< draft-ietf-suit-architecture-00.txt		draft-ietf-suit-architecture-01.txt >
	SUIT B. Moran		SUIT B. Moran
	Internet-Draft Arm Limited		Internet-Draft Arm Limited
	Intended status: Informational M. Meriac		Intended status: Informational M. Meriac
	Expires: December 5, 2018 Consultant		Expires: January 3, 2019 Consultant
	H. Tschofenig		H. Tschofenig
	Arm Limited		Arm Limited
	June 03, 2018		D. Brown
			Linaro
			July 02, 2018
	A Firmware Update Architecture for Internet of Things Devices		A Firmware Update Architecture for Internet of Things Devices
	draft-ietf-suit-architecture-00		draft-ietf-suit-architecture-01
	Abstract		Abstract
	Vulnerabilities with Internet of Things (IoT) devices have raised the		Vulnerabilities with Internet of Things (IoT) devices have raised the
	need for a solid and secure firmware update mechanism that is also		need for a solid and secure firmware update mechanism that is also
	suitable for constrained devices. Incorporating such update		suitable for constrained devices. Incorporating such update
	mechanism to fix vulnerabilities, to update configuration settings as		mechanism to fix vulnerabilities, to update configuration settings as
	well as adding new functionality is recommended by security experts.		well as adding new functionality is recommended by security experts.
	This document lists requirements and describes an architecture for a		This document lists requirements and describes an architecture for a
	skipping to change at page 1, line 46 ¶		skipping to change at page 1, line 48 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on December 5, 2018.		This Internet-Draft will expire on January 3, 2019.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 IETF Trust and the persons identified as the		Copyright (c) 2018 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 36 ¶		skipping to change at page 2, line 36 ¶
	the copyright in such materials, this document may not be modified		the copyright in such materials, this document may not be modified
	outside the IETF Standards Process, and derivative works of it may		outside the IETF Standards Process, and derivative works of it may
	not be created outside the IETF Standards Process, except to format		not be created outside the IETF Standards Process, except to format
	it for publication as an RFC or to translate it into languages other		it for publication as an RFC or to translate it into languages other
	than English.		than English.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3		2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
	3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4		3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
	3.1. Agnostic to how firmware images are distributed . . . . . 5		3.1. Agnostic to how firmware images are distributed . . . . . 6
	3.2. Friendly to broadcast delivery . . . . . . . . . . . . . 5		3.2. Friendly to broadcast delivery . . . . . . . . . . . . . 6
	3.3. Use state-of-the-art security mechanisms . . . . . . . . 5		3.3. Use state-of-the-art security mechanisms . . . . . . . . 7
	3.4. Rollback attacks must be prevented . . . . . . . . . . . 6		3.4. Rollback attacks must be prevented . . . . . . . . . . . 7
	3.5. High reliability . . . . . . . . . . . . . . . . . . . . 6		3.5. High reliability . . . . . . . . . . . . . . . . . . . . 7
	3.6. Operate with a small bootloader . . . . . . . . . . . . . 6		3.6. Operate with a small bootloader . . . . . . . . . . . . . 8
	3.7. Small Parsers . . . . . . . . . . . . . . . . . . . . . . 7		3.7. Small Parsers . . . . . . . . . . . . . . . . . . . . . . 8
	3.8. Minimal impact on existing firmware formats . . . . . . . 7		3.8. Minimal impact on existing firmware formats . . . . . . . 8
	3.9. Robust permissions . . . . . . . . . . . . . . . . . . . 7		3.9. Robust permissions . . . . . . . . . . . . . . . . . . . 8
	3.10. Operating modes . . . . . . . . . . . . . . . . . . . . . 8		3.10. Operating modes . . . . . . . . . . . . . . . . . . . . . 9
	4. Claims . . . . . . . . . . . . . . . . . . . . . . . . . . . 9		4. Claims . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
	5. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 10		5. Communication Architecture . . . . . . . . . . . . . . . . . 11
	6. Manifest . . . . . . . . . . . . . . . . . . . . . . . . . . 12		6. Manifest . . . . . . . . . . . . . . . . . . . . . . . . . . 14
	7. Example Flow . . . . . . . . . . . . . . . . . . . . . . . . 13		7. Device Firmware Update Examples . . . . . . . . . . . . . . . 15
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15		7.1. Single CPU SoC . . . . . . . . . . . . . . . . . . . . . 16
	9. Security Considerations . . . . . . . . . . . . . . . . . . . 15		7.2. Single CPU with Secure - Normal Mode Partitioning . . . . 16
	10. Mailing List Information . . . . . . . . . . . . . . . . . . 16		7.3. Dual CPU, shared memory . . . . . . . . . . . . . . . . . 16
	11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16		7.4. Dual CPU, other bus . . . . . . . . . . . . . . . . . . . 16
	12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17		8. Example Flow . . . . . . . . . . . . . . . . . . . . . . . . 17
	12.1. Normative References . . . . . . . . . . . . . . . . . . 17		9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
	12.2. Informative References . . . . . . . . . . . . . . . . . 17		10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
	12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 18		11. Mailing List Information . . . . . . . . . . . . . . . . . . 19
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18		12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
			13. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
			13.1. Normative References . . . . . . . . . . . . . . . . . . 21
			13.2. Informative References . . . . . . . . . . . . . . . . . 21
			13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 21
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
	1. Introduction		1. Introduction
	When developing IoT devices, one of the most difficult problems to		When developing IoT devices, one of the most difficult problems to
	solve is how to update the firmware on the device. Once the device		solve is how to update the firmware on the device. Once the device
	is deployed, firmware updates play a critical part in its lifetime,		is deployed, firmware updates play a critical part in its lifetime,
	particularly when devices have a long lifetime, are deployed in		particularly when devices have a long lifetime, are deployed in
	remote or inaccessible areas or where manual intervention is cost		remote or inaccessible areas or where manual intervention is cost
	prohibitive or otherwise difficult. The need for a firmware update		prohibitive or otherwise difficult. The need for a firmware update
	may be to fix bugs in software, to add new functionality, or to re-		may be to fix bugs in software, to add new functionality, or to re-
	configure the device.		configure the device.
	The firmware update process has to ensure that		The firmware update process, among other goals, has to ensure that
	- The firmware image is authenticated and attempts to flash a		- The firmware image is authenticated and attempts to flash a
	malicious firmware image are prevented.		malicious firmware image are prevented.
	- The firmware image can be confidentiality protected so that		- The firmware image can be confidentiality protected so that
	attempts by an adversary to recover the plaintext binary can be		attempts by an adversary to recover the plaintext binary can be
	prevented. Obtaining the plaintext binary is often one of the		prevented. Obtaining the plaintext binary is often one of the
	first steps for an attack to mount an attack.		first steps for an attack to mount an attack.
	2. Conventions and Terminology		2. Conventions and Terminology
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	offer very basic functionality in the first stage and resides in		offer very basic functionality in the first stage and resides in
	ROM whereas the second stage may implement more complex		ROM whereas the second stage may implement more complex
	functionality and resides in flash memory so that it can be		functionality and resides in flash memory so that it can be
	updated in the future (in case bugs have been found). The exact		updated in the future (in case bugs have been found). The exact
	split of components into the different stages, the number of		split of components into the different stages, the number of
	firmware images stored by an IoT device, and the detailed		firmware images stored by an IoT device, and the detailed
	functionality varies throughout different implementations.		functionality varies throughout different implementations.
	The following entities are used:		The following entities are used:
	- Author: The author is the entity that creates the firmware image,		- Author: The author is the entity that creates the firmware image.
	signs and/or encrypts it. The author is most likely a developer		There may be multiple authors in a system either when a device
	using a set of tools.		consists of multiple micro-controllers or when the the final
			firmware image consists of software components from multiple
			companies.
	- Device: The device is the recipient of the firmware image and the		- Device: The device is the recipient of the firmware image and the
	manifest. The goal is to update the firmware of the device.		manifest. The goal is to update the firmware of the device. A
			single device may need to obtain more than one firmware image and
			manifest to succesfully perform an update.
	- Untrusted Storage: Firmware images and manifests may be stored on		- Communicator: The communicator component of the device interacts
	untrusted fileservers or cloud storage infrastructure. Some		with the firmware update server. It receives firmware images and
	deployments may require storage of the firmware images/manifests		triggers an update, if needed. The communicator either polls a
	to be stored on various entities before they reach the device.		firmware update server for the most recent manifest/firmware or
			manifests/firmware images are pushed to it. Note that the
			firmware update process may involve multiple stages since one or
			multiple manifests may need to be downloaded before the
			communicator can fetch one or multiple firmware images/software
			components.
			- Status Tracker: The status tracker offers device management
			functionality that includes keep track of the firmware update
			process. This includes fine-grained monitoring of changes at the
			device, for example, what state of the firmware update cycle the
			device is currently in.
			- Firmware Server: Entity that stores firmware images and manifests.
			Some deployments may require storage of the firmware images/
			manifests on more than one entities before they reach the device.
			- Device Operator: The actor responsible for the day-to-day
			operation of a fleet of IoT devices.
			- Network Operator: The actor responsible for the operation of a
			network to which IoT devices connect.
			In addition to the entities in the list above there is an orthogonal
			infrastructure with a Trust Provisioning Authority (TPA) distributing
			trust anchors and authorization permissions to various entities in
			the system. The TPA may also delegate rights to install, update,
			enhance, or delete trust anchors and authorization permissions to
			other parties in the system. This infrastructure overlaps the
			communication architecture and different deployments may empower
			certain entities while other deployments may not. For example, in
			some cases, the Original Design Manufacturer (ODM), which is a
			company that designs and manufactures a product, may act as a TPA and
			may decide to remain in full control over the firmware update process
			of their products.
			The terms 'trust anchor' and 'trust anchor store' are defined in
			[RFC6024]:
			- "A trust anchor represents an authoritative entity via a public
			key and associated data. The public key is used to verify digital
			signatures, and the associated data is used to constrain the types
			of information for which the trust anchor is authoritative."
			- "A trust anchor store is a set of one or more trust anchors stored
			in a device. A device may have more than one trust anchor store,
			each of which may be used by one or more applications."
			Furthermore, the following abbreviations are used in this document:
			- Microcontroller (MCU for microcontroller unit) is a small computer
			on a single integrated circuit, which is often used for mass
			volumne IoT devices.
			- System on Chip (SoC) is an integrated circuit that integrates all
			components of a computer, such as CPU, memory, input/output ports,
			secondary storage, etc.
			- Homogeneous Storage Architecture (HoSA): A device that stores all
			firmware components in the same way, for example in a file system
			or in flash memory.
			- Heterogeneous Storage Architecture (HeSA): A device that stores at
			least one firmware component differently from the rest, for
			example a device with an external, updatable radio, or a device
			with internal and external flash memory.
	3. Requirements		3. Requirements
	The firmware update mechanism described in this specification was		The firmware update mechanism described in this specification was
	designed with the following requirements in mind:		designed with the following requirements in mind:
	- Agnostic to how firmware images are distributed		- Agnostic to how firmware images are distributed
	- Friendly to broadcast delivery		- Friendly to broadcast delivery
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	- Friendly to broadcast delivery		- Friendly to broadcast delivery
	- Use state-of-the-art security mechanisms		- Use state-of-the-art security mechanisms
	- Rollback attacks must be prevented		- Rollback attacks must be prevented
	- High reliability		- High reliability
	- Operate with a small bootloader		- Operate with a small bootloader
	- Small Parsers		- Small Parsers
	- Minimal impact on existing firmware formats		- Minimal impact on existing firmware formats
	- Robust permissions		- Robust permissions
	- Diverse modes of operation		- Diverse modes of operation
	3.1. Agnostic to how firmware images are distributed		3.1. Agnostic to how firmware images are distributed
	Firmware images can be conveyed to devices in a variety of ways,		Firmware images can be conveyed to devices in a variety of ways,
	including USB, UART, WiFi, BLE, low-power WAN technologies, etc. and		including USB, UART, WiFi, BLE, low-power WAN technologies, etc. and
	use different protocols (e.g., CoAP, HTTP). The specified mechanism		use different protocols (e.g., CoAP, HTTP). The specified mechanism
	needs to be agnostic to the distribution of the firmware images and		needs to be agnostic to the distribution of the firmware images and
	manifests.		manifests.
	3.2. Friendly to broadcast delivery		3.2. Friendly to broadcast delivery
	This architecture does not specify any specific broadcast protocol		This architecture does not specify any specific broadcast protocol
	however, given that broadcast may be desirable for some networks,		however, given that broadcast may be desirable for some networks,
	updates must cause the least disruption possible both in metadata and		updates must cause the least disruption possible both in metadata and
	payload transmission.		payload transmission.
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	required to validate at least one signature or MAC with minimal		required to validate at least one signature or MAC with minimal
	exposure.		exposure.
	3.8. Minimal impact on existing firmware formats		3.8. Minimal impact on existing firmware formats
	The design of the firmware update mechanism must not require changes		The design of the firmware update mechanism must not require changes
	to existing firmware formats.		to existing firmware formats.
	3.9. Robust permissions		3.9. Robust permissions
	A device may have many modules that require updating individually.		When a device obtains a monolithic firmware image from a single
	It may also need to trust several actors in order to authorize an		author without any additional approval steps then the authorization
	update. These actors might include the following (this is not a		flow is relatively simple. There are, however, other cases where
	comprehensive list).		more complex policy decisions need to be made before updating a
			device.
	* A firmware author
	* A device OEM
	* A device operator
	* A network operator
	* A device owner
	These actors exert their authority on the device by making claims (as
	in Section 4).
	For example, a firmware author may not have the authority to install		In this architecture the authorization policy is separated from the
	firmware on a device in critical infrastructure without the		underlying communication architecture. This is accomplished by
	authorization of a device operator. In this case, the device may be		separating the entities from their permissions. For example, an
	programmed to reject firmware updates unless they are signed both by		author may not have the authority to install a firmware image on a
	the firmware author and by the device operator. To facilitate		device in critical infrastructure without the authorization of a
	complex use-cases such as this, updates require several claims.		device operator. In this case, the device may be programmed to
			reject firmware updates unless they are signed both by the firmware
			author and by the device operator.
	Alternatively, a device may trust precisely one authority, which does		Alternatively, a device may trust precisely one entity, which does
	all permission management and coordination. Effectively, the		all permission management and coordination. This entity allows the
	authority allows the device to offload complex permissions		device to offload complex permissions calculations for the device.
	calculations for the device.
	3.10. Operating modes		3.10. Operating modes
	There are three broad classifications of update operating modes.		There are three broad classifications of update operating modes.
	* Self initiated		- Client-initiated Update
	* Third-party initiated
	* Hybrid
	Self initiated updates take the form of a proactive IoT device that		- Server-initiated Update
	checks for updates. Third-party initiated updates are triggered by
	an actor other than the IoT device, be it a server, a peer, or a
	user. Hybrid updates are those that require agreement from both the
	target IoT device and another actor.
	Third-party initiated updates are important to consider because		- Hybrid Update
	timing of updates may need to be tightly controlled in some high-
	reliability environments.
	An IoT device goes through several steps in the course of an update,		Client-initiated updates take the form of a communicator on a device
	each of which can be self-initiated or third-party initiated, or		proactively checking for new firmware imagines provided by firmware
	hybrid. An IoT device may go through the following steps, though		servers.
	this is not a comprehensive list.
	* Notification		Server-initiated updates are important to consider because timing of
	* Pre-authorisation		updates may need to be tightly controlled in some high- reliability
	* Dependency resolution		environments. In this case the communicator, potentially in
	* Download		coordination with the status tracker, determines what devices qualify
	* Installation		for a firmware update. Once those devices have been selected the
			firmware server distributes updates to those devices.
	The notification step consists informing an IoT device that an update		Note: This assumes that the firmware server is able to reach the
	is available. This can be accomplished via polling (self-initiated),		device, which may require devices to keep reachability information at
	push notifications (third-party initiated), or more complex		the communicator and / or at the firmware server up-to-date. This
	mechanisms.		may also require keeping state at NATs and stateful packet filtering
			firewalls alive.
	The pre-authorisation step involves verifying the update authority		Hybrid updates are those that require an interaction between the
	and making a determination that the device is prepared to initiate		device and the firmware server / communicator. The communicator
	the fetching and processing of updates. If the device has all		pushes notifications of availability of an update to the device, and
	information that is necessary to make this determination, then the		the device then downloads the image from the firmware server when it
	pre-authorisation may be self-initiated. However, the device can		wants.
	wait for instruction to begin (third-party initiated). Hybrid
	approaches are possible as well.		An alternative approach is to consider the steps a device has to go
			through in the course of an update:
			- Notification
			- Pre-authorisation
			- Dependency resolution
			- Download
			- Installation
			The notification step consists of the communicator informing the
			device that an update is available. This can be accomplished via
			polling (client-initiated), push notifications (server-initiated), or
			more complex mechanisms.
			The pre-authorisation step involves verifying whether the entity
			signing the manifest is indeed authorized to perform an update. The
			device must also determine whether it should fetching and processing
			of the firmware image (unless it has been attached already to the
			manifest itself).
	A dependency resolution phase is needed when more than one component		A dependency resolution phase is needed when more than one component
	can be updated or when a differential update is used. The necessary		can be updated or when a differential update is used. The necessary
	dependencies must be available prior to installation.		dependencies must be available prior to installation.
	The download step is the process of acquiring a local copy of the		The download step is the process of acquiring a local copy of the
	payload. When the download is self-initiated, this means that the		firmware image. When the download is client-initiated, this means
	IoT device chooses when a download occurs and initiates the download		that the device chooses when a download occurs and initiates the
	process. When a download is third-party initiated, this means that		download process. When a download is server-party initiated, this
	either the remote service tells the IoT device when to download or		means that either the communicator / firmware server tells the device
	that it initiates the transfer directly to the IoT device. For		when to download or that it initiates the transfer directly to the
	example, a download from an HTTP server is initiated locally. A		device. For example, a download from an HTTP-based firmware server
	transfer to a LwM2M Firmware Update resource [LwM2M] is initiated		is client-initiated. A transfer to a LwM2M Firmware Update resource
	remotely.		[LwM2M] is server-initiated.
			If the Device has downloaded a new firmware image and is ready to
			install it it may need to wait for a trigger from a Communicator to
			install the firmware update, may trigger the update automatically, or
			may go through a more complex decision making process to determine
			the appropriate timing for an update (such as delaying the update
			process to a later time when end users are less impacted by the
			update process).
	Installation is the act of processing the payload into a format that		Installation is the act of processing the payload into a format that
	the IoT device can recognise.		the IoT device can recognise and the bootloader is responsible for
			then booting from the newly installed firmware image.
	Each of these steps may require different permissions expressed in		Each of these steps may require different permissions.
	claims and may be implemented in a variety of ways.
	4. Claims		4. Claims
	When a simple set of permissions fails to encapsulate the rules		Claims in the manifest offer a way to convey instructions to a device
	required for a device to make decisions about firmware, claims can be		that impact the firmware update process. To have any value the
	used instead. Claims represent a form of policy. Several claims can		manifest containing those claims must be authenticated and integrity
	be used together, when multiple actors should have the rights to set		protected. The credential used to must be directly or indirectly
	policies.		related to the trust anchor installed at the device by the Trust
			Provisioning Authority.
	Some example claims are:
	- Trust the actor identified by the referenced public key.
	- Trust the actor with access to the referenced shared secret (MAC).
	- Three actors are trusted identified by their public keys.
	Signatures from at least two of these actors are required to trust
	a manifest.
	- The actor identified by the referenced public key is authorized to
	create secondary policies
	The baseline claims for all manifests are described in [SUIT-IM]. In		The baseline claims for all manifests are described in
	summary, they are:		[I-D.ietf-suit-information-model]. For example, there are:
	- Do not install firmware with earlier metadata than the current		- Do not install firmware with earlier metadata than the current
	metadata.		metadata.
	- Only install firmware with a matching vendor, model, hardware		- Only install firmware with a matching vendor, model, hardware
	revision, software version, etc.		revision, software version, etc.
	- Only install firmware that is before its best-before timestamp.		- Only install firmware that is before its best-before timestamp.
	- Only install firmware with metadata signed/authenticated by a		- Only allow a firmware installation if dependencies have been met.
	trusted actor.
	- Only allow an actor to exercise rights on the device via a
	manifest if that actor has signed the manifest.
	- Only allow a firmware installation if all required rights have		- Choose the mechanism to install the firmware, based on the type of
	been met through signatures/MACs (one or more) or manifest		firmware it is.
	dependencies (one or more).
	- Use the instructions provided by the manifest to install the		5. Communication Architecture
	firmware.
	- Install any and all firmware images that are linked together with		Figure 1 shows the communication architecture where a firmware image
	manifest dependencies.		is created by an author, and uploaded to a firmware server. The
			firmware image/manifest is distributed to the device either in a push
			or pull manner using the communicator residing on the device. The
			device operator keeps track of the process using the status tracker.
			This allows the device operator to know and control what devices have
			received an update and which of them are still pending an update.
	- Choose the mechanism to install the firmware, based on the type of		Firmware + +----------+ Firmware + +-----------+
	firmware it is.		Manifest | |-+ Manifest | |-+
			+--------->| Firmware | |<---------------| | |
			| | Server | | | Author | |
			| | | | | | |
			| +----------+ | +-----------+ |
			| +----------+ +-----------+
			|
			|
			|
			-+-- ------
			---- | ---- ---- ----
			// | \\ // \\
			/ | \ / \
			/ | \ / \
			/ | \ / \
			/ | \ / \
			| v | | |
			| +------------+ |
			| |Communicator| | | |
			| +--------+---+ | Device | +--------+ |
			| | | | Management| | | |
			| | Device |<----------------------------->| Status | |
			| | | | | | Tracker| |
			| +--------+ | || | | |
			| | || +--------+ |
			| | | |
			| | \ /
			\ / \ /
			\ / \ Device /
			\ Network / \ Operator /
			\ Operator / \\ //
			\\ // ---- ----
			---- ---- ------
			-----
	5. Architecture		Figure 1: Architecture.
	We start the architectural description with the security model. It		End-to-end security mechanisms are used to protect the firmware image
	is based on end-to-end security. In Figure 1 a firmware image is		and the manifest although Figure 2 does not show the manifest itself
	created by an author, sent to the device and subsequently installed.		since it may be distributed independently.
	When the author is ready to distribute the firmware image it is
	conveyed using some communication channel to the device, which will
	typically involve the use of untrusted storage. Examples of
	untrusted storage are FTP servers, Web servers or USB sticks. End-
	to-end security mechanisms are used to protect the firmware image.
	Figure 1 does not show the manifest itself, which provides the meta-
	data about the firmware image and offers the security protection. It
	may bundled with the firmware image or travel as a standalone item.
	+-----------+		+-----------+
	+--------+ | | +--------+		+--------+ | | +--------+
	| | Firmware Image | Untrusted | Firmware Image | |		| | Firmware Image | Firmware | Firmware Image | |
	| Device |<-----------------| Storage |<------------------| Author |		| Device |<-----------------| Server |<------------------| Author |
	| | | | | |		| | | | | |
	+--------+ +-----------+ +--------+		+--------+ +-----------+ +--------+
	^ *		^ *
	* *		* *
	************************************************************		************************************************************
	End-to-End Security		End-to-End Security
	Figure 1: End-to-End Security.		Figure 2: End-to-End Security.
	Whether the firmware image and the manifest is pushed to the device		Whether the firmware image and the manifest is pushed to the device
	or fetched by the device is outside the scope of this work and		or fetched by the device is a deployment specific decision.
	existing device management protocols can be used for efficiently
	distributing this information.
	The following assumptions are made to allow the device to verify the		The following assumptions are made to allow the device to verify the
	received firmware image and manifest before updating software:		received firmware image and manifest before updating software:
	- To accept an update, a device needs to decide whether the author		- To accept an update, a device needs to verify the signature
	signing the firmware image and the manifest is authorized to make		covering the manifest. There may be one or multiple manifests
	the updates. We use public key cryptography to accomplish this.		that need to be validated, potentially signed by different
	The device verifies the signature covering the manifest using a		parties. The device needs to be in possession of the trust
	digital signature algorithm OR the device verifies the MAC		anchors to verify those signatures. Installing trust anchors to
	covering the manifest using a MAC algorithm. The device is		devices via the Trust Provisioning Authority happens in an out-of-
	provisioned with a trust anchor that is used to validate the		band fashion prior to the firmware update process.
	digital signature or MAC produced by the author. This trust
	anchor is potentially different from the trust anchor used to		- Not all entities creating and signing manifests have the same
	validate the digital signature produced for other protocols (such		permissions. A device needs to determine whether the requested
	as device management protocols). This trust anchor may be		action is indeed covered by the permission of the party that
	provisioned to the device during manufacturing or during		signed the manifest. Informing the device about the permissions
	commissioning.		of the different parties also happens in an out-of-band fashion
			and is also a duty of the Trust Provisioning Authority.
	- For confidentiality protection of firmware images the author needs		- For confidentiality protection of firmware images the author needs
	to be in possession of the certificate/public key or a pre-shared		to be in possession of the certificate/public key or a pre-shared
	key of a device.		key of a device. The use of confidentiality protection of
			firmware images is deployment specific.
	There are different types of delivery modes, which are illustrated		There are different types of delivery modes, which are illustrated
	based on examples below.		based on examples below.
	There is an option for embedding a firmware image into a manifest.		There is an option for embedding a firmware image into a manifest.
	This is a useful approach for deployments where devices are not		This is a useful approach for deployments where devices are not
	connected to the Internet and cannot contact a dedicated server for		connected to the Internet and cannot contact a dedicated server for
	download of the firmware. It is also applicable when the firmware		download of the firmware. It is also applicable when the firmware
	update happens via a USB stick or via Bluetooth Smart. Figure 2		update happens via a USB stick or via Bluetooth Smart. Figure 3
	shows this delivery mode graphically.		shows this delivery mode graphically.
	/------------\ /------------\		/------------\ /------------\
	/Manifest with \ /Manifest with \		/Manifest with \ /Manifest with \
	|attached | |attached |		|attached | |attached |
	\firmware image/ \firmware image/		\firmware image/ \firmware image/
	\------------/ +-----------+ \------------/		\------------/ +-----------+ \------------/
	+--------+ | | +--------+		+--------+ | | +--------+
	| |<.................| Untrusted |<................| |		| |<.................| Firmware |<................| |
	| Device | | Storage | | Author |		| Device | | Server | | Author |
	| | | | | |		| | | | | |
	+--------+ +-----------+ +--------+		+--------+ +-----------+ +--------+
	Figure 2: Manifest with attached firmware.		Figure 3: Manifest with attached firmware.
	Figure 3 shows an option for remotely updating a device where the		Figure 4 shows an option for remotely updating a device where the
	device fetches the firmware image from some file server. The		device fetches the firmware image from some file server. The
	manifest itself is delivered independently and provides information		manifest itself is delivered independently and provides information
	about the firmware image(s) to download.		about the firmware image(s) to download.
	/------------\		/------------\
	/ \		/ \
	| Manifest |		| Manifest |
	\ /		\ /
	+--------+ \------------/ +--------+		+--------+ \------------/ +--------+
	| |<..............................................>| |		| |<..............................................>| |
	| Device | -- | Author |		| Device | -- | Author |
	| |<- --- | |		| |<- --- | |
	+--------+ -- --- +--------+		+--------+ -- --- +--------+
	-- ---		-- ---
	--- ---		--- ---
	-- +-----------+ --		-- +-----------+ --
	-- | | --		-- | | --
	/------------\ -- | Untrusted |<- /------------\		/------------\ -- | Firmware |<- /------------\
	/ \ -- | Storage | / \		/ \ -- | Server | / \
	| Firmware | | | | Firmware |		| Firmware | | | | Firmware |
	\ / +-----------+ \ /		\ / +-----------+ \ /
	\------------/ \------------/		\------------/ \------------/
	Figure 3: Independent retrieval of the firmware image.		Figure 4: Independent retrieval of the firmware image.
	This architecture does not mandate a specific delivery mode but a		This architecture does not mandate a specific delivery mode but a
	solution must support both types.		solution must support both types.
	6. Manifest		6. Manifest
	In order for a device to apply an update, it has to make several		In order for a device to apply an update, it has to make several
	decisions about the update:		decisions about the update:
	- Does it trust the author of the update?		- Does it trust the author of the update?
	skipping to change at page 13, line 4 ¶		skipping to change at page 15, line 19 ¶
	- When should the device apply the update?		- When should the device apply the update?
	- How should the device apply the update?		- How should the device apply the update?
	- What kind of firmware binary is it?		- What kind of firmware binary is it?
	- Where should the update be obtained?		- Where should the update be obtained?
	- Where should the firmware be stored?		- Where should the firmware be stored?
	The manifest encodes the information that devices need in order to		The manifest encodes the information that devices need in order to
	make these decisions. It is a data structure that contains the		make these decisions. It is a data structure that contains the
	following information:		following information:
	- information about the device(s) the firmware image is intended to		- information about the device(s) the firmware image is intended to
	be applied to,		be applied to,
	- information about when the firmware update has to be applied,		- information about when the firmware update has to be applied,
	- information about when the manifest was created,		- information about when the manifest was created,
	- dependencies on other manifests,		- dependencies on other manifests,
	- pointers to the firmware image and information about the format,		- pointers to the firmware image and information about the format,
	- information about where to store the firmware image,		- information about where to store the firmware image,
	- cryptographic information, such as digital signatures or message		- cryptographic information, such as digital signatures or message
	authentication codes (MACs).		authentication codes (MACs).
	The manifest information model is described in [SUIT-IM].		The manifest information model is described in
			[I-D.ietf-suit-information-model].
	7. Example Flow		7. Device Firmware Update Examples
			Although these documents attempt to define a firmware update
			architecture that is applicable to both existing systems, as well as
			yet-to-be-conceived systems; it is still helpful to consider existing
			architectures.
			7.1. Single CPU SoC
			The simplest, and currently most common, architecture consists of a
			single MCU along with its own peripherals. These SoCs generally
			contain some amount of flash memory for code and fixed data, as well
			as RAM for working storage. These systems either have a single
			firmware image, or an immutable bootloader that runs a single image.
			A notable characteristic of these SoCs is that the primary code is
			generally execute in place (XIP). Combined with the non-relocatable
			nature of the code, firmware updates need to be done in place.
			7.2. Single CPU with Secure - Normal Mode Partitioning
			Another configuration consists of a similar architecture to the
			previous, with a single CPU. However, this CPU supports a security
			partitioning scheme that allows memory (in addition to other things)
			to be divided into secure and normal mode. There will generally be
			two images, one for secure mode, and one for normal mode. In this
			configuration, firmware upgrades will generally be done by the CPU in
			secure mode, which is able to write to both areas of the flash
			device. In addition, there are requirements to be able to update
			either image independently, as well as to update them together
			atomically, as specified in the associated manifests.
			7.3. Dual CPU, shared memory
			This configuration has two or more CPUs in a single SoC that share
			memory (flash and RAM). Generally, they will be a protection
			mechanism to prevent one CPU from accessing the other's memory.
			Upgrades in this case will typically be done by one of the CPUs, and
			is similar to the single CPU with secure mode.
			7.4. Dual CPU, other bus
			This configuration has two or more CPUs, each having their own
			memory. There will be a communication channel between them, but it
			will be used as a peripheral, not via shared memory. In this case,
			each CPU will have to be responsible for its own firmware upgrade.
			It is likely that one of the CPUs will be considered a master, and
			will direct the other CPU to do the upgrade. This configuration is
			commonly used to offload specific work to other CPUs. Firmware
			dependencies are similar to the other solutions above, sometimes
			allowing only one image to be upgraded, other times requiring several
			to be upgraded atomically. Because the updates are happening on
			multiple CPUs, upgrading the two images atomically is challenging.
			8. Example Flow
	The following example message flow illustrates the interaction for		The following example message flow illustrates the interaction for
	distributing a firmware image to a device starting with an author		distributing a firmware image to a device starting with an author
	uploading the new firmware to untrusted storage and creating a		uploading the new firmware to Firmware Server and creating a
	manifest. The firmware and manifest are stored on the same untrusted		manifest. The firmware and manifest are stored on the same Firmware
	storage.		Server.
	+--------+ +-----------------+ +------+		+--------+ +-----------------+ +------------+ +----------+
	| Author | |Untrusted Storage| |Device|		| Author | | Firmware Server | |Communicator| |Bootloader|
	+--------+ +-----------------+ +------+		+--------+ +-----------------+ +------------+ +----------+
	| | |		| | | +
	| Create Firmware | |		| Create Firmware | | |
	|--------------- | |		|--------------- | | |
	| | | |		| | | | |
	|<-------------- | |		|<-------------- | | |
	| | |		| | | |
	| Upload Firmware | |		| Upload Firmware | | |
	|------------------>| |		|------------------>| | |
	| | |		| | | |
	| Create Manifest | |		| Create Manifest | | |
	|---------------- | |		|---------------- | | |
	| | | |		| | | | |
	|<--------------- | |		|<--------------- | | |
	| | |		| | | |
	| Sign Manifest | |		| Sign Manifest | | |
	|-------------- | |		|-------------- | | |
	| | | |		| | | | |
	|<------------- | |		|<------------- | | |
	| | |		| | | |
	| Upload Manifest | |		| Upload Manifest | | |
	|------------------>| |		|------------------>| | |
	| | |		| | | |
	| | Query Manifest |		| | Query Manifest | |
	| |<--------------------|		| |<--------------------| |
	| | |		| | | |
	| | Send Manifest |		| | Send Manifest | |
	| |-------------------->|		| |-------------------->| |
	| | |		| | | Validate |
	| | | Validate Manifest		| | | Manifest |
	| | |------------------		| | |---------+ |
	| | | |		| | | | |
	| | |<-----------------		| | |<--------+ |
	| | |		| | | |
	| | Request Firmware |		| | Request Firmware | |
	| |<--------------------|		| |<--------------------| |
	| | |		| | | |
	| | Send Firmware |		| | Send Firmware | |
	| |-------------------->|		| |-------------------->| |
	| | |		| | | Verify |
	| | | Verify Firmware		| | | Firmware |
	| | |---------------		| | |--------------- |
	| | | |		| | | | |
	| | |<--------------		| | |<-------------- |
	| | |		| | | |
	| | | Store Firmware		| | | Store |
	| | |--------------		| | | Firmware |
	| | | |		| | |-------------- |
	| | |<-------------		| | | | |
	| | |		| | |<------------- |
	| | | Reboot		| | | |
	| | |-------		| | | |
	| | | |		| | | Reboot |
	| | |<------		| | |--------------->|
	| | |		| | | |
	| | | Bootloader validates		| | | Validate |
	| | | Firmware		| | | Firmware |
	| | |----------------------		| | | ---------------|
	| | | |		| | | | |
	| | |<---------------------		| | | -------------->|
	| | |		| | | |
	| | | Bootloader activates		| | | Activate new |
	| | | Firmware		| | | Firmware |
	| | |----------------------		| | | ---------------|
	| | | |		| | | | |
	| | |<---------------------		| | | -------------->|
	| | |		| | | |
	| | | Bootloader transfers		| | | Boot new |
	| | | control to new Firmware		| | | Firmware |
	| | |----------------------		| | | ---------------|
	| | | |		| | | | |
	| | |<---------------------		| | | -------------->|
	| | |		| | | |
	Figure 4: Example Flow for a Firmware Upate.		Figure 5: Example Flow for a Firmware Upate.
	8. IANA Considerations		9. IANA Considerations
	This document does not require any actions by IANA.		This document does not require any actions by IANA.
	9. Security Considerations		10. Security Considerations
	Firmware updates fix security vulnerabilities and are considered to		Firmware updates fix security vulnerabilities and are considered to
	be an important building block in securing IoT devices. Due to the		be an important building block in securing IoT devices. Due to the
	importance of firmware updates for IoT devices the Internet		importance of firmware updates for IoT devices the Internet
	Architecture Board (IAB) organized a 'Workshop on Internet of Things		Architecture Board (IAB) organized a 'Workshop on Internet of Things
	(IoT) Software Update (IOTSU)', which took place at Trinity College		(IoT) Software Update (IOTSU)', which took place at Trinity College
	Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at		Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at
	the big picture. A report about this workshop can be found at		the big picture. A report about this workshop can be found at
	[RFC8240]. A standardized firmware manifest format providing end-to-		[RFC8240]. A standardized firmware manifest format providing end-to-
	end security from the author to the device will be specified in a		end security from the author to the device will be specified in a
	skipping to change at page 16, line 13 ¶		skipping to change at page 19, line 38 ¶
	involvement.		involvement.
	- energy efficiency and battery lifetime considerations.		- energy efficiency and battery lifetime considerations.
	- key management required for verifying the digital signature		- key management required for verifying the digital signature
	protecting the manifest.		protecting the manifest.
	- incentives for manufacturers to offer a firmware update mechanism		- incentives for manufacturers to offer a firmware update mechanism
	as part of their IoT products.		as part of their IoT products.
	10. Mailing List Information		11. Mailing List Information
	The discussion list for this document is located at the e-mail		The discussion list for this document is located at the e-mail
	address suit@ietf.org [1]. Information on the group and information		address suit@ietf.org [1]. Information on the group and information
	on how to subscribe to the list is at		on how to subscribe to the list is at
	https://www1.ietf.org/mailman/listinfo/suit		https://www1.ietf.org/mailman/listinfo/suit
	Archives of the list can be found at: https://www.ietf.org/mail-		Archives of the list can be found at: https://www.ietf.org/mail-
	archive/web/suit/current/index.html		archive/web/suit/current/index.html
	11. Acknowledgements		12. Acknowledgements
	We would like to thank the following persons for their feedback:		We would like to thank the following persons for their feedback:
	- Geraint Luff		- Geraint Luff
	- Amyas Phillips		- Amyas Phillips
	- Dan Ros		- Dan Ros
	- Thomas Eichinger		- Thomas Eichinger
	- Michael Richardson		- Michael Richardson
	- Emmanuel Baccelli		- Emmanuel Baccelli
	- Ned Smith		- Ned Smith
	- David Brown
	- Jim Schaad		- Jim Schaad
	- Carsten Bormann		- Carsten Bormann
	- Cullen Jennings		- Cullen Jennings
	- Olaf Bergmann		- Olaf Bergmann
	- Suhas Nandakumar		- Suhas Nandakumar
	- Phillip Hallam-Baker		- Phillip Hallam-Baker
	- Marti Bolivar		- Marti Bolivar
	- Andrzej Puzdrowski		- Andrzej Puzdrowski
	- Markus Gueller		- Markus Gueller
	- Henk Birkholz		- Henk Birkholz
	- Jintao Zhu		- Jintao Zhu
	We would also like to thank the WG chairs, Russ Housley, David		We would also like to thank the WG chairs, Russ Housley, David
	Waltermire, Dave Thaler for their support and their reviews.		Waltermire, Dave Thaler for their support and their reviews.
	Kathleen Moriarty was the responsible security area director when		Kathleen Moriarty was the responsible security area director when
	this work was started.		this work was started.
	12. References		13. References
	12.1. Normative References		13.1. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997, <https://www.rfc-		DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
	editor.org/info/rfc2119>.		editor.org/info/rfc2119>.
	[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer		[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
	Security (TLS) / Datagram Transport Layer Security (DTLS)		Security (TLS) / Datagram Transport Layer Security (DTLS)
	Profiles for the Internet of Things", RFC 7925,		Profiles for the Internet of Things", RFC 7925,
	DOI 10.17487/RFC7925, July 2016, <https://www.rfc-		DOI 10.17487/RFC7925, July 2016, <https://www.rfc-
	editor.org/info/rfc7925>.		editor.org/info/rfc7925>.
	12.2. Informative References		13.2. Informative References
			[I-D.ietf-suit-information-model]
			Moran, B., Tschofenig, H., Birkholz, H., and J. Jimenez,
			"Firmware Updates for Internet of Things Devices - An
			Information Model for Manifests", draft-ietf-suit-
			information-model-00 (work in progress), June 2018.
	[LwM2M] OMA, ., "Lightweight Machine to Machine Technical		[LwM2M] OMA, ., "Lightweight Machine to Machine Technical
	Specification, Version 1.0.2", February 2018,		Specification, Version 1.0.2", February 2018,
	<http://www.openmobilealliance.org/release/LightweightM2M/		<http://www.openmobilealliance.org/release/LightweightM2M/
	V1_0_2-20180209-A/		V1_0_2-20180209-A/
	OMA-TS-LightweightM2M-V1_0_2-20180209-A.pdf>.		OMA-TS-LightweightM2M-V1_0_2-20180209-A.pdf>.
	[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard		[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
	(AES) Key Wrap with Padding Algorithm", RFC 5649,		(AES) Key Wrap with Padding Algorithm", RFC 5649,
	DOI 10.17487/RFC5649, September 2009, <https://www.rfc-		DOI 10.17487/RFC5649, September 2009, <https://www.rfc-
	editor.org/info/rfc5649>.		editor.org/info/rfc5649>.
			[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
			Requirements", RFC 6024, DOI 10.17487/RFC6024, October
			2010, <https://www.rfc-editor.org/info/rfc6024>.
	[RFC8240] Tschofenig, H. and S. Farrell, "Report from the Internet		[RFC8240] Tschofenig, H. and S. Farrell, "Report from the Internet
	of Things Software Update (IoTSU) Workshop 2016",		of Things Software Update (IoTSU) Workshop 2016",
	RFC 8240, DOI 10.17487/RFC8240, September 2017,		RFC 8240, DOI 10.17487/RFC8240, September 2017,
	<https://www.rfc-editor.org/info/rfc8240>.		<https://www.rfc-editor.org/info/rfc8240>.
	[SUIT-IM] Moran, B., Tschofenig, H., Birkholz, H., and J. Jimenez,		13.3. URIs
	"Firmware Updates for Internet of Things Devices - An
	Information Model for Manifests", June 2018.
	12.3. URIs
	[1] mailto:suit@ietf.org		[1] mailto:suit@ietf.org
	Authors' Addresses		Authors' Addresses
	Brendan Moran		Brendan Moran
	Arm Limited		Arm Limited
	EMail: Brendan.Moran@arm.com		EMail: Brendan.Moran@arm.com
	Milosch Meriac		Milosch Meriac
	Consultant		Consultant
	EMail: milosch@meriac.com		EMail: milosch@meriac.com
	Hannes Tschofenig		Hannes Tschofenig
	Arm Limited		Arm Limited
	EMail: hannes.tschofenig@gmx.net		EMail: hannes.tschofenig@gmx.net
			David Brown
			Linaro
			EMail: david.brown@linaro.org
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