< draft-ietf-suit-architecture-12.txt   draft-ietf-suit-architecture-13.txt >
SUIT B. Moran SUIT B. Moran
Internet-Draft H. Tschofenig Internet-Draft H. Tschofenig
Intended status: Informational Arm Limited Intended status: Informational Arm Limited
Expires: March 21, 2021 D. Brown Expires: April 19, 2021 D. Brown
Linaro Linaro
M. Meriac M. Meriac
Consultant Consultant
September 17, 2020 October 16, 2020
A Firmware Update Architecture for Internet of Things A Firmware Update Architecture for Internet of Things
draft-ietf-suit-architecture-12 draft-ietf-suit-architecture-13
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 reliable and secure firmware update mechanism suitable for
suitable for constrained devices. Incorporating such update devices with resource constraints. Incorporating such an update
mechanism to fix vulnerabilities, to update configuration settings as mechanism is a fundamental requirement for fixing vulnerabilities but
well as adding new functionality is recommended by security experts. it also enables other important capabilities such as updating
configuration settings as well as adding new functionality.
This document lists requirements and describes an architecture for a In addition to the definition of terminology and an architecture this
firmware update mechanism suitable for IoT devices. The architecture document motivates the standardization of a manifest format as a
is agnostic to the transport of the firmware images and associated transport-agnostic means for describing and protecting firmware
meta-data. updates.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on March 21, 2021. This Internet-Draft will expire on April 19, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 5
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Agnostic to how firmware images are distributed . . . . . 7 2.2. Stakeholders . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Friendly to broadcast delivery . . . . . . . . . . . . . 8 2.3. Functions . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Use state-of-the-art security mechanisms . . . . . . . . 8 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Rollback attacks must be prevented . . . . . . . . . . . 9 4. Invoking the Firmware . . . . . . . . . . . . . . . . . . . . 12
3.5. High reliability . . . . . . . . . . . . . . . . . . . . 9 4.1. The Bootloader . . . . . . . . . . . . . . . . . . . . . 14
3.6. Operate with a small bootloader . . . . . . . . . . . . . 9 5. Types of IoT Devices . . . . . . . . . . . . . . . . . . . . 15
3.7. Small Parsers . . . . . . . . . . . . . . . . . . . . . . 10 5.1. Single MCU . . . . . . . . . . . . . . . . . . . . . . . 15
3.8. Minimal impact on existing firmware formats . . . . . . . 10 5.2. Single CPU with Secure - Normal Mode Partitioning . . . . 16
3.9. Robust permissions . . . . . . . . . . . . . . . . . . . 10 5.3. Symmetric Multiple CPUs . . . . . . . . . . . . . . . . . 16
3.10. Operating modes . . . . . . . . . . . . . . . . . . . . . 11 5.4. Dual CPU, shared memory . . . . . . . . . . . . . . . . . 16
3.11. Suitability to software and personalization data . . . . 13 5.5. Dual CPU, other bus . . . . . . . . . . . . . . . . . . . 17
4. Claims . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6. Manifests . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5. Communication Architecture . . . . . . . . . . . . . . . . . 14 7. Securing Firmware Updates . . . . . . . . . . . . . . . . . . 19
6. Manifest . . . . . . . . . . . . . . . . . . . . . . . . . . 18 8. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7. Device Firmware Update Examples . . . . . . . . . . . . . . . 19 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
7.1. Single CPU SoC . . . . . . . . . . . . . . . . . . . . . 19 10. Security Considerations . . . . . . . . . . . . . . . . . . . 25
7.2. Single CPU with Secure - Normal Mode Partitioning . . . . 19 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
7.3. Symmetric Multiple CPUs . . . . . . . . . . . . . . . . . 19 12. Informative References . . . . . . . . . . . . . . . . . . . 26
7.4. Dual CPU, shared memory . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
7.5. Dual CPU, other bus . . . . . . . . . . . . . . . . . . . 20
8. Bootloader . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11. Security Considerations . . . . . . . . . . . . . . . . . . . 26
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
13. Informative References . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction 1. Introduction
When developing Internet of Things (IoT) devices, one of the most Firmware updates can help to fix security vulnerabilities and are
difficult problems to solve is how to update firmware on the device. considered to be an important building block in securing IoT devices.
Once the device is deployed, firmware updates play a critical part in Due to rising concerns about insecure IoT devices the Internet
its lifetime, particularly when devices have a long lifetime, are Architecture Board (IAB) organized a 'Workshop on Internet of Things
deployed in remote or inaccessible areas where manual intervention is (IoT) Software Update (IOTSU)', which took place at Trinity College
cost prohibitive or otherwise difficult. Updates to the firmware of Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at
an IoT device are done to fix bugs in software, to add new the bigger picture. A report about this workshop can be found at
functionality, and to re-configure the device to work in new [RFC8240]. The workshop revealed a number of challenges for
developers and led to the formation of the IETF Software Updates for
Internet of Things (SUIT) working group.
Developing secure Internet of Things (IoT) devices is not an easy
task and supporting a firmware update solution requires skillful
engineers. Once devices are deployed, firmware updates play a
critical part in their lifecycle management, particularly when
devices have a long lifetime, or are deployed in remote or
inaccessible areas where manual intervention is cost prohibitive or
otherwise difficult. Firmware updates
for IoT devices are expected to work automatically, i.e. without user
involvement. Automatic updates that do not require human
intervention are key to a scalable solution for fixing software
vulnerabilities.
Firmware updates are not only done to fix bugs, but they can also add
new functionality, and re-configure the device to work in new
environments or to behave differently in an already deployed context. environments or to behave differently in an already deployed context.
The firmware update process, among other goals, has to ensure that The firmware update process has to ensure that
- The firmware image is authenticated and integrity protected. - The firmware image is authenticated and integrity protected.
Attempts to flash a modified firmware image or an image from an Attempts to flash a maliciously modified firmware image or an
unknown source are prevented. image from an unknown, untrusted source must be prevented. In
examples this document uses asymmetric cryptography because it is
the preferred approach by many IoT deployments. The use of
symmetric credentials is also supported and can be used by very
constrained IoT devices.
- 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 firmware is often one of the first steps mitigated or at least made more difficult. Obtaining the firmware
to mount an attack since it gives the adversary valuable insights is often one of the first steps to mount an attack since it gives
into used software libraries, configuration settings and generic the adversary valuable insights into the software libraries used,
functionality (even though reverse engineering the binary can be a configuration settings and generic functionality. Even though
tedious process). reverse engineering the binary can be a tedious process modern
reverse engineering frameworks have made this task a lot easier.
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.
While the standardization work has been informed by and optimised for While the standardization work has been informed by and optimized for
firmware update use cases of Class 1 devices (according to the device firmware update use cases of Class 1 devices (according to the device
class definitions in RFC 7228 [RFC7228]) devices, there is nothing in class definitions in RFC 7228 [RFC7228]) devices, there is nothing in
the architecture that restricts its use to only these constrained IoT the architecture that restricts its use to only these constrained IoT
devices. Moreover, this architecture is not limited to managing devices. Moreover, this architecture is not limited to managing
software updates, but can also be applied to managing the delivery of software updates, but can also be applied to managing the delivery of
arbitrary data, such as configuration information and keys. arbitrary data, such as configuration information and keys. Unlike
higher end devices, like laptops and desktop PCs, many IoT devices do
not have user interfaces and support for unattended updates is,
therefore, essential for the design of a practical solution.
Constrained IoT devices often use a software engineering model where
a developer is responsible for creating and compiling all software
running on the device into a single, monolithic firmware image. On
higher end devices application software is, on the other hand, often
downloaded separately and even obtained from developers different to
the developers of the lower level software. The details for how to
obtain those application layer software binaries then depends heavily
on the platform, programming language uses and the sandbox the
software is executed in.
More details about the security goals are discussed in Section 5 and While the IETF standardization work has been focused on the manifest
requirements are described in Section 3. format, a fully interoperable solution needs more than a standardized
manifest. For example, protocols for transferring firmware images
and manifests to the device need to be available as well as the
status tracker functionality. Devices also require a mechanism to
discover the status tracker(s) and/or firmware servers. These
building blocks have been developed by various organizations under
the umbrella of an IoT device management solution. The LwM2M
protocol is one IoT device management protocol.
There are, however, several areas that (partially) fall outside the
scope of the IETF and other standards organizations but need to be
considered by firmware authors, as well as device and network
operators. Here are some of them, as highlighted during the IOTSU
workshop:
- Installing firmware updates in a robust fashion so that the update
does not break the device functionality of the environment this
device operates in. This requires proper testing and offering
recovery strategies when a firmware update is unsuccessful.
- Making firmware updates available in a timely fashion considering
the complexity of the decision making process for updating
devices, potential re-certification requirements, the length of a
supply chain an update needs to go through before it reaches the
end customer, and the need for user consent to install updates.
- Ensuring an energy efficient design of a battery-powered IoT
devices because a firmware update, particularly writing the
firmware image to flash, is a heavy task for a device.
- Creating incentives for device operators to use a firmware update
mechanism and to demand the integration of it from IoT device
vendors.
- Ensuring that firmware updates addressing critical flaws can be
obtained even after a product is discontinued or a vendor goes out
of business.
This document starts with a terminology followed by the description
of the architecture. We then explain the bootloader and how it
integrates with the firmware update mechanism. Subsequently, we
offer a categorization of IoT devices in terms of their hardware
capabilities relevant for firmware updates. Next, we talk about the
manifest structure and how to use it to secure firmware updates. We
conclude with a more detailed example.
2. Conventions and Terminology 2. Conventions and Terminology
This document uses the following terms: 2.1. Terms
- Manifest: The manifest contains meta-data about the firmware This document uses the following terms:
image. The manifest is protected against modification and
provides information about the author.
- Firmware Image: The firmware image, or image, is a binary that may - Firmware Image: The firmware image, or image, is a binary that may
contain the complete software of a device or a subset of it. The contain the complete software of a device or a subset of it. The
firmware image may consist of multiple images, if the device firmware image may consist of multiple images, if the device
contains more than one microcontroller. Often it is also a contains more than one microcontroller. Often it is also a
compressed archive that contains code, configuration data, and compressed archive that contains code, configuration data, and
even the entire file system. The image may consist of a even the entire file system. The image may consist of a
differential update for performance reasons. Firmware is the more differential update for performance reasons.
universal term. The terms, firmware image, firmware, and image,
are used in this document and are interchangeable.
- Software: The terms "software" and "firmware" are used The terms, firmware image, firmware, and image, are used in this
interchangeably. document and are interchangeable. We use the term application
firmware image to differentiate it from a firmware image that
contains the bootloader. An application firmware image, as the
name indicates, contains the application program often including
all the necessary code to run it (such as protocol stacks, and
embedded operating system).
- Bootloader: A bootloader is a piece of software that is executed - Manifest: The manifest contains meta-data about the firmware
once a microcontroller has been reset. It is responsible for image. The manifest is protected against modification and
deciding whether to boot a firmware image that is present or provides information about the author.
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. A more
detailed discussion is provided in Section 8.
- Microcontroller (MCU for microcontroller unit): An MCU is a - Microcontroller (MCU for microcontroller unit): An MCU is a
compact integrated circuit designed for use in embedded systems. compact integrated circuit designed for use in embedded systems.
A typical microcontroller includes a processor, memory (RAM and A typical microcontroller includes a processor, memory (RAM and
flash), input/output (I/O) ports and other features connected via flash), input/output (I/O) ports and other features connected via
some bus on a single chip. The term 'system on chip (SoC)' is some bus on a single chip. The term 'system on chip (SoC)' is
often used for these types of devices. often used interchangeably with MCU, but MCU tends to imply more
limited peripheral functions.
- System on Chip (SoC): An 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.
- Trusted Execution Environments (TEEs): An execution environment
that runs alongside of, but is isolated from, an REE.
- Rich Execution Environment (REE): An environment that is provided - Rich Execution Environment (REE): An environment that is provided
and governed by a typical OS (e.g., Linux, Windows, Android, iOS), and governed by a typical OS (e.g., Linux, Windows, Android, iOS),
potentially in conjunction with other supporting operating systems potentially in conjunction with other supporting operating systems
and hypervisors; it is outside of the TEE. This environment and and hypervisors; it is outside of the TEE. This environment and
applications running on it are considered un-trusted. applications running on it are considered un-trusted.
- Trusted applications (TAs): An application component that runs in - System on Chip (SoC): An SoC is an integrated circuit that
contains all components of a computer, such as CPU, memory, input/
output ports, secondary storage, a bus to connect the components,
and other hardware blocks of logic.
- Trust Anchor: A trust anchor, as defined in [RFC6024], 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."
- Trust Anchor Store: A trust anchor store, as defined in [RFC6024],
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. A trust anchor store
must resist modification against unauthorized insertion, deletion,
and modification.
- Trusted Applications (TAs): An application component that runs in
a TEE. a TEE.
For more information about TEEs see [I-D.ietf-teep-architecture]. - Trusted Execution Environments (TEEs): An execution environment
TEEP requires the use of SUIT for delivering TAs. that runs alongside of, but is isolated from, an REE. For more
information about TEEs see [I-D.ietf-teep-architecture].
The following entities are used: 2.2. Stakeholders
The following stakeholders are used in this document:
- Author: The author is the entity that creates the firmware image. - Author: The author is the entity that creates the firmware image.
There may be multiple authors in a system either when a device There may be multiple authors involved in producing firmware
consists of multiple micro-controllers or when the the final running on an IoT device. Section 5 talks about those IoT device
firmware image consists of software components from multiple deployment cases.
companies.
- Firmware Consumer: The firmware consumer is the recipient of the - Device Operator: The device operator is responsible for the day-
firmware image and the manifest. It is responsible for parsing to-day operation of a fleet of IoT devices. Customers of IoT
and verifying the received manifest and for storing the obtained devices, as the owners of IoT devices - such as enterprise
firmware image. The firmware consumer plays the role of the customers or end users, interact with their IoT devices indirectly
update component on the IoT device typically running in the through the device operator via web or smart phone apps.
application firmware. It interacts with the firmware server and
with the status tracker, if present. - Network Operator: The network operator is responsible for the
operation of a network to which IoT devices connect.
- Trust Provisioning Authority (TPA): The TPA distributes trust
anchors and authorization policies to various stakeholders. The
TPA may also delegate rights to stakeholders. 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.
2.3. Functions
- (IoT) Device: A device refers to the entire IoT product, which - (IoT) Device: A device refers to the entire IoT product, which
consists of one or many MCUs, sensors and/or actuators. Many IoT consists of one or many MCUs, sensors and/or actuators. Many IoT
devices sold today contain multiple MCUs and therefore a single devices sold today contain multiple MCUs and therefore a single
device may need to obtain more than one firmware image and device may need to obtain more than one firmware image and
manifest to succesfully perform an update. The terms device and manifest to successfully perform an update.
firmware consumer are used interchangably since the firmware
consumer is one software component running on an MCU on the
device.
- Status Tracker: The status tracker offers device management - Status Tracker: The status tracker has a client and a server
functionality to retrieve information about the installed firmware component and performs three tasks: 1) It communicates the
on a device and other device characteristics (including free availability of a new firmware version. This information will
memory and hardware components), to obtain the state of the flow from the server to the client.
firmware update cycle the device is currently in, and to trigger 2) It conveys information about software and hardware
the update process. The deployment of status trackers is flexible characteristics of the device. The information flow is from the
and they may be used as cloud-based servers, on-premise servers, client to the server.
embedded in edge computing device (such as Internet access 3) It can remotely trigger the firmware update process. The
gateways or protocol translation gateways), or even in smart information flow is from the server to the client.
phones and tablets. IoT devices that self-initiate updates may
run a status tracker. Similarly, IoT devices that act as a proxy For example, a device operator may want to read the installed
for other IoT devices in a protocol translation or edge computing firmware version number running on the device and information
device node may also run a status tracker. However, if the device about available flash memory. Once an update has been triggered,
contains multiple MCUs, the main MCU may act as a limited status the device operator may want to obtain information about the state
tracker towards the other MCUs if updates are to be synchronized of the firmware update. If errors occurred, the device operator
across MCUs. How much functionality a status tracker includes may want to troubleshoot problems by first obtaining diagnostic
depends on the selected configuration of the device management information (typically using a device management protocol).
functionality and the communication environment it is used in. In
a generic networking environment the protocol used between the We make no assumptions about where the server-side component is
client and the server-side of the status tracker need to deal with deployed. The deployment of status trackers is flexible and may
Internet communication challenges involving firewall and NAT be found at
traversal. In other cases, the communication interaction may be cloud-based servers, on-premise servers, or may be embedded in
rather simple. This architecture document does not impose edge computing device. A status tracker server component may even
requirements on the status tracker. be deployed on an IoT device. For example, if the IoT device
contains multiple MCUs, then the main MCU may act as a status
tracker towards the other MCUs. Such deployment is useful when
updates have to be synchronized across MCUs.
The status tracker may be operated by any suitable stakeholder;
typically the Author, Device Operator, or Network Operator.
- Firmware Consumer: The firmware consumer is the recipient of the
firmware image and the manifest. It is responsible for parsing
and verifying the received manifest and for storing the obtained
firmware image. The firmware consumer plays the role of the
update component on the IoT device typically running in the
application firmware. It interacts with the firmware server and
with the status tracker client (locally).
- Firmware Server: The firmware server stores firmware images and - Firmware Server: The firmware server stores firmware images and
manifests and distributes them to IoT devices. Some deployments manifests and distributes them to IoT devices. Some deployments
may require a store-and-forward concept, which requires storing may require a store-and-forward concept, which requires storing
the firmware images/manifests on more than one entity before the firmware images/manifests on more than one entity before
they reach the device. There is typically some interaction they reach the device. There is typically some interaction
between the firmware server and the status tracker but those between the firmware server and the status tracker and these two
entities are often physically separated on different devices for entities are often physically separated on different devices for
scalability reasons. scalability reasons.
- Device Operator: The actor responsible for the day-to-day - Bootloader: A bootloader is a piece of software that is executed
operation of a fleet of IoT devices. once a microcontroller has been reset. It is responsible for
deciding what code to execute.
- Network Operator: The actor responsible for the operation of a
network to which IoT devices connect.
- Claim: A piece of information asserted about a recipient or
payload.
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 3. Architecture
in a device. A device may have more than one trust anchor store,
each of which may be used by one or more applications." A trust
anchor store must resist modification against unauthorized
insertion, deletion, and modification.
3. Requirements More devices today than ever before are connected to the Internet,
which drives the need for firmware updates to be provided over the
Internet rather than through traditional interfaces, such as USB or
RS-232. Updating updates over the Internet requires the device to
fetch the new firmware image as well as the manifest.
The firmware update mechanism described in this specification was Hence, the following components are necessary on a device for a
designed with the following requirements in mind: firmware update solution:
- Agnostic to how firmware images are distributed - the Internet protocol stack for firmware downloads. Because
firmware images are often multiple kilobytes, sometimes exceeding
one hundred kilobytes, in size for low end IoT devices and even
several megabytes large for IoT devices running full-fledged
operating systems like Linux, the protocol mechanism for
retrieving these images needs to offer features like congestion
control, flow control, fragmentation and reassembly, and
mechanisms to resume interrupted or corrupted transfers.
- Friendly to broadcast delivery - the capability to write the received firmware image to persistent
storage (most likely flash memory).
- Use state-of-the-art security mechanisms - a manifest parser with code to verify a digital signature or a
message authentication code.
- Rollback attacks must be prevented - the ability to unpack, to decompress and/or to decrypt the
received firmware image.
- High reliability - (optionally) a status tracker.
- Operate with a small bootloader The features listed above are most likely offered by code in the
application firmware image running on the device rather than by the
bootloader itself. Note that cryptographic algorithms will likely
run in a trusted execution environment, on a separate MCU, in a
hardware security module, or in a secure element rather than in the
same context with the application code.
- Small Parsers Figure 1 shows the architecture where a firmware image is created by
an author, and made available to a firmware server. For security
reasons, the author will not have the permissions to upload firmware
images to the firmware server and to initiate an update him- or
herself. Instead, authors will make firmware images available to the
device operators. Note that there may be a longer supply chain
involved to pass software updates from the author all the way to the
party that can then finally make a decision to deploy it with IoT
devices.
- Minimal impact on existing firmware formats As a first step in the firmware update process, the status tracker
client need to be made aware of the availability of a new firmware
update by the status tracker server. This can be accomplished via
polling (client-initiated), push notifications (server-initiated), or
more complex mechanisms (such as a hybrid approach):
- Robust permissions - Client-initiated updates take the form of a status tracker client
proactively checking (polling) for updates.
- Diverse modes of operation - With Server-initiated updates the server-side component of the
status tracker learns about a new firmware version and determines
what devices qualify for a firmware update. Once the relevant
devices have been selected, the status tracker informs these
devices and the firmware consumers obtain those images and
manifests. Server-initiated updates are important because they
allow a quick response time. Note that the client-side status
tracker needs to be reachable by the server-side component. This
may require devices to keep reachability information on the
server-side up-to-date and state at NATs and stateful packet
filtering firewalls alive.
- Suitability to software and personalization data - Using a hybrid approach the server-side of the status tracker
pushes notifications of availability of an update to the client
side and requests the firmware consumer to pull the manifest and
the firmware image from the firmware server.
3.1. Agnostic to how firmware images are distributed Once the device operator triggers update via the status tracker, it
will keep track of the update process on the device. This allows the
device operator to know what devices have received an update and
which of them are still pending an update.
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, mesh
use different protocols (e.g., CoAP, HTTP). The specified mechanism networks and many more. At the application layer a variety of
needs to be agnostic to the distribution of the firmware images and protocols are also available: MQTT, CoAP, and HTTP are the most
manifests. popular application layer protocols used by IoT devices. This
architecture does not make assumptions about how the firmware images
3.2. Friendly to broadcast delivery are distributed to the devices and therefore aims to support all
these technologies.
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
firmware transmission.
For an update to be broadcast friendly, it cannot rely on link layer, In some cases it may be desirable to distribute firmware images using
a multicast or broadcast protocol. This architecture does not make
recommendations for any such protocol. However, given that broadcast
may be desirable for some networks, updates must cause the least
disruption possible both in metadata and firmware transmission. For
an update to be broadcast friendly, it cannot rely on link layer,
network layer, or transport layer security. A solution has to rely network layer, or transport layer security. A solution has to rely
on security protection applied to the manifest and firmware image on security protection applied to the manifest and firmware image
instead. In addition, the same manifest must be deliverable to many instead. In addition, the same manifest must be deliverable to many
devices, both those to which it applies and those to which it does 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. not, without a chance that the wrong device will accept the update.
Considerations that apply to network broadcasts apply equally to the Considerations that apply to network broadcasts apply equally to the
use of third-party content distribution networks for payload use of third-party content distribution networks for payload
distribution. distribution.
3.3. Use state-of-the-art security mechanisms +----------+
| |
End-to-end security between the author and the device is shown in | Author |
Section 5. | |
+----------+
Authentication ensures that the device can cryptographically identify Firmware + Manifest |
the author(s) creating firmware images and manifests. Authenticated +----------------------------------+ | Firmware +
identities may be used as input to the authorization process. | | | Manifest
| ---+------- |
Integrity protection ensures that no third party can modify the | ---- | --|-
manifest or the firmware image. | //+----------+ | \\
-+-- // | | | \
For confidentiality protection of the firmware image, it must be done ----/ | ---- |/ | Firmware |<-+ | \
in such a way that every intended recipient can decrypt it. The // | \\ | | Server | | | \
information that is encrypted individually for each device must / | \ / | | + + \
maintain friendliness to Content Distribution Networks, bulk storage, / | \ / +----------+ \ / |
and broadcast protocols. / +--------+--------+ \ / | |
/ | v | \ / v |
A manifest specification must support different cryptographic | | +------------+ | | | +----------------+ |
algorithms and algorithm extensibility. Due of the nature of | | | Firmware | | | Device | |
unchangeable code in ROM for use with bootloaders the use of post- | | | Consumer | | | | | Management | |
quantum secure signature mechanisms, such as hash-based signatures | | +------------+ | | | | | |
[RFC8778], are attractive. These algorithms maintain security in | | +------------+ | | | | +--------+ | |
presence of quantum computers. | | | Status |<-+--------------------+-> | | | |
| | | Tracker | | | | | | Status | | |
A mandatory-to-implement set of algorithms will be specified in the | | | Client | | | | | | Tracker| | |
manifest specification [I-D.ietf-suit-manifest]}. | | +------------+ | | | | | Server | | |
| | Device | | | | +--------+ | |
3.4. Rollback attacks must be prevented | +-----------------+ | \ | | /
\ / \ +----------------+ /
A device presented with an old, but valid manifest and firmware must \ Network / \ /
not be tricked into installing such firmware since a vulnerability in \ Operator / \ Device Operator /
the old firmware image may allow an attacker to gain control of the \\ // \ \ //
device. ---- ---- ---- ----
----- -----------
3.5. High reliability
A power failure at any time must not cause a failure of the device.
Equally, adverse network conditions during an update must not cause
the 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.
3.6. Operate with a small bootloader
Throughout this document we assume that the bootloader itself is
distinct from the role of the firmware consumer and therefore does
not manage the firmware update process. This may give the impression
that the bootloader itself is a completely separate component, which
is mainly responsible for selecting a firmware image to boot.
The overlap between the firmware update process and the bootloader
functionality comes in two forms, namely
- First, a bootloader must verify the firmware image it boots as
part of the secure boot process. Doing so requires meta-data to
be stored alongside the firmware image so that the bootloader can
cryptographically verify the firmware image before booting it to
ensure it has not been tampered with or replaced. This meta-data
used by the bootloader may well be the same manifest obtained with
the firmware image during the update process (with the severable
fields stripped off).
- Second, an IoT device needs a recovery strategy in case the
firmware update / boot process fails. The recovery strategy may
include storing two or more firmware images on the device or
offering the ability to have a second stage bootloader perform the
firmware update process again using firmware updates over serial,
USB or even wireless connectivity like a limited version of
Bluetooth Smart. In the latter case the firmware consumer
functionality is contained in the second stage bootloader and
requires the necessary functionality for executing the firmware
update process, including manifest parsing.
In general, it is assumed that the bootloader itself, or a minimal
part of it, will not be updated since a failed update of the
bootloader poses a risk in reliability.
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.
This is typically not a difficult requirement to accomplish because
there are not other task/processing running while the bootloader is
active (unlike it may be the case when running the application
firmware).
Note: This is an implementation requirement.
3.7. Small Parsers
Since parsers are known sources of bugs, any parsers used to process
the manifest 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.
3.8. Minimal impact on existing firmware formats
The design of the firmware update mechanism must not require changes
to existing firmware formats.
3.9. Robust permissions
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.
3.10. Operating modes
There are three broad classifications of update operating modes.
- Client-initiated Update
- Server-initiated Update
- Hybrid Update
Client-initiated updates take the form of a firmware consumer on a
device proactively checking (polling) for new firmware images.
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 status tracker determines what
devices qualify for a firmware update. Once those devices have been
selected the firmware server distributes updates to the firmware
consumers.
Note: This assumes that the status tracker is able to reach the
device, which may require devices to keep reachability information at
the status tracker 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
firmware consumer and the status tracker. The status tracker pushes
notifications of availability of an update to the firmware consumer,
and it then downloads the image from a firmware server as soon as
possible.
While these broad classifications encompass the majority of operating
modes, some may not be covered in these classifications. By
reinterpreting these modes as a set of operations performed by the
system as a whole, all operating modes can be represented.
The steps performed in the course of an update by the system
containing an updatable device are:
- Notification
- Pre-authorisation
- Dependency resolution Figure 1: Architecture.
- Download
- Installation Firmware images and manifests may be conveyed as a bundle or
detached. The manifest must support both approaches.
This is a coarse-grained high level view of steps required to install For distribution as a bundle, the firmware image is embedded into the
a new firmware. By considering where in the system each of these manifest. This is a useful approach for deployments where devices
steps is performed, each operating mode can be represented. Each of are not connected to the Internet and cannot contact a dedicated
these steps is broken down into smaller constituent parts. Section 5 firmware server for the firmware download. It is also applicable
defines the steps taken from the perspective of the communication when the firmware update happens via a USB sticks or short range
between actors in the system. Section 8 describes some additional radio technologies (such as Bluetooth Smart).
steps that a bootloader takes in addition to those described here.
Section 9 shows an example of the steps undertaken by each party in
the course of an update.
The notification step consists of the status tracker informing the Alternatively, the manifest is distributed detached from the firmware
firmware consumer that an update is available. This can be image. Using this approach, the firmware consumer is presented with
accomplished via polling (client-initiated), push notifications the manifest first and then needs to obtain one or more firmware
(server-initiated), or more complex mechanisms. images as dictated in the manifest.
The pre-authorisation step involves verifying whether the entity The pre-authorisation step involves verifying whether the entity
signing the manifest is indeed authorized to perform an update. The signing the manifest is indeed authorized to perform an update. The
firmware consumer must also determine whether it should fetch and firmware consumer must also determine whether it should fetch and
process a firmware image, which is referenced in a manifest. process a firmware image, which is referenced in a manifest.
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
firmware image. When the download is client-initiated, this means firmware image. When the download is client-initiated, this means
that the firmware consumer chooses when a download occurs and that the firmware consumer chooses when a download occurs and
initiates the download process. When a download is server-initiated, initiates the download process. When a download is server-initiated,
this means that the status tracker tells the device when to download this means that the status tracker tells the device when to download
or that it initiates the transfer directly to the firmware consumer. or that it initiates the transfer directly to the firmware consumer.
For example, a download from an HTTP-based firmware server is client- For example, a download from an HTTP/1.1-based firmware server is
initiated. Pushing a manifest and firmware image to the transfer to client-initiated. Pushing a manifest and firmware image to the
the Package resource of the LwM2M Firmware Update object [LwM2M] is Package resource of the LwM2M Firmware Update object [LwM2M] is
server-initiated. server-initiated update.
If the firmware consumer has downloaded a new firmware image and is If the firmware consumer has downloaded a new firmware image and is
ready to install it, it may need to wait for a trigger from the ready to install it, to initiate the installation, it may - either
status tracker to initiate the installation, may trigger the update need to wait for a trigger from the status tracker, - or trigger the
automatically, or may go through a more complex decision making update automatically, - or go through a more complex decision making
process to determine the appropriate timing for an update (such as process to determine the appropriate timing for an update. Sometimes
delaying the update process to a later time when end users are less the final decision may require confirmation of the user of the device
impacted by the update process). for safety reasons.
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 and the bootloader is responsible for the IoT device can recognize and the bootloader is responsible for
then booting from the newly installed firmware image. then booting from the newly installed firmware image. This process
is different when a bootloader is not involved. For example, when an
Each of these steps may require different permissions. application is updated in a full-featured operating system, the
updater may halt and restart the application in isolation. Devices
3.11. Suitability to software and personalization data must not fail when a disruption occurs during the update process.
For example, a power failure or network disruption during the update
The work on a standardized manifest format initially focused on the process must not cause the device to fail.
most constrained IoT devices and those devices contain code put
together by a single author (although that author may obtain code
from other developers, some of it only in binary form).
Later it turns out that other use cases may benefit from a
standardized manifest format also for conveying software and even
personalization data alongside software. Trusted Execution
Environments (TEEs), for example, greatly benefit from a protocol for
managing the lifecycle of trusted applications (TAs) running inside a
TEE. TEEs may obtain TAs from different authors and those TAs may
require personalization data, such as payment information, to be
securely conveyed to the TEE.
To support this wider range of use cases the manifest format should
therefore be extensible to convey other forms of payloads as well.
4. Claims
The information conveyed from an Author to a Firmware Consumer can be
considered to be Claims as described in [RFC7519] and [RFC8392]. The
same security considerations apply to the Claims expressed in the
manifest. The chief difference between manifest Claims and CWT or
JWT claims is that a manifest has multiple subjects. The manifest
contains:
1. Claims about the Firmware, including its dependencies
2. Claims about the Firmware Consumer's physical or software
properties
3. Claims about the Author, or the Author's delegate
The credential used to authenticate these Claims 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
[I-D.ietf-suit-information-model].
5. Communication Architecture
Figure 1 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 firmware consumer 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.
Firmware + +----------+ Firmware + +-----------+
Manifest | |-+ Manifest | |-+
+--------->| Firmware | |<---------------| | |
| | Server | | | Author | |
| | | | | | |
| +----------+ | +-----------+ |
| +----------+ +-----------+
|
|
|
-+-- ------
---- | ---- ---- ----
// | \\ // \\
/ | \ / \
/ | \ / \
/ | \ / \
/ | \ / \
| v | | |
| +------------+ |
| | Firmware | | | |
| | Consumer | | Device | +--------+ |
| +------------+ | Management| | | |
| | |<------------------------->| Status | |
| | Device | | | | Tracker| |
| +------------+ | || | | |
| | || +--------+ |
| | | |
| | \ /
\ / \ /
\ / \ Device /
\ Network / \ Operator /
\ Operator / \\ //
\\ // ---- ----
---- ---- ------
-----
Figure 1: Architecture.
End-to-end security mechanisms are used to protect the firmware image
and the manifest although Figure 2 does not show the manifest itself
since it may be distributed independently.
+-----------+
+--------+ | | +--------+
| | Firmware Image | Firmware | Firmware Image | |
| Device |<-----------------| Server |<------------------| Author |
| | | | | |
+--------+ +-----------+ +--------+
^ *
* *
************************************************************
End-to-End Security
Figure 2: End-to-End Security.
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 firmware consumer 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. 4. Invoking the Firmware
This is a useful approach for deployments where devices are not
connected to the Internet and cannot contact a dedicated firmware
server for the firmware download. It is also applicable when the
firmware update happens via a USB stick or via Bluetooth Smart.
Figure 3 shows this delivery mode graphically.
/------------\ /------------\ Section 3 describes the steps for getting the firmware image and the
/Manifest with \ /Manifest with \ manifest from the author to the firmware consumer on the IoT device.
|attached | |attached | Once the firmware consumer has retrieved and successfully processed
\firmware image/ \firmware image/ the manifest and the firmware image it needs to invoke the new
\------------/ +-----------+ \------------/ firmware image. This is managed in many different ways, depending on
+--------+ | | +--------+ the type of device, but it typically involves halting the current
| |<.................| Firmware |<................| | version of the firmware, handing control over to a firmware with a
| Device | | Server | | Author | higher privilege/trust level (the firmware verifier) verifying the
| | | | | | new firmware's authenticity & integrity, and then invoking it.
+--------+ +-----------+ +--------+
Figure 3: Manifest with attached firmware. In an execute-in-place microcontroller, this is often done by
rebooting into a bootloader (simultaneously halting the application &
handing over to the higher privilege level) then executing a secure
boot process (verifying and invoking the new image).
Figure 4 shows an option for remotely updating a device where the In a rich OS, this may be done by halting one or more processes, then
device fetches the firmware image from some file server. The invoking new applications. In some OSs, this implicitly involves the
manifest itself is delivered independently and provides information kernel verifying the code signatures on the new applications.
about the firmware image(s) to download.
/--------\ /--------\ The invocation process is security sensitive. An attacker will
/ \ / \ typically try to retrieve a firmware image from the device for
| Manifest | | Manifest | reverse engineering or will try to get the firmware verifier to
\ / \ / execute an attacker-modified firmware image. The firmware verifier
\--------/ \--------/ will therefore have to perform security checks on the firmware image
+-----------+ before it can be invoked. These security checks by the firmware
+--------+ | | +--------+ verifier happen in addition to the security checks that took place
| |<.................| Status |................>| | when the firmware image and the manifest were downloaded by the
| Device | | Tracker | -- | Author | firmware consumer.
| |<- | | --- | |
+--------+ -- +-----------+ --- +--------+
-- ---
--- ---
-- +-----------+ --
-- | | --
/------------\ -- | Firmware |<- /------------\
/ \ -- | Server | / \
| Firmware | | | | Firmware |
\ / +-----------+ \ /
\------------/ \------------/
Figure 4: Independent retrieval of the firmware image. The overlap between the firmware consumer and the firmware verifier
functionality comes in two forms, namely
This architecture does not mandate a specific delivery mode but a - A firmware verifier must verify the firmware image it boots as
solution must support both types. part of the secure boot process. Doing so requires meta-data to
be stored alongside the firmware image so that the firmware
verifier can cryptographically verify the firmware image before
booting it to ensure it has not been tampered with or replaced.
This meta-data used by the firmware verifier may well be the same
manifest obtained with the firmware image during the update
process.
6. Manifest - An IoT device needs a recovery strategy in case the firmware
update / invocation process fails. The recovery strategy may
include storing two or more application firmware images on the
device or offering the ability to invoke a recovery image to
perform the firmware update process again using firmware updates
over serial, USB or even wireless connectivity like Bluetooth
Smart. In the latter case the firmware consumer functionality is
contained in the recovery image and requires the necessary
functionality for executing the firmware update process, including
manifest parsing.
In order for a device to apply an update, it has to make several While this document assumes that the firmware verifier itself is
decisions about the update: distinct from the role of the firmware consumer and therefore does
not manage the firmware update process, this is not a requirement and
these roles may be combined in practice.
- Does it trust the author of the update? Using a bootloader as the firmware verifier requires some special
considerations, particularly when the bootloader implements the
robustness requirements identified by the IOTSU workshop [RFC8240].
- Has the firmware been corrupted? 4.1. The Bootloader
- Does the firmware update apply to this device? In most cases the MCU must restart in order to hand over control to
the bootloader. Once the MCU has initiated a restart, the bootloader
determines whether a newly available firmware image should be
executed. If the bootloader concludes that the newly available
firmware image is invalid, a recovery strategy is necessary. There
are only two approaches recovering from an invalid firmware: either
the bootloader must be able to select a different, valid firmware, or
it must be able to obtain a new, valid firmware. Both of these
approaches have implications for the architecture of the update
system.
- Is the update older than the active firmware? Assuming the first approach, there are (at least) three firmware
images available on the device:
- When should the device apply the update? - First, the bootloader is also firmware. If a bootloader is
updatable then its firmware image is treated like any other
application firmware image.
- How should the device apply the update? - Second, the firmware image that has to be replaced is still
available on the device as a backup in case the freshly downloaded
firmware image does not boot or operate correctly.
- What kind of firmware binary is it? - Third, there is the newly downloaded firmware image.
- Where should the update be obtained? Therefore, the firmware consumer must know where to store the new
firmware. In some cases, this may be implicit, for example replacing
the least-recently-used firmware image. In other cases, the storage
location of the new firmware must be explicit, for example when a
device has one or more application firmware images and a recovery
image with limited functionality, sufficient only to perform an
update.
- Where should the firmware be stored? Since many low end IoT devices use non-relocatable code, either the
bootloader needs to copy the newly downloaded application firmware
image into the location of the old application firmware image and
vice versa or multiple versions of the firmware need to be prepared
for different locations.
The manifest encodes the information that devices need in order to In general, it is assumed that the bootloader itself, or a minimal
make these decisions. It is a data structure that contains the part of it, will not be updated since a failed update of the
following information: bootloader poses a reliability risk.
- information about the device(s) the firmware image is intended to For a bootloader to offer a secure boot functionality it needs to
be applied to, implement the following functionality:
- information about when the firmware update has to be applied, - The bootloader needs to fetch the manifest (or manifest-alike
headers) from nonvolatile storage and parse its contents for
subsequent cryptographic verification.
- information about when the manifest was created, - Cryptographic libraries with hash functions, digital signatures
(for asymmetric crypto), keyed message digests (for symmetric
crypto) need to be accessible.
- dependencies on other manifests, - The device needs to have a trust anchor store to verify the
digital signature. (Alternatively, access to a key store for use
with the keyed message digest.)
- pointers to the firmware image and information about the format, - Ability to expose boot process-related data to the application
firmware (such as to the status tracker). This allows to share
information about the current firmware version, and the status of
the firmware update process and whether errors have occurred.
- information about where to store the firmware image, - Produce boot measurements as part of an attestation solution. See
[I-D.ietf-rats-architecture] for more information. (optional)
- cryptographic information, such as digital signatures or message - Ability to decrypt firmware images, in case confidentiality
authentication codes (MACs). protection was applied). This requires a solution for key
management. (optional)
The manifest information model is described in 5. Types of IoT Devices
[I-D.ietf-suit-information-model].
7. Device Firmware Update Examples There are billions of MCUs used in devices today produced by a large
number of silicon manufacturers. While MCUs can vary significantly
in their characteristics, there are a number of similiaries allowing
us to categorize in groups.
Although these documents attempt to define a firmware update The firmware update architecture, and the manifest format in
architecture that is applicable to both existing systems, as well as particular, needs to offer enough flexibility to cover these common
yet-to-be-conceived systems; it is still helpful to consider existing deployment cases.
architectures.
7.1. Single CPU SoC 5.1. Single MCU
The simplest, and currently most common, architecture consists of a The simplest, and currently most common, architecture consists of a
single MCU along with its own peripherals. These SoCs generally single MCU along with its own peripherals. These SoCs generally
contain some amount of flash memory for code and fixed data, as well contain some amount of flash memory for code and fixed data, as well
as RAM for working storage. These systems either have a single as RAM for working storage. A notable characteristic of these SoCs
firmware image, or an immutable bootloader that runs a single image. is that the primary code is generally execute in place (XIP). Due to
A notable characteristic of these SoCs is that the primary code is the non-relocatable nature of the code, the firmware image needs to
generally execute in place (XIP). Combined with the non-relocatable be placed in a specific location in flash since the code cannot be
nature of the code, firmware updates need to be done in place. executed from an arbitrary location in flash. Hence, then the
firmware image is updated it is necessary to swap the old and the new
image.
7.2. Single CPU with Secure - Normal Mode Partitioning 5.2. Single CPU with Secure - Normal Mode Partitioning
Another configuration consists of a similar architecture to the Another configuration consists of a similar architecture to the
previous, with a single CPU. However, this CPU supports a security previous, with a single CPU. However, this CPU supports a security
partitioning scheme that allows memory (in addition to other things) partitioning scheme that allows memory (in addition to other things)
to be divided into secure and normal mode. There will generally be 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 two images, one for secure mode, and one for normal mode. In this
configuration, firmware upgrades will generally be done by the CPU in configuration, firmware upgrades will generally be done by the CPU in
secure mode, which is able to write to both areas of the flash secure mode, which is able to write to both areas of the flash
device. In addition, there are requirements to be able to update device. In addition, there are requirements to be able to update
either image independently, as well as to update them together either image independently, as well as to update them together
atomically, as specified in the associated manifests. atomically, as specified in the associated manifests.
7.3. Symmetric Multiple CPUs 5.3. Symmetric Multiple CPUs
In more complex SoCs with symmetric multi-processing support, In more complex SoCs with symmetric multi-processing support,
advanced operating systems, such as Linux, are often used. These advanced operating systems, such as Linux, are often used. These
SoCs frequently use an external storage medium such as raw NAND flash SoCs frequently use an external storage medium, such as raw NAND
or eMMC. Due to the higher quantity of resources, these devices are flash or eMMC. Due to the higher quantity of resources, these
often capable of storing multiple copies of their firmware images and devices are often capable of storing multiple copies of their
selecting the most appropriate one to boot. Many SoCs also support firmware images and selecting the most appropriate one to boot. Many
bootloaders that are capable of updating the firmware image, however SoCs also support bootloaders that are capable of updating the
this is typically a last resort because it requires the device to be firmware image, however this is typically a last resort because it
held in the bootloader while the new firmware is downloaded and requires the device to be held in the bootloader while the new
installed, which results in down-time for the device. Firmware firmware is downloaded and installed, which results in down-time for
updates in this class of device are typically not done in-place. the device. Firmware updates in this class of device are typically
not done in-place.
7.4. Dual CPU, shared memory 5.4. Dual CPU, shared memory
This configuration has two or more heterogeneous CPUs in a single SoC This configuration has two or more heterogeneous CPUs in a single SoC
that share memory (flash and RAM). Generally, they will be a that share memory (flash and RAM). Generally, there will be a
protection mechanism to prevent one CPU from accessing the other's mechanism to prevent one CPU from unintentionally accessing memory
memory. Upgrades in this case will typically be done by one of the currently allocated to the other. Upgrades in this case will
CPUs, and is similar to the single CPU with secure mode. typically be done by one of the CPUs, and is similar to the single
CPU with secure mode.
7.5. Dual CPU, other bus 5.5. Dual CPU, other bus
This configuration has two or more heterogeneous CPUs, each having This configuration has two or more heterogeneous CPUs, each having
their own memory. There will be a communication channel between their own memory. There will be a communication channel between
them, but it will be used as a peripheral, not via shared memory. In 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 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 the upgrade. It is likely that one of the CPUs will be considered the
primary CPU, and will direct the other CPU to do the upgrade. This primary CPU, and will direct the other CPU to do the upgrade. This
configuration is commonly used to offload specific work to other configuration is commonly used to offload specific work to other
CPUs. Firmware dependencies are similar to the other solutions CPUs. Firmware dependencies are similar to the other solutions
above, sometimes allowing only one image to be upgraded, other times above, sometimes allowing only one image to be upgraded, other times
requiring several to be upgraded atomically. Because the updates are requiring several to be upgraded atomically. Because the updates are
happening on multiple CPUs, upgrading the two images atomically is happening on multiple CPUs, upgrading the two images atomically is
challenging. challenging.
8. Bootloader 6. Manifests
More devices today than ever before are being connected to the
Internet, which drives the need for firmware updates to be provided
over the Internet rather than through traditional interfaces, such as
USB or RS232. Updating a device over the Internet requires the
device to fetch not only the firmware image but also the manifest.
Hence, the following building blocks are necessary for a firmware
update solution:
- the Internet protocol stack for firmware downloads (*),
- the capability to write the received firmware image to persistent In order for a firmware consumer to apply an update, it has to make
storage (most likely flash memory) prior to performing the update, several decisions using manifest-provided information and data
available on the device itself. For more detailed information and a
longer list of information elements in the manifest consult the
information model specification [I-D.ietf-suit-information-model],
which offers justifications for each element, and the manifest, see
[I-D.ietf-suit-manifest], for details about how this information is
included in the manifest.
- the ability to unpack, decompress or otherwise process the Table 1 provides examples of decisions to be made.
received firmware image,
- the features to verify an image and a manifest, including digital +----------------------------+--------------------------------------+
signature verification or checking a message authentication code, | Decision | Information Elements |
+----------------------------+--------------------------------------+
| Should I trust the author | Trust anchors and authorization |
| of the firmware? | policies on the device |
| | |
| Has the firmware been | Digital signature and MAC covering |
| corrupted? | the firmware image |
| | |
| Does the firmware update | Conditions with Vendor ID, Class ID |
| apply to this device? | and Device ID |
| | |
| Is the update older than | Sequence number in the manifest (1) |
| the active firmware? | |
| | |
| When should the device | Wait directive |
| apply the update? | |
| | |
| How should the device | Manifest commands |
| apply the update? | |
| | |
| What kind of firmware | Unpack algorithms to interpret a |
| binary is it? | format. |
| | |
| Where should the update be | Dependencies on other manifests and |
| obtained? | firmware image URI in Manifest |
| | |
| Where should the firmware | Storage Location and Component |
| be stored? | Identifier |
+----------------------------+--------------------------------------+
- a manifest parsing library, and Table 1: Firmware Update Decisions.
- integration of the device into a device management server to (1): A device presented with an old, but valid manifest and firmware
perform automatic firmware updates and to track their progress. 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.
(*) Because firmware images are often multiple kilobytes, sometimes Keeping the code size and complexity of a manifest parsers small is
exceeding one hundred kilobytes, in size for low end IoT devices and important for constrained IoT devices. Since the manifest parsing
even several megabytes large for IoT devices running full-fledged code may also be used by the bootloader it is part of the trusted
operating systems like Linux, the protocol mechanism for retrieving computing base.
these images needs to offer features like congestion control, flow
control, fragmentation and reassembly, and mechanisms to resume
interrupted or corrupted transfers.
All these features are most likely offered by the application, i.e. A manifest may not only be used to protect firmware images but also
firmware consumer, running on the device (except for basic security configuration data such as network credentials or personalization
algorithms that may run either on a trusted execution environment or data related to firmware or software. Personalization data
on a separate hardware security MCU/module) rather than by the demonstrates the need for mutually-distrustful delivery of two or
bootloader itself. more images into a device. Personalization data is used with Trusted
Execution Environments (TEEs), which benefit from a protocol for
managing the lifecycle of trusted applications (TAs) running inside a
TEE. TEEs may obtain TAs from different authors and those TAs may
require personalization data, such as payment information, to be
securely conveyed to the TEE. The TA's author does not want to
expose the TA to the user, and the user does not want to expose the
payment information to the TA's author.
Once manifests have been processed and firmware images successfully 7. Securing Firmware Updates
downloaded and verified the device needs to hand control over to the
bootloader. In most cases this requires the MCU to restart. Once
the MCU has initiated a restart, the bootloader takes over control
and determines whether the newly downloaded firmware image should be
executed.
The boot process is security sensitive because the firmware images Using firmware updates to fix vulnerabilities in devices is important
may, for example, be stored in off-chip flash memory giving attackers but securing this update mechanism is equally important since
easy access to the image for reverse engineering and potentially also security problems are exacerbated by the update mechanism: update is
for modifying the binary. The bootloader will therefore have to essentially authorized remote code execution, so any security
perform security checks on the firmware image before it can be problems in the update process expose that remote code execution
booted. These security checks by the bootloader happen in addition system. Failure to secure the firmware update process will help
to the security checks that happened when the firmware image and the attackers to take control over devices.
manifest were downloaded.
The manifest may have been stored alongside the firmware image to End-to-end security mechanisms are used to protect the firmware image
allow re-verification of the firmware image during every boot and the manifest. The following assumptions are made to allow the
attempt. Alternatively, secure boot-specific meta-data may have been firmware consumer to verify the received firmware image and manifest
created by the application after a successful firmware download and before updating software:
verification process. Whether to re-use the standardized manifest
format that was used during the initial firmware retrieval process or
whether it is better to use a different format for the secure boot-
specific meta-data depends on the system design. The manifest format
does, however, have the capability to serve also as a building block
for secure boot with its severable elements that allow shrinking the
size of the manifest by stripping elements that are no longer needed.
In order to satisfy the reliability requirements defined in - Authentication ensures that the device can cryptographically
Section 3.5, devices must always be able to return to a working identify the author(s) creating firmware images and manifests.
firmware image. This has implications for the design of the Authenticated identities may be used as input to the authorization
bootloader: If the firmware image contains the firmware consumer process. Not all entities creating and signing manifests have the
functionality, as described above, then the bootloader must be able same permissions. A device needs to determine whether the
to roll back to a working firmware image. Alternatively, the requested action is indeed covered by the permission of the party
bootloader may have enough functionality to fetch a firmware image that signed the manifest. Informing the device about the
plus manifest from a firmware server over the Internet. A multi- permissions of the different parties also happens in an out-of-
stage bootloader may soften this requirement at the expense of a more band fashion and is also a duty of the Trust Provisioning
sophisticated boot process. Authority.
For a bootloader to offer a secure boot mechanism it needs to provide - Integrity protection ensures that no third party can modify the
the following features: manifest or the firmware image. 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.
- ability to access security algorithms, such as SHA-256 to compute - For confidentiality protection of the firmware image, it must be
a fingerprint over the firmware image and a digital signature done in such a way that the intended firmware consumer(s), other
algorithm. authorized parties, and no one else can decrypt it. The
information that is encrypted individually for each device/
recipient must maintain friendliness to Content Distribution
Networks, bulk storage, and broadcast protocols. 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 optional.
- access keying material directly or indirectly to utilize the A manifest specification must support different cryptographic
digital signature. The device needs to have a trust anchor store. algorithms and algorithm extensibility. Moreover, since RSA- and
ECC-based signature schemes may become vulnerable to quantum-
accelerated key extraction in the future, unchangeable bootloader
code in ROM is recommended to use post-quantum secure signature
schemes such as hash-based signatures [RFC8778]. A bootloader author
must carefully consider the service lifetime of their product and the
time horizon for quantum-accelerated key extraction. The worst-case
estimate, at time of writing, for the time horizon to key extraction
with quantum acceleration is approximately 2030, based on current
research [quantum-factorization].
- ability to expose boot process-related data to the application When a device obtains a monolithic firmware image from a single
firmware (such as to the device management software). This allows author without any additional approval steps then the authorization
a device management server to determine whether the firmware flow is relatively simple. There are, however, other cases where
update has been successful and, if not, what errors occurred. more complex policy decisions need to be made before updating a
device.
- to (optionally) offer attestation information (such as In this architecture the authorization policy is separated from the
measurements). 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.
While the software architecture of the bootloader and its security Alternatively, a device may trust precisely one entity, which does
mechanisms are implementation-specific, the manifest can be used to all permission management and coordination. This entity allows the
control the firmware download from the Internet in addition to device to offload complex permissions calculations for the device.
augmenting secure boot process. These building blocks are highly
relevant for the design of the manifest.
9. Example 8. Example
Figure 5 illustrates an example message flow for distributing a Figure 2 illustrates an example message flow for distributing a
firmware image to a device starting with an author uploading the new firmware image to a device. The firmware and manifest are stored on
firmware to firmware server and creating a manifest. The firmware the same firmware server and distributed in a detached manner.
and manifest are stored on the same firmware server. This setup does
not use a status tracker and the firmware consumer component is
therefore responsible for periodically checking whether a new
firmware image is available for download.
+--------+ +-----------------+ +------------+ +----------+ +--------+ +-----------------+ +-----------------------------+
| | | | | Firmware | | | | | | Firmware Server | | IoT Device |
| Author | | Firmware Server | | Consumer | |Bootloader| | Author | | Status Tracker | | +------------+ +----------+ |
+--------+ +-----------------+ +------------+ +----------+ +--------+ | Server | | | Firmware | |Bootloader| |
| | | + | +-----------------+ | | Consumer | | | |
| Create Firmware | | | | | | +------------+ +----------+ |
|--------------+ | | | | | | | | |
| | | | | | | | +-----------------------+ |
|<-------------+ | | | | Create Firmware | | | Status Tracker Client | |
| | | | |--------------+ | | +-----------------------+ |
| Upload Firmware | | | | | | `''''''''''''''''''''''''''''
|------------------>| | | |<-------------+ | | | |
| | | | | | | | |
| Create Manifest | | | | Upload Firmware | | | |
|---------------+ | | | |------------------>| | | |
| | | | | | | | | |
|<--------------+ | | | | Create Manifest | | | |
| | | | |---------------+ | | | |
| Sign Manifest | | | | | | | | |
|-------------+ | | | |<--------------+ | | | |
| | | | | | | | | |
|<------------+ | | | | Sign Manifest | | | |
| | | | |-------------+ | | | |
| Upload Manifest | | | | | | | | |
|------------------>| | | |<------------+ | | | |
| | | | | | | | |
| | Query Manifest | | | Upload Manifest | | | |
| |<--------------------| | |------------------>| Notification of | | |
| | | | | | new firmware image | | |
| | Send Manifest | | | |----------------------------->| |
| |-------------------->| | | | | | |
| | |Initiate| |
| | | Update | |
| | |<-------| |
| | | | |
| | Query Manifest | | |
| |<--------------------| . |
| | | . |
| | Send Manifest | . |
| |-------------------->| . |
| | | Validate | | | | Validate |
| | | Manifest | | | | Manifest |
| | |---------+ | | | |--------+ |
| | | | | | | | | |
| | |<--------+ | | | |<-------+ |
| | | | | | | . |
| | Request Firmware | | | | Request Firmware | . |
| |<--------------------| | | |<--------------------| . |
| | | | | | | . |
| | Send Firmware | | | | Send Firmware | . |
| |-------------------->| | | |-------------------->| . |
| | | Verify | | | | Verify . |
| | | Firmware | | | | Firmware |
| | |--------------+ | | | |--------+ |
| | | | | | | | | |
| | |<-------------+ | | | |<-------+ |
| | | | | | | . |
| | | Store | | | | Store . |
| | | Firmware | | | | Firmware |
| | |-------------+ | | | |--------+ |
| | | | | | | | | |
| | |<------------+ | | | |<-------+ |
| | | | | | | . |
| | | | | | | . |
| | | Trigger Reboot | | | | . |
| | |--------------->| | | | | |
| | | | | | | Update | |
| | | | | | |Complete| |
| | |------->| |
| | | |
| | Firmware Update Completed | |
| |<-----------------------------| |
| | | |
| | Reboot | |
| |----------------------------->| |
| | | | |
| | | | |
| | |Reboot |
| | | |------>|
| | | | |
| | | . |
| | +---+----------------+--+ | | +---+----------------+--+
| | S| | | | | | S| | | |
| | E| | Verify | | | | E| | Verify | |
| | C| | Firmware | | | | C| | Firmware | |
| | U| | +--------------| | | | U| | +--------------| |
| | R| | | | | | | R| | | | |
| | E| | +------------->| | | | E| | +------------->| |
| | | | | | | | | | | |
| | B| | Activate new | | | | B| | Activate new | |
| | O| | Firmware | | | | O| | Firmware | |
skipping to change at page 24, line 27 skipping to change at page 23, line 5
| | T| | | | | | | T| | | | |
| | | | +------------->| | | | | | +------------->| |
| | P| | | | | | P| | | |
| | R| | Boot new | | | | R| | Boot new | |
| | O| | Firmware | | | | O| | Firmware | |
| | C| | +--------------| | | | C| | +--------------| |
| | E| | | | | | | E| | | | |
| | S| | +------------->| | | | S| | +------------->| |
| | S| | | | | | S| | | |
| | +---+----------------+--+ | | +---+----------------+--+
| | | | | | | . |
| | | | |
| | . | |
| | Device running new firmware | |
| |<-----------------------------| |
| | . | |
| | | |
Figure 5: First Example Flow for a Firmware Upate. Figure 2: First Example Flow for a Firmware Update.
Figure 6 shows an example follow with the device using a status Figure 3 shows an exchange that starts with the status tracker
tracker. For editorial reasons the author publishing the manifest at querying the device for its current firmware version. Later, a new
the status tracker and the firmware image at the firmware server is firmware version becomes available and since this device is running
not shown. Also omitted is the secure boot process following the an older version the status tracker server interacts with the device
successful firmware update process. to initiate an update.
The exchange starts with the device interacting with the status The manifest and the firmware are stored on different servers in this
tracker; the details of such exchange will vary with the different example. When the device processes the manifest it learns where to
device management systems being used. In any case, the status download the new firmware version. The firmware consumer downloads
tracker learns about the firmware version of the devices it manages. the firmware image with the newer version X.Y.Z after successful
In our example, the device under management is using firmware version validation of the manifest. Subsequently, a reboot is initiated and
A.B.C. At a later point in time the author uploads a new firmware the secure boot process starts. Finally, the device reports the
along with the manifest to the firmware server and the status successful boot of the new firmware version.
tracker, respectively. While there is no need to store the manifest
and the firmware on different servers this example shows a common
pattern used in the industry. The status tracker may then
automatically, based on human intervention or based on a more complex
policy decide to inform the device about the newly available firmware
image. In our example, it does so by pushing the manifest to the
firmware consumer. The firmware consumer downloads the firmware
image with the newer version X.Y.Z after successful validation of the
manifest. Subsequently, a reboot is initiated and the secure boot
process starts.
+---------+ +-----------------+ +-----------------------------+ +---------+ +-----------------+ +-----------------------------+
| Status | | | | +------------+ +----------+ | | Status | | Firmware Server | | +------------+ +----------+ |
| Tracker | | Firmware Server | | | Firmware | |Bootloader| | | Tracker | | Status Tracker | | | Firmware | |Bootloader| |
| | | | | | Consumer | | | | | Server | | Server | | | Consumer | | | |
+---------+ +-----------------+ | +------------+ +----------+ | +---------+ +-----------------+ | | +Status | +----------+ |
| | | | Tracker | | |
| | | | Client | | |
| | | +------------+ | |
| | | | IoT Device | | | | | | IoT Device | |
| | `'''''''''''''''''''''''''''' | | `''''''''''''''''''''''''''''
| | | | | | | |
| Query Firmware Version | | | Query Firmware Version | |
|------------------------------------->| | |------------------------------------->| |
| Firmware Version A.B.C | | | Firmware Version A.B.C | |
|<-------------------------------------| | |<-------------------------------------| |
| | | | | | | |
| <<some time later>> | | | <<some time later>> | |
| | | | | | | |
skipping to change at page 26, line 19 skipping to change at page 24, line 44
| | | Trigger Reboot | | | | Trigger Reboot |
| | |--------------->| | | |--------------->|
| | | | | | | |
| | | | | | | |
| | | __..-------..._' | | | __..-------..._'
| | ,-' `-. | | ,-' `-.
| | | Secure Boot | | | | Secure Boot |
| | `-. _/ | | `-. _/
| | |`--..._____,,.,-' | | |`--..._____,,.,-'
| | | | | | | |
| Device running firmware X.Y.Z | |
|<-------------------------------------| |
| | | |
| | | |
Figure 6: Second Example Flow for a Firmware Upate. Figure 3: Second Example Flow for a Firmware Update.
10. IANA Considerations 9. IANA Considerations
This document does not require any actions by IANA. This document does not require any actions by IANA.
11. Security Considerations 10. Security Considerations
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
[RFC8240]. 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, namely
- installing 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 This document describes terminology, requirements and an architecture
protecting the manifest. for firmware updates of IoT devices. The content of the document is
thereby focused on improving security of IoT devices via firmware
update mechanisms and informs the standardization of a manifest
format.
- incentives for manufacturers to offer a firmware update mechanism An in-depth examination of the security considerations of the
as part of their IoT products. architecture is presented in [I-D.ietf-suit-information-model].
12. Acknowledgements 11. 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
skipping to change at page 28, line 16 skipping to change at page 26, line 16
- Henk Birkholz - Henk Birkholz
- Jintao Zhu - Jintao Zhu
- Takeshi Takahashi - Takeshi Takahashi
- Jacob Beningo - Jacob Beningo
- Kathleen Moriarty - Kathleen Moriarty
- Bob Briscoe
- Roman Danyliw
- Brian Carpenter
- Theresa Enghardt
- Rich Salz
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.
13. Informative References 12. Informative References
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture",
draft-ietf-rats-architecture-06 (work in progress),
September 2020.
[I-D.ietf-suit-information-model] [I-D.ietf-suit-information-model]
Moran, B., Tschofenig, H., and H. Birkholz, "An Moran, B., Tschofenig, H., and H. Birkholz, "An
Information Model for Firmware Updates in IoT Devices", Information Model for Firmware Updates in IoT Devices",
draft-ietf-suit-information-model-07 (work in progress), draft-ietf-suit-information-model-07 (work in progress),
June 2020. June 2020.
[I-D.ietf-suit-manifest] [I-D.ietf-suit-manifest]
Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg, Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
"A Concise Binary Object Representation (CBOR)-based "A Concise Binary Object Representation (CBOR)-based
skipping to change at page 28, line 46 skipping to change at page 27, line 17
"Trusted Execution Environment Provisioning (TEEP) "Trusted Execution Environment Provisioning (TEEP)
Architecture", draft-ietf-teep-architecture-12 (work in Architecture", draft-ietf-teep-architecture-12 (work in
progress), July 2020. progress), July 2020.
[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/OMA-TS-LightweightM2M- V1_0_2-20180209-A/OMA-TS-LightweightM2M-
V1_0_2-20180209-A.pdf>. V1_0_2-20180209-A.pdf>.
[quantum-factorization]
Department of Computer Science, Purdue University, .,
Quantum Computing Institute, Oak Ridge National
Laboratory, ., Quantum Computing Institute, Oak Ridge
National Laboratory, ., Quantum Computing Institute, Oak
Ridge National Laboratory, ., and . Department of
Chemistry, Physics and Birck Nanotechnology Center, Purdue
University, "Quantum Annealing for Prime Factorization",
n.d.,
<https://www.nature.com/articles/s41598-018-36058-z>.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management [RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <https://www.rfc-editor.org/info/rfc6024>. 2010, <https://www.rfc-editor.org/info/rfc6024>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014, DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>. <https://www.rfc-editor.org/info/rfc7228>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
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