A Manifest Information Model for Firmware Updates in IoT Devices
draft-ietf-suit-information-model-11
SUIT B. Moran
Internet-Draft H. Tschofenig
Intended status: Informational Arm Limited
Expires: October 8, 2021 H. Birkholz
Fraunhofer SIT
April 06, 2021
A Manifest Information Model for Firmware Updates in IoT Devices
draft-ietf-suit-information-model-11
Abstract
Vulnerabilities with Internet of Things (IoT) devices have raised the
need for a reliable and secure firmware update mechanism that is also
suitable for constrained devices. Ensuring that devices function and
remain secure over their service life requires such an update
mechanism to fix vulnerabilities, to update configuration settings,
as well as adding new functionality.
One component of such a firmware update is a concise and machine-
processable meta-data document, or manifest, that describes the
firmware image(s) and offers appropriate protection. This document
describes the information that must be present in the manifest.
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 https://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 October 8, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements and Terminology . . . . . . . . . . . . . . . . 6
2.1. Requirements Notation . . . . . . . . . . . . . . . . . . 6
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
3. Manifest Information Elements . . . . . . . . . . . . . . . . 6
3.1. Version ID of the Manifest Structure . . . . . . . . . . 7
3.2. Monotonic Sequence Number . . . . . . . . . . . . . . . . 7
3.3. Vendor ID . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Class ID . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4.1. Example 1: Different Classes . . . . . . . . . . . . 9
3.4.2. Example 2: Upgrading Class ID . . . . . . . . . . . . 9
3.4.3. Example 3: Shared Functionality . . . . . . . . . . . 9
3.4.4. Example 4: White-labelling . . . . . . . . . . . . . 10
3.5. Precursor Image Digest Condition . . . . . . . . . . . . 10
3.6. Required Image Version List . . . . . . . . . . . . . . . 10
3.7. Expiration Time . . . . . . . . . . . . . . . . . . . . . 11
3.8. Payload Format . . . . . . . . . . . . . . . . . . . . . 11
3.9. Processing Steps . . . . . . . . . . . . . . . . . . . . 11
3.10. Storage Location . . . . . . . . . . . . . . . . . . . . 12
3.10.1. Example 1: Two Storage Locations . . . . . . . . . . 12
3.10.2. Example 2: File System . . . . . . . . . . . . . . . 12
3.10.3. Example 3: Flash Memory . . . . . . . . . . . . . . 12
3.11. Component Identifier . . . . . . . . . . . . . . . . . . 12
3.12. Payload Indicator . . . . . . . . . . . . . . . . . . . . 13
3.13. Payload Digests . . . . . . . . . . . . . . . . . . . . . 13
3.14. Size . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.15. Manifest Envelope Element: Signature . . . . . . . . . . 14
3.16. Additional Installation Instructions . . . . . . . . . . 14
3.17. Aliases . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.18. Dependencies . . . . . . . . . . . . . . . . . . . . . . 15
3.19. Encryption Wrapper . . . . . . . . . . . . . . . . . . . 15
3.20. XIP Address . . . . . . . . . . . . . . . . . . . . . . . 15
3.21. Load-time Metadata . . . . . . . . . . . . . . . . . . . 15
3.22. Run-time metadata . . . . . . . . . . . . . . . . . . . . 16
3.23. Payload . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.24. Manifest Envelope Element: Delegation Chain . . . . . . . 16
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4. Security Considerations . . . . . . . . . . . . . . . . . . . 17
4.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 17
4.2. Threat Descriptions . . . . . . . . . . . . . . . . . . . 17
4.2.1. THREAT.IMG.EXPIRED: Old Firmware . . . . . . . . . . 17
4.2.2. THREAT.IMG.EXPIRED.OFFLINE : Offline device + Old
Firmware . . . . . . . . . . . . . . . . . . . . . . 18
4.2.3. THREAT.IMG.INCOMPATIBLE: Mismatched Firmware . . . . 18
4.2.4. THREAT.IMG.FORMAT: The target device misinterprets
the type of payload . . . . . . . . . . . . . . . . . 19
4.2.5. THREAT.IMG.LOCATION: The target device installs the
payload to the wrong location . . . . . . . . . . . . 19
4.2.6. THREAT.NET.REDIRECT: Redirection to inauthentic
payload hosting . . . . . . . . . . . . . . . . . . . 20
4.2.7. THREAT.NET.ONPATH: Traffic interception . . . . . . . 20
4.2.8. THREAT.IMG.REPLACE: Payload Replacement . . . . . . . 20
4.2.9. THREAT.IMG.NON_AUTH: Unauthenticated Images . . . . . 21
4.2.10. THREAT.UPD.WRONG_PRECURSOR: Unexpected Precursor
images . . . . . . . . . . . . . . . . . . . . . . . 21
4.2.11. THREAT.UPD.UNAPPROVED: Unapproved Firmware . . . . . 21
4.2.12. THREAT.IMG.DISCLOSURE: Reverse Engineering Of
Firmware Image for Vulnerability Analysis . . . . . . 23
4.2.13. THREAT.MFST.OVERRIDE: Overriding Critical Manifest
Elements . . . . . . . . . . . . . . . . . . . . . . 23
4.2.14. THREAT.MFST.EXPOSURE: Confidential Manifest Element
Exposure . . . . . . . . . . . . . . . . . . . . . . 24
4.2.15. THREAT.IMG.EXTRA: Extra data after image . . . . . . 24
4.2.16. THREAT.KEY.EXPOSURE: Exposure of signing keys . . . . 24
4.2.17. THREAT.MFST.MODIFICATION: Modification of manifest or
payload prior to signing . . . . . . . . . . . . . . 24
4.2.18. THREAT.MFST.TOCTOU: Modification of manifest between
authentication and use . . . . . . . . . . . . . . . 25
4.3. Security Requirements . . . . . . . . . . . . . . . . . . 25
4.3.1. REQ.SEC.SEQUENCE: Monotonic Sequence Numbers . . . . 25
4.3.2. REQ.SEC.COMPATIBLE: Vendor, Device-type Identifiers . 26
4.3.3. REQ.SEC.EXP: Expiration Time . . . . . . . . . . . . 26
4.3.4. REQ.SEC.AUTHENTIC: Cryptographic Authenticity . . . . 26
4.3.5. REQ.SEC.AUTH.IMG_TYPE: Authenticated Payload Type . . 27
4.3.6. Security Requirement REQ.SEC.AUTH.IMG_LOC:
Authenticated Storage Location . . . . . . . . . . . 27
4.3.7. REQ.SEC.AUTH.REMOTE_LOC: Authenticated Remote Payload 27
4.3.8. REQ.SEC.AUTH.EXEC: Secure Execution . . . . . . . . . 27
4.3.9. REQ.SEC.AUTH.PRECURSOR: Authenticated precursor
images . . . . . . . . . . . . . . . . . . . . . . . 28
4.3.10. REQ.SEC.AUTH.COMPATIBILITY: Authenticated Vendor and
Class IDs . . . . . . . . . . . . . . . . . . . . . . 28
4.3.11. REQ.SEC.RIGHTS: Rights Require Authenticity . . . . . 28
4.3.12. REQ.SEC.IMG.CONFIDENTIALITY: Payload Encryption . . . 28
4.3.13. REQ.SEC.ACCESS_CONTROL: Access Control . . . . . . . 29
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4.3.14. REQ.SEC.MFST.CONFIDENTIALITY: Encrypted Manifests . . 29
4.3.15. REQ.SEC.IMG.COMPLETE_DIGEST: Whole Image Digest . . . 29
4.3.16. REQ.SEC.REPORTING: Secure Reporting . . . . . . . . . 30
4.3.17. REQ.SEC.KEY.PROTECTION: Protected storage of signing
keys . . . . . . . . . . . . . . . . . . . . . . . . 30
4.3.18. REQ.SEC.MFST.CHECK: Validate manifests prior to
deployment . . . . . . . . . . . . . . . . . . . . . 30
4.3.19. REQ.SEC.MFST.TRUSTED: Construct manifests in a
trusted environment . . . . . . . . . . . . . . . . . 30
4.3.20. REQ.SEC.MFST.CONST: Manifest kept immutable between
check and use . . . . . . . . . . . . . . . . . . . . 31
4.4. User Stories . . . . . . . . . . . . . . . . . . . . . . 31
4.4.1. USER_STORY.INSTALL.INSTRUCTIONS: Installation
Instructions . . . . . . . . . . . . . . . . . . . . 31
4.4.2. USER_STORY.MFST.FAIL_EARLY: Fail Early . . . . . . . 31
4.4.3. USER_STORY.OVERRIDE: Override Non-Critical Manifest
Elements . . . . . . . . . . . . . . . . . . . . . . 32
4.4.4. USER_STORY.COMPONENT: Component Update . . . . . . . 32
4.4.5. USER_STORY.MULTI_AUTH: Multiple Authorizations . . . 32
4.4.6. USER_STORY.IMG.FORMAT: Multiple Payload Formats . . . 33
4.4.7. USER_STORY.IMG.CONFIDENTIALITY: Prevent Confidential
Information Disclosures . . . . . . . . . . . . . . . 33
4.4.8. USER_STORY.IMG.UNKNOWN_FORMAT: Prevent Devices from
Unpacking Unknown Formats . . . . . . . . . . . . . . 33
4.4.9. USER_STORY.IMG.CURRENT_VERSION: Specify Version
Numbers of Target Firmware . . . . . . . . . . . . . 33
4.4.10. USER_STORY.IMG.SELECT: Enable Devices to Choose
Between Images . . . . . . . . . . . . . . . . . . . 34
4.4.11. USER_STORY.EXEC.MFST: Secure Execution Using
Manifests . . . . . . . . . . . . . . . . . . . . . . 34
4.4.12. USER_STORY.EXEC.DECOMPRESS: Decompress on Load . . . 34
4.4.13. USER_STORY.MFST.IMG: Payload in Manifest . . . . . . 34
4.4.14. USER_STORY.MFST.PARSE: Simple Parsing . . . . . . . . 34
4.4.15. USER_STORY.MFST.DELEGATION: Delegated Authority in
Manifest . . . . . . . . . . . . . . . . . . . . . . 35
4.4.16. USER_STORY.MFST.PRE_CHECK: Update Evaluation . . . . 35
4.5. Usability Requirements . . . . . . . . . . . . . . . . . 35
4.5.1. REQ.USE.MFST.PRE_CHECK: Pre-Installation Checks . . . 35
4.5.2. REQ.USE.MFST.OVERRIDE_REMOTE: Override Remote
Resource Location . . . . . . . . . . . . . . . . . . 35
4.5.3. REQ.USE.MFST.COMPONENT: Component Updates . . . . . . 36
4.5.4. REQ.USE.MFST.MULTI_AUTH: Multiple authentications . . 37
4.5.5. REQ.USE.IMG.FORMAT: Format Usability . . . . . . . . 37
4.5.6. REQ.USE.IMG.NESTED: Nested Formats . . . . . . . . . 37
4.5.7. REQ.USE.IMG.VERSIONS: Target Version Matching . . . . 38
4.5.8. REQ.USE.IMG.SELECT: Select Image by Destination . . . 38
4.5.9. REQ.USE.EXEC: Executable Manifest . . . . . . . . . . 38
4.5.10. REQ.USE.LOAD: Load-Time Information . . . . . . . . . 38
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4.5.11. REQ.USE.PAYLOAD: Payload in Manifest Envelope . . . . 38
4.5.12. REQ.USE.PARSE: Simple Parsing . . . . . . . . . . . . 39
4.5.13. REQ.USE.DELEGATION: Delegation of Authority in
Manifest . . . . . . . . . . . . . . . . . . . . . . 40
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 40
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
7.1. Normative References . . . . . . . . . . . . . . . . . . 40
7.2. Informative References . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
Vulnerabilities with Internet of Things (IoT) devices have raised the
need for a reliable and secure firmware update mechanism that is also
suitable for constrained devices. Ensuring that devices function and
remain secure over their service life requires such an update
mechanism to fix vulnerabilities, to update configuration settings,
as well as adding new functionality.
One component of such a firmware update is a concise and machine-
processable meta-data document, or manifest, that describes the
firmware image(s) and offers appropriate protection. This document
describes the information that must be present in the manifest.
This document describes all the information elements required in a
manifest to secure firmware updates of IoT devices. Each information
element is motiviated by user stories and threats it aims to
mitigate. These threats and user stories are not intended to be an
exhaustive list of the threats against IoT devices, nor of the
possible user stories that describe how to conduct a firmware update.
Instead they are intended to describe the threats against firmware
updates in isolation and provide sufficient motivation to specify the
information elements that cover a wide range of user stories.
To distinguish information elements from their encoding and
serialization over the wire this document presents an information
model. RFC 3444 [RFC3444] describes the differences between
information and data models.
Because this document covers a wide range of user stories and a wide
range of threats, not all information elements apply to all
scenarios. As a result, various information elements are optional to
implement and optional to use, depending on which threats exist in a
particular domain of application and which user stories are important
for deployments.
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2. Requirements and Terminology
2.1. Requirements Notation
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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Unless otherwise stated these words apply to the design of the
manifest format, not its implementation or application. Hence,
whenever an information is declared as "REQUIRED" this implies that
the manifest format document has to include support for it.
2.2. Terminology
This document uses terms defined in [I-D.ietf-suit-architecture].
The term 'Operator' refers to both Device and Network Operator.
Secure time and secure clock refer to a set of requirements on time
sources. For local time sources, this primarily means that the clock
must be monotonically increasing, including across power cycles,
firmware updates, etc. For remote time sources, the provided time
must be guaranteed to be correct to within some predetermined bounds,
whenever the time source is accessible.
The term Envelope is used to describe an encoding that allows the
bundling of a manifest with related information elements that are not
directly contained within the manifest.
The term Payload is used to describe the data that is delivered to a
device during an update. This is distinct from a "firmware image",
as described in [I-D.ietf-suit-architecture], because the payload is
often in an intermediate state, such as being encrypted, compressed
and/or encoded as a differential update. The payload, taken in
isolation, is often not the final firmware image.
3. Manifest Information Elements
Each manifest information element is anchored in a security
requirement or a usability requirement. The manifest elements are
described below, justified by their requirements.
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3.1. Version ID of the Manifest Structure
An identifier that describes which iteration of the manifest format
is contained in the structure. This allows devices to identify the
version of the manifest data model that is in use.
This element is REQUIRED.
3.2. Monotonic Sequence Number
A monotonically increasing sequence number to prevent malicious
actors from reverting a firmware update against the policies of the
relevant authority.
For convenience, the monotonic sequence number may be a UTC
timestamp. This allows global synchronisation of sequence numbers
without any additional management.
This element is REQUIRED.
Implements: REQ.SEC.SEQUENCE (Section 4.3.1)
3.3. Vendor ID
The Vendor ID element helps to distinguish between identically named
products from different vendors. Vendor ID is not intended to be a
human-readable element. It is intended for binary match/mismatch
comparison only.
Recommended practice is to use [RFC4122] version 5 UUIDs with the
vendor's domain name and the DNS name space ID. Other options
include type 1 and type 4 UUIDs.
This element is RECOMMENDED.
Implements: REQ.SEC.COMPATIBLE (Section 4.3.2),
REQ.SEC.AUTH.COMPATIBILITY (Section 4.3.10).
Here is an example for a domain name-based UUID. Vendor A creates a
UUID based on a domain name it controls, such as vendorId =
UUID5(DNS, "vendor-a.example")
Because the DNS infrastructure prevents multiple registrations of the
same domain name, this UUID is (with very high probability)
guaranteed to be unique. Because the domain name is known, this UUID
is reproducible. Type 1 and type 4 UUIDs produce similar guarantees
of uniqueness, but not reproducibility.
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This approach creates a contention when a vendor changes its name or
relinquishes control of a domain name. In this scenario, it is
possible that another vendor would start using that same domain name.
However, this UUID is not proof of identity; a device's trust in a
vendor must be anchored in a cryptographic key, not a UUID.
3.4. Class ID
A device "Class" is a set of different device types that can accept
the same firmware update without modification. It thereby allows
devices to determine applicability of a firmware in an unambiguous
way. Class IDs must be unique within the scope of a Vendor ID. This
is to prevent similarly, or identically named devices colliding in
their customer's infrastructure.
Recommended practice is to use [RFC4122] version 5 UUIDs with as much
information as necessary to define firmware compatibility. Possible
information used to derive the class UUID includes:
o model name or number
o hardware revision
o runtime library version
o bootloader version
o ROM revision
o silicon batch number
The Class ID UUID should use the Vendor ID as the name space
identifier. Other options include version 1 and 4 UUIDs. Classes
may be more fine-grained granular than is required to identify
firmware compatibility. Classes must not be less granular than is
required to identify firmware compatibility. Devices may have
multiple Class IDs.
Class ID is not intended to be a human-readable element. It is
intended for binary match/mismatch comparison only.
If Class ID is not implemented, then each logical device class must
use a unique trust anchor for authorization.
This element is RECOMMENDED.
Implements: Security Requirement REQ.SEC.COMPATIBLE (Section 4.3.2),
REQ.SEC.AUTH.COMPATIBILITY (Section 4.3.10).
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3.4.1. Example 1: Different Classes
Vendor A creates product Z and product Y. The firmware images of
products Z and Y are not interchangeable. Vendor A creates UUIDs as
follows:
o vendorId = UUID5(DNS, "vendor-a.com")
o ZclassId = UUID5(vendorId, "Product Z")
o YclassId = UUID5(vendorId, "Product Y")
This ensures that Vendor A's Product Z cannot install firmware for
Product Y and Product Y cannot install firmware for Product Z.
3.4.2. Example 2: Upgrading Class ID
Vendor A creates product X. Later, Vendor A adds a new feature to
product X, creating product X v2. Product X requires a firmware
update to work with firmware intended for product X v2.
Vendor A creates UUIDs as follows:
o vendorId = UUID5(DNS, "vendor-a.com")
o XclassId = UUID5(vendorId, "Product X")
o Xv2classId = UUID5(vendorId, "Product X v2")
When product X receives the firmware update necessary to be
compatible with product X v2, part of the firmware update changes the
class ID to Xv2classId.
3.4.3. Example 3: Shared Functionality
Vendor A produces two products, product X and product Y. These
components share a common core (such as an operating system), but
have different applications. The common core and the applications
can be updated independently. To enable X and Y to receive the same
common core update, they require the same class ID. To ensure that
only product X receives application X and only product Y receives
application Y, product X and product Y must have different class IDs.
The vendor creates Class IDs as follows:
o vendorId = UUID5(DNS, "vendor-a.com")
o XclassId = UUID5(vendorId, "Product X")
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o YclassId = UUID5(vendorId, "Product Y")
o CommonClassId = UUID5(vendorId, "common core")
Product X matches against both XclassId and CommonClassId. Product Y
matches against both YclassId and CommonClassId.
3.4.4. Example 4: White-labelling
Vendor A creates a product A and its firmware. Vendor B sells the
product under its own name as Product B with some customised
configuration. The vendors create the Class IDs as follows:
o vendorIdA = UUID5(DNS, "vendor-a.com")
o classIdA = UUID5(vendorIdA, "Product A-Unlabelled")
o vendorIdB = UUID5(DNS, "vendor-b.com")
o classIdB = UUID5(vendorIdB, "Product B")
The product will match against each of these class IDs. If Vendor A
and Vendor B provide different components for the device, the
implementor may choose to make ID matching scoped to each component.
Then, the vendorIdA, classIdA match the component ID supplied by
Vendor A, and the vendorIdB, classIdB match the component ID supplied
by Vendor B.
3.5. Precursor Image Digest Condition
This element provides information about the payload that needs to be
present on the device for an update to apply. This may, for example,
be the case with differential updates.
This element is OPTIONAL.
Implements: REQ.SEC.AUTH.PRECURSOR (Section 4.3.9)
3.6. Required Image Version List
Payloads may only be applied to a specific firmware version or
firmware versions. For example, a payload containing a differential
update may be applied only to a specific firmware version.
When a payload applies to multiple versions of a firmware, the
required image version list specifies which firmware versions must be
present for the update to be applied. This allows the update author
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to target specific versions of firmware for an update, while
excluding those to which it should not or cannot be applied.
This element is OPTIONAL.
Implements: REQ.USE.IMG.VERSIONS (Section 4.5.7)
3.7. Expiration Time
This element tells a device the time at which the manifest expires
and should no longer be used. This element should be used where a
secure source of time is provided and firmware is intended to expire
predictably. This element may also be displayed (e.g. via an app)
for user confirmation since users typically have a reliable knowledge
of the date.
Special consideration is required for end-of-life if a firmware will
not be updated again, for example if a business stops issuing updates
to a device. In this case the last valid firmware should not have an
expiration time.
This element is OPTIONAL.
Implements: REQ.SEC.EXP (Section 4.3.3)
3.8. Payload Format
This element describes the payload format within the signed metadata.
It is used to enable devices to decode payloads correctly.
This element is REQUIRED.
Implements: REQ.SEC.AUTH.IMG_TYPE (Section 4.3.5), REQ.USE.IMG.FORMAT
(Section 4.5.5)
3.9. Processing Steps
A representation of the Processing Steps required to decode a
payload, in particular those that are compressed, packed, or
encrypted. The representation must describe which algorithms are
used and must convey any additional parameters required by those
algorithms.
A Processing Step may indicate the expected digest of the payload
after the processing is complete.
This element is RECOMMENDED.
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Implements: REQ.USE.IMG.NESTED (Section 4.5.6)
3.10. Storage Location
This element tells the device where to store a payload within a given
component. The device can use this to establish which permissions
are necessary and the physical storage location to use.
This element is REQUIRED.
Implements: REQ.SEC.AUTH.IMG_LOC (Section 4.3.6)
3.10.1. Example 1: Two Storage Locations
A device supports two components: an OS and an application. These
components can be updated independently, expressing dependencies to
ensure compatibility between the components. The Author chooses two
storage identifiers:
o "OS"
o "APP"
3.10.2. Example 2: File System
A device supports a full-featured filesystem. The Author chooses to
use the storage identifier as the path at which to install the
payload. The payload may be a tarball, in which case, it unpacks the
tarball into the specified path.
3.10.3. Example 3: Flash Memory
A device supports flash memory. The Author chooses to make the
storage identifier the offset where the image should be written.
3.11. Component Identifier
In a device with more than one storage subsystem, a storage
identifier is insufficient to identify where and how to store a
payload. To resolve this, a component identifier indicates to which
part of the storage subsystem the payload shall be placed.
A serialization may choose to combine Component Identifier and
Storage Location (Section 3.10).
This element is OPTIONAL.
Implements: REQ.USE.MFST.COMPONENT (Section 4.5.3)
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3.12. Payload Indicator
This element provides the information required for the device to
acquire the payload. This functionality is only needed when the
target device does not intrinsically know where to find the payload.
This can be encoded in several ways:
o One URI
o A list of URIs
o A prioritised list of URIs
o A list of signed URIs
This element is OPTIONAL.
Implements: REQ.SEC.AUTH.REMOTE_LOC (Section 4.3.7)
3.13. Payload Digests
This element contains one or more digests of one or more payloads.
This allows the target device to ensure authenticity of the
payload(s) when combined with the Signature (Section 3.15) element.
A manifest format must provide a mechanism to select one payload from
a list based on system parameters, such as Execute-In-Place
Installation Address.
This element is REQUIRED. Support for more than one digest is
OPTIONAL.
Implements: REQ.SEC.AUTHENTIC (Section 4.3.4), REQ.USE.IMG.SELECT
(Section 4.5.8)
3.14. Size
The size of the payload in bytes, which informs the target device how
big of a payload to expect. Without it, devices are exposed to some
classes of denial of service attack.
This element is REQUIRED.
Implements: REQ.SEC.AUTH.EXEC (Section 4.3.8)
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3.15. Manifest Envelope Element: Signature
The Signature element contains all the information necessary to
protect the contents of the manifest against modification and to
offer authentication of the signer. Because the Signature element
authenticates the manifest, it cannot be contained within the
manifest. Instead, the manifest is either contained within the
signature element, or the signature element is a member of the
Manifest Envelope and bundled with the manifest.
The Signature element represents the foundation of all security
properties of the manifest. Manifests, which are included as
dependencies by another manifests, should include a signature so that
the recipient can distinguish between different actors with different
permissions.
The Signature element must support multiple signers and multiple
signing algorithms. A manifest format may allow multiple manifests
to be covered by a single Signature element.
This element is REQUIRED in non-dependency manifests.
Implements: REQ.SEC.AUTHENTIC (Section 4.3.4), REQ.SEC.RIGHTS
(Section 4.3.11), REQ.USE.MFST.MULTI_AUTH (Section 4.5.4)
3.16. Additional Installation Instructions
Additional installation instructions are machine-readable commands
the device should execute when processing the manifest. This
information is distinct from the information necessary to process a
payload. Additional installation instructions include information
such as update timing (for example, install only on Sunday, at 0200),
procedural considerations (for example, shut down the equipment under
control before executing the update), pre- and post-installation
steps (for example, run a script). Other installation instructions
could include requesting user confirmation before installing.
This element is OPTIONAL.
Implements: REQ.USE.MFST.PRE_CHECK (Section 4.5.1)
3.17. Aliases
A mechanism for a manifest to augment or replace URIs or URI lists
defined by one or more of its dependencies.
This element is OPTIONAL.
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Implements: REQ.USE.MFST.OVERRIDE_REMOTE (Section 4.5.2)
3.18. Dependencies
A list of other manifests that are required by the current manifest.
Manifests are identified an unambiguous way, such as a cryptographic
digest.
This element is REQUIRED to support deployments that include both
multiple authorities and multiple payloads.
Implements: REQ.USE.MFST.COMPONENT (Section 4.5.3)
3.19. Encryption Wrapper
Encrypting firmware images requires symmetric content encryption
keys. The encryption wrapper provides the information needed for a
device to obtain or locate a key that it uses to decrypt the
firmware.
This element is REQUIRED for encrypted payloads.
Implements: REQ.SEC.IMG.CONFIDENTIALITY (Section 4.3.12)
3.20. XIP Address
In order to support execute in place (XIP) systems with multiple
possible base addresses, it is necessary to specify which address the
payload is linked for.
For example a microcontroller may have a simple bootloader that
chooses one of two images to boot. That microcontroller then needs
to choose one of two firmware images to install, based on which of
its two images is older.
This element is OPTIONAL.
Implements: REQ.USE.IMG.SELECT (Section 4.5.8)
3.21. Load-time Metadata
Load-time metadata provides the device with information that it needs
in order to load one or more images. This metadata may include any
of:
o the source
o the destination
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o cryptographic information
o decompression information
o unpacking information
Typically, loading is done by copying an image from its permanent
storage location into its active use location. The metadata allows
operations such as decryption, decompression, and unpacking to be
performed during that copy.
This element is OPTIONAL.
Implements: REQ.USE.LOAD (Section 4.5.10)
3.22. Run-time metadata
Run-time metadata provides the device with any extra information
needed to boot the device. This may include the entry-point of an
XIP image or the kernel command-line to boot a Linux image.
This element is OPTIONAL.
Implements: REQ.USE.EXEC (Section 4.5.9)
3.23. Payload
The Payload element is contained within the manifest or manifest
envelope and enables the manifest and payload to be delivered
simultaneously. This is used for delivering small payloads, such as
cryptographic keys or configuration data.
This element is OPTIONAL.
Implements: REQ.USE.PAYLOAD (Section 4.5.11)
3.24. Manifest Envelope Element: Delegation Chain
The delegation chain offers enhanced authorization functionality via
authorization tokens. Each token itself is protected and does not
require another layer of protection. Because the delegation chain is
needed to verify the signature, it must be placed in the Manifest
Envelope, rather than the Manifest.
This element is OPTIONAL.
Implements: REQ.USE.DELEGATION (Section 4.5.13)
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4. Security Considerations
The following sub-sections describe the threat model, user stories,
security requirements, and usability requirements. This section also
provides the motivations for each of the manifest information
elements.
Note that it is worthwhile to recall that a firmware update is, by
definition, remote code execution. Hence, if a device is configured
to trust an entity to provide firmware, it trusts this entity to do
the "right thing". Many classes of attacks can be mitigated by
verifying that a firmware update came from a trusted party and that
no rollback is taking place. However, if the trusted entity has been
compromised and distributes attacker-provided firmware to devices
then the possibilities for deference are limited.
4.1. Threat Model
The following sub-sections aim to provide information about the
threats that were considered, the security requirements that are
derived from those threats and the fields that permit implementation
of the security requirements. This model uses the S.T.R.I.D.E.
[STRIDE] approach. Each threat is classified according to:
o Spoofing identity
o Tampering with data
o Repudiation
o Information disclosure
o Denial of service
o Elevation of privilege
This threat model only covers elements related to the transport of
firmware updates. It explicitly does not cover threats outside of
the transport of firmware updates. For example, threats to an IoT
device due to physical access are out of scope.
4.2. Threat Descriptions
4.2.1. THREAT.IMG.EXPIRED: Old Firmware
Classification: Elevation of Privilege
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An attacker sends an old, but valid manifest with an old, but valid
firmware image to a device. If there is a known vulnerability in the
provided firmware image, this may allow an attacker to exploit the
vulnerability and gain control of the device.
Threat Escalation: If the attacker is able to exploit the known
vulnerability, then this threat can be escalated to ALL TYPES.
Mitigated by: REQ.SEC.SEQUENCE (Section 4.3.1)
4.2.2. THREAT.IMG.EXPIRED.OFFLINE : Offline device + Old Firmware
Classification: Elevation of Privilege
An attacker targets a device that has been offline for a long time
and runs an old firmware version. The attacker sends an old, but
valid manifest to a device with an old, but valid firmware image.
The attacker-provided firmware is newer than the installed one but
older than the most recently available firmware. If there is a known
vulnerability in the provided firmware image then this may allow an
attacker to gain control of a device. Because the device has been
offline for a long time, it is unaware of any new updates. As such
it will treat the old manifest as the most current.
The exact mitigation for this threat depends on where the threat
comes from. This requires careful consideration by the implementor.
If the threat is from a network actor, including an on-path attacker,
or an intruder into a management system, then a user confirmation can
mitigate this attack, simply by displaying an expiration date and
requesting confirmation. On the other hand, if the user is the
attacker, then an online confirmation system (for example a trusted
timestamp server) can be used as a mitigation system.
Threat Escalation: If the attacker is able to exploit the known
vulnerability, then this threat can be escalated to ALL TYPES.
Mitigated by: REQ.SEC.EXP (Section 4.3.3), REQ.USE.MFST.PRE_CHECK
(Section 4.5.1),
4.2.3. THREAT.IMG.INCOMPATIBLE: Mismatched Firmware
Classification: Denial of Service
An attacker sends a valid firmware image, for the wrong type of
device, signed by an actor with firmware installation permission on
both types of device. The firmware is verified by the device
positively because it is signed by an actor with the appropriate
permission. This could have wide-ranging consequences. For devices
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that are similar, it could cause minor breakage, or expose security
vulnerabilities. For devices that are very different, it is likely
to render devices inoperable.
Mitigated by: REQ.SEC.COMPATIBLE (Section 4.3.2)
For example, suppose that two vendors, Vendor A and Vendor B, adopt
the same trade name in different geographic regions, and they both
make products with the same names, or product name matching is not
used. This causes firmware from Vendor A to match devices from
Vendor B.
If the vendors are the firmware authorities, then devices from Vendor
A will reject images signed by Vendor B since they use different
credentials. However, if both devices trust the same Author, then,
devices from Vendor A could install firmware intended for devices
from Vendor B.
4.2.4. THREAT.IMG.FORMAT: The target device misinterprets the type of
payload
Classification: Denial of Service
If a device misinterprets the format of the firmware image, it may
cause a device to install a firmware image incorrectly. An
incorrectly installed firmware image would likely cause the device to
stop functioning.
Threat Escalation: An attacker that can cause a device to
misinterpret the received firmware image may gain elevation of
privilege and potentially expand this to all types of threat.
Mitigated by: REQ.SEC.AUTH.IMG_TYPE (Section 4.3.5)
4.2.5. THREAT.IMG.LOCATION: The target device installs the payload to
the wrong location
Classification: Denial of Service
If a device installs a firmware image to the wrong location on the
device, then it is likely to break. For example, a firmware image
installed as an application could cause a device and/or an
application to stop functioning.
Threat Escalation: An attacker that can cause a device to
misinterpret the received code may gain elevation of privilege and
potentially expand this to all types of threat.
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Mitigated by: REQ.SEC.AUTH.IMG_LOC (Section 4.3.6)
4.2.6. THREAT.NET.REDIRECT: Redirection to inauthentic payload hosting
Classification: Denial of Service
If a device is tricked into fetching a payload for an attacker
controlled site, the attacker may send corrupted payloads to devices.
Mitigated by: REQ.SEC.AUTH.REMOTE_LOC (Section 4.3.7)
4.2.7. THREAT.NET.ONPATH: Traffic interception
Classification: Spoofing Identity, Tampering with Data
An attacker intercepts all traffic to and from a device. The
attacker can monitor or modify any data sent to or received from the
device. This can take the form of: manifests, payloads, status
reports, and capability reports being modified or not delivered to
the intended recipient. It can also take the form of analysis of
data sent to or from the device, either in content, size, or
frequency.
Mitigated by: REQ.SEC.AUTHENTIC (Section 4.3.4),
REQ.SEC.IMG.CONFIDENTIALITY (Section 4.3.12), REQ.SEC.AUTH.REMOTE_LOC
(Section 4.3.7), REQ.SEC.MFST.CONFIDENTIALITY (Section 4.3.14),
REQ.SEC.REPORTING (Section 4.3.16)
4.2.8. THREAT.IMG.REPLACE: Payload Replacement
Classification: Elevation of Privilege
An attacker replaces a newly downloaded firmware after a device
finishes verifying a manifest. This could cause the device to
execute the attacker's code. This attack likely requires physical
access to the device. However, it is possible that this attack is
carried out in combination with another threat that allows remote
execution. This is a typical Time Of Check/Time Of Use (TICTOC)
attack.
Threat Escalation: If the attacker is able to exploit a known
vulnerability, or if the attacker can supply their own firmware, then
this threat can be escalated to ALL TYPES.
Mitigated by: REQ.SEC.AUTH.EXEC (Section 4.3.8)
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4.2.9. THREAT.IMG.NON_AUTH: Unauthenticated Images
Classification: Elevation of Privilege / All Types
If an attacker can install their firmware on a device, for example by
manipulating either payload or metadata, then they have complete
control of the device.
Mitigated by: REQ.SEC.AUTHENTIC (Section 4.3.4)
4.2.10. THREAT.UPD.WRONG_PRECURSOR: Unexpected Precursor images
Classification: Denial of Service / All Types
Modifications of payloads and metadata allow an attacker to introduce
a number of denial of service attacks. Below are some examples.
An attacker sends a valid, current manifest to a device that has an
unexpected precursor image. If a payload format requires a precursor
image (for example, delta updates) and that precursor image is not
available on the target device, it could cause the update to break.
An attacker that can cause a device to install a payload against the
wrong precursor image could gain elevation of privilege and
potentially expand this to all types of threat. However, it is
unlikely that a valid differential update applied to an incorrect
precursor would result in a functional, but vulnerable firmware.
Mitigated by: REQ.SEC.AUTH.PRECURSOR (Section 4.3.9)
4.2.11. THREAT.UPD.UNAPPROVED: Unapproved Firmware
Classification: Denial of Service, Elevation of Privilege
This threat can appear in several ways, however it is ultimately
about ensuring that devices retain the behaviour required by their
owner, or operator. The owner or operator of a device typically
requires that the device maintain certain features, functions,
capabilities, behaviours, or interoperability constraints (more
generally, behaviour). If these requirements are broken, then a
device will not fulfill its purpose. Therefore, if any party other
than the device's owner or the owner's contracted Device Operator has
the ability to modify device behaviour without approval, then this
constitutes an elevation of privilege.
Similarly, a Network Operator may require that devices behave in a
particular way in order to maintain the integrity of the network. If
devices behaviour on a network can be modified without the approval
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of the Network Operator, then this constitutes an elevation of
privilege with respect to the network.
For example, if the owner of a device has purchased that device
because of features A, B, and C, and a firmware update is issued by
the manufacturer, which removes feature A, then the device may not
fulfill the owner's requirements any more. In certain circumstances,
this can cause significantly greater threats. Suppose that feature A
is used to implement a safety-critical system, whether the
manufacturer intended this behaviour or not. When unapproved
firmware is installed, the system may become unsafe.
In a second example, the owner or operator of a system of two or more
interoperating devices needs to approve firmware for their system in
order to ensure interoperability with other devices in the system.
If the firmware is not qualified, the system as a whole may not work.
Therefore, if a device installs firmware without the approval of the
device owner or operator, this is a threat to devices or the system
as a whole.
Similarly, the operator of a network may need to approve firmware for
devices attached to the network in order to ensure favourable
operating conditions within the network. If the firmware is not
qualified, it may degrade the performance of the network. Therefore,
if a device installs firmware without the approval of the Network
Operator, this is a threat to the network itself.
Threat Escalation: If the firmware expects configuration that is
present in devices deployed in Network A, but not in devices deployed
in Network B, then the device may experience degraded security,
leading to threats of All Types.
Mitigated by: REQ.SEC.RIGHTS (Section 4.3.11), REQ.SEC.ACCESS_CONTROL
(Section 4.3.13)
4.2.11.1. Example 1: Multiple Network Operators with a Single Device
Operator
In this example, assume that Device Operators expect the rights to
create firmware but that Network Operators expect the rights to
qualify firmware as fit-for-purpose on their networks. Additionally,
assume that Device Operators manage devices that can be deployed on
any network, including Network A and B in our example.
An attacker may obtain a manifest for a device on Network A. Then,
this attacker sends that manifest to a device on Network B. Because
Network A and Network B are under control of different Operators, and
the firmware for a device on Network A has not been qualified to be
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deployed on Network B, the target device on Network B is now in
violation of the Operator B's policy and may be disabled by this
unqualified, but signed firmware.
This is a denial of service because it can render devices inoperable.
This is an elevation of privilege because it allows the attacker to
make installation decisions that should be made by the Operator.
4.2.11.2. Example 2: Single Network Operator with Multiple Device
Operators
Multiple devices that interoperate are used on the same network and
communicate with each other. Some devices are manufactured and
managed by Device Operator A and other devices by Device Operator B.
A new firmware is released by Device Operator A that breaks
compatibility with devices from Device Operator B. An attacker sends
the new firmware to the devices managed by Device Operator A without
approval of the Network Operator. This breaks the behaviour of the
larger system causing denial of service and possibly other threats.
Where the network is a distributed SCADA system, this could cause
misbehaviour of the process that is under control.
4.2.12. THREAT.IMG.DISCLOSURE: Reverse Engineering Of Firmware Image
for Vulnerability Analysis
Classification: All Types
An attacker wants to mount an attack on an IoT device. To prepare
the attack he or she retrieves the provided firmware image and
performs reverse engineering of the firmware image to analyze it for
specific vulnerabilities.
Mitigated by: REQ.SEC.IMG.CONFIDENTIALITY (Section 4.3.12)
4.2.13. THREAT.MFST.OVERRIDE: Overriding Critical Manifest Elements
Classification: Elevation of Privilege
An authorized actor, but not the Author, uses an override mechanism
(USER_STORY.OVERRIDE (Section 4.4.3)) to change an information
element in a manifest signed by the Author. For example, if the
authorized actor overrides the digest and URI of the payload, the
actor can replace the entire payload with a payload of their choice.
Threat Escalation: By overriding elements such as payload
installation instructions or firmware digest, this threat can be
escalated to all types.
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Mitigated by: REQ.SEC.ACCESS_CONTROL (Section 4.3.13)
4.2.14. THREAT.MFST.EXPOSURE: Confidential Manifest Element Exposure
Classification: Information Disclosure
A third party may be able to extract sensitive information from the
manifest.
Mitigated by: REQ.SEC.MFST.CONFIDENTIALITY (Section 4.3.14)
4.2.15. THREAT.IMG.EXTRA: Extra data after image
Classification: All Types
If a third party modifies the image so that it contains extra code
after a valid, authentic image, that third party can then use their
own code in order to make better use of an existing vulnerability.
Mitigated by: REQ.SEC.IMG.COMPLETE_DIGEST (Section 4.3.15)
4.2.16. THREAT.KEY.EXPOSURE: Exposure of signing keys
Classification: All Types
If a third party obtains a key or even indirect access to a key, for
example in an hardware security module (HSM), then they can perform
the same actions as the legitimate owner of the key. If the key is
trusted for firmware update, then the third party can perform
firmware updates as though they were the legitimate owner of the key.
For example, if manifest signing is performed on a server connected
to the internet, an attacker may compromise the server and then be
able to sign manifests, even if the keys for manifest signing are
held in an HSM that is accessed by the server.
Mitigated by: REQ.SEC.KEY.PROTECTION (Section 4.3.17)
4.2.17. THREAT.MFST.MODIFICATION: Modification of manifest or payload
prior to signing
Classification: All Types
If an attacker can alter a manifest or payload before it is signed,
they can perform all the same actions as the manifest author. This
allows the attacker to deploy firmware updates to any devices that
trust the manifest author. If an attacker can modify the code of a
payload before the corresponding manifest is created, they can insert
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their own code. If an attacker can modify the manifest before it is
signed, they can redirect the manifest to their own payload.
For example, the attacker deploys malware to the developer's computer
or signing service that watches manifest creation activities and
inserts code into any binary that is referenced by a manifest.
For example, the attacker deploys malware to the developer's computer
or signing service that replaces the referenced binary (digest) and
URI with the attacker's binary (digest) and URI.
Mitigated by: REQ.SEC.MFST.CHECK (Section 4.3.18),
REQ.SEC.MFST.TRUSTED (Section 4.3.19)
4.2.18. THREAT.MFST.TOCTOU: Modification of manifest between
authentication and use
Classification: All Types
If an attacker can modify a manifest after it is authenticated (Time
Of Check) but before it is used (Time Of Use), then the attacker can
place any content whatsoever in the manifest.
Mitigated by: REQ.SEC.MFST.CONST (Section 4.3.20)
4.3. Security Requirements
The security requirements here are a set of policies that mitigate
the threats described in Section 4.1.
4.3.1. REQ.SEC.SEQUENCE: Monotonic Sequence Numbers
Only an actor with firmware installation authority is permitted to
decide when device firmware can be installed. To enforce this rule,
manifests MUST contain monotonically increasing sequence numbers.
Manifests may use UTC epoch timestamps to coordinate monotonically
increasing sequence numbers across many actors in many locations. If
UTC epoch timestamps are used, they must not be treated as times,
they must be treated only as sequence numbers. Devices must reject
manifests with sequence numbers smaller than any onboard sequence
number, i.e. there is no sequence number roll over.
Note: This is not a firmware version field. It is a manifest
sequence number. A firmware version may be rolled back by creating a
new manifest for the old firmware version with a later sequence
number.
Mitigates: THREAT.IMG.EXPIRED (Section 4.2.1)
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Implemented by: Monotonic Sequence Number (Section 3.2)
4.3.2. REQ.SEC.COMPATIBLE: Vendor, Device-type Identifiers
Devices MUST only apply firmware that is intended for them. Devices
must know that a given update applies to their vendor, model,
hardware revision, and software revision. Human-readable identifiers
are often error-prone in this regard, so unique identifiers should be
used instead.
Mitigates: THREAT.IMG.INCOMPATIBLE (Section 4.2.3)
Implemented by: Vendor ID Condition (Section 3.3), Class ID Condition
(Section 3.4)
4.3.3. REQ.SEC.EXP: Expiration Time
A firmware manifest MAY expire after a given time and devices may
have a secure clock (local or remote). If a secure clock is provided
and the Firmware manifest has an expiration timestamp, the device
must reject the manifest if current time is later than the expiration
time.
Special consideration is required for end-of-life in case device will
not be updated again, for example if a business stops issuing updates
for a device. The last valid firmware should not have an expiration
time.
Mitigates: THREAT.IMG.EXPIRED.OFFLINE (Section 4.2.2)
Implemented by: Expiration Time (Section 3.7)
4.3.4. REQ.SEC.AUTHENTIC: Cryptographic Authenticity
The authenticity of an update MUST be demonstrable. Typically, this
means that updates must be digitally signed. Because the manifest
contains information about how to install the update, the manifest's
authenticity must also be demonstrable. To reduce the overhead
required for validation, the manifest contains the cryptographic
digest of the firmware image, rather than a second digital signature.
The authenticity of the manifest can be verified with a digital
signature or Message Authentication Code. The authenticity of the
firmware image is tied to the manifest by the use of a cryptographic
digest of the firmware image.
Mitigates: THREAT.IMG.NON_AUTH (Section 4.2.9), THREAT.NET.ONPATH
(Section 4.2.7)
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Implemented by: Signature (Section 3.15), Payload Digest
(Section 3.13)
4.3.5. REQ.SEC.AUTH.IMG_TYPE: Authenticated Payload Type
The type of payload MUST be authenticated. For example, the target
must know whether the payload is XIP firmware, a loadable module, or
configuration data.
Mitigates: THREAT.IMG.FORMAT (Section 4.2.4)
Implemented by: Payload Format (Section 3.8), Signature
(Section 3.15)
4.3.6. Security Requirement REQ.SEC.AUTH.IMG_LOC: Authenticated Storage
Location
The location on the target where the payload is to be stored MUST be
authenticated.
Mitigates: THREAT.IMG.LOCATION (Section 4.2.5)
Implemented by: Storage Location (Section 3.10)
4.3.7. REQ.SEC.AUTH.REMOTE_LOC: Authenticated Remote Payload
The location where a target should find a payload MUST be
authenticated. Remote resources need to receive an equal amount of
cryptographic protection as the manifest itself, when dereferencing
URIs. The security considerations of Uniform Resource Identifiers
(URIs) are applicable [RFC3986].
Mitigates: THREAT.NET.REDIRECT (Section 4.2.6), THREAT.NET.ONPATH
(Section 4.2.7)
Implemented by: Payload Indicator (Section 3.12)
4.3.8. REQ.SEC.AUTH.EXEC: Secure Execution
The target SHOULD verify firmware at time of boot. This requires
authenticated payload size, and digest.
Mitigates: THREAT.IMG.REPLACE (Section 4.2.8)
Implemented by: Payload Digest (Section 3.13), Size (Section 3.14)
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4.3.9. REQ.SEC.AUTH.PRECURSOR: Authenticated precursor images
If an update uses a differential compression method, it MUST specify
the digest of the precursor image and that digest MUST be
authenticated.
Mitigates: THREAT.UPD.WRONG_PRECURSOR (Section 4.2.10)
Implemented by: Precursor Image Digest (Section 3.5)
4.3.10. REQ.SEC.AUTH.COMPATIBILITY: Authenticated Vendor and Class IDs
The identifiers that specify firmware compatibility MUST be
authenticated to ensure that only compatible firmware is installed on
a target device.
Mitigates: THREAT.IMG.INCOMPATIBLE (Section 4.2.3)
Implemented By: Vendor ID Condition (Section 3.3), Class ID Condition
(Section 3.4)
4.3.11. REQ.SEC.RIGHTS: Rights Require Authenticity
If a device grants different rights to different actors, exercising
those rights MUST be accompanied by proof of those rights, in the
form of proof of authenticity. Authenticity mechanisms, such as
those required in REQ.SEC.AUTHENTIC (Section 4.3.4), can be used to
prove authenticity.
For example, if a device has a policy that requires that firmware
have both an Authorship right and a Qualification right and if that
device grants Authorship and Qualification rights to different
parties, such as a Device Operator and a Network Operator,
respectively, then the firmware cannot be installed without proof of
rights from both the Device Operator and the Network Operator.
Mitigates: THREAT.UPD.UNAPPROVED (Section 4.2.11)
Implemented by: Signature (Section 3.15)
4.3.12. REQ.SEC.IMG.CONFIDENTIALITY: Payload Encryption
The manifest information model MUST enable encrypted payloads.
Encryption helps to prevent third parties, including attackers, from
reading the content of the firmware image. This can protect against
confidential information disclosures and discovery of vulnerabilities
through reverse engineering. Therefore the manifest must convey the
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information required to allow an intended recipient to decrypt an
encrypted payload.
Mitigates: THREAT.IMG.DISCLOSURE (Section 4.2.12), THREAT.NET.ONPATH
(Section 4.2.7)
Implemented by: Encryption Wrapper (Section 3.19)
4.3.13. REQ.SEC.ACCESS_CONTROL: Access Control
If a device grants different rights to different actors, then an
exercise of those rights MUST be validated against a list of rights
for the actor. This typically takes the form of an Access Control
List (ACL). ACLs are applied to two scenarios:
1. An ACL decides which elements of the manifest may be overridden
and by which actors.
2. An ACL decides which component identifier/storage identifier
pairs can be written by which actors.
Mitigates: THREAT.MFST.OVERRIDE (Section 4.2.13),
THREAT.UPD.UNAPPROVED (Section 4.2.11)
Implemented by: Client-side code, not specified in manifest.
4.3.14. REQ.SEC.MFST.CONFIDENTIALITY: Encrypted Manifests
A manifest format MUST allow encryption of selected parts of the
manifest or encryption of the entire manifest to prevent sensitive
content of the firmware metadata to be leaked.
Mitigates: THREAT.MFST.EXPOSURE (Section 4.2.14), THREAT.NET.ONPATH
(Section 4.2.7)
Implemented by: Manifest Encryption Wrapper / Transport Security
4.3.15. REQ.SEC.IMG.COMPLETE_DIGEST: Whole Image Digest
The digest SHOULD cover all available space in a fixed-size storage
location. Variable-size storage locations MUST be restricted to
exactly the size of deployed payload. This prevents any data from
being distributed without being covered by the digest. For example,
XIP microcontrollers typically have fixed-size storage. These
devices should deploy a digest that covers the deployed firmware
image, concatenated with the default erased value of any remaining
space.
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Mitigates: THREAT.IMG.EXTRA (Section 4.2.15)
Implemented by: Payload Digests (Section 3.13)
4.3.16. REQ.SEC.REPORTING: Secure Reporting
Status reports from the device to any remote system MUST be performed
over an authenticated, confidential channel in order to prevent
modification or spoofing of the reports.
Mitigates: THREAT.NET.ONPATH (Section 4.2.7)
4.3.17. REQ.SEC.KEY.PROTECTION: Protected storage of signing keys
Cryptographic keys for signing/authenticating manifests SHOULD be
stored in a manner that is inaccessible to networked devices, for
example in an HSM, or an air-gapped computer. This protects against
an attacker obtaining the keys.
Keys SHOULD be stored in a way that limits the risk of a legitimate,
but compromised, entity (such as a server or developer computer)
issuing signing requests.
Mitigates: THREAT.KEY.EXPOSURE (Section 4.2.16)
4.3.18. REQ.SEC.MFST.CHECK: Validate manifests prior to deployment
Manifests SHOULD be verified prior to deployment. This reduces
problems that may arise with devices installing firmware images that
damage devices unintentionally.
Mitigates: THREAT.MFST.MODIFICATION (Section 4.2.17)
4.3.19. REQ.SEC.MFST.TRUSTED: Construct manifests in a trusted
environment
For high risk deployments, such as large numbers of devices or
critical function devices, manifests SHOULD be constructed in an
environment that is protected from interference, such as an air-
gapped computer. Note that a networked computer connected to an HSM
does not fulfill this requirement (see THREAT.MFST.MODIFICATION
(Section 4.2.17)).
Mitigates: THREAT.MFST.MODIFICATION (Section 4.2.17)
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4.3.20. REQ.SEC.MFST.CONST: Manifest kept immutable between check and
use
Both the manifest and any data extracted from it MUST be held
immutable between its authenticity verification (time of check) and
its use (time of use). To make this guarantee, the manifest MUST fit
within an internal memory or a secure memory, such as encrypted
memory. The recipient SHOULD defend the manifest from tampering by
code or hardware resident in the recipient, for example other
processes or debuggers.
If an application requires that the manifest is verified before
storing it, then this means the manifest MUST fit in RAM.
Mitigates: THREAT.MFST.TOCTOU (Section 4.2.18)
4.4. User Stories
User stories provide expected use cases. These are used to feed into
usability requirements.
4.4.1. USER_STORY.INSTALL.INSTRUCTIONS: Installation Instructions
As a Device Operator, I want to provide my devices with additional
installation instructions so that I can keep process details out of
my payload data.
Some installation instructions might be:
o Use a table of hashes to ensure that each block of the payload is
validated before writing.
o Do not report progress.
o Pre-cache the update, but do not install.
o Install the pre-cached update matching this manifest.
o Install this update immediately, overriding any long-running
tasks.
Satisfied by: REQ.USE.MFST.PRE_CHECK (Section 4.5.1)
4.4.2. USER_STORY.MFST.FAIL_EARLY: Fail Early
As a designer of a resource-constrained IoT device, I want bad
updates to fail as early as possible to preserve battery life and
limit consumed bandwidth.
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Satisfied by: REQ.USE.MFST.PRE_CHECK (Section 4.5.1)
4.4.3. USER_STORY.OVERRIDE: Override Non-Critical Manifest Elements
As a Device Operator, I would like to be able to override the non-
critical information in the manifest so that I can control my devices
more precisely. The authority to override this information is
provided via the installation of a limited trust anchor by another
authority.
Some examples of potentially overridable information:
o URIs (Section 3.12): this allows the Device Operator to direct
devices to their own infrastructure in order to reduce network
load.
o Conditions: this allows the Device Operator to pose additional
constraints on the installation of the manifest.
o Directives (Section 3.16): this allows the Device Operator to add
more instructions such as time of installation.
o Processing Steps (Section 3.9): If an intermediary performs an
action on behalf of a device, it may need to override the
processing steps. It is still possible for a device to verify the
final content and the result of any processing step that specifies
a digest. Some processing steps should be non-overridable.
Satisfied by: REQ.USE.MFST.COMPONENT (Section 4.5.3)
4.4.4. USER_STORY.COMPONENT: Component Update
As a Device Operator, I want to divide my firmware into components,
so that I can reduce the size of updates, make different parties
responsible for different components, and divide my firmware into
frequently updated and infrequently updated components.
Satisfied by: REQ.USE.MFST.COMPONENT (Section 4.5.3)
4.4.5. USER_STORY.MULTI_AUTH: Multiple Authorizations
As a Device Operator, I want to ensure the quality of a firmware
update before installing it, so that I can ensure interoperability of
all devices in my product family. I want to restrict the ability to
make changes to my devices to require my express approval.
Satisfied by: REQ.USE.MFST.MULTI_AUTH (Section 4.5.4),
REQ.SEC.ACCESS_CONTROL (Section 4.3.13)
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4.4.6. USER_STORY.IMG.FORMAT: Multiple Payload Formats
As a Device Operator, I want to be able to send multiple payload
formats to suit the needs of my update, so that I can optimise the
bandwidth used by my devices.
Satisfied by: REQ.USE.IMG.FORMAT (Section 4.5.5)
4.4.7. USER_STORY.IMG.CONFIDENTIALITY: Prevent Confidential Information
Disclosures
As a firmware author, I want to prevent confidential information in
the manifest from being disclosed when distributing manifests and
firmware images. Confidential information may include information
about the device these updates are being applied to as well as
information in the firmware image itself.
Satisfied by: REQ.SEC.IMG.CONFIDENTIALITY (Section 4.3.12)
4.4.8. USER_STORY.IMG.UNKNOWN_FORMAT: Prevent Devices from Unpacking
Unknown Formats
As a Device Operator, I want devices to determine whether they can
process a payload prior to downloading it.
In some cases, it may be desirable for a third party to perform some
processing on behalf of a target. For this to occur, the third party
MUST indicate what processing occurred and how to verify it against
the Trust Provisioning Authority's intent.
This amounts to overriding Processing Steps (Section 3.9) and Payload
Indicator (Section 3.12).
Satisfied by: REQ.USE.IMG.FORMAT (Section 4.5.5), REQ.USE.IMG.NESTED
(Section 4.5.6), REQ.USE.MFST.OVERRIDE_REMOTE (Section 4.5.2)
4.4.9. USER_STORY.IMG.CURRENT_VERSION: Specify Version Numbers of
Target Firmware
As a Device Operator, I want to be able to target devices for updates
based on their current firmware version, so that I can control which
versions are replaced with a single manifest.
Satisfied by: REQ.USE.IMG.VERSIONS (Section 4.5.7)
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4.4.10. USER_STORY.IMG.SELECT: Enable Devices to Choose Between Images
As a developer, I want to be able to sign two or more versions of my
firmware in a single manifest so that I can use a very simple
bootloader that chooses between two or more images that are executed
in-place.
Satisfied by: REQ.USE.IMG.SELECT (Section 4.5.8)
4.4.11. USER_STORY.EXEC.MFST: Secure Execution Using Manifests
As a signer for both secure execution/boot and firmware deployment, I
would like to use the same signed document for both tasks so that my
data size is smaller, I can share common code, and I can reduce
signature verifications.
Satisfied by: REQ.USE.EXEC (Section 4.5.9)
4.4.12. USER_STORY.EXEC.DECOMPRESS: Decompress on Load
As a developer of firmware for a run-from-RAM device, I would like to
use compressed images and to indicate to the bootloader that I am
using a compressed image in the manifest so that it can be used with
secure execution/boot.
Satisfied by: REQ.USE.LOAD (Section 4.5.10)
4.4.13. USER_STORY.MFST.IMG: Payload in Manifest
As an operator of devices on a constrained network, I would like the
manifest to be able to include a small payload in the same packet so
that I can reduce network traffic.
Small payloads may include, for example, wrapped content encryption
keys, configuration information, public keys, authorization tokens,
or X.509 certificates.
Satisfied by: REQ.USE.PAYLOAD (Section 4.5.11)
4.4.14. USER_STORY.MFST.PARSE: Simple Parsing
As a developer for constrained devices, I want a low complexity
library for processing updates so that I can fit more application
code on my device.
Satisfied by: REQ.USE.PARSE (Section 4.5.12)
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4.4.15. USER_STORY.MFST.DELEGATION: Delegated Authority in Manifest
As a Device Operator that rotates delegated authority more often than
delivering firmware updates, I would like to delegate a new authority
when I deliver a firmware update so that I can accomplish both tasks
in a single transmission.
Satisfied by: REQ.USE.DELEGATION (Section 4.5.13)
4.4.16. USER_STORY.MFST.PRE_CHECK: Update Evaluation
As an operator of a constrained network, I would like devices on my
network to be able to evaluate the suitability of an update prior to
initiating any large download so that I can prevent unnecessary
consumption of bandwidth.
Satisfied by: REQ.USE.MFST.PRE_CHECK (Section 4.5.1)
4.5. Usability Requirements
The following usability requirements satisfy the user stories listed
above.
4.5.1. REQ.USE.MFST.PRE_CHECK: Pre-Installation Checks
A manifest format MUST be able to carry all information required to
process an update.
For example: Information about which precursor image is required for
a differential update must be placed in the manifest.
Satisfies: [USER_STORY.MFST.PRE_CHECK(#user-story-mfst-pre-check),
USER_STORY.INSTALL.INSTRUCTIONS (Section 4.4.1)
Implemented by: Additional installation instructions (Section 3.16)
4.5.2. REQ.USE.MFST.OVERRIDE_REMOTE: Override Remote Resource Location
A manifest format MUST be able to redirect payload fetches. This
applies where two manifests are used in conjunction. For example, a
Device Operator creates a manifest specifying a payload and signs it,
and provides a URI for that payload. A Network Operator creates a
second manifest, with a dependency on the first. They use this
second manifest to override the URIs provided by the Device Operator,
directing them into their own infrastructure instead. Some devices
may provide this capability, while others may only look at canonical
sources of firmware. For this to be possible, the device must fetch
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the payload, whereas a device that accepts payload pushes will ignore
this feature.
Satisfies: USER_STORY.OVERRIDE (Section 4.4.3)
Implemented by: Aliases (Section 3.17)
4.5.3. REQ.USE.MFST.COMPONENT: Component Updates
A manifest format MUST be able to express the requirement to install
one or more payloads from one or more authorities so that a multi-
payload update can be described. This allows multiple parties with
different permissions to collaborate in creating a single update for
the IoT device, across multiple components.
This requirement implies that it must be possible to construct a tree
of manifests on a multi-image target.
In order to enable devices with a heterogeneous storage architecture,
the manifest must enable specification of both storage system and the
storage location within that storage system.
Satisfies: USER_STORY.OVERRIDE (Section 4.4.3), USER_STORY.COMPONENT
(Section 4.4.4)
Implemented by: Dependencies, StorageIdentifier, ComponentIdentifier
4.5.3.1. Example 1: Multiple Microcontrollers
An IoT device with multiple microcontrollers in the same physical
device will likely require multiple payloads with different component
identifiers.
4.5.3.2. Example 2: Code and Configuration
A firmware image can be divided into two payloads: code and
configuration. These payloads may require authorizations from
different actors in order to install (see REQ.SEC.RIGHTS
(Section 4.3.11) and REQ.SEC.ACCESS_CONTROL (Section 4.3.13)). This
structure means that multiple manifests may be required, with a
dependency structure between them.
4.5.3.3. Example 3: Multiple Software Modules
A firmware image can be divided into multiple functional blocks for
separate testing and distribution. This means that code would need
to be distributed in multiple payloads. For example, this might be
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desirable in order to ensure that common code between devices is
identical in order to reduce distribution bandwidth.
4.5.4. REQ.USE.MFST.MULTI_AUTH: Multiple authentications
A manifest format MUST be able to carry multiple signatures so that
authorizations from multiple parties with different permissions can
be required in order to authorize installation of a manifest.
Satisfies: USER_STORY.MULTI_AUTH (Section 4.4.5)
Implemented by: Signature (Section 3.15)
4.5.5. REQ.USE.IMG.FORMAT: Format Usability
The manifest format MUST accommodate any payload format that an
Operator wishes to use. This enables the recipient to detect which
format the Operator has chosen. Some examples of payload format are:
o Binary
o Executable and Linkable Format (ELF)
o Differential
o Compressed
o Packed configuration
o Intel HEX
o Motorola S-Record
Satisfies: USER_STORY.IMG.FORMAT (Section 4.4.6)
USER_STORY.IMG.UNKNOWN_FORMAT (Section 4.4.8)
Implemented by: Payload Format (Section 3.8)
4.5.6. REQ.USE.IMG.NESTED: Nested Formats
The manifest format MUST accommodate nested formats, announcing to
the target device all the nesting steps and any parameters used by
those steps.
Satisfies: USER_STORY.IMG.CONFIDENTIALITY (Section 4.4.7)
Implemented by: Processing Steps (Section 3.9)
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4.5.7. REQ.USE.IMG.VERSIONS: Target Version Matching
The manifest format MUST provide a method to specify multiple version
numbers of firmware to which the manifest applies, either with a list
or with range matching.
Satisfies: USER_STORY.IMG.CURRENT_VERSION (Section 4.4.9)
Implemented by: Required Image Version List (Section 3.6)
4.5.8. REQ.USE.IMG.SELECT: Select Image by Destination
The manifest format MUST provide a mechanism to list multiple
equivalent payloads by Execute-In-Place Installation Address,
including the payload digest and, optionally, payload URIs.
Satisfies: USER_STORY.IMG.SELECT (Section 4.4.10)
Implemented by: XIP Address (Section 3.20)
4.5.9. REQ.USE.EXEC: Executable Manifest
The manifest format MUST allow to describe an executable system with
a manifest on both Execute-In-Place microcontrollers and on complex
operating systems. In addition, the manifest format MUST be able to
express metadata, such as a kernel command-line, used by any loader
or bootloader.
Satisfies: USER_STORY.EXEC.MFST (Section 4.4.11)
Implemented by: Run-time metadata (Section 3.22)
4.5.10. REQ.USE.LOAD: Load-Time Information
The manifest format MUST enable carrying additional metadata for load
time processing of a payload, such as cryptographic information,
load-address, and compression algorithm. Note that load comes before
execution/boot.
Satisfies: USER_STORY.EXEC.DECOMPRESS (Section 4.4.12)
Implemented by: Load-time metadata (Section 3.21)
4.5.11. REQ.USE.PAYLOAD: Payload in Manifest Envelope
The manifest format MUST allow placing a payload in the same
structure as the manifest. This may place the payload in the same
packet as the manifest.
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Integrated payloads may include, for example, binaries as well as
configuration information, and keying material.
When an integrated payload is provided, this increases the size of
the manifest. Manifest size can cause several processing and storage
concerns that require careful consideration. The payload can prevent
the whole manifest from being contained in a single network packet,
which can cause fragmentation and the loss of portions of the
manifest in lossy networks. This causes the need for reassembly and
retransmission logic. The manifest MUST be held immutable between
verification and processing (see REQ.SEC.MFST.CONST
(Section 4.3.20)), so a larger manifest will consume more memory with
immutability guarantees, for example internal RAM or NVRAM, or
external secure memory. If the manifest exceeds the available
immutable memory, then it MUST be processed modularly, evaluating
each of: delegation chains, the security container, and the actual
manifest, which includes verifying the integrated payload. If the
security model calls for downloading the manifest and validating it
before storing to NVRAM in order to prevent wear to NVRAM and energy
expenditure in NVRAM, then either increasing memory allocated to
manifest storage or modular processing of the received manifest may
be required. While the manifest has been organised to enable this
type of processing, it creates additional complexity in the parser.
If the manifest is stored in NVRAM prior to processing, the
integrated payload may cause the manifest to exceed the available
storage. Because the manifest is received prior to validation of
applicability, authority, or correctness, integrated payloads cause
the recipient to expend network bandwidth and energy that may not be
required if the manifest is discarded and these costs vary with the
size of the integrated payload.
See also: REQ.SEC.MFST.CONST (Section 4.3.20).
Satisfies: USER_STORY.MFST.IMG (Section 4.4.13)
Implemented by: Payload (Section 3.23)
4.5.12. REQ.USE.PARSE: Simple Parsing
The structure of the manifest MUST be simple to parse to reduce the
attack vectors against manifest parsers.
Satisfies: USER_STORY.MFST.PARSE (Section 4.4.14)
Implemented by: N/A
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4.5.13. REQ.USE.DELEGATION: Delegation of Authority in Manifest
A manifest format MUST enable the delivery of delegation information.
This information delivers a new key with which the recipient can
verify the manifest.
Satisfies: USER_STORY.MFST.DELEGATION (Section 4.4.15)
Implemented by: Delegation Chain (Section 3.24)
5. IANA Considerations
This document does not require any actions by IANA.
6. Acknowledgements
We would like to thank our working group chairs, Dave Thaler, Russ
Housley and David Waltermire, for their review comments and their
support.
We would like to thank the participants of the 2018 Berlin SUIT
Hackathon and the June 2018 virtual design team meetings for their
discussion input. In particular, we would like to thank Koen
Zandberg, Emmanuel Baccelli, Carsten Bormann, David Brown, Markus
Gueller, Frank Audun Kvamtro, Oyvind Ronningstad, Michael Richardson,
Jan-Frederik Rieckers, Francisco Acosta, Anton Gerasimov, Matthias
Waehlisch, Max Groening, Daniel Petry, Gaetan Harter, Ralph Hamm,
Steve Patrick, Fabio Utzig, Paul Lambert, Benjamin Kaduk, Said
Gharout, and Milen Stoychev.
We would like to thank those who contributed to the development of
this information model. In particular, we would like to thank
Milosch Meriac, Jean-Luc Giraud, Dan Ros, Amyas Philips, and Gary
Thomson.
Finally, we would like to thank the following IESG members for their
review feedback: Erik Kline, Murray Kucherawy, Barry Leiba, Alissa
Cooper, Stephen Farrell and Benjamin Kaduk.
7. References
7.1. Normative References
[I-D.ietf-suit-architecture]
Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things",
draft-ietf-suit-architecture-15 (work in progress),
January 2021.
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[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>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[RFC3444] Pras, A. and J. Schoenwaelder, "On the Difference between
Information Models and Data Models", RFC 3444,
DOI 10.17487/RFC3444, January 2003,
<https://www.rfc-editor.org/info/rfc3444>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[STRIDE] Microsoft, "The STRIDE Threat Model", May 2018,
<https://msdn.microsoft.com/en-us/library/
ee823878(v=cs.20).aspx>.
Authors' Addresses
Brendan Moran
Arm Limited
EMail: Brendan.Moran@arm.com
Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@gmx.net
Henk Birkholz
Fraunhofer SIT
EMail: henk.birkholz@sit.fraunhofer.de
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