wdiff rfc9124v1.txt rfc9124.txt

Internet Engineering Task Force (IETF) B. Moran
Request for Comments: 9124 H. Tschofenig
Category: Informational Arm Limited
ISSN: 2070-1721 H. Birkholz
Fraunhofer SIT
September 2021
January 2022

A Manifest Information Model for Firmware Updates in Internet of Things
(IoT) Devices

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 lifetime requires such an update
mechanism to fix vulnerabilities, update configuration settings, and
add new functionality.

One component of such a firmware update is a concise and machine-
processable metadata 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 document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9124.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Simplified Revised BSD License.

Table of Contents

1. Introduction
2. Requirements and Terminology
2.1. Requirements Notation
2.2. Terminology
3. Manifest Information Elements
3.1. Version ID of the Manifest Structure
3.2. Monotonic Sequence Number
3.3. Vendor ID
3.4. Class ID
3.4.1. Example 1: Different Classes
3.4.2. Example 2: Upgrading Class ID
3.4.3. Example 3: Shared Functionality
3.4.4. Example 4: White-Labelling Rebranding
3.5. Precursor Image Digest Condition
3.6. Required Image Version List
3.7. Expiration Time
3.8. Payload Format
3.9. Processing Steps
3.10. Storage Location
3.10.1. Example 1: Two Storage Locations
3.10.2. Example 2: File System Filesystem
3.10.3. Example 3: Flash Memory
3.11. Component Identifier
3.12. Payload Indicator
3.13. Payload Digests
3.14. Size
3.15. Manifest Envelope Element: Signature
3.16. Additional Installation Instructions
3.17. Manifest Text Information
3.18. Aliases
3.19. Dependencies
3.20. Encryption Wrapper
3.21. XIP Address
3.22. Load-Time Metadata
3.23. Run-Time Runtime Metadata
3.24. Payload
3.25. Manifest Envelope Element: Delegation Chain
4. Security Considerations
4.1. Threat Model
4.2. Threat Descriptions
4.2.1. THREAT.IMG.EXPIRED: Old Firmware
4.2.2. THREAT.IMG.EXPIRED.OFFLINE: Offline Device + Old
Firmware
4.2.3. THREAT.IMG.INCOMPATIBLE: Mismatched Firmware
4.2.4. THREAT.IMG.FORMAT: The Target Device Misinterprets the
Type of Payload
4.2.5. THREAT.IMG.LOCATION: The Target Device Installs the
Payload to the Wrong Location
4.2.6. THREAT.NET.REDIRECT: Redirection to Inauthentic Payload
Hosting
4.2.7. THREAT.NET.ONPATH: Traffic Interception
4.2.8. THREAT.IMG.REPLACE: Payload Replacement
4.2.9. THREAT.IMG.NON_AUTH: Unauthenticated Images
4.2.10. THREAT.UPD.WRONG_PRECURSOR: Unexpected Precursor Images
4.2.11. THREAT.UPD.UNAPPROVED: Unapproved Firmware
4.2.12. THREAT.IMG.DISCLOSURE: Reverse Engineering of Firmware
Image for Vulnerability Analysis
4.2.13. THREAT.MFST.OVERRIDE: Overriding Critical Manifest
Elements
4.2.14. THREAT.MFST.EXPOSURE: Confidential Manifest Element
Exposure
4.2.15. THREAT.IMG.EXTRA: Extra Data after Image
4.2.16. THREAT.KEY.EXPOSURE: Exposure of Signing Keys
4.2.17. THREAT.MFST.MODIFICATION: Modification of Manifest or
Payload prior to Signing
4.2.18. THREAT.MFST.TOCTOU: Modification of Manifest between
Authentication and Use
4.3. Security Requirements
4.3.1. REQ.SEC.SEQUENCE: Monotonic Sequence Numbers
4.3.2. REQ.SEC.COMPATIBLE: Vendor, Device-Type Identifiers
4.3.3. REQ.SEC.EXP: Expiration Time
4.3.4. REQ.SEC.AUTHENTIC: Cryptographic Authenticity
4.3.5. REQ.SEC.AUTH.IMG_TYPE: Authenticated Payload Type
4.3.6. REQ.SEC.AUTH.IMG_LOC: Authenticated Storage Location
4.3.7. REQ.SEC.AUTH.REMOTE_LOC: Authenticated Remote Payload
4.3.8. REQ.SEC.AUTH.EXEC: Secure Execution
4.3.9. REQ.SEC.AUTH.PRECURSOR: Authenticated Precursor Images
4.3.10. REQ.SEC.AUTH.COMPATIBILITY: Authenticated Vendor and
Class IDs
4.3.11. REQ.SEC.RIGHTS: Rights Require Authenticity
4.3.12. REQ.SEC.IMG.CONFIDENTIALITY: Payload Encryption
4.3.13. REQ.SEC.ACCESS_CONTROL: Access Control
4.3.14. REQ.SEC.MFST.CONFIDENTIALITY: Encrypted Manifests
4.3.15. REQ.SEC.IMG.COMPLETE_DIGEST: Whole Image Digest
4.3.16. REQ.SEC.REPORTING: Secure Reporting
4.3.17. REQ.SEC.KEY.PROTECTION: Protected Storage of Signing
Keys
4.3.18. REQ.SEC.KEY.ROTATION: Protected Storage of Signing Keys
4.3.19. REQ.SEC.MFST.CHECK: Validate Manifests prior to
Deployment
4.3.20. REQ.SEC.MFST.TRUSTED: Construct Manifests in a Trusted
Environment
4.3.21. REQ.SEC.MFST.CONST: Manifest Kept Immutable between
Check and Use
4.4. User Stories
4.4.1. USER_STORY.INSTALL.INSTRUCTIONS: Installation
Instructions
4.4.2. USER_STORY.MFST.FAIL_EARLY: Fail Early
4.4.3. USER_STORY.OVERRIDE: Override Non-critical Manifest
Elements
4.4.4. USER_STORY.COMPONENT: Component Update
4.4.5. USER_STORY.MULTI_AUTH: Multiple Authorizations
4.4.6. USER_STORY.IMG.FORMAT: Multiple Payload Formats
4.4.7. USER_STORY.IMG.CONFIDENTIALITY: Prevent Confidential
Information Disclosures
4.4.8. USER_STORY.IMG.UNKNOWN_FORMAT: Prevent Devices from
Unpacking Unknown Formats
4.4.9. USER_STORY.IMG.CURRENT_VERSION: Specify Version Numbers
of Target Firmware
4.4.10. USER_STORY.IMG.SELECT: Enable Devices to Choose between
Images
4.4.11. USER_STORY.EXEC.MFST: Secure Execution Using Manifests
4.4.12. USER_STORY.EXEC.DECOMPRESS: Decompress on Load
4.4.13. USER_STORY.MFST.IMG: Payload in Manifest
4.4.14. USER_STORY.MFST.PARSE: Simple Parsing
4.4.15. USER_STORY.MFST.DELEGATION: Delegated Authority in
Manifest
4.4.16. USER_STORY.MFST.PRE_CHECK: Update Evaluation
4.4.17. USER_STORY.MFST.ADMINISTRATION: Administration of
Manifests
4.5. Usability Requirements
4.5.1. REQ.USE.MFST.PRE_CHECK: Pre-installation Checks
4.5.2. REQ.USE.MFST.TEXT: Descriptive Manifest Information
4.5.3. REQ.USE.MFST.OVERRIDE_REMOTE: Override Remote Resource
Location
4.5.4. REQ.USE.MFST.COMPONENT: Component Updates
4.5.5. REQ.USE.MFST.MULTI_AUTH: Multiple Authentications
4.5.6. REQ.USE.IMG.FORMAT: Format Usability
4.5.7. REQ.USE.IMG.NESTED: Nested Formats
4.5.8. REQ.USE.IMG.VERSIONS: Target Version Matching
4.5.9. REQ.USE.IMG.SELECT: Select Image by Destination
4.5.10. REQ.USE.EXEC: Executable Manifest
4.5.11. REQ.USE.LOAD: Load-Time Information
4.5.12. REQ.USE.PAYLOAD: Payload in Manifest Envelope
4.5.13. REQ.USE.PARSE: Simple Parsing
4.5.14. REQ.USE.DELEGATION: Delegation of Authority in Manifest
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

This document describes all the information elements required in a
manifest to secure firmware updates of IoT devices. Each information
element is motivated 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 and 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 models 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.

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 element 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 [RFC9019]. The term "Operator"
refers to either a Device Operator device operator or a Network Operator. 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 both authenticated and guaranteed to be correct to
within some predetermined bounds, whenever the time source is
accessible.

The term "Envelope" (or "Manifest 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" "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 [RFC9019], 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.

3.1. Version ID of the Manifest Structure

This is 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

This element provides a monotonically increasing (unsigned) sequence
number to prevent malicious actors from reverting a firmware update
against the policies of the relevant authority. This number must not
wrap around.

For convenience, the monotonic sequence number may be a UTC
timestamp. This allows global synchronisation synchronization 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. The 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 version 5 Universally Unique
Identifiers (UUIDs) [RFC4122] with the vendor's domain name and the
DNS name space ID. Other options include type 1 and type 4 UUIDs.

Fixed-size binary identifiers are preferred because they are simple
to match, unambiguous in length, explicitly non-parsable, and require
no issuing authority. Guaranteed unique integers are preferred
because they are small and simple to match; however, they may not be
fixed length, and they may require an issuing authority to ensure
uniqueness. Free-form text is avoided because it is variable length,
prone to error, and often requires parsing outside the scope of the
manifest serialization.

If human-readable content is required, it SHOULD be contained in a
separate manifest information element: Manifest Text Information
(Section 3.17).

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.

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 the applicability of the 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
from colliding in their customer's infrastructure.

Recommended practice is to use version 5 UUIDs [RFC4122] with as much
information as necessary to define firmware compatibility. Possible
information used to derive the class Class ID UUID includes:

* Model name or number

* Hardware revision

* Runtime library version

* Bootloader version

* ROM revision

* Silicon batch number

The Class ID UUID should use the Vendor ID as the name space
identifier. 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.

The Class ID is not intended to be a human-readable element. It is
intended for binary match/mismatch comparison only. A manifest
serialization SHOULD NOT permit free-form text content to be used for
the Class ID. A fixed-size binary identifier SHOULD be used.

Some organizations desire to keep the same product naming across
multiple, incompatible hardware revisions for ease of user
experience. If this naming is propagated into the firmware, then
matching a specific hardware version becomes a challenge. An opaque,
non-readable binary identifier has no naming implications and so is
more likely to be usable for distinguishing among incompatible device
groupings, regardless of naming.

Fixed-size binary identifiers are preferred because they are simple
to match, unambiguous in length, opaque and free from naming
implications, and explicitly non-parsable. Free-form text is avoided
because it is variable length, prone to error, often requires parsing
outside the scope of the manifest serialization, and may be
homogenized across incompatible device groupings.

If the Class ID is not implemented, then each logical device class
must use a unique trust anchor for authorization.

This element is RECOMMENDED.

Implements: REQ.SEC.COMPATIBLE (Section 4.3.2),
REQ.SEC.AUTH.COMPATIBILITY (Section 4.3.10)

3.4.1. Example 1: Different Classes

Vendor A creates product Product Z and product Product Y. The firmware images of
products
Products Z and Y are not interchangeable. Vendor A creates UUIDs as
follows:

* vendorId = UUID5(DNS, "vendor-a.example")

* ZclassId = UUID5(vendorId, "Product Z")

* 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 Product X. Later, Vendor A adds a new feature to
product
Product X, creating product Product X v2. Product X requires a firmware
update to work with firmware intended for product Product X v2.

Vendor A creates UUIDs as follows:

* vendorId = UUID5(DNS, "vendor-a.example")

* XclassId = UUID5(vendorId, "Product X")

* Xv2classId = UUID5(vendorId, "Product X v2")

When product Product X receives the firmware update necessary to be
compatible with product 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 Product X and product Product Y. These
components share a common core (such as an operating system (OS)) 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 Product X receives application Application X and only product Product Y receives
application
Application Y, product Product X and product Product Y must have different Class IDs.
The vendor creates Class IDs as follows:

* vendorId = UUID5(DNS, "vendor-a.example")

* XclassId = UUID5(vendorId, "Product X")

* YclassId = UUID5(vendorId, "Product Y")

* 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 Rebranding

Vendor A creates a product Product A and its firmware. Vendor B sells the
product under its own name as Product B with some customised customized
configuration. The vendors create the Class IDs as follows:

* vendorIdA = UUID5(DNS, "vendor-a.example")

* classIdA = UUID5(vendorIdA, "Product A-Unlabelled") A-Unlabeled")

* vendorIdB = UUID5(DNS, "vendor-b.example")

* 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
multiple 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 firmware, the required
image version list specifies which firmware versions must be present
for the update to be applied. This allows the update author 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 4.5.8)

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 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 4.5.6)

3.9. Processing Steps

This element provides a representation of the Processing Steps 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 processing step may indicate the expected digest of the payload
after the processing is complete.

This element is RECOMMENDED.

Implements: REQ.USE.IMG.NESTED (Section 4.5.7)

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 author chooses two
storage identifiers:

* "OS"

* "APP"

3.10.2. Example 2: File System Filesystem

A device supports a full-featured filesystem. The Author 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 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 the use of a component
identifier and storage location (Section 3.10).

This element is OPTIONAL.

Implements: REQ.USE.MFST.COMPONENT (Section 4.5.4)

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:

* One URI

* A list of URIs

* A prioritised prioritized list of URIs

* 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 an execute-in-place (XIP)
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 4.5.9)

3.14. Size

This element provides 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 attacks.

This element is REQUIRED.

Implements: REQ.SEC.AUTH.EXEC (Section 4.3.8)

3.15. Manifest Envelope Element: Signature

The Signature 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 signature element
authenticates the manifest, it cannot be contained within the
manifest. Instead, either the manifest is contained within the
signature element or the signature element is a member of the
Manifest Envelope and bundled with the manifest.

The Signature signature element represents the foundation of all security
properties of the manifest. Manifests, which are included as
dependencies by another other manifests, should include a signature so that
the recipient can distinguish between different actors with different
permissions.

The Signature signature element must support multiple signers and multiple
signing algorithms. A manifest format may allow multiple manifests
to be covered by a single Signature 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 4.5.5)

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), and 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. Manifest Text Information

This is textual information pertaining to the update described by the
manifest. This information is for human consumption only. It MUST
NOT be the basis of any decision made by the recipient.

This element is OPTIONAL.

Implements: REQ.USE.MFST.TEXT (Section 4.5.2)

3.18. Aliases

Aliases provide 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.

Implements: REQ.USE.MFST.OVERRIDE_REMOTE (Section 4.5.3)

3.19. Dependencies

This is a list of other manifests that are required by the current
manifest. Manifests are identified in 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 4.5.4)

3.20. 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)

3.21. XIP Address

In order to support 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 4.5.9)

3.22. 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 the following:

* The source (e.g., non-volatile storage)

* The destination (e.g., an address in RAM)

* Cryptographic information

* Decompression information

* 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 4.5.11)

3.23. Run-Time Runtime Metadata

Run-time

Runtime 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 4.5.10)

3.24. Payload

The Payload element is contained within the manifest or manifest
envelope 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 4.5.12)

3.25. Manifest Envelope Element: Delegation Chain

The delegation chain offers enhanced authorization functionality via
authorization tokens, such as Concise Binary Object Representation
(CBOR) Web Tokens [RFC8392] with Proof-of-Possession Key Semantics
[RFC8747]. Each token itself is protected and does not require
another layer of protection. Each authorization token typically
includes a public key or a public key fingerprint; however, this is
dependent on the tokens used. Each token MAY include additional
metadata, such as key usage information. Because the delegation
chain is needed to verify the signature, it must be placed in the
Manifest Envelope, rather than the manifest.

The first token in any delegation chain MUST be authenticated by the
recipient's Trust Anchor. trust anchor. Each subsequent token MUST be
authenticated using the previous token. This allows a recipient to
discard each antecedent token after it has authenticated the
subsequent token. The final token MUST enable authentication of the
manifest. More than one delegation chain MAY be used if more than
one signature is used. Note that no restriction is placed on the
encoding order of these tokens; the order of elements is logical
only.

This element is OPTIONAL.

Implements: REQ.USE.DELEGATION (Section 4.5.14),
REQ.SEC.KEY.ROTATION (Section 4.3.18)

4. Security Considerations

The following subsections 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".
behave correctly. 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 defense are limited.

4.1. Threat Model

The following subsections 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 Spoofing,
Tampering, Repudiation, Information Disclosure, Denial of Service,
and Elevation of Privilege (STRIDE) approach [STRIDE]. Each threat
is classified according to the following:

* Spoofing identity

* Tampering with data

* Repudiation

* Information disclosure

* Denial of service

* 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

Many of the threats detailed in this section contain a "threat
escalation" description. This explains how the described threat
might fit together with other threats and produce a high-severity
threat. This is important because some of the described threats may
seem low severity but could be used with others to construct a high-
severity compromise.

4.2.1. THREAT.IMG.EXPIRED: Old Firmware

Classification: Elevation of Privilege

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. 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 firmware
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. 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 device types. 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 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, 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 threats.

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 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 threats.

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, 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 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) (TOCTOU)
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. all types.

Mitigated by: REQ.SEC.AUTH.EXEC (Section 4.3.8)

4.2.9. THREAT.IMG.NON_AUTH: Unauthenticated Images

Classification: Elevation of Privilege / All Types 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 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 threats. However, it is
unlikely that a valid differential update applied to an incorrect
precursor would result in 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 behavior required by their
owner or operator. Operator. The owner or operator Operator of a device typically
requires that the device maintain certain features, functions,
capabilities, behaviours, behaviors, or interoperability constraints (more
generally, behaviour). behavior). 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 device operator has
the ability to modify device behaviour behavior without approval, then this
constitutes an elevation of privilege.

Similarly, a Network Operator network operator may require that devices behave in a
particular way in order to maintain the integrity of the network. If
device behaviour behavior on a network can be modified without the approval of
the Network Operator, 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 Features A, B, and C, and a firmware update that removes
feature
Feature A is issued by the manufacturer, then the device may not
fulfill the owner's requirements any more. In certain circumstances,
this can cause significantly greater threats. Suppose that feature Feature A
is used to implement a safety-critical system, whether the
manufacturer intended this behaviour behavior or not. When unapproved firmware
is installed, the system may become unsafe.

In a second example, the owner or operator 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, Operator, this is a threat to devices or the system
as a whole.

Similarly, the operator Operator of a network may need to approve firmware for
devices attached to the network in order to ensure favourable favorable
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, network
operator, this is a threat to the network itself.

Threat Escalation: If the firmware network operator 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. 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 device operators expect the rights to
create firmware but that Network Operators network operators expect the rights to
qualify firmware as "fit for purpose" on their networks.
Additionally, assume that Device Operators device operators manage devices that can be
deployed on any network, including Network A and Network 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 the control of different Operators,
and the firmware for a device on Network A has not been qualified to
be deployed on Network B, the target device on Network B is now in
violation of 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.
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
the approval of the Network Operator. network operator. This breaks the behaviour behavior of
the larger system, causing denial of service and, possibly, other
threats. Where the network is a distributed Supervisory Control and
Data Acquisition (SCADA) system, this could cause misbehaviour misbehavior of the
process that is under control.

4.2.12. THREAT.IMG.DISCLOSURE: Reverse Engineering of Firmware Image
for Vulnerability Analysis

Classification: All Types 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 is reverse engineering of the firmware image to analyze it engineered and
analyzed 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, author, uses an override mechanism
(USER_STORY.OVERRIDE (Section 4.4.3)) to change an information
element in a manifest signed by the Author. 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 a firmware digest, this threat can be
escalated to all types.

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 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 all types

If a third party obtains a key or even indirect access to a key --
for example, in a 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 updates, 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),
REQ.SEC.KEY.ROTATION (Section 4.3.18)

4.2.17. THREAT.MFST.MODIFICATION: Modification of Manifest or Payload
prior to Signing

Classification: All Types 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
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 4.3.19),
REQ.SEC.MFST.TRUSTED (Section 4.3.20)

4.2.18. THREAT.MFST.TOCTOU: Modification of Manifest between
Authentication and Use

Classification: All Types all types

If an attacker can modify a manifest after it is authenticated (Time
Of Check) (time
of check) but before it is used (Time Of Use), (time of use), then the attacker can
place any content whatsoever in the manifest.

Mitigated by: REQ.SEC.MFST.CONST (Section 4.3.21)

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 rollover.

| 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)

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 prone to error 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 the current time is later than the
expiration time.

Special consideration is required for end-of-life in cases where a
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.

If a device has a flawed time source (either local or remote), an old
update can be deployed as new.

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)

Implemented by: Signature (Section 3.15), Payload Digests
(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. 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 the time of boot. This requires
authenticated payload size and firmware digest.

Mitigates: THREAT.IMG.REPLACE (Section 4.2.8)

Implemented by: Payload Digests (Section 3.13), Size (Section 3.14)

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 device operator and a Network Operator, network operator,
respectively, then the firmware cannot be installed without proof of
rights from both the Device Operator device operator and the Network Operator. 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
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 3.20)

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 from being 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.

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)

Implemented by: Transport Security / Manifest format triggering
generation of reports

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)

Implemented by: Hardware-assisted isolation technologies, which are
outside the scope of the manifest format

4.3.18. REQ.SEC.KEY.ROTATION: Protected Storage of Signing Keys

Cryptographic keys for signing/authenticating manifests SHOULD be
replaced from time to time. Because it is difficult and risky to
replace a Trust Anchor, trust anchor, keys used for signing updates SHOULD be
delegates of that Trust Anchor. trust anchor.

If key expiration is performed based on time, then a secure clock is
needed. If the time source used by a recipient to check for
expiration is flawed, an old signing key can be used as current,
which compounds THREAT.KEY.EXPOSURE (Section 4.2.16).

Mitigates: THREAT.KEY.EXPOSURE (Section 4.2.16)

Implemented by: Secure storage technology, which is a system design/
implementation aspect outside the scope of the manifest format

4.3.19. 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)

Implemented by: Testing infrastructure. While outside the scope of
the manifest format, proper testing of low-level software is
essential for avoiding unnecessary downtime or worse situations.

4.3.20. REQ.SEC.MFST.TRUSTED: Construct Manifests in a Trusted
Environment

For high-risk deployments, such as large numbers of devices or
devices that provide critical functions, 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)

Implemented by: Physical and network security for protecting the
environment where firmware updates are prepared to avoid
unauthorized access to this infrastructure

4.3.21. 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 internal memory or 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 be verified before
storing it, then this means the manifest MUST fit in RAM.

Mitigates: THREAT.MFST.TOCTOU (Section 4.2.18)

Implemented by: Proper system design with sufficient resources and
implementation avoiding TOCTOU attacks

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, 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 as follows:

* Use a table of hashes to ensure that each block of the payload is
validated before writing.

* Do not report progress.

* Pre-cache the update, but do not install.

* Install the pre-cached update matching this manifest.

* 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.

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, 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:

URIs (Section 3.12): This allows the Device Operator device operator to direct
devices to their own infrastructure in order to reduce network
load.

Conditions: This allows the Device Operator device operator to impose additional
constraints on the installation of the manifest.

Directives (Section 3.16): This allows the Device Operator device operator to add
more instructions, such as time of installation.

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 4.5.4)

4.4.4. USER_STORY.COMPONENT: Component Update

As a Device Operator, 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 4.5.4)

4.4.5. USER_STORY.MULTI_AUTH: Multiple Authorizations

As a Device Operator, 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 4.5.5),
REQ.SEC.ACCESS_CONTROL (Section 4.3.13)

4.4.6. USER_STORY.IMG.FORMAT: Multiple Payload Formats

As a Device Operator, device operator, I want to be able to send multiple payload
formats to suit the needs of my update, so that I can optimise optimize the
bandwidth used by my devices.

Satisfied by: REQ.USE.IMG.FORMAT (Section 4.5.6)

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, 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 4.5.6), REQ.USE.IMG.NESTED
(Section 4.5.7), REQ.USE.MFST.OVERRIDE_REMOTE (Section 4.5.3)

4.4.9. USER_STORY.IMG.CURRENT_VERSION: Specify Version Numbers of
Target Firmware

As a Device Operator, 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 4.5.8)

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 4.5.9)

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 4.5.10)

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 4.5.11)

4.4.13. USER_STORY.MFST.IMG: Payload in Manifest

As an operator 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 4.5.12)

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 4.5.13)

4.4.15. USER_STORY.MFST.DELEGATION: Delegated Authority in Manifest

As a Device Operator 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 4.5.14)

4.4.16. USER_STORY.MFST.PRE_CHECK: Update Evaluation

As an operator 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)

4.4.17. USER_STORY.MFST.ADMINISTRATION: Administration of Manifests

As a Device Operator, device operator, I want to understand what an update will do and
to which devices it applies so that I can make informed choices about
which updates to apply, when to apply them, and which devices should
be updated.

Satisfied by: REQ.USE.MFST.TEXT (Section 4.5.2)

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 (Section 4.4.16),
USER_STORY.INSTALL.INSTRUCTIONS (Section 4.4.1)

Implemented by: Additional Installation Instructions (Section 3.16)

4.5.2. REQ.USE.MFST.TEXT: Descriptive Manifest Information

It MUST be possible for a Device Operator device operator to determine what a
manifest will do and which devices will accept it prior to
distribution.

Satisfies: USER_STORY.MFST.ADMINISTRATION (Section 4.4.17)

Implemented by: Manifest Text Information (Section 3.17)

4.5.3. 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
device operator creates a manifest specifying a payload and signs it,
and provides a URI for that payload. A Network Operator 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, 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
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 3.18)

4.5.4. 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 a 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

4.5.4.1. Example 1: Multiple Microcontrollers

An IoT device with multiple microcontrollers in the same physical
device will likely require multiple payloads with different component
identifiers.

4.5.4.2. 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.

4.5.4.3. 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
desirable in order to ensure that common code between devices is
identical in order to reduce distribution bandwidth.

4.5.5. 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)

4.5.6. 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
as follows:

* Binary

* Executable and Linkable Format (ELF)

* Differential

* Compressed

* Packed configuration

* Intel HEX

* 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)

4.5.7. 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)

4.5.8. 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)

4.5.9. REQ.USE.IMG.SELECT: Select Image by Destination

The manifest format MUST provide a mechanism to list multiple
equivalent payloads by execute-in-place (XIP) installation address,
including the payload digest and, optionally, payload URIs.

Satisfies: USER_STORY.IMG.SELECT (Section 4.4.10)

Implemented by: XIP Address (Section 3.21)

4.5.10. REQ.USE.EXEC: Executable Manifest

The manifest format MUST allow the description of an executable
system with a manifest on both XIP microcontrollers and 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 Runtime Metadata (Section 3.23)

4.5.11. 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 3.22)

4.5.12. 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.

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 4.3.21)), 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 the following: 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 organized 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 4.3.21)

Satisfies: USER_STORY.MFST.IMG (Section 4.4.13)

Implemented by: Payload (Section 3.24)

4.5.13. 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

4.5.14. 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 3.25)

5. IANA Considerations

This document has no IANA actions.

6. References

6.1. 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>.

[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>.

[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.

[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.

[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things",
RFC 9019, DOI 10.17487/RFC9019, April 2021,
<https://www.rfc-editor.org/info/rfc9019>.

6.2. 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>. November 2009,
<https://docs.microsoft.com/en-us/previous-versions/
commerce-server/ee823878(v=cs.20)?redirectedfrom=MSDN>.

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 Software
Updates for Internet of Things (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
Kvamtrø, Øyvind Rønningstad, Michael Richardson, Jan-Frederik
Rieckers, Francisco Acosta, Anton Gerasimov, Matthias Wählisch, Max
Gröning, Daniel Petry, Gaëtan Harter, Ralph Hamm, Steve Patrick,
Fabio Utzig, Paul Lambert, Saïd 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 Phillips, 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.

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