< draft-ietf-suit-architecture-14.txt   draft-ietf-suit-architecture-15.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: April 24, 2021 D. Brown Expires: July 22, 2021 D. Brown
Linaro Linaro
M. Meriac M. Meriac
Consultant Consultant
October 21, 2020 January 18, 2021
A Firmware Update Architecture for Internet of Things A Firmware Update Architecture for Internet of Things
draft-ietf-suit-architecture-14 draft-ietf-suit-architecture-15
Abstract Abstract
Vulnerabilities in Internet of Things (IoT) devices have raised the Vulnerabilities with Internet of Things (IoT) devices have raised the
need for a reliable and secure firmware update mechanism suitable for need for a solid and secure firmware update mechanism that is also
devices with resource constraints. Incorporating such an update suitable for constrained devices. Incorporating such update
mechanism is a fundamental requirement for fixing vulnerabilities but mechanism to fix vulnerabilities, to update configuration settings as
it also enables other important capabilities such as updating well as adding new functionality is recommended by security experts.
configuration settings as well as adding new functionality.
In addition to the definition of terminology and an architecture this This document lists requirements and describes an architecture for a
document motivates the standardization of a manifest format as a firmware update mechanism suitable for IoT devices. The architecture
transport-agnostic means for describing and protecting firmware is agnostic to the transport of the firmware images and associated
updates. meta-data.
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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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on April 24, 2021. This Internet-Draft will expire on July 22, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 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 . . . . . . . . . . . . . . . . . 5 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
2.1. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Stakeholders . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Agnostic to how firmware images are distributed . . . . . 7
2.3. Functions . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2. Friendly to broadcast delivery . . . . . . . . . . . . . 7
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8 3.3. Use state-of-the-art security mechanisms . . . . . . . . 8
4. Invoking the Firmware . . . . . . . . . . . . . . . . . . . . 12 3.4. Rollback attacks must be prevented . . . . . . . . . . . 8
4.1. The Bootloader . . . . . . . . . . . . . . . . . . . . . 14 3.5. High reliability . . . . . . . . . . . . . . . . . . . . 8
5. Types of IoT Devices . . . . . . . . . . . . . . . . . . . . 15 3.6. Operate with a small bootloader . . . . . . . . . . . . . 9
5.1. Single MCU . . . . . . . . . . . . . . . . . . . . . . . 15 3.7. Small Parsers . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Single CPU with Secure - Normal Mode Partitioning . . . . 16 3.8. Minimal impact on existing firmware formats . . . . . . . 10
5.3. Symmetric Multiple CPUs . . . . . . . . . . . . . . . . . 16 3.9. Robust permissions . . . . . . . . . . . . . . . . . . . 10
5.4. Dual CPU, shared memory . . . . . . . . . . . . . . . . . 16 3.10. Operating modes . . . . . . . . . . . . . . . . . . . . . 10
5.5. Dual CPU, other bus . . . . . . . . . . . . . . . . . . . 17 3.11. Suitability to software and personalization data . . . . 12
6. Manifests . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4. Claims . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Securing Firmware Updates . . . . . . . . . . . . . . . . . . 19 5. Communication Architecture . . . . . . . . . . . . . . . . . 13
8. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6. Manifest . . . . . . . . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 7. Device Firmware Update Examples . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 25 7.1. Single CPU SoC . . . . . . . . . . . . . . . . . . . . . 18
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25 7.2. Single CPU with Secure - Normal Mode Partitioning . . . . 18
12. Informative References . . . . . . . . . . . . . . . . . . . 26 7.3. Dual CPU, shared memory . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 7.4. Dual CPU, other bus . . . . . . . . . . . . . . . . . . . 18
8. Bootloader . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
11. Security Considerations . . . . . . . . . . . . . . . . . . . 25
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
13. Informative References . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction 1. Introduction
Firmware updates can help to fix security vulnerabilities and are When developing Internet of Things (IoT) devices, one of the most
considered to be an important building block in securing IoT devices. difficult problems to solve is how to update firmware on the device.
Due to rising concerns about insecure IoT devices the Internet Once the device is deployed, firmware updates play a critical part in
Architecture Board (IAB) organized a 'Workshop on Internet of Things its lifetime, particularly when devices have a long lifetime, are
(IoT) Software Update (IOTSU)', which took place at Trinity College deployed in remote or inaccessible areas where manual intervention is
Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at cost prohibitive or otherwise difficult. Updates to the firmware of
the bigger picture. A report about this workshop can be found at an IoT device are done to fix bugs in software, to add new
[RFC8240]. The workshop revealed a number of challenges for functionality, and to re-configure the device to work in new
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. Conversely, IoT devices are expected to account for
user preferences and consent when scheduling updates. 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 has to ensure that The firmware update process, among other goals, has to ensure that
- The firmware image is authenticated and integrity protected. - The firmware image is authenticated and integrity protected.
Attempts to flash a maliciously modified firmware image or an Attempts to flash a modified firmware image or an image from an
image from an unknown, untrusted source must be prevented. In unknown source are prevented.
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
mitigated or at least made more difficult. Obtaining the firmware prevented. Obtaining the firmware is often one of the first steps
is often one of the first steps to mount an attack since it gives to mount an attack since it gives the adversary valuable insights
the adversary valuable insights into the software libraries used, into used software libraries, configuration settings and generic
configuration settings and generic functionality. Even though functionality (even though reverse engineering the binary can be a
reverse engineering the binary can be a tedious process modern tedious process).
reverse engineering frameworks have made this task a lot easier.
While the standardization work has been informed by and optimized for
firmware update use cases of Class 1 devices (according to the device
class definitions in RFC 7228 [RFC7228]) devices, there is nothing in
the architecture that restricts its use to only these constrained IoT
devices. Moreover, this architecture is not limited to managing
firmware and software updates, but can also be applied to managing
the delivery of 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 used and the
sandbox in which the software is executed.
While the IETF standardization work has been focused on the manifest
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, for example
using pre-configured hostnames or [RFC6763] DNS-SD. 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
device because a firmware update, particularly radio communication
and writing the firmware image to flash, is an energy-intensive
task for a device.
- Creating incentives for device operators to use a firmware update This version of the document assumes asymmetric cryptography and a
mechanism and to demand the integration of it from IoT device public key infrastructure. Future versions may also describe a
vendors. symmetric key approach for very constrained devices.
- Ensuring that firmware updates addressing critical flaws can be While the standardization work has been informed by and optimised for
obtained even after a product is discontinued or a vendor goes out firmware update use cases of Class 1 devices (according to the device
of business. class definitions in RFC 7228 [RFC7228]), there is nothing in the
architecture that restricts its use to only these constrained IoT
devices. Software update and delivery of arbitrary data, such as
configuration information and keys, can equally be managed by
manifests.
This document starts with a terminology followed by the description More details about the security goals are discussed in Section 5 and
of the architecture. We then explain the bootloader and how it requirements are described in Section 3.
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
2.1. Terms
This document uses the following terms: This document uses the following terms:
- Manifest: The manifest contains meta-data about the firmware
image. The manifest is protected against modification and
provides information about the author.
- Firmware Image: The firmware image, 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. differential update for performance reasons. Firmware is the more
universal term. The terms, firmware image, firmware, and image,
are used in this document and are interchangeable.
The terms, firmware image, firmware, and image, are used in this - Software: The terms "software" and "firmware" are used
document and are interchangeable. We use the term application interchangeably.
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).
- Manifest: The manifest contains meta-data about the firmware - Bootloader: A bootloader is a piece of software that is executed
image. The manifest is protected against modification and once a microcontroller has been reset. It is responsible for
provides information about the author. deciding whether to boot a firmware image that is present or
whether to obtain and verify a new firmware image. Since the
bootloader is a security critical component its functionality may
be split into separate stages. Such a multi-stage bootloader may
offer very basic functionality in the first stage and resides in
ROM whereas the second stage may implement more complex
functionality and resides in flash memory so that it can be
updated in the future (in case bugs have been found). The exact
split of components into the different stages, the number of
firmware images stored by an IoT device, and the detailed
functionality varies throughout different implementations. 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 interchangeably with MCU, but MCU tends to imply more often used for these types of devices.
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.
- Software: Similar to firmware, but typically dynamically loaded by - Trusted applications (TAs): An application component that runs in
an Operating System. Used interchangeably with firmware in this
document.
- 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.
- Trusted Execution Environments (TEEs): An execution environment For more information about TEEs see [I-D.ietf-teep-architecture].
that runs alongside of, but is isolated from, an REE. For more
information about TEEs see [I-D.ietf-teep-architecture].
2.2. Stakeholders
The following stakeholders are used in this document: The following entities are used:
- Author: The author is the entity that creates the firmware image. - Author: The author is the entity that creates the firmware image.
There may be multiple authors involved in producing firmware There may be multiple authors in a system either when a device
running on an IoT device. Section 5 talks about those IoT device consists of multiple micro-controllers or when the the final
deployment cases. firmware image consists of software components from multiple
companies.
- Device Operator: The device operator is responsible for the day-
to-day operation of a fleet of IoT devices. Customers of IoT
devices, as the owners of IoT devices - such as enterprise
customers or end users, interact with their IoT devices indirectly
through the device operator via web or smart phone apps.
- 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 devices and various
stakeholders. The TPA may also delegate rights to stakeholders.
Typically, the Original Equipment Manufacturer (OEM) or Original
Design Manufacturer (ODM) will act as a TPA, however complex
supply chains may require a different design. In some cases, the
TPA may decide to remain in full control over the firmware update
process of their products.
- User: The end-user of a device. The user may interact with
devices via web or smart phone apps, as well as through direct
user interfaces.
2.3. Functions
- (IoT) Device: A device refers to the entire IoT product, which
consists of one or many MCUs, sensors and/or actuators. Many IoT
devices sold today contain multiple MCUs and therefore a single
device may need to obtain more than one firmware image and
manifest to successfully perform an update.
- Status Tracker: The status tracker has a client and a server
component and performs three tasks: 1) It communicates the
availability of a new firmware version. This information will
flow from the server to the client.
2) It conveys information about software and hardware
characteristics of the device. The information flow is from the
client to the server.
3) It can remotely trigger the firmware update process. The
information flow is from the server to the client.
For example, a device operator may want to read the installed
firmware version number running on the device and information
about available flash memory. Once an update has been triggered,
the device operator may want to obtain information about the state
of the firmware update. If errors occurred, the device operator
may want to troubleshoot problems by first obtaining diagnostic
information (typically using a device management protocol).
We make no assumptions about where the server-side component is
deployed. The deployment of status trackers is flexible and may
be found at
cloud-based servers, on-premise servers, or may be embedded in
edge computing devices. A status tracker server component may
even 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 Consumer: The firmware consumer is the recipient of the
firmware image and the manifest. It is responsible for parsing firmware image and the manifest. It is responsible for parsing
and verifying the received manifest and for storing the obtained and verifying the received manifest and for storing the obtained
firmware image. The firmware consumer plays the role of the firmware image. The firmware consumer plays the role of the
update component on the IoT device typically running in the update component on the IoT device typically running in the
application firmware. It interacts with the firmware server and application firmware. It interacts with the firmware server and
with the status tracker client (locally). with the status tracker, if present.
- (IoT) Device: A device refers to the entire IoT product, which
consists of one or many MCUs, sensors and/or actuators. Many IoT
devices sold today contain multiple MCUs and therefore a single
device may need to obtain more than one firmware image and
manifest to succesfully perform an update. The terms device and
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
functionality to retrieve information about the installed firmware
on a device and other device characteristics (including free
memory and hardware components), to obtain the state of the
firmware update cycle the device is currently in, and to trigger
the update process. The deployment of status trackers is flexible
and they may be used as cloud-based servers, on-premise servers,
embedded in edge computing device (such as Internet access
gateways or protocol translation gateways), or even in smart
phones and tablets. While the IoT device itself runs the client-
side of the status tracker it will most likely not run a status
tracker itself unless it acts as a proxy for other IoT devices in
a protocol translation or edge computing device node. How much
functionality a status tracker includes depends on the selected
configuration of the device management functionality and the
communication environment it is used in. In a generic networking
environment the protocol used between the client and the server-
side of the status tracker need to deal with Internet
communication challenges involving firewall and NAT traversal. In
other cases, the communication interaction may be rather simple.
This architecture document does not impose requirements on the
status tracker.
- 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 and these two between the firmware server and the status tracker but those
entities are often physically separated on different devices for entities are often physically separated on different devices for
scalability reasons. scalability reasons.
- Bootloader: A bootloader is a piece of software that is executed - Device Operator: The actor responsible for the day-to-day
once a microcontroller has been reset. It is responsible for operation of a fleet of IoT devices.
deciding what code to execute.
3. Architecture - Network Operator: The actor responsible for the operation of a
network to which IoT devices connect.
More devices today than ever before are connected to the Internet, In addition to the entities in the list above there is an orthogonal
which drives the need for firmware updates to be provided over the infrastructure with a Trust Provisioning Authority (TPA) distributing
Internet rather than through traditional interfaces, such as USB or trust anchors and authorization permissions to various entities in
RS-232. Sending updates over the Internet requires the device to the system. The TPA may also delegate rights to install, update,
fetch the new firmware image as well as the manifest. 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.
Hence, the following components are necessary on a device for a The terms 'trust anchor' and 'trust anchor store' are defined in
firmware update solution: [RFC6024]:
- the Internet protocol stack for firmware downloads. Because - "A trust anchor represents an authoritative entity via a public
firmware images are often multiple kilobytes, sometimes exceeding key and associated data. The public key is used to verify digital
one hundred kilobytes, in size for low end IoT devices and even signatures, and the associated data is used to constrain the types
several megabytes large for IoT devices running full-fledged of information for which the trust anchor is authoritative."
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.
- the capability to write the received firmware image to persistent - "A trust anchor store is a set of one or more trust anchors stored
storage (most likely flash memory). 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.
- a manifest parser with code to verify a digital signature or a 3. Requirements
message authentication code.
- the ability to unpack, to decompress and/or to decrypt the The firmware update mechanism described in this specification was
received firmware image. designed with the following requirements in mind:
- a status tracker. - Agnostic to how firmware images are distributed
The features listed above are most likely offered by code in the - Friendly to broadcast delivery
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.
Figure 1 shows the architecture where a firmware image is created by - Use state-of-the-art security mechanisms
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.
As a first step in the firmware update process, the status tracker - Rollback attacks must be prevented
client needs 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):
- Client-initiated updates take the form of a status tracker client - High reliability
proactively checking (polling) for updates.
- With Server-initiated updates the server-side component of the - Operate with a small bootloader
status tracker learns about a new firmware version and determines
which 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.
- Using a hybrid approach the server-side of the status tracker - Small Parsers
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.
Once the device operator triggers update via the status tracker, it - Minimal impact on existing firmware formats
will keep track of the update process on the device. This allows the
device operator to know what devices have received an update and - Robust permissions
which of them are still pending an update.
- Diverse modes of operation
- Suitability to software and personalization data
3.1. Agnostic to how firmware images are distributed
Firmware images can be conveyed to devices in a variety of ways, Firmware images can be conveyed to devices in a variety of ways,
including USB, UART, WiFi, BLE, low-power WAN technologies, mesh including USB, UART, WiFi, BLE, low-power WAN technologies, etc. and
networks and many more. At the application layer a variety of use different protocols (e.g., CoAP, HTTP). The specified mechanism
protocols are also available: MQTT, CoAP, and HTTP are the most needs to be agnostic to the distribution of the firmware images and
popular application layer protocols used by IoT devices. This manifests.
architecture does not make assumptions about how the firmware images
are distributed to the devices and therefore aims to support all
these technologies.
In some cases it may be desirable to distribute firmware images using 3.2. Friendly to broadcast delivery
a multicast or broadcast protocol. This architecture does not make
recommendations for any such protocol. However, given that broadcast This architecture does not specify any specific broadcast protocol.
may be desirable for some networks, updates must cause the least However, given that broadcast may be desirable for some networks,
disruption possible both in metadata and firmware transmission. For updates must cause the least disruption possible both in metadata and
an update to be broadcast friendly, it cannot rely on link layer, 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
| |
| Author |
| |
+----------+
Firmware + Manifest |
+----------------------------------+ | Firmware +
| | | Manifest
| ---+------- |
| ---- | --|-
| //+----------+ | \\
-+-- // | | | \
----/ | ---- |/ | Firmware |<-+ | \
// | \\ | | Server | | | \
/ | \ / | | + + \
/ | \ / +----------+ \ / |
/ +--------+--------+ \ / | |
/ | v | \ / v |
| | +------------+ | | | +----------------+ |
| | | Firmware | | | Device | |
| | | Consumer | | | | | Management | |
| | +------------+ | | | | | |
| | +------------+ | | | | +--------+ | |
| | | Status |<-+--------------------+-> | | | |
| | | Tracker | | | | | | Status | | |
| | | Client | | | | | | Tracker| | |
| | +------------+ | | | | | Server | | |
| | Device | | | | +--------+ | |
| +-----------------+ | \ | | /
\ / \ +----------------+ /
\ Network / \ /
\ Operator / \ Device Operator /
\\ // \ \ //
---- ---- ---- ----
----- -----------
Figure 1: Architecture. End-to-end security between the author and the device is shown in
Section 5.
Firmware images and manifests may be conveyed as a bundle or Authentication ensures that the device can cryptographically identify
detached. The manifest must support both approaches. the author(s) creating firmware images and manifests. Authenticated
identities may be used as input to the authorization process.
For distribution as a bundle, the firmware image is embedded into the Integrity protection ensures that no third party can modify the
manifest. This is a useful approach for deployments where devices manifest or the firmware image.
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 sticks or short range
radio technologies (such as Bluetooth Smart).
Alternatively, the manifest is distributed detached from the firmware For confidentiality protection of the firmware image, it must be done
image. Using this approach, the firmware consumer is presented with in such a way that every intended recipient can decrypt it. The
the manifest first and then needs to obtain one or more firmware information that is encrypted individually for each device must
images as dictated in the manifest. maintain friendliness to Content Distribution Networks, bulk storage,
and broadcast protocols.
A manifest specification must support different cryptographic
algorithms and algorithm extensibility. Due of the nature of
unchangeable code in ROM for use with bootloaders the use of post-
quantum secure signature mechanisms, such as hash-based signatures
[RFC8778], are attractive. These algorithms maintain security in
presence of quantum computers.
A mandatory-to-implement set of algorithms will be specified in the
manifest specification [I-D.ietf-suit-manifest]}.
3.4. Rollback attacks must be prevented
A device presented with an old, but valid manifest and firmware must
not be tricked into installing such firmware since a vulnerability in
the old firmware image may allow an attacker to gain control of the
device.
3.5. High reliability
A power failure at any time must not cause a failure of the device.
A failure to validate any part of an update must not cause a failure
of the device. One way to achieve this functionality is to provide a
minimum of two storage locations for firmware and one bootable
location for firmware. An alternative approach is to use a 2nd stage
bootloader with build-in full featured firmware update functionality
such that it is possible to return to the update process after power
down.
Note: This is an implementation requirement rather than a requirement
on the manifest format.
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 they must be minimal.
Additionally, it must be easy to parse only those fields that are
required to validate at least one signature or MAC with minimal
exposure.
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.
An alternative view to the operating modes is to consider the steps a
device has to go through in the course of an update:
- Notification
- Pre-authorisation
- Dependency resolution
- Download
- Installation
The notification step consists of the status tracker informing the
firmware consumer that an update is available. This can be
accomplished via polling (client-initiated), push notifications
(server-initiated), or more complex mechanisms.
The pre-authorisation step involves verifying whether the entity 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/1.1-based firmware server is For example, a download from an HTTP-based firmware server is client-
client-initiated. Pushing a manifest and firmware image to the initiated. Pushing a manifest and firmware image to the transfer to
Package resource of the LwM2M Firmware Update object [LwM2M] is the Package resource of the LwM2M Firmware Update object [LwM2M] is
server-initiated update. server-initiated.
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, to initiate the installation, it may - either ready to install it, it may need to wait for a trigger from the
need to wait for a trigger from the status tracker, - or trigger the status tracker to initiate the installation, may trigger the update
update automatically, - or go through a more complex decision making automatically, or may go through a more complex decision making
process to determine the appropriate timing for an update. Sometimes process to determine the appropriate timing for an update (such as
the final decision may require confirmation of the user of the device delaying the update process to a later time when end users are less
for safety reasons. impacted by the update process).
Installation is the act of processing the payload into a format that Installation is the act of processing the payload into a format that
the IoT device can recognize and the bootloader is responsible for the IoT device can recognise and the bootloader is responsible for
then booting from the newly installed firmware image. This process then booting from the newly installed firmware image.
is different when a bootloader is not involved. For example, when an
application is updated in a full-featured operating system, the
updater may halt and restart the application in isolation. Devices
must not fail when a disruption occurs during the update process.
For example, a power failure or network disruption during the update
process must not cause the device to fail.
4. Invoking the Firmware Each of these steps may require different permissions.
Section 3 describes the steps for getting the firmware image and the 3.11. Suitability to software and personalization data
manifest from the author to the firmware consumer on the IoT device.
Once the firmware consumer has retrieved and successfully processed
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
version of the firmware, handing control over to a firmware with a
higher privilege/trust level (the firmware verifier) verifying the
new firmware's authenticity & integrity, and then invoking it.
In an execute-in-place microcontroller, this is often done by The work on a standardized manifest format initially focused on the
rebooting into a bootloader (simultaneously halting the application & most constrained IoT devices and those devices contain code put
handing over to the higher privilege level) then executing a secure together by a single author (although that author may obtain code
boot process (verifying and invoking the new image). from other developers, some of it only in binary form).
In a rich OS, this may be done by halting one or more processes, then Later it turns out that other use cases may benefit from a
invoking new applications. In some OSs, this implicitly involves the standardized manifest format also for conveying software and even
kernel verifying the code signatures on the new applications. 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.
The invocation process is security sensitive. An attacker will To support this wider range of use cases the manifest format should
typically try to retrieve a firmware image from the device for therefore be extensible to convey other forms of payloads as well.
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
when the firmware image and the manifest were downloaded by the
firmware consumer.
The overlap between the firmware consumer and the firmware verifier 4. Claims
functionality comes in two forms, namely
- A firmware verifier must verify the firmware image it boots as Claims in the manifest offer a way to convey instructions to a device
part of the secure boot process. Doing so requires meta-data to that impact the firmware update process. To have any value the
be stored alongside the firmware image so that the firmware manifest containing those claims must be authenticated and integrity
verifier can cryptographically verify the firmware image before protected. The credential used must be directly or indirectly
booting it to ensure it has not been tampered with or replaced. related to the trust anchor installed at the device by the Trust
This meta-data used by the firmware verifier may well be the same Provisioning Authority.
manifest obtained with the firmware image during the update
process.
- An IoT device needs a recovery strategy in case the firmware The baseline claims for all manifests are described in
update / invocation process fails. The recovery strategy may [I-D.ietf-suit-information-model]. For example, there are:
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.
While this document assumes that the firmware verifier itself is - Do not install firmware with earlier metadata than the current
distinct from the role of the firmware consumer and therefore does metadata.
not manage the firmware update process, this is not a requirement and
these roles may be combined in practice.
Using a bootloader as the firmware verifier requires some special - Only install firmware with a matching vendor, model, hardware
considerations, particularly when the bootloader implements the revision, software version, etc.
robustness requirements identified by the IOTSU workshop [RFC8240].
4.1. The Bootloader - Only install firmware that is before its best-before timestamp.
In most cases the MCU must restart in order to hand over control to - Only allow a firmware installation if dependencies have been met.
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 for 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.
Assuming the first approach, there are (at least) three firmware - Choose the mechanism to install the firmware, based on the type of
images available on the device: firmware it is.
- First, the bootloader is also firmware. If a bootloader is 5. Communication Architecture
updatable then its firmware image is treated like any other
application firmware image.
- Second, the firmware image that has to be replaced is still Figure 1 shows the communication architecture where a firmware image
available on the device as a backup in case the freshly downloaded is created by an author, and uploaded to a firmware server. The
firmware image does not boot or operate correctly. 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.
- Third, there is the newly downloaded firmware image. Firmware + +----------+ Firmware + +-----------+
Manifest | |-+ Manifest | |-+
+--------->| Firmware | |<---------------| | |
| | Server | | | Author | |
| | | | | | |
| +----------+ | +-----------+ |
| +----------+ +-----------+
|
|
|
-+-- ------
---- | ---- ---- ----
// | \\ // \\
/ | \ / \
/ | \ / \
/ | \ / \
/ | \ / \
| v | | |
| +------------+ |
| | Firmware | | | |
| | Consumer | | Device | +--------+ |
| +------------+ | Management| | | |
| | |<------------------------->| Status | |
| | Device | | | | Tracker| |
| +------------+ | || | | |
| | || +--------+ |
| | | |
| | \ /
\ / \ /
\ / \ Device /
\ Network / \ Operator /
\ Operator / \\ //
\\ // ---- ----
---- ---- ------
-----
Therefore, the firmware consumer must know where to store the new Figure 1: Architecture.
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.
Since many low end IoT devices do not use position-independent code, End-to-end security mechanisms are used to protect the firmware image
either the bootloader needs to copy the newly downloaded application and the manifest although Figure 2 does not show the manifest itself
firmware image into the location of the old application firmware since it may be distributed independently.
image and vice versa or multiple versions of the firmware need to be
prepared for different locations.
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 reliability risk. | | Firmware Image | Firmware | Firmware Image | |
| Device |<-----------------| Server |<------------------| Author |
| | | | | |
+--------+ +-----------+ +--------+
^ *
* *
************************************************************
End-to-End Security
For a bootloader to offer a secure boot functionality it needs to Figure 2: End-to-End Security.
implement the following functionality:
- The bootloader needs to fetch the manifest (or manifest-alike Whether the firmware image and the manifest is pushed to the device
headers) from nonvolatile storage and parse its contents for or fetched by the device is a deployment specific decision.
subsequent cryptographic verification.
- Cryptographic libraries with hash functions, digital signatures The following assumptions are made to allow the firmware consumer to
(for asymmetric crypto), message authentication codes (for verify the received firmware image and manifest before updating
symmetric crypto) need to be accessible. software:
- The device needs to have a trust anchor store to verify the - To accept an update, a device needs to verify the signature
digital signature. (Alternatively, access to a key store for use covering the manifest. There may be one or multiple manifests
with the message authentication code.) 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 expose boot process-related data to the application - Not all entities creating and signing manifests have the same
firmware (such as to the status tracker). This allows to share permissions. A device needs to determine whether the requested
information about the current firmware version, and the status of action is indeed covered by the permission of the party that
the firmware update process and whether errors have occurred. 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.
- Produce boot measurements as part of an attestation solution. See - For confidentiality protection of firmware images the author needs
[I-D.ietf-rats-architecture] for more information. (optional) 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.
- Ability to decrypt firmware images, in case confidentiality There are different types of delivery modes, which are illustrated
protection was applied. This requires a solution for key based on examples below.
management. (optional)
5. Types of IoT Devices There is an option for embedding a firmware image into a manifest.
This is a useful approach for deployments where devices are not
connected to the Internet and cannot contact a dedicated 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.
There are billions of MCUs used in devices today produced by a large /------------\ /------------\
number of silicon manufacturers. While MCUs can vary significantly /Manifest with \ /Manifest with \
in their characteristics, there are a number of similiaries allowing |attached | |attached |
us to categorize in groups. \firmware image/ \firmware image/
\------------/ +-----------+ \------------/
+--------+ | | +--------+
| |<.................| Firmware |<................| |
| Device | | Server | | Author |
| | | | | |
+--------+ +-----------+ +--------+
The firmware update architecture, and the manifest format in Figure 3: Manifest with attached firmware.
particular, needs to offer enough flexibility to cover these common
deployment cases.
5.1. Single MCU Figure 4 shows an option for remotely updating a device where the
device fetches the firmware image from some file server. The
manifest itself is delivered independently and provides information
about the firmware image(s) to download.
/--------\ /--------\
/ \ / \
| Manifest | | Manifest |
\ / \ /
\--------/ \--------/
+-----------+
+--------+ | | +--------+
| |<.................| Status |................>| |
| Device | | Tracker | -- | Author |
| |<- | | --- | |
+--------+ -- +-----------+ --- +--------+
-- ---
--- ---
-- +-----------+ --
-- | | --
/------------\ -- | Firmware |<- /------------\
/ \ -- | Server | / \
| Firmware | | | | Firmware |
\ / +-----------+ \ /
\------------/ \------------/
Figure 4: Independent retrieval of the firmware image.
This architecture does not mandate a specific delivery mode but a
solution must support both types.
6. Manifest
In order for a device to apply an update, it has to make several
decisions about the update:
- Does it trust the author of the update?
- Has the firmware been corrupted?
- Does the firmware update apply to this device?
- Is the update older than the active firmware?
- When should the device apply the update?
- How should the device apply the update?
- What kind of firmware binary is it?
- Where should the update be obtained?
- Where should the firmware be stored?
The manifest encodes the information that devices need in order to
make these decisions. It is a data structure that contains the
following information:
- information about the device(s) the firmware image is intended to
be applied to,
- information about when the firmware update has to be applied,
- information about when the manifest was created,
- dependencies on other manifests,
- pointers to the firmware image and information about the format,
- information about where to store the firmware image,
- cryptographic information, such as digital signatures or message
authentication codes (MACs).
The manifest information model is described in
[I-D.ietf-suit-information-model].
7. Device Firmware Update Examples
Although these documents attempt to define a firmware update
architecture that is applicable to both existing systems, as well as
yet-to-be-conceived systems; it is still helpful to consider existing
architectures.
7.1. Single CPU SoC
The simplest, and currently most common, architecture consists of a 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. A notable characteristic of these SoCs as RAM for working storage. These systems either have a single
is that the primary code is generally execute in place (XIP). Due to firmware image, or an immutable bootloader that runs a single image.
the non-relocatable nature of the code, the firmware image needs to A notable characteristic of these SoCs is that the primary code is
be placed in a specific location in flash since the code cannot be generally execute in place (XIP). Combined with the non-relocatable
executed from an arbitrary location in flash. Hence, when the nature of the code, firmware updates need to be done in place.
firmware image is updated it is necessary to swap the old and the new
image.
5.2. Single CPU with Secure - Normal Mode Partitioning 7.2. 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.
5.3. Symmetric Multiple CPUs 7.3. Dual CPU, shared memory
In more complex SoCs with symmetric multi-processing support,
advanced operating systems, such as Linux, are often used. These
SoCs frequently use an external storage medium, such as raw NAND
flash or eMMC. Due to the higher quantity of resources, these
devices are often capable of storing multiple copies of their
firmware images and selecting the most appropriate one to boot. Many
SoCs also support bootloaders that are capable of updating the
firmware image, however this is typically a last resort because it
requires the device to be held in the bootloader while the new
firmware is downloaded and installed, which results in down-time for
the device. Firmware updates in this class of device are typically
not done in-place.
5.4. Dual CPU, shared memory This configuration has two or more CPUs in a single SoC that share
memory (flash and RAM). Generally, they will be a protection
mechanism to prevent one CPU from accessing the other's memory.
Upgrades in this case will typically be done by one of the CPUs, and
is similar to the single CPU with secure mode.
This configuration has two or more heterogeneous CPUs in a single SoC 7.4. Dual CPU, other bus
that share memory (flash and RAM). Generally, there will be a
mechanism to prevent one CPU from unintentionally accessing memory
currently allocated to the other. Upgrades in this case will
typically be done by one of the CPUs, and is similar to the single
CPU with secure mode.
5.5. Dual CPU, other bus This configuration has two or more CPUs, each having their own
memory. There will be a communication channel between them, but it
will be used as a peripheral, not via shared memory. In this case,
each CPU will have to be responsible for its own firmware upgrade.
It is likely that one of the CPUs will be considered a master, and
will direct the other CPU to do the upgrade. This configuration is
commonly used to offload specific work to other CPUs. Firmware
dependencies are similar to the other solutions above, sometimes
allowing only one image to be upgraded, other times requiring several
to be upgraded atomically. Because the updates are happening on
multiple CPUs, upgrading the two images atomically is challenging.
This configuration has two or more heterogeneous CPUs, each having 8. Bootloader
their own memory. There will be a communication channel between
them, but it will be used as a peripheral, not via shared memory. In
this case, each CPU will have to be responsible for its own firmware
upgrade. It is likely that one of the CPUs will be considered the
primary CPU, and will direct the other CPU to do the upgrade. This
configuration is commonly used to offload specific work to other
CPUs. Firmware dependencies are similar to the other solutions
above, sometimes allowing only one image to be upgraded, other times
requiring several to be upgraded atomically. Because the updates are
happening on multiple CPUs, upgrading the two images atomically is
challenging.
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:
In order for a firmware consumer to apply an update, it has to make - the Internet protocol stack for firmware downloads (*),
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.
Table 1 provides examples of decisions to be made. - the capability to write the received firmware image to persistent
storage (most likely flash memory) prior to performing the update,
+----------------------------+--------------------------------------+ - the ability to unpack, decompress or otherwise process the
| Decision | Information Elements | received firmware image,
+----------------------------+--------------------------------------+
| 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 |
+----------------------------+--------------------------------------+
Table 1: Firmware Update Decisions. - the features to verify an image and a manifest, including digital
signature verification or checking a message authentication code,
(1): A device presented with an old, but valid manifest and firmware - a manifest parsing library, and
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.
Keeping the code size and complexity of a manifest parsers small is - integration of the device into a device management server to
important for constrained IoT devices. Since the manifest parsing perform automatic firmware updates and to track their progress.
code may also be used by the bootloader it is part of the trusted
computing base.
A manifest may not only be used to protect firmware images but also (*) Because firmware images are often multiple kilobytes, sometimes
configuration data such as network credentials or personalization exceeding one hundred kilobytes, in size for low end IoT devices and
data related to firmware or software. Personalization data even several megabytes large for IoT devices running full-fledged
demonstrates the need for confidentiality to be maintained between operating systems like Linux, the protocol mechanism for retrieving
two or more stakeholders that both deliver images to the same device. these images needs to offer features like congestion control, flow
control, fragmentation and reassembly, and mechanisms to resume
interrupted or corrupted transfers.
Personalization data is used with Trusted Execution Environments All these features are most likely offered by the application, i.e.
(TEEs), which benefit from a protocol for managing the lifecycle of firmware consumer, running on the device (except for basic security
trusted applications (TAs) running inside a TEE. TEEs may obtain TAs algorithms that may run either on a trusted execution environment or
from different authors and those TAs may require personalization on a separate hardware security MCU/module) rather than by the
data, such as payment information, to be securely conveyed to the bootloader itself.
TEE. The TA's author does not want to expose the TA's code to any
other stakeholder or third party. The user does not want to expose
the payment information to any other stakeholder or third party.
7. Securing Firmware Updates Once manifests have been processed and firmware images successfully
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.
Using firmware updates to fix vulnerabilities in devices is important The boot process is security sensitive because the firmware images
but securing this update mechanism is equally important since may, for example, be stored in off-chip flash memory giving attackers
security problems are exacerbated by the update mechanism: update is easy access to the image for reverse engineering and potentially also
essentially authorized remote code execution, so any security for modifying the binary. The bootloader will therefore have to
problems in the update process expose that remote code execution perform security checks on the firmware image before it can be
system. Failure to secure the firmware update process will help booted. These security checks by the bootloader happen in addition
attackers to take control over devices. to the security checks that happened when the firmware image and the
manifest were downloaded.
End-to-end security mechanisms are used to protect the firmware image The manifest may have been stored alongside the firmware image to
and the manifest. The following assumptions are made to allow the allow re-verification of the firmware image during every boot
firmware consumer to verify the received firmware image and manifest attempt. Alternatively, secure boot-specific meta-data may have been
before updating software: created by the application after a successful firmware download and
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.
- Authentication ensures that the device can cryptographically If the application image contains the firmware consumer
identify the author(s) creating firmware images and manifests. functionality, as described above, then it is necessary that a
Authenticated identities may be used as input to the authorization working image is left on the device. This allows the bootloader to
process. Not all entities creating and signing manifests have the roll back to a working firmware image to execute a firmware download
same permissions. A device needs to determine whether the if the bootloader itself does not have enough functionality to fetch
requested action is indeed covered by the permission of the party a firmware image plus manifest from a firmware server over the
that signed the manifest. Informing the device about the Internet. A multi-stage bootloader may soften this requirement at
permissions of the different parties also happens in an out-of- the expense of a more sophisticated boot process.
band fashion and is also a duty of the Trust Provisioning
Authority.
- Integrity protection ensures that no third party can modify the For a bootloader to offer a secure boot mechanism it needs to provide
manifest or the firmware image. To accept an update, a device the following features:
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.
- For confidentiality protection of the firmware image, it must be - ability to access security algorithms, such as SHA-256 to compute
done in such a way that the intended firmware consumer(s), other a fingerprint over the firmware image and a digital signature
authorized parties, and no one else can decrypt it. The algorithm.
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.
A manifest specification must support different cryptographic - access keying material directly or indirectly to utilize the
algorithms and algorithm extensibility. Moreover, since RSA- and digital signature. The device needs to have a trust anchor store.
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].
When a device obtains a monolithic firmware image from a single - ability to expose boot process-related data to the application
author without any additional approval steps then the authorization firmware (such as to the device management software). This allows
flow is relatively simple. There are, however, other cases where a device management server to determine whether the firmware
more complex policy decisions need to be made before updating a update has been successful and, if not, what errors occurred.
device.
In this architecture the authorization policy is separated from the - to (optionally) offer attestation information (such as
underlying communication architecture. This is accomplished by measurements).
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 While the software architecture of the bootloader and its security
all permission management and coordination. This entity allows the mechanisms are implementation-specific, the manifest can be used to
device to offload complex permissions calculations for the device. control the firmware download from the Internet in addition to
augmenting secure boot process. These building blocks are highly
relevant for the design of the manifest.
8. Example 9. Example
Figure 2 illustrates an example message flow for distributing a Figure 5 illustrates an example message flow for distributing a
firmware image to a device. The firmware and manifest are stored on firmware image to a device starting with an author uploading the new
the same firmware server and distributed in a detached manner. firmware to firmware server and creating a manifest. The firmware
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 Server | | IoT Device | | | | | | Firmware | | |
| Author | | Status Tracker | | +------------+ +----------+ | | Author | | Firmware Server | | Consumer | |Bootloader|
+--------+ | Server | | | Firmware | |Bootloader| | +--------+ +-----------------+ +------------+ +----------+
| +-----------------+ | | Consumer | | | | | | | +
| | | +------------+ +----------+ | | Create Firmware | | |
| | | | | | |--------------+ | | |
| | | +-----------------------+ | | | | | |
| Create Firmware | | | Status Tracker Client | | |<-------------+ | | |
|--------------+ | | +-----------------------+ | | | | |
| | | `'''''''''''''''''''''''''''' | Upload Firmware | | |
|<-------------+ | | | | |------------------>| | |
| | | | | | | | |
| Upload Firmware | | | | | Create Manifest | | |
|------------------>| | | | |---------------+ | | |
| | | | | | | | | |
| Create Manifest | | | | |<--------------+ | | |
|---------------+ | | | | | | | |
| | | | | | | Sign Manifest | | |
|<--------------+ | | | | |-------------+ | | |
| | | | | | | | | |
| Sign Manifest | | | | |<------------+ | | |
|-------------+ | | | | | | | |
| | | | | | | Upload Manifest | | |
|<------------+ | | | | |------------------>| | |
| | | | | | | | |
| Upload Manifest | | | | | | Query 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 23, line 6 skipping to change at page 23, line 7
| | 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 2: First Example Flow for a Firmware Update. Figure 5: First Example Flow for a Firmware Upate.
Figure 3 shows an exchange that starts with the status tracker Figure 6 shows an example follow with the device using a status
querying the device for its current firmware version. Later, a new tracker. For editorial reasons the author publishing the manifest at
firmware version becomes available and since this device is running the status tracker and the firmware image at the firmware server is
an older version the status tracker server interacts with the device not shown. Also omitted is the secure boot process following the
to initiate an update. successful firmware update process.
The manifest and the firmware are stored on different servers in this The exchange starts with the device interacting with the status
example. When the device processes the manifest it learns where to tracker; the details of such exchange will vary with the different
download the new firmware version. The firmware consumer downloads device management systems being used. In any case, the status
the firmware image with the newer version X.Y.Z after successful tracker learns about the firmware version of the devices it manages.
validation of the manifest. Subsequently, a reboot is initiated and In our example, the device under management is using firmware version
the secure boot process starts. Finally, the device reports the A.B.C. At a later point in time the author uploads a new firmware
successful boot of the new firmware version. along with the manifest to the firmware server and the status
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 | | Firmware Server | | +------------+ +----------+ | | Status | | | | +------------+ +----------+ |
| Tracker | | Status Tracker | | | Firmware | |Bootloader| | | Tracker | | Firmware Server | | | Firmware | |Bootloader| |
| Server | | Server | | | Consumer | | | | | | | | | | 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 24, line 45 skipping to change at page 24, line 48
| | | Trigger Reboot | | | | Trigger Reboot |
| | |--------------->| | | |--------------->|
| | | | | | | |
| | | | | | | |
| | | __..-------..._' | | | __..-------..._'
| | ,-' `-. | | ,-' `-.
| | | Secure Boot | | | | Secure Boot |
| | `-. _/ | | `-. _/
| | |`--..._____,,.,-' | | |`--..._____,,.,-'
| | | | | | | |
| Device running firmware X.Y.Z | |
|<-------------------------------------| |
| | | |
| | | |
Figure 3: Second Example Flow for a Firmware Update. Figure 6: Second Example Flow for a Firmware Upate.
9. IANA Considerations 10. IANA Considerations
This document does not require any actions by IANA. This document does not require any actions by IANA.
10. Security Considerations 11. Security Considerations
This document describes terminology, requirements and an architecture Firmware updates fix security vulnerabilities and are considered to
for firmware updates of IoT devices. The content of the document is be an important building block in securing IoT devices. Due to the
thereby focused on improving security of IoT devices via firmware importance of firmware updates for IoT devices the Internet
update mechanisms and informs the standardization of a manifest Architecture Board (IAB) organized a 'Workshop on Internet of Things
format. (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.
An in-depth examination of the security considerations of the There are, however, many other considerations raised during the
architecture is presented in [I-D.ietf-suit-information-model]. 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
11. Acknowledgements - 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
protecting the manifest.
- incentives for manufacturers to offer a firmware update mechanism
as part of their IoT products.
12. Acknowledgements
We would like to thank the following persons for their feedback: We would like to thank the following persons for their feedback:
- Geraint Luff - Geraint Luff
- Amyas Phillips - Amyas Phillips
- Dan Ros - Dan Ros
- Thomas Eichinger - Thomas Eichinger
skipping to change at page 26, line 4 skipping to change at page 26, line 38
- Olaf Bergmann - Olaf Bergmann
- Suhas Nandakumar - Suhas Nandakumar
- Phillip Hallam-Baker - Phillip Hallam-Baker
- Marti Bolivar - Marti Bolivar
- Andrzej Puzdrowski - Andrzej Puzdrowski
- Markus Gueller - Markus Gueller
- Henk Birkholz - Henk Birkholz
- Jintao Zhu - Jintao Zhu
- Takeshi Takahashi - Takeshi Takahashi
- Jacob Beningo - Jacob Beningo
- Kathleen Moriarty - Kathleen Moriarty
- Bob Briscoe
- Roman Danyliw
- Brian Carpenter
- Theresa Enghardt
- Rich Salz
- Mohit Sethi
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.
12. Informative References 13. 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-08 (work in progress),
June 2020. October 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
Serialization Format for the Software Updates for Internet Serialization Format for the Software Updates for Internet
of Things (SUIT) Manifest", draft-ietf-suit-manifest-09 of Things (SUIT) Manifest", draft-ietf-suit-manifest-11
(work in progress), July 2020. (work in progress), December 2020.
[I-D.ietf-teep-architecture] [I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler, Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP) "Trusted Execution Environment Provisioning (TEEP)
Architecture", draft-ietf-teep-architecture-12 (work in Architecture", draft-ietf-teep-architecture-13 (work in
progress), July 2020. progress), November 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/
V1_0_2-20180209-A.pdf>. OMA-TS-LightweightM2M-V1_0_2-20180209-A.pdf>.
[quantum-factorization]
Jiang, S., Britt, K., McCaskey, A., Humble, T., and S.
Kais, "Quantum Annealing for Prime Factorization",
December 2018,
<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>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[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>.
[RFC8240] Tschofenig, H. and S. Farrell, "Report from the Internet [RFC8240] Tschofenig, H. and S. Farrell, "Report from the Internet
of Things Software Update (IoTSU) Workshop 2016", of Things Software Update (IoTSU) Workshop 2016",
RFC 8240, DOI 10.17487/RFC8240, September 2017, RFC 8240, DOI 10.17487/RFC8240, September 2017,
<https://www.rfc-editor.org/info/rfc8240>. <https://www.rfc-editor.org/info/rfc8240>.
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