A Firmware Update Architecture for Internet of Things DevicesArm LimitedBrendan.Moran@arm.comConsultantmilosch@meriac.comArm Limitedhannes.tschofenig@gmx.netLinarodavid.brown@linaro.org
Security
SUITInternet-DraftVulnerabilities with Internet of Things (IoT) devices have raised
the need for a solid and secure firmware update mechanism that is
also suitable for constrained devices. Incorporating such update
mechanism to fix vulnerabilities, to update configuration settings
as well as adding new functionality is recommended by security
experts.This document lists requirements and describes an architecture for
a firmware update mechanism suitable for IoT devices. The
architecture is agnostic to the transport of the firmware images
and associated meta-data.This version of the document assumes asymmetric cryptography and
a public key infrastructure. Future versions may also describe
a symmetric key approach for very constrained devices.When developing IoT devices, one of the most difficult problems
to solve is how to update the firmware on the device. Once the
device is deployed, firmware updates play a critical part in its
lifetime, particularly when devices have a long lifetime, are
deployed in remote or inaccessible areas or where manual
intervention is cost prohibitive or otherwise difficult. The need
for a firmware update may be to fix bugs in software, to add new
functionality, or to re-configure the device.The firmware update process, among other goals, has to ensure thatThe firmware image is authenticated and attempts to flash a
malicious firmware image are prevented.The firmware image can be confidentiality protected so that
attempts by an adversary to recover the plaintext binary can
be prevented. Obtaining the plaintext binary is often one of
the first steps for an attack to mount an attack.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 RFC 2119 .This document uses the following terms:Manifest: The manifest contains meta-data about the firmware
image. The manifest is protected against modification and
provides information about the author.Firmware Image: The firmware image is a binary that may
contain the complete software of a device or a subset of
it. The firmware image may consist of multiple images, if
the device contains more than one microcontroller. The
image may consist of a differential update for performance
reasons. Firmware is the more universal term. Both terms
are used in this document and are interchangeable.Bootloader: A bootloader is a piece of software that is
executed once a microcontroller has been reset. It is
responsible for deciding whether to boot a firmware image
that is present or whether to obtain and verify a new
firmware image. Since the bootloader is a security critical
component its functionality may be split into separate stages.
Such a multi-stage bootloader may offer very basic functionality
in the first stage and resides in ROM whereas the second stage
may implement more complex functionality and resides in flash
memory so that it can be updated in the future (in case bugs
have been found). The exact split of components into the
different stages, the number of firmware images stored by an
IoT device, and the detailed functionality varies throughout
different implementations.The following entities are used:Author: The author is the entity that creates the firmware image.
There may be multiple authors in a system either when a device
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 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.Communicator: The communicator component of the device interacts
with the firmware update server. It receives firmware images and
triggers an update, if needed.
The communicator either polls a 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
:“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.The firmware update mechanism described in this specification
was designed with the following requirements in mind:Agnostic to how firmware images are distributedFriendly to broadcast deliveryUse state-of-the-art security mechanismsRollback attacks must be preventedHigh reliabilityOperate with a small bootloaderSmall ParsersMinimal impact on existing firmware formatsRobust permissionsDiverse modes of operationFirmware images can be conveyed to devices in a variety of ways,
including USB, UART, WiFi, BLE, low-power WAN technologies, etc.
and use different protocols (e.g., CoAP, HTTP). The specified
mechanism needs to be agnostic to the distribution of the
firmware images and manifests.This architecture does not specify any specific broadcast protocol
however, given that broadcast may be desirable for some networks,
updates must cause the least disruption possible both in metadata
and payload transmission.For an update to be broadcast friendly, it cannot rely on link
layer, network layer, or transport layer security. In addition,
the same message must be deliverable to many devices, both those
to which it applies and those to which it does not, without a
chance that the wrong device will accept the update. Considerations
that apply to network broadcasts apply equally to the use of
third-party content distribution networks for payload distribution.End-to-end security between the author and the device, as shown
in , is used to ensure that the device can verify
firmware images and manifests produced by authorized authors.The use of post-quantum secure signature mechanisms, such as
hash-based signatures, should be explored. A migration to post-quantum
secure signatures would require significant effort, therefore,
mandatory-to-implement support for post-quantum secure signatures
is a goal.A mandatory-to-implement set of algorithms has to be defined offering
a key length of 112-bit symmetric key or security or more, as outlined
in Section 20 of RFC 7925 . This corresponds to a 233 bit
ECC key or a 2048 bit RSA key.If the firmware image is to be encrypted, it must be done in such a
way that every intended recipient can decrypt it. The information
that is encrypted individually for each device must be an absolute
minimum, for example AES Key Wrap , in order to maintain
friendliness to Content Distribution Networks, bulk storage, and
broadcast protocols.A device presented with an old, but valid manifest and firmware
must not be tricked into installing such firmware since a
vulnerability in the old firmware image may allow an attacker to
gain control of the device.A power failure at any time must not cause a failure of the device.
A failure to validate any part of an update must not cause a
failure of the device. One way to achieve this functionality is
to provide a minimum of two storage locations for firmware and one
bootable location for firmware. An alternative approach is to use a
2nd stage bootloader with build-in full featured firmware update
functionality such that it is possible to return to the update
process after power down.Note: This is an implementation requirement rather than a requirement
on the manifest format.The bootloader must be minimal, containing only flash support,
cryptographic primitives and optionally a recovery mechanism. The
recovery mechanism is used in case the update process failed and
may include support for firmware updates over serial, USB or even
a limited version of wireless connectivity standard like a limited
Bluetooth Smart. Such a recovery mechanism must provide security
at least at the same level as the full featured firmware update
functionalities.The bootloader needs to verify the received manifest and to install
the bootable firmware image. The bootloader should not require
updating since a failed update poses a risk in reliability. If more
functionality is required in the bootloader, it must use a two-stage
bootloader, with the first stage comprising the functionality defined
above.All information necessary for a device to make a decision about the
installation of a firmware update must fit into the available RAM of
a constrained IoT device. This prevents flash write exhaustion.Note: This is an implementation requirement.Since parsers are known sources of bugs they must be minimal.
Additionally, it must be easy to parse only those fields that are
required to validate at least one signature or MAC with minimal
exposure.The design of the firmware update mechanism must not require
changes to existing firmware formats.When a device obtains a monolithic firmware image from a single author
without any additional approval steps then the authorization flow is
relatively simple. There are, however, other cases where more complex
policy decisions need to be made before updating a device.In this architecture the authorization policy is separated from
the underlying communication architecture. This is accomplished
by separating the entities from their permissions. For example,
an author may not have the authority to install a firmware image
on a device in critical infrastructure without the authorization
of a 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 entity, which
does all permission management and coordination. This entity
allows the device to offload complex permissions
calculations for the device.There are three broad classifications of update operating modes.Client-initiated UpdateServer-initiated UpdateHybrid UpdateClient-initiated updates take the form of a communicator on
a device proactively checking for new firmware imagines provided
by firmware servers.Server-initiated updates are important to consider because
timing of updates may need to be tightly controlled in some high-
reliability environments. In this case the communicator, potentially
in coordination with the status tracker, determines what devices
qualify for a firmware update. Once those devices have been
selected the firmware server distributes updates to those devices.Note: This assumes that the firmware server is able to reach the
device, which may require devices to keep reachability
information at the communicator and / or at the firmware server
up-to-date. This may also require keeping state at NATs and stateful
packet filtering firewalls alive.Hybrid updates are those that require an interaction between the
device and the firmware server / communicator. The communicator
pushes notifications of availability of an update to the device,
and the device then downloads the image from the firmware server
when it wants.An alternative approach is to consider the steps a device has
to go through in the course of an update:NotificationPre-authorisationDependency resolutionDownloadInstallationThe 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 can be updated or when a differential update is used.
The necessary dependencies must be available prior to installation.The download step is the process of acquiring a local copy of the
firmware image. When the download is client-initiated, this means
that the device chooses when a download occurs and initiates
the download process. When a download is server-party initiated,
this means that either the communicator / firmware server tells
the device when to download or that it initiates the transfer
directly to the device. For example, a download from an
HTTP-based firmware server is client-initiated. A transfer to a
LwM2M Firmware Update resource 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
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.Claims in the manifest offer a way to convey instructions to
a device that impact the firmware update process. To have any
value the manifest containing those claims must be authenticated
and integrity protected. The credential used to must be directly
or indirectly related to the trust anchor installed at the device
by the Trust Provisioning Authority.The baseline claims for all manifests are described in .
For example, there are:Do not install firmware with earlier metadata than the current
metadata.Only install firmware with a matching vendor, model, hardware
revision, software version, etc.Only install firmware that is before its best-before timestamp.Only allow a firmware installation if dependencies have been met.Choose the mechanism to install the firmware, based on the type
of firmware it is. shows the communication architecture where a
firmware image 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.End-to-end security mechanisms are used to protect the firmware
image and the manifest although does not show the
manifest itself since it may be distributed independently.Whether the firmware image and the manifest is pushed to the device or
fetched by the device is a deployment specific decision.The following assumptions are made to allow the device to verify the
received firmware image and manifest before updating software:To accept an update, a device needs to verify the signature covering
the manifest. There may be one or multiple manifests that need to be
validated, potentially signed by different parties. The device needs
to be in possession of the trust anchors to verify those signatures.
Installing trust anchors to devices via the Trust Provisioning Authority
happens in an out-of-band fashion prior to the firmware update process.Not all entities creating and signing manifests have the same
permissions. A device needs to determine whether the requested action
is indeed covered by the permission of the party that signed the manifest.
Informing the device about the permissions 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
to be in possession of the certificate/public key or a pre-shared 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
based on examples below.There is an option for embedding a firmware image into a manifest.
This is a useful approach for deployments where devices are not connected
to the Internet and cannot contact a dedicated server for download of
the firmware. It is also applicable when the firmware update happens via a
USB stick or via Bluetooth Smart. shows this
delivery mode graphically. shows an option for remotely updating a device
where the device fetches the firmware image from some file server. The
manifest itself is delivered independently and provides information about
the firmware image(s) to download.This architecture does not mandate a specific delivery mode but a solution
must support both types.In order for a device to apply an update, it has to make several decisions
about the update:Does it trust the author of the update?Has the firmware been corrupted?Does the firmware update apply to this device?Is the update older than the active firmware?When should the device apply the update?How should the device apply the update?What kind of firmware binary is it?Where should the update be obtained?Where should the firmware be stored?The manifest encodes the information that devices need in order to
make these decisions. It is a data structure that contains the
following information:information about the device(s) the firmware image is intended to
be applied to,information about when the firmware update has to be applied,information about when the manifest was created,dependencies on other manifests,pointers to the firmware image and information about the format,information about where to store the firmware image,cryptographic information, such as digital signatures or message
authentication codes (MACs).The manifest information model is described in .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.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.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.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.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.The following example message flow illustrates the
interaction for distributing a firmware image to a device
starting with an author uploading the new firmware to
Firmware Server and creating a manifest. The firmware
and manifest are stored on the same Firmware Server.This document does not require any actions by IANA.Firmware updates fix security vulnerabilities and are considered to be
an important building block in securing IoT devices. Due to the
importance of firmware updates for IoT devices the Internet
Architecture Board (IAB) organized a ‘Workshop on Internet of Things
(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
the big picture. A report about this workshop can be found at
. A standardized firmware manifest format providing
end-to-end security from the author to the device will be specified
in a separate document.There are, however, many other considerations raised during the
workshop. Many of them are outside the scope of standardization
organizations since they fall into the realm of product engineering,
regulatory frameworks, and business models. The following
considerations are outside the scope of this document, namelyinstalling firmware updates in a robust fashion so that the
update does not break the device functionality of the environment
this device operates in.installing firmware updates in a timely fashion considering the
complexity of the decision making process of updating devices,
potential re-certification requirements, and the need for user
consent to install updates.the distribution of the actual firmware update, potentially in
an efficient manner to a large number of devices without human
involvement.energy efficiency and battery lifetime considerations.key management required for verifying the digital signature
protecting the manifest.incentives for manufacturers to offer a firmware update mechanism
as part of their IoT products.The discussion list for this document is located at the e-mail
address suit@ietf.org. Information on the group and information on how to
subscribe to the list is at https://www1.ietf.org/mailman/listinfo/suitArchives of the list can be found at:
https://www.ietf.org/mail-archive/web/suit/current/index.htmlWe would like to thank the following persons for their feedback:Geraint LuffAmyas PhillipsDan RosThomas EichingerMichael RichardsonEmmanuel BaccelliNed SmithJim SchaadCarsten BormannCullen JenningsOlaf BergmannSuhas NandakumarPhillip Hallam-BakerMarti BolivarAndrzej PuzdrowskiMarkus GuellerHenk BirkholzJintao ZhuWe would also like to thank the WG chairs, Russ Housley, David Waltermire,
Dave Thaler for their support and their reviews. Kathleen Moriarty was the
responsible security area director when this work was started.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Transport Layer Security (TLS) / Datagram Transport Layer Security (DTLS) Profiles for the Internet of ThingsA common design pattern in Internet of Things (IoT) deployments is the use of a constrained device that collects data via sensors or controls actuators for use in home automation, industrial control systems, smart cities, and other IoT deployments.This document defines a Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) 1.2 profile that offers communications security for this data exchange thereby preventing eavesdropping, tampering, and message forgery. The lack of communication security is a common vulnerability in IoT products that can easily be solved by using these well-researched and widely deployed Internet security protocols.Report from the Internet of Things Software Update (IoTSU) Workshop 2016This document provides a summary of the Internet of Things Software Update (IoTSU) Workshop that took place at Trinity College Dublin, Ireland on the 13th and 14th of June, 2016. The main goal of the workshop was to foster a discussion on requirements, challenges, and solutions for bringing software and firmware updates to IoT devices. This report summarizes the discussions and lists recommendations to the standards community.Note that this document is a report on the proceedings of the workshop. The views and positions documented in this report are those of the workshop participants and do not necessarily reflect IAB views and positions.Trust Anchor Management RequirementsA 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 relying party uses trust anchors to determine if a digitally signed object is valid by verifying a digital signature using the trust anchor's public key, and by enforcing the constraints expressed in the associated data for the trust anchor. This document describes some of the problems associated with the lack of a standard trust anchor management mechanism and defines requirements for data formats and push-based protocols designed to address these problems. This document is not an Internet Standards Track specification; it is published for informational purposes.Advanced Encryption Standard (AES) Key Wrap with Padding AlgorithmThis document specifies a padding convention for use with the AES Key Wrap algorithm specified in RFC 3394. This convention eliminates the requirement that the length of the key to be wrapped be a multiple of 64 bits, allowing a key of any practical length to be wrapped. This memo provides information for the Internet community.Firmware Updates for Internet of Things Devices - An Information Model for ManifestsVulnerabilities with Internet of Things (IoT) devices have raised the need for a solid and secure firmware update mechanism that is also suitable for constrained devices. Incorporating such update mechanism to fix vulnerabilities, to update configuration settings as well as adding new functionality is recommended by security experts. One component of such a firmware update is the meta-data, or manifest, that describes the firmware image(s) and offers appropriate protection. This document describes all the information that must be present in the manifest.Lightweight Machine to Machine Technical Specification, Version 1.0.2