]> 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. The MUD 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 a 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 pattern 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 URL to the MUD file to a MUD Manager in the operational network, which then interacts with 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 such as: DHCP, IEEE 802.1AB Link Layer Discovery Protocol (LLDP), and IEEE 802.1X whereby the URL to the MUD file would be contained in the certificate used in an EAP method. The MUD Manager must trust the MUD File Server from which the MUD file is fetched to return an authentic copy of the MUD file. It must also trust the device to report the correct MUD URL. In case of DHCP and LLDP the URL is likely 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, a certificate created for use with network access authentication is typically not signed by the entity that wrote the software and configured the device, which leads to a conflation of rights. There is a need to bind the entity that creates the software and configuration to the MUD file. Only the developer can attest the communication requirements of the device. This specification defines an extension to the Software Updates for Internet of Things (SUIT) manifest format to include a MUD URL. When combining a MUD URL with a manifest used for software/firmware updates then a network operator can gain 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. Attestation-related terminology is defined in .

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 | Serverdig | . . ^ 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 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) . The MUD Manager can then (with the help of the Verifier) validate the Evidence in order 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. If the SUIT_MUD_container, see , has been severed, the MUD Manager can use the suit-reference-uri to retrieve the complete SUIT manifest. The manifest authenticity is verified by the MUD Manager, which enforces that the MUD file presented is also authentic and as intended by the device software vendor. The MUD Manager acquires the MUD file from the MUD URL found in the SUIT manifest. 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

Pros The approach described in this document has several advantages over other 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 onus is placed on the software/firmware author to provide a MUD file that describes the behavior of the software running on a device. 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 they do not attest a known software/firmware version. 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. The SUIT manifest must then be conveyed to the network during onboarding or during the network access authentication step. To accomplish the transport of the manifest Evidence is used, which needs to be available at the protocol of choice.

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). 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 . The following Concise Data Definition Language (CDDL) describes the extension to the SUIT_Manifest structure:

SUIT_Digest / SUIT_MUD_container ) ]]>

The SUIT_Envelope is also amended:

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. 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.

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]] IANA is requested to add a new value to the SUIT envelope elements registry created with : Label: TBD2 [[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 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 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. Transmute This specification defines a method for computing a hash value over a COSE Key. It defines which fields in a COSE Key structure are used in the hash computation, the method of creating a canonical form of the fields, and how to hash the byte sequence. The resulting hash value can be used for identifying or selecting a key that is the subject of the thumbprint. 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 Attestation is the process by which an entity produces evidence about itself that another party can use to evaluate the trustworthiness of that entity. In use cases that require the use of remote attestation, such as confidential computing or device onboarding, an attester has to convey evidence or attestation results to a relying party. This information exchange may happen at different layers in the protocol stack. This specification provides a generic way of passing evidence and attestation results in the TLS handshake. Functionality-wise this is accomplished with the help of key attestation. FIDO Alliance

Acknowledgements We would like to thank Roman Danyliw for his excellent review as the responsible security area director, Bahcet Sarikaya for his Genart review, and Susan Hares for her Opsdir review.