLS with TLS 1.3 (EAP-TLS 1.3) further improves security and privacy by always providing forward secrecy, never disclosing the peer identity, and by mandating use of revocation checking when compared to EAP-TLS with earlier versions of TLS. This document also provides guidance on authentication, authorization, and resumption for EAP-TLS in general (regardless of the underlying TLS version used). This document updates RFC 5216. Intuit Arm Limited Arm Limited Arm Limited Huawei Linaro The TLS handshake protocol allows authentication of one or both peers using static, long-term credentials. In some cases, it is also desirable to ensure that the peer runtime environment is in a secure state. Such an assurance can be achieved using attestation which is a process by which an entity produces evidence about itself that another party can use to appraise whether that entity is found in a secure state. This document describes a series of protocol extensions to the TLS 1.3 handshake that enables the binding of the TLS authentication key to a remote attestation session. This enables an entity capable of producing attestation evidence, such as a confidential workload running in a Trusted Execution Environment (TEE), or an IoT device that is trying to authenticate itself to a network access point, to present a more comprehensive set of security metrics to its peer. These extensions have been designed to allow the peers to use any attestation technology, in any remote attestation topology, and mutually. FIDO Alliance Sandelman Software Works Huawei Technologies Cisco Systems This document provides a way for an RFC8520 Manufacturer Usage Description (MUD) definitions to declare what are acceptable replacement MUD URLs for a device. RFCEDITOR-please-remove: this document is being worked on at: https://github.com/mcr/iot-mud-acceptable-urls This document specifies automated bootstrapping of an Autonomic Control Plane. To do this, a Secure Key Infrastructure is bootstrapped. This is done using manufacturer-installed X.509 certificates, in combination with a manufacturer's authorizing service, both online and offline. We call this process the Bootstrapping Remote Secure Key Infrastructure (BRSKI) protocol. Bootstrapping a new device can occur when using a routable address and a cloud service, only link-local connectivity, or limited/disconnected networks. Support for deployment models with less stringent security requirements is included. Bootstrapping is complete when the cryptographic identity of the new key infrastructure is successfully deployed to the device. The established secure connection can be used to deploy a locally issued certificate to the device as well. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK]

Acknowledgements We would like to thank Roman Danyliw for his excellent review as the responsible security area director, Bahcet Sarikaya for his Genart review, Michael Richardson for his IoT directorate review and Susan Hares for her Opsdir review. During the IESG review Robert Wilton, Eliot Lear, Zaheduzzaman Sarker, Francesca Palombini, John Scudder, Paul Wouters, Éric Vyncke, and Murray Kucherawy.