A Firmware Update Architecture for Internet of Things Devices
draft-ietf-suit-architecture-00
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draft-ietf-suit-architecture-00
SUIT B. Moran
Internet-Draft Arm Limited
Intended status: Informational M. Meriac
Expires: December 5, 2018 Consultant
H. Tschofenig
Arm Limited
June 03, 2018
A Firmware Update Architecture for Internet of Things Devices
draft-ietf-suit-architecture-00
Abstract
Vulnerabilities with Internet of Things (IoT) devices have raised the
need for a solid and secure firmware update mechanism that is also
suitable for constrained devices. Incorporating such update
mechanism to fix vulnerabilities, to update configuration settings as
well as adding new functionality is recommended by security experts.
This document lists requirements and describes an architecture for a
firmware update mechanism suitable for IoT devices. The architecture
is agnostic to the transport of the firmware images and associated
meta-data.
This version of the document assumes asymmetric cryptography and a
public key infrastructure. Future versions may also describe a
symmetric key approach for very constrained devices.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 5, 2018.
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Copyright Notice
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than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Agnostic to how firmware images are distributed . . . . . 5
3.2. Friendly to broadcast delivery . . . . . . . . . . . . . 5
3.3. Use state-of-the-art security mechanisms . . . . . . . . 5
3.4. Rollback attacks must be prevented . . . . . . . . . . . 6
3.5. High reliability . . . . . . . . . . . . . . . . . . . . 6
3.6. Operate with a small bootloader . . . . . . . . . . . . . 6
3.7. Small Parsers . . . . . . . . . . . . . . . . . . . . . . 7
3.8. Minimal impact on existing firmware formats . . . . . . . 7
3.9. Robust permissions . . . . . . . . . . . . . . . . . . . 7
3.10. Operating modes . . . . . . . . . . . . . . . . . . . . . 8
4. Claims . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Manifest . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Example Flow . . . . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
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10. Mailing List Information . . . . . . . . . . . . . . . . . . 16
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1. Normative References . . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . 17
12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
When developing IoT devices, one of the most difficult problems to
solve is how to update the firmware on the device. Once the device
is deployed, firmware updates play a critical part in its lifetime,
particularly when devices have a long lifetime, are deployed in
remote or inaccessible areas or where manual intervention is cost
prohibitive or otherwise difficult. The need for a firmware update
may be to fix bugs in software, to add new functionality, or to re-
configure the device.
The firmware update process has to ensure that
- The firmware image is authenticated and attempts to flash a
malicious firmware image are prevented.
- The firmware image can be confidentiality protected so that
attempts by an adversary to recover the plaintext binary can be
prevented. Obtaining the plaintext binary is often one of the
first steps for an attack to mount an attack.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
This document uses the following terms:
- Manifest: The manifest contains meta-data about the firmware
image. The manifest is protected against modification and
provides information about the author.
- Firmware Image: The firmware image is a binary that may contain
the complete software of a device or a subset of it. The firmware
image may consist of multiple images, if the device contains more
than one microcontroller. The image may consist of a differential
update for performance reasons. Firmware is the more universal
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term. Both terms are used in this document and are
interchangeable.
- Bootloader: A bootloader is a piece of software that is executed
once a microcontroller has been reset. It is responsible for
deciding whether to boot a firmware image that is present or
whether to obtain and verify a new firmware image. Since the
bootloader is a security critical component its functionality may
be split into separate stages. Such a multi-stage bootloader may
offer very basic functionality in the first stage and resides in
ROM whereas the second stage may implement more complex
functionality and resides in flash memory so that it can be
updated in the future (in case bugs have been found). The exact
split of components into the different stages, the number of
firmware images stored by an IoT device, and the detailed
functionality varies throughout different implementations.
The following entities are used:
- Author: The author is the entity that creates the firmware image,
signs and/or encrypts it. The author is most likely a developer
using a set of tools.
- Device: The device is the recipient of the firmware image and the
manifest. The goal is to update the firmware of the device.
- Untrusted Storage: Firmware images and manifests may be stored on
untrusted fileservers or cloud storage infrastructure. Some
deployments may require storage of the firmware images/manifests
to be stored on various entities before they reach the device.
3. Requirements
The firmware update mechanism described in this specification was
designed with the following requirements in mind:
- Agnostic to how firmware images are distributed
- Friendly to broadcast delivery
- Use state-of-the-art security mechanisms
- Rollback attacks must be prevented
- High reliability
- Operate with a small bootloader
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- Small Parsers
- Minimal impact on existing firmware formats
- Robust permissions
- Diverse modes of operation
3.1. Agnostic to how firmware images are distributed
Firmware images can be conveyed to devices in a variety of ways,
including USB, UART, WiFi, BLE, low-power WAN technologies, etc. and
use different protocols (e.g., CoAP, HTTP). The specified mechanism
needs to be agnostic to the distribution of the firmware images and
manifests.
3.2. Friendly to broadcast delivery
This architecture does not specify any specific broadcast protocol
however, given that broadcast may be desirable for some networks,
updates must cause the least disruption possible both in metadata and
payload transmission.
For an update to be broadcast friendly, it cannot rely on link layer,
network layer, or transport layer security. In addition, the same
message must be deliverable to many devices, both those to which it
applies and those to which it does not, without a chance that the
wrong device will accept the update. Considerations that apply to
network broadcasts apply equally to the use of third-party content
distribution networks for payload distribution.
3.3. Use state-of-the-art security mechanisms
End-to-end security between the author and the device, as shown in
Section 5, is used to ensure that the device can verify firmware
images and manifests produced by authorized authors.
The use of post-quantum secure signature mechanisms, such as hash-
based signatures, should be explored. A migration to post-quantum
secure signatures would require significant effort, therefore,
mandatory-to-implement support for post-quantum secure signatures is
a goal.
A mandatory-to-implement set of algorithms has to be defined offering
a key length of 112-bit symmetric key or security or more, as
outlined in Section 20 of RFC 7925 [RFC7925]. This corresponds to a
233 bit ECC key or a 2048 bit RSA key.
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If the firmware image is to be encrypted, it must be done in such a
way that every intended recipient can decrypt it. The information
that is encrypted individually for each device must be an absolute
minimum, for example AES Key Wrap [RFC5649], in order to maintain
friendliness to Content Distribution Networks, bulk storage, and
broadcast protocols.
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
The bootloader must be minimal, containing only flash support,
cryptographic primitives and optionally a recovery mechanism. The
recovery mechanism is used in case the update process failed and may
include support for firmware updates over serial, USB or even a
limited version of wireless connectivity standard like a limited
Bluetooth Smart. Such a recovery mechanism must provide security at
least at the same level as the full featured firmware update
functionalities.
The bootloader needs to verify the received manifest and to install
the bootable firmware image. The bootloader should not require
updating since a failed update poses a risk in reliability. If more
functionality is required in the bootloader, it must use a two-stage
bootloader, with the first stage comprising the functionality defined
above.
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All information necessary for a device to make a decision about the
installation of a firmware update must fit into the available RAM of
a constrained IoT device. This prevents flash write exhaustion.
Note: This is an implementation requirement.
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
A device may have many modules that require updating individually.
It may also need to trust several actors in order to authorize an
update. These actors might include the following (this is not a
comprehensive list).
* A firmware author
* A device OEM
* A device operator
* A network operator
* A device owner
These actors exert their authority on the device by making claims (as
in Section 4).
For example, a firmware author may not have the authority to install
firmware 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. To facilitate
complex use-cases such as this, updates require several claims.
Alternatively, a device may trust precisely one authority, which does
all permission management and coordination. Effectively, the
authority allows the device to offload complex permissions
calculations for the device.
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3.10. Operating modes
There are three broad classifications of update operating modes.
* Self initiated
* Third-party initiated
* Hybrid
Self initiated updates take the form of a proactive IoT device that
checks for updates. Third-party initiated updates are triggered by
an actor other than the IoT device, be it a server, a peer, or a
user. Hybrid updates are those that require agreement from both the
target IoT device and another actor.
Third-party initiated updates are important to consider because
timing of updates may need to be tightly controlled in some high-
reliability environments.
An IoT device goes through several steps in the course of an update,
each of which can be self-initiated or third-party initiated, or
hybrid. An IoT device may go through the following steps, though
this is not a comprehensive list.
* Notification
* Pre-authorisation
* Dependency resolution
* Download
* Installation
The notification step consists informing an IoT device that an update
is available. This can be accomplished via polling (self-initiated),
push notifications (third-party initiated), or more complex
mechanisms.
The pre-authorisation step involves verifying the update authority
and making a determination that the device is prepared to initiate
the fetching and processing of updates. If the device has all
information that is necessary to make this determination, then the
pre-authorisation may be self-initiated. However, the device can
wait for instruction to begin (third-party initiated). Hybrid
approaches are possible as well.
A dependency resolution phase is needed when more than one component
can be updated or when a differential update is used. The necessary
dependencies must be available prior to installation.
The download step is the process of acquiring a local copy of the
payload. When the download is self-initiated, this means that the
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IoT device chooses when a download occurs and initiates the download
process. When a download is third-party initiated, this means that
either the remote service tells the IoT device when to download or
that it initiates the transfer directly to the IoT device. For
example, a download from an HTTP server is initiated locally. A
transfer to a LwM2M Firmware Update resource [LwM2M] is initiated
remotely.
Installation is the act of processing the payload into a format that
the IoT device can recognise.
Each of these steps may require different permissions expressed in
claims and may be implemented in a variety of ways.
4. Claims
When a simple set of permissions fails to encapsulate the rules
required for a device to make decisions about firmware, claims can be
used instead. Claims represent a form of policy. Several claims can
be used together, when multiple actors should have the rights to set
policies.
Some example claims are:
- Trust the actor identified by the referenced public key.
- Trust the actor with access to the referenced shared secret (MAC).
- Three actors are trusted identified by their public keys.
Signatures from at least two of these actors are required to trust
a manifest.
- The actor identified by the referenced public key is authorized to
create secondary policies
The baseline claims for all manifests are described in [SUIT-IM]. In
summary, they are:
- Do not install firmware with earlier metadata than the current
metadata.
- Only install firmware with a matching vendor, model, hardware
revision, software version, etc.
- Only install firmware that is before its best-before timestamp.
- Only install firmware with metadata signed/authenticated by a
trusted actor.
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- Only allow an actor to exercise rights on the device via a
manifest if that actor has signed the manifest.
- Only allow a firmware installation if all required rights have
been met through signatures/MACs (one or more) or manifest
dependencies (one or more).
- Use the instructions provided by the manifest to install the
firmware.
- Install any and all firmware images that are linked together with
manifest dependencies.
- Choose the mechanism to install the firmware, based on the type of
firmware it is.
5. Architecture
We start the architectural description with the security model. It
is based on end-to-end security. In Figure 1 a firmware image is
created by an author, sent to the device and subsequently installed.
When the author is ready to distribute the firmware image it is
conveyed using some communication channel to the device, which will
typically involve the use of untrusted storage. Examples of
untrusted storage are FTP servers, Web servers or USB sticks. End-
to-end security mechanisms are used to protect the firmware image.
Figure 1 does not show the manifest itself, which provides the meta-
data about the firmware image and offers the security protection. It
may bundled with the firmware image or travel as a standalone item.
+-----------+
+--------+ | | +--------+
| | Firmware Image | Untrusted | Firmware Image | |
| Device |<-----------------| Storage |<------------------| Author |
| | | | | |
+--------+ +-----------+ +--------+
^ *
* *
************************************************************
End-to-End Security
Figure 1: End-to-End Security.
Whether the firmware image and the manifest is pushed to the device
or fetched by the device is outside the scope of this work and
existing device management protocols can be used for efficiently
distributing this information.
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The following assumptions are made to allow the device to verify the
received firmware image and manifest before updating software:
- To accept an update, a device needs to decide whether the author
signing the firmware image and the manifest is authorized to make
the updates. We use public key cryptography to accomplish this.
The device verifies the signature covering the manifest using a
digital signature algorithm OR the device verifies the MAC
covering the manifest using a MAC algorithm. The device is
provisioned with a trust anchor that is used to validate the
digital signature or MAC produced by the author. This trust
anchor is potentially different from the trust anchor used to
validate the digital signature produced for other protocols (such
as device management protocols). This trust anchor may be
provisioned to the device during manufacturing or during
commissioning.
- 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.
There are different types of delivery modes, which are illustrated
based on examples below.
There is an option for embedding a firmware image into a manifest.
This is a useful approach for deployments where devices are not
connected to the Internet and cannot contact a dedicated server for
download of the firmware. It is also applicable when the firmware
update happens via a USB stick or via Bluetooth Smart. Figure 2
shows this delivery mode graphically.
/------------\ /------------\
/Manifest with \ /Manifest with \
|attached | |attached |
\firmware image/ \firmware image/
\------------/ +-----------+ \------------/
+--------+ | | +--------+
| |<.................| Untrusted |<................| |
| Device | | Storage | | Author |
| | | | | |
+--------+ +-----------+ +--------+
Figure 2: Manifest with attached firmware.
Figure 3 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.
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/------------\
/ \
| Manifest |
\ /
+--------+ \------------/ +--------+
| |<..............................................>| |
| Device | -- | Author |
| |<- --- | |
+--------+ -- --- +--------+
-- ---
--- ---
-- +-----------+ --
-- | | --
/------------\ -- | Untrusted |<- /------------\
/ \ -- | Storage | / \
| Firmware | | | | Firmware |
\ / +-----------+ \ /
\------------/ \------------/
Figure 3: 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?
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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 [SUIT-IM].
7. Example Flow
The following example message flow illustrates the interaction for
distributing a firmware image to a device starting with an author
uploading the new firmware to untrusted storage and creating a
manifest. The firmware and manifest are stored on the same untrusted
storage.
+--------+ +-----------------+ +------+
| Author | |Untrusted Storage| |Device|
+--------+ +-----------------+ +------+
| | |
| Create Firmware | |
|--------------- | |
| | | |
|<-------------- | |
| | |
| Upload Firmware | |
|------------------>| |
| | |
| Create Manifest | |
|---------------- | |
| | | |
|<--------------- | |
| | |
| Sign Manifest | |
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|-------------- | |
| | | |
|<------------- | |
| | |
| Upload Manifest | |
|------------------>| |
| | |
| | Query Manifest |
| |<--------------------|
| | |
| | Send Manifest |
| |-------------------->|
| | |
| | | Validate Manifest
| | |------------------
| | | |
| | |<-----------------
| | |
| | Request Firmware |
| |<--------------------|
| | |
| | Send Firmware |
| |-------------------->|
| | |
| | | Verify Firmware
| | |---------------
| | | |
| | |<--------------
| | |
| | | Store Firmware
| | |--------------
| | | |
| | |<-------------
| | |
| | | Reboot
| | |-------
| | | |
| | |<------
| | |
| | | Bootloader validates
| | | Firmware
| | |----------------------
| | | |
| | |<---------------------
| | |
| | | Bootloader activates
| | | Firmware
| | |----------------------
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| | | |
| | |<---------------------
| | |
| | | Bootloader transfers
| | | control to new Firmware
| | |----------------------
| | | |
| | |<---------------------
| | |
Figure 4: Example Flow for a Firmware Upate.
8. IANA Considerations
This document does not require any actions by IANA.
9. Security Considerations
Firmware updates fix security vulnerabilities and are considered to
be an important building block in securing IoT devices. Due to the
importance of firmware updates for IoT devices the Internet
Architecture Board (IAB) organized a 'Workshop on Internet of Things
(IoT) Software Update (IOTSU)', which took place at Trinity College
Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at
the big picture. A report about this workshop can be found at
[RFC8240]. A standardized firmware manifest format providing end-to-
end security from the author to the device will be specified in a
separate document.
There are, however, many other considerations raised during the
workshop. Many of them are outside the scope of standardization
organizations since they fall into the realm of product engineering,
regulatory frameworks, and business models. The following
considerations are outside the scope of this document, namely
- installing firmware updates in a robust fashion so that the update
does not break the device functionality of the environment this
device operates in.
- installing firmware updates in a timely fashion considering the
complexity of the decision making process of updating devices,
potential re-certification requirements, and the need for user
consent to install updates.
- the distribution of the actual firmware update, potentially in an
efficient manner to a large number of devices without human
involvement.
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- 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.
10. Mailing List Information
The discussion list for this document is located at the e-mail
address suit@ietf.org [1]. Information on the group and information
on how to subscribe to the list is at
https://www1.ietf.org/mailman/listinfo/suit
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/suit/current/index.html
11. Acknowledgements
We would like to thank the following persons for their feedback:
- Geraint Luff
- Amyas Phillips
- Dan Ros
- Thomas Eichinger
- Michael Richardson
- Emmanuel Baccelli
- Ned Smith
- David Brown
- Jim Schaad
- Carsten Bormann
- Cullen Jennings
- Olaf Bergmann
- Suhas Nandakumar
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- Phillip Hallam-Baker
- Marti Bolivar
- Andrzej Puzdrowski
- Markus Gueller
- Henk Birkholz
- Jintao Zhu
We would also like to thank the WG chairs, Russ Housley, David
Waltermire, Dave Thaler for their support and their reviews.
Kathleen Moriarty was the responsible security area director when
this work was started.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016, <https://www.rfc-
editor.org/info/rfc7925>.
12.2. Informative References
[LwM2M] OMA, ., "Lightweight Machine to Machine Technical
Specification, Version 1.0.2", February 2018,
<http://www.openmobilealliance.org/release/LightweightM2M/
V1_0_2-20180209-A/
OMA-TS-LightweightM2M-V1_0_2-20180209-A.pdf>.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
DOI 10.17487/RFC5649, September 2009, <https://www.rfc-
editor.org/info/rfc5649>.
Moran, et al. Expires December 5, 2018 [Page 17]
Internet-Draft IoT Firmware Update Architecture June 2018
[RFC8240] Tschofenig, H. and S. Farrell, "Report from the Internet
of Things Software Update (IoTSU) Workshop 2016",
RFC 8240, DOI 10.17487/RFC8240, September 2017,
<https://www.rfc-editor.org/info/rfc8240>.
[SUIT-IM] Moran, B., Tschofenig, H., Birkholz, H., and J. Jimenez,
"Firmware Updates for Internet of Things Devices - An
Information Model for Manifests", June 2018.
12.3. URIs
[1] mailto:suit@ietf.org
Authors' Addresses
Brendan Moran
Arm Limited
EMail: Brendan.Moran@arm.com
Milosch Meriac
Consultant
EMail: milosch@meriac.com
Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@gmx.net
Moran, et al. Expires December 5, 2018 [Page 18]