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Versions: 00                                                            
SUIT                                                            B. Moran
Internet-Draft                                               Arm Limited
Intended status: Standards Track                         26 October 2021
Expires: 29 April 2022


          SUIT Manifest Extensions for Multiple Trust Domains
                   draft-moran-suit-trust-domains-00

Abstract

   This specification describes extensions to the SUIT manifest format
   (as defined in [I-D.ietf-suit-manifest]) for use in deployments with
   multiple trust domains.  A device has more than one trust domain when
   it uses different trust anchors for different purposes or components
   in the context of firmware update.

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 29 April 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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 BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Changes to SUIT Workflow Model  . . . . . . . . . . . . . . .   5
   4.  Changes to Manifest Metadata Structure  . . . . . . . . . . .   5
   5.  Delegation Chains . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Delegation Chains . . . . . . . . . . . . . . . . . . . .   7
   6.  Dependencies  . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.    Changes to Required Checks  . . . . . . . . . . . . . .   8
     6.2.  Changes to Abstract Machine Description . . . . . . . . .   9
     6.3.  Changes to Special Cases of Component Index and Dependency
           Index . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.4.  Processing Dependencies . . . . . . . . . . . . . . . . .  10
       6.4.1.  Multiple Manifest Processors  . . . . . . . . . . . .  11
     6.5.  Added and Modified Commands . . . . . . . . . . . . . . .  12
       6.5.1.  suit-directive-set-component-index  . . . . . . . . .  12
       6.5.2.  suit-directive-set-dependency-index . . . . . . . . .  13
       6.5.3.  suit-directive-process-dependency . . . . . . . . . .  14
       6.5.4.  suit-directive-unlink . . . . . . . . . . . . . . . .  14
     6.6.  SUIT_Dependency Manifest Element  . . . . . . . . . . . .  15
   7.  Creating Manifests  . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Dependency Template . . . . . . . . . . . . . . . . . . .  16
       7.1.1.  Composite Manifests . . . . . . . . . . . . . . . . .  16
     7.2.  Encrypted Manifest Template . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  SUIT Commands . . . . . . . . . . . . . . . . . . . . . .  18
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  A.  Full CDDL  . . . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Devices that go beyond single-signer update require more complex
   rules for deploying firmware updates.  For example, devices may
   require:

   *  long-term trust anchors with a mechanism to delegate trust to
      short term keys.

   *  software components from multiple software signing authorities.

   *  a mechanism to remove an uneeded component

   *  single-object dependencies



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   *  a partly encrypted manifest so that distribution does not reveal
      private information

   These mechanisms are not part of the core manifest specification, but
   they are needed for more advanced use cases, such as the architecture
   described in [I-D.ietf-teep-architecture].

   This specification extends the SUIT Manifest specification
   ([I-D.ietf-suit-manifest]).

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Additionally, the following terminology is used throughout this
   document:

   *  SUIT: Software Update for the Internet of Things, also the IETF
      working group for this standard.

   *  Payload: A piece of information to be delivered.  Typically
      Firmware for the purposes of SUIT.

   *  Resource: A piece of information that is used to construct a
      payload.

   *  Manifest: A manifest is a bundle of metadata about the firmware
      for an IoT device, where to find the firmware, and the devices to
      which it applies.

   *  Envelope: A container with the manifest, an authentication wrapper
      with cryptographic information protecting the manifest,
      authorization information, and severable elements (see: TBD).

   *  Update: One or more manifests that describe one or more payloads.

   *  Update Authority: The owner of a cryptographic key used to sign
      updates, trusted by Recipients.

   *  Recipient: The system, typically an IoT device, that receives and
      processes a manifest.

   *  Manifest Processor: A component of the Recipient that consumes
      Manifests and executes the commands in the Manifest.



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   *  Component: An updatable logical block of the Firmware, Software,
      configuration, or data of the Recipient.

   *  Component Set: A group of interdependent Components that must be
      updated simultaneously.

   *  Command: A Condition or a Directive.

   *  Condition: A test for a property of the Recipient or its
      Components.

   *  Directive: An action for the Recipient to perform.

   *  Trusted Invocation: A process by which a system ensures that only
      trusted code is executed, for example secure boot or launching a
      Trusted Application.

   *  A/B images: Dividing a Recipient's storage into two or more
      bootable images, at different offsets, such that the active image
      can write to the inactive image(s).

   *  Record: The result of a Command and any metadata about it.

   *  Report: A list of Records.

   *  Procedure: The process of invoking one or more sequences of
      commands.

   *  Update Procedure: A procedure that updates a Recipient by fetching
      dependencies and images, and installing them.

   *  Invocation Procedure: A procedure in which a Recipient verifies
      dependencies and images, loading images, and invokes one or more
      image.

   *  Software: Instructions and data that allow a Recipient to perform
      a useful function.

   *  Firmware: Software that is typically changed infrequently, stored
      in nonvolatile memory, and small enough to apply to [RFC7228]
      Class 0-2 devices.

   *  Image: Information that a Recipient uses to perform its function,
      typically firmware/software, configuration, or resource data such
      as text or images.  Also, a Payload, once installed is an Image.

   *  Slot: One of several possible storage locations for a given
      Component, typically used in A/B image systems



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   *  Abort: An event in which the Manifest Processor immediately halts
      execution of the current Procedure.  It creates a Record of an
      error condition.

3.  Changes to SUIT Workflow Model

   The use of the features presented for use with multiple trust domains
   requires some augmentation of the workflow presented in the SUIT
   Manifest specification ([I-D.ietf-suit-manifest]):

   One additional assumption is added for the Update Procedure:

   *  All dependency manifests should be present before any payload is
      fetched.

   One additional assumption is added to the Invocation Procedure:

   *  All dependencies must be validated prior to loading.

   Two steps are added to the expected installation workflow of a
   Recipient:

   1.  *Verify delegation chains*

   2.  Verify the signature of the manifest.

   3.  Verify the applicability of the manifest.

   4.  *Resolve dependencies.*

   5.  Fetch payload(s).

   6.  Install payload(s).

   In addition, when multiple manifests are used for an update, each
   manifest's steps occur in a lockstep fashion; all manifests have
   dependency resolution performed before any manifest performs a
   payload fetch, etc.

4.  Changes to Manifest Metadata Structure

   To accomodate the additional metadata needed to enable these
   features, the envelope and manifest have several new elements added.

   The Envelope gains two more elements: Delegation chains and
   Integrated Dependencies The Common metadata section in the Manifest
   also gains a list of dependencies.




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   The new metadata structure is shown below.

   +-------------------------+
   | Envelope                |
   +-------------------------+
   | Delegation Chains       |
   | Authentication Block    |
   | Manifest           --------------> +------------------------------+
   | Severable Elements      |          | Manifest                     |
   | Human-Readable Text     |          +------------------------------+
   | COSWID                  |          | Structure Version            |
   | Integrated Dependencies |          | Sequence Number              |
   | Integrated Payloads     |          | Reference to Full Manifest   |
   +-------------------------+    +------ Common Structure             |
                                  | +---- Command Sequences            |
   +-------------------------+    | |   | Digests of Envelope Elements |
   | Common Structure        | <--+ |   +------------------------------+
   +-------------------------+      |
   | Dependencies            |      +-> +-----------------------+
   | Component IDs           |          | Command Sequence      |
   | Common Command Sequence ---------> +-----------------------+
   +-------------------------+          | List of ( pairs of (  |
                                        |   * command code      |
                                        |   * argument /        |
                                        |      reporting policy |
                                        | ))                    |
                                        +-----------------------+

5.  Delegation Chains

   Delegation Chains allow a Recipient to establish a chain of trust
   from a Trust Anchor to the signer of a manifest by validating
   delegation claims.  Each delegation claim is a [RFC8392] CBOR Web
   Tokens (CWTs).  The first claim in each list is signed by a Trust
   Anchor.  Each subsequent claim in a list is signed by the public key
   claimed in the preceding list element.  The last element in each list
   claims a public key that can be used to verify a signature in the
   Authentication Block (See Sectino 5.2 of [I-D.ietf-suit-manifest]).

   See Section 5.1 for more detail.











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5.1.  Delegation Chains

   The suit-delegation element MAY carry one or more CBOR Web Tokens
   (CWTs) [RFC8392], with [RFC8747] cnf claims.  They can be used to
   perform enhanced authorization decisions.  The CWTs are arranged into
   a list of lists.  Each list starts with a CWT authorized by a Trust
   Anchor, and finishes with a key used to authenticate the Manifest
   (see Section 8.3 of [I-D.ietf-suit-manifest]).  This allows an Update
   Authority to delegate from a long term Trust Anchor, down through
   intermediaries, to a delegate without any out-of-band provisioning of
   Trust Anchors or intermediary keys.

   A Recipient MAY choose to cache intermediaries and/or delegates.  If
   an Update Distributor knows that a targeted Recipient has cached some
   intermediaries or delegates, it MAY choose to strip any cached
   intermediaries or delegates from the Delegation Chains in order to
   reduce bandwidth and energy.

6.  Dependencies

   A dependency is another SUIT_Envelope that describes additional
   components.

   Dependency manifests enable several additional use cases.  In
   particular, they enable two or more entities who are trusted for
   different privileges to coordinate.  This can be used in many
   scenarios, for example:

   *  An IoT device may contain a processor in its radio in addition to
      the primary processor.  These two processors may have separate
      firmware with separate signing authorities.  Dependencies allow
      the firmware for the primary processor to reference a manifest
      signed by a different authority.

   *  A network operator may wish to provide local caching of update
      payloads.  The network operator overrides the URI of payload by
      providing a dependent manifest that references the original
      manifest, but replaces its URI.

   *  A device operator provides a device with some additional
      configuration.  The device operator wants to test their
      configuration with each new firmware version before releasing it.
      The configuration is delivered as a binary in the same way as a
      firmware image.  The device operator references the firmware
      manifest from the firmware author in their own manifest which also
      defines the configuration.





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   By using dependencies, components such as software, configuration,
   models, and other resoruces authenticated by different trust anchors
   can be delivered to devices.

6.1.    Changes to Required Checks

   This section augments the definitions in Required Checks
   (Section 6.2) of [I-D.ietf-suit-manifest].

   More checks are required when handling dependencies.  By default, any
   signature of a dependency MUST be verified.  However, there are some
   exceptions to this rule: where a device supports only one level of
   access (no ACLs defining which authorities have access to different
   componetns), it MAY choose to skip signature verification of
   dependencies, since they are referenced by digest.  Where a device
   differentiates between trust levels, such as with an ACL, it MAY
   choose to defer the verification of signatures of dependencies until
   the list of affected components is known so that it can skip
   redundant signature verifications.  For example, a dependency signed
   by the same author as the dependent does not require a signature
   verification.  Similarly, if the signer of the dependent has full
   rights to the device, according to the ACL, then no signature
   verification is necessary on the dependency.

   If the manifest contains more than one component and/or dependency,
   each command sequence MUST begin with a Set Component Index or Set
   Dependency Index command.

   If a dependency is specified, then the manifest processor MUST
   perform the following checks:

   1.  At the beginning of each section in the dependent: all previous
       sections of each dependency have been executed.

   2.  At the end of each section in the dependent: The corresponding
       section in each dependency has been executed.

   If the interpreter does not support dependencies and a manifest
   specifies a dependency, then the interpreter MUST reject the
   manifest.

   If a Recipient supports groups of interdependent components (a
   Component Set), then it SHOULD verify that all Components in the
   Component Set are specified by one update, that is: a single manifest
   and all its dependencies that together:

   1.  have sufficient permissions imparted by their signatures




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   2.  specify a digest and a payload for every Component in the
       Component Set.

   The single dependent manifest is sometimes called a Root Manifest.

6.2.  Changes to Abstract Machine Description

   This section augments the Abstract Machine Description (Section 6.4)
   in [I-D.ietf-suit-manifest] With the addition of dependencies, some
   changes are necessary to the abstract machine, outside the typical
   scope of added commands.  These changes alter the behaviour of an
   existing command and way that the parser processes manifests:

   *  All commands may target dependency manifests as well as
      components.  To support this behaviour, there is a new command
      instroduced: Set Dependency Index.  This change works together
      with Set Component Index to choose the object on which the
      manifest is operating.

   *  Dependencies are processed in lock-step with the Root Manifest.
      This means that every dependency's current command sequence must
      be executed before a dependent's later command sequence may be
      executed.  For example, every dependency's Dependency Resolution
      step MUST be executed before any dependent's payload fetch step.

   A new command, Set Dependency Index, is added and the logic of Set
   Componment Index is modified as below:

   As in [I-D.ietf-suit-manifest], To simplify the logic describing the
   command semantics, the object "current" is used.  It represents the
   component identified by the Component Index or the dependency
   identified by the Dependency Index:

   current := components\[component-index\]
       if component-index is not false
       else dependencies\[dependency-index\]

   As a result, Set Component Index is described as current :=
   components[arg].  The actual operation performed for Set Component
   Index is described by the following pseudocode, however, because of
   the definition of current (above), these are semantically equivalent.

   component-index := arg
   dependency-index := false

   Similarly, Set Dependency Index is semantically equivalent to current
   := dependencies[arg], but the actual operation performed is:




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   dependency-index := arg
   component-index := false

   Dependencies are identified by digest, but referenced in commands by
   Dependency Index, the index into the array of Dependencies.

6.3.  Changes to Special Cases of Component Index and Dependency Index

   The considerations that apply in Special Cases of Component Index and
   Dependency Index (Section 6.5) of [I-D.ietf-suit-manifest] are
   augmented to include Dependency Index as well as Component Index.

   The target(s) assigned for each command are defined by the following
   pseudocode.

   if component-index is true:
       current-list = components
   else if component-index is array:
       current-list = [ components[idx] for idx in component-index ]
   else if component-index is integer:
       current-list = [ components[component-index] ]
   else if dependency-index is true:
       current-list = dependencies
   else if dependency-index is array:
       current-list = [ dependencies[idx] for idx in dependency-index ]
   else:
       current-list = [ dependencies[dependency-index] ]
   for current in current-list:
       cmd(current)

6.4.  Processing Dependencies

   As described in Section 6.1, each manifest must invoke each of its
   dependencies' sections from the corresponding section of the
   dependent.  Any changes made to parameters by the dependency persist
   in the dependent.

   When a Process Dependency command is encountered, the interpreter
   loads the dependency identified by the Current Dependency Index.  The
   interpreter first executes the common-sequence section of the
   identified dependency, then it executes the section of the dependency
   that corresponds to the currently executing section of the dependent.

   If the specified dependency does not contain the current section,
   Process Dependency succeeds immediately.

   The Manifest Processor MUST also support a Dependency Index of True,
   which applies to every dependency, as described in Section 6.3



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   The interpreter also performs the checks described in Section 6.1 to
   ensure that the dependent is processing the dependency correctly.

6.4.1.  Multiple Manifest Processors

   When a system has multiple security domains, each domain might
   require independent verification of authenticity or security
   policies.  Security domains might be divided by separation technology
   such as Arm TrustZone, Intel SGX, or another TEE technology.
   Security domains might also be divided into separate processors and
   memory spaces, with a communication interface between them.

   For example, an application processor may have an attached
   communications module that contains a processor.  The communications
   module might require metadata signed by a specific Trust Authority
   for regulatory approval.  This may be a different Trust Authority
   than the application processor.

   When there are two or more security domains (see
   [I-D.ietf-teep-architecture]), a manifest processor might be required
   in each.  The first manifest processor is the normal manifest
   processor as described for the Recipient in Section 6 of
   [I-D.ietf-suit-manifest].  The second manifest processor only
   executes sections when the first manifest processor requests it.  An
   API interface is provided from the second manifest processor to the
   first.  This allows the first manifest processor to request a limited
   set of operations from the second.  These operations are limited to:
   setting parameters, inserting an Envelope, invoking a Manifest
   Command Sequence.  The second manifest processor declares a prefix to
   the first, which tells the first manifest processor when it should
   delegate to the second.  These rules are enforced by underlying
   separation of privilege infrastructure, such as TEEs, or physical
   separation.

   When the first manifest processor encounters a dependency prefix,
   that informs the first manifest processor that it should provide the
   second manifest processor with the corresponding dependency Envelope.
   This is done when the dependency is fetched.  The second manifest
   processor immediately verifies any authentication information in the
   dependency Envelope.  When a parameter is set for any component that
   matches the prefix, this parameter setting is passed to the second
   manifest processor via an API.  As the first manifest processor works
   through the Procedure (set of command sequences) it is executing,
   each time it sees a Process Dependency command that is associated
   with the prefix declared by the second manifest processor, it uses
   the API to ask the second manifest processor to invoke that
   dependency section instead.




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   This mechanism ensures that the two or more manifest processors do
   not need to trust each other, except in a very limited case.  When
   parameter setting across security domains is used, it must be very
   carefully considered.  Only parameters that do not have an effect on
   security properties should be allowed.  The dependency manifest MAY
   control which parameters are allowed to be set by using the Override
   Parameters directive.  The second manifest processor MAY also control
   which parameters may be set by the first manifest processor by means
   of an ACL that lists the allowed parameters.  For example, a URI may
   be set by a dependent without a substantial impact on the security
   properties of the manifest.

6.5.  Added and Modified Commands

   All commands are modified in that they can also target dependencies.
   However, Set Component Index has a larger modification.

        +======================+=================================+
        | Command Name         | Semantic of the Operation       |
        +======================+=================================+
        | Set Component Index  | current := components[arg]      |
        +----------------------+---------------------------------+
        | Set Dependency Index | current := dependencies[arg]    |
        +----------------------+---------------------------------+
        | Set Parameters       | current.params[k] := v if not k |
        |                      | in params for-each k,v in arg   |
        +----------------------+---------------------------------+
        | Process Dependency   | exec(current[common]);          |
        |                      | exec(current[current-segment])  |
        +----------------------+---------------------------------+
        | Unlink               | unlink(current)                 |
        +----------------------+---------------------------------+

                                 Table 1

6.5.1.  suit-directive-set-component-index

   Set Component Index defines the component to which successive
   directives and conditions will apply.  The supplied argument MUST be
   one of three types:

   1.  An unsigned integer (REQUIRED to implement in parser)

   2.  A boolean (REQUIRED to implement in parser ONLY IF 2 or more
       components supported)

   3.  An array of unsigned integers (REQUIRED to implement in parser
       ONLY IF 3 or more components supported)



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   If the following commands apply to ONE component, an unsigned integer
   index into the component list is used.  If the following commands
   apply to ALL components, then the boolean value "True" is used
   instead of an index.  If the following commands apply to more than
   one, but not all components, then an array of unsigned integer
   indices into the component list is used.  See Section 6.3 for more
   details.

   If the following commands apply to NO components, then the boolean
   value "False" is used.  When suit-directive-set-dependency-index is
   used, suit-directive-set-component-index = False is implied.  When
   suit-directive-set-component-index is used, suit-directive-set-
   dependency-index = False is implied.

   If component index is set to True when a command is invoked, then the
   command applies to all components, in the order they appear in suit-
   common-components.  When the Manifest Processor invokes a command
   while the component index is set to True, it must execute the command
   once for each possible component index, ensuring that the command
   receives the parameters corresponding to that component index.

6.5.2.  suit-directive-set-dependency-index

   Set Dependency Index defines the manifest to which successive
   directives and conditions will apply.  The supplied argument MUST be
   either a boolean or an unsigned integer index into the dependencies,
   or an array of unsigned integer indices into the list of
   dependencies.  If the following directives apply to ALL dependencies,
   then the boolean value "True" is used instead of an index.  If the
   following directives apply to NO dependencies, then the boolean value
   "False" is used.  When suit-directive-set-component-index is used,
   suit-directive-set-dependency-index = False is implied.  When suit-
   directive-set-dependency-index is used, suit-directive-set-component-
   index = False is implied.

   If dependency index is set to True when a command is invoked, then
   the command applies to all dependencies, in the order they appear in
   suit-common-components.  When the Manifest Processor invokes a
   command while the dependency index is set to True, the Manifest
   Processor MUST execute the command once for each possible dependency
   index, ensuring that the command receives the parameters
   corresponding to that dependency index.  If the dependency index is
   set to an array of unsigned integers, then the Manifest Processor
   MUST execute the command once for each listed dependency index,
   ensuring that the command receives the parameters corresponding to
   that dependency index.

   See Section 6.3 for more details.



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   Typical operations that require suit-directive-set-dependency-index
   include setting a source URI or Encryption Information, invoking
   "Fetch," or invoking "Process Dependency" for an individual
   dependency.

6.5.3.  suit-directive-process-dependency

   Execute the commands in the common section of the current dependency,
   followed by the commands in the equivalent section of the current
   dependency.  For example, if the current section is "fetch payload,"
   this will execute "common" in the current dependency, then "fetch
   payload" in the current dependency.  Once this is complete, the
   command following suit-directive-process-dependency will be
   processed.

   If the current dependency is False, this directive has no effect.  If
   the current dependency is True, then this directive applies to all
   dependencies.  If the current section is "common," then the command
   sequence MUST be terminated with an error.

   When SUIT_Process_Dependency completes, it forwards the last status
   code that occurred in the dependency.

6.5.3.1.  suit-directive-set-parameters

   suit-directive-set-parameters allows the manifest to configure
   behavior of future directives by changing parameters that are read by
   those directives.  When dependencies are used, suit-directive-set-
   parameters also allows a manifest to modify the behavior of its
   dependencies.

   If a parameter is already set, suit-directive-set-parameters will
   skip setting the parameter to its argument.  This provides the core
   of the override mechanism, allowing dependent manifests to change the
   behavior of a manifest.

   suit-directive-set-parameters does not specify a reporting policy.

6.5.4.  suit-directive-unlink

   suit-directive-unlink marks the current component as unused in the
   current manifest.  This can be used to remove temporary storage or
   remove components that are no longer needed.  Example use cases:

   *  Temporary storage for encrypted download

   *  Temporary storage for verifying decompressed file before writing
      to flash



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   *  Removing Trusted Service no longer needed by Trusted Application

   Once the current Command Sequence is complete, the manifest
   processors checks each marked component to see whether any other
   manifests have referenced it.  Those marked components with no other
   references are deleted.  The manifest processor MAY choose to ignore
   a Unlink directive depending on device policy.

   suit-directive-unlink is OPTIONAL to implement in manifest
   processors.

6.6.  SUIT_Dependency Manifest Element

   SUIT_Dependency specifies a manifest that describes a dependency of
   the current manifest.  The Manifest is identified, but the Recipient
   should expect an Envelope when it acquires the dependency.  This is
   because the Manifest is the one invariant element of the Envelope,
   where other elements may change by countersigning, adding
   authentication blocks, or severing elements.

   The suit-dependency-digest specifies the dependency manifest uniquely
   by identifying a particular Manifest structure.  This is identical to
   the digest that would be present as the payload of any suit-
   authentication-block in the dependency's Envelope.  The digest is
   calculated over the Manifest structure instead of the COSE
   Sig_structure or Mac_structure.  This is necessary to ensure that
   removing a signature from a manifest does not break dependencies due
   to missing signature elements.  This is also necessary to support the
   trusted intermediary use case, where an intermediary re-signs the
   Manifest, removing the original signature, potentially with a
   different algorithm, or trading COSE_Sign for COSE_Mac.

   The suit-dependency-prefix element contains a
   SUIT_Component_Identifier (see Section 8.4.5.1 of
   [I-D.ietf-suit-manifest]).  This specifies the scope at which the
   dependency operates.  This allows the dependency to be forwarded on
   to a component that is capable of parsing its own manifests.  It also
   allows one manifest to be deployed to multiple dependent Recipients
   without those Recipients needing consistent component hierarchy.
   This element is OPTIONAL for Recipients to implement.

   A dependency prefix can be used with a component identifier.  This
   allows complex systems to understand where dependencies need to be
   applied.  The dependency prefix can be used in one of two ways.  The
   first simply prepends the prefix to all Component Identifiers in the
   dependency.





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   A dependency prefix can also be used to indicate when a dependency
   manifest needs to be processed by a secondary manifest processor, as
   described in Section 6.4.1.

7.  Creating Manifests

   This section details a set of templates for creating manifests.
   These templates explain which parameters, commands, and orders of
   commands are necessary to achieve a stated goal.

7.1.  Dependency Template

   The goal of the Dependency template is to obtain, verify, and process
   a dependency manifest as appropriate.

   The following commands are placed into the dependency resolution
   sequence:

   *  Set Dependency Index directive (see Section 6.5.2)

   *  Set Parameters directive (see Section 6.5.3.1) for URI (see
      Section 8.4.8.9 of [I-D.ietf-suit-manifest])

   *  Fetch directive (see Section 8.4.10.4 of [I-D.ietf-suit-manifest])

   *  Check Image Match condition (see Section 8.4.9.2 of
      [I-D.ietf-suit-manifest] of [I-D.ietf-suit-manifest])

   *  Process Dependency directive (see Section 6.5.3)

   Then, the validate sequence contains the following operations:

   *  Set Dependency Index directive (see Section 6.5.2)

   *  Check Image Match condition (see Section 8.4.9.2 of
      [I-D.ietf-suit-manifest])

   *  Process Dependency directive (see Section 6.5.3)

   NOTE: Any changes made to parameters in a dependency persist in the
   dependent.

7.1.1.  Composite Manifests

   An implementer MAY choose to place a dependency's envelope in the
   envelope of its dependent.  The dependent envelope key for the
   dependency envelope MUST NOT be a value between 0 and 24 and it MUST
   NOT be used by any other envelope element in the dependent manifest.



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   The URI for a dependency enclosed in this way MUST be expressed as a
   fragment-only reference, as defined in [RFC3986], Section 4.4.  The
   fragment identifier is the stringified envelope key of the
   dependency.  For example, an envelope that contains a dependency at
   key 42 would use a URI "#42", key -73 would use a URI "#-73".

7.2.  Encrypted Manifest Template

   The goal of the Encrypted Manifest template is to fetch and decrypt a
   manifest so that it can be used as a dependency.  To use an encrypted
   manifest, create a plaintext dependent, and add the encrypted
   manifest as a dependency.  The dependent can include very little
   information.

   NOTE: This template also requires the extensions defined in
   [I-D.ietf-suit-firmware-encryption]

   The following operations are placed into the dependency resolution
   block:

   *  Set Dependency Index directive (see Section 6.5.2)

   *  Set Parameters directive (see Section 6.5.3.1) for

      -  URI (see Section 8.4.8.9 of [I-D.ietf-suit-manifest])

      -  Encryption Info (See [I-D.ietf-suit-firmware-encryption])

   *  Fetch directive (see Section 8.4.10.4 of [I-D.ietf-suit-manifest])

   *  Check Image Match condition (see Section 8.4.9.2 of
      [I-D.ietf-suit-manifest])

   *  Process Dependency directive (see Section 6.5.3)

   Then, the validate block contains the following operations:

   *  Set Dependency Index directive (see Section 6.5.2)

   *  Check Image Match condition (see Section 8.4.9.2 of
      [I-D.ietf-suit-manifest])

   *  Process Dependency directive (see Section 6.5.3)

   A plaintext manifest and its encrypted dependency may also form a
   composite manifest (Section 7.1.1).





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8.  IANA Considerations

   IANA is requested to allocate the following numbers in the listed
   registries:

8.1.  SUIT Commands

   +=======+============+===================================+=========+
   | Label | Name       | Reference                         |         |
   +=======+============+===================================+=========+
   | 13    | Set        | Section 6.5.2                     |         |
   |       | Dependency |                                   |         |
   |       | Index      |                                   |         |
   +-------+------------+-----------------------------------+---------+
   | 18    | Process    | suit-directive-process-dependency | Section |
   |       | Dependency |                                   | 6.5.3   |
   +-------+------------+-----------------------------------+---------+
   | 19    | Set        | Section 6.5.3.1                   |         |
   |       | Parameters |                                   |         |
   +-------+------------+-----------------------------------+---------+
   | 33    | Unlink     | Section 6.5.4                     |         |
   +-------+------------+-----------------------------------+---------+

                                 Table 2

9.  Security Considerations

   This document is about a manifest format protecting and describing
   how to retrieve, install, and invoke firmware images and as such it
   is part of a larger solution for delivering firmware updates to IoT
   devices.  A detailed security treatment can be found in the
   architecture [RFC9019] and in the information model
   [I-D.ietf-suit-information-model] documents.

10.  References

10.1.  Normative References

   [I-D.ietf-suit-manifest]
              Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
              "A Concise Binary Object Representation (CBOR)-based
              Serialization Format for the Software Updates for Internet
              of Things (SUIT) Manifest", Work in Progress, Internet-
              Draft, draft-ietf-suit-manifest-14, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-suit-manifest-
              14.txt>.





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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>.

   [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>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [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>.

10.2.  Informative References

   [I-D.ietf-suit-firmware-encryption]
              Tschofenig, H., Housley, R., and B. Moran, "Firmware
              Encryption with SUIT Manifests", Work in Progress,
              Internet-Draft, draft-ietf-suit-firmware-encryption-02, 25
              October 2021, <https://www.ietf.org/archive/id/draft-ietf-
              suit-firmware-encryption-02.txt>.










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   [I-D.ietf-suit-information-model]
              Moran, B., Tschofenig, H., and H. Birkholz, "A Manifest
              Information Model for Firmware Updates in IoT Devices",
              Work in Progress, Internet-Draft, draft-ietf-suit-
              information-model-13, 8 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-suit-
              information-model-13.txt>.

   [I-D.ietf-teep-architecture]
              Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-teep-architecture-15, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-teep-
              architecture-15.txt>.

Appendix A.  A.  Full CDDL

   To be valid, the following CDDL MUST be appended to the SUIT Manifest
   CDDL.  The SUIT CDDL is defined in Appendix A of
   [I-D.ietf-suit-manifest]

   $$SUIT_Envelope_Extensions //=
       (suit-delegation => bstr .cbor SUIT_Delegation)
   $$SUIT_Envelope_Extensions //= SUIT_Integrated_Dependency

   SUIT_Delegation = [ + [ + bstr .cbor CWT ] ]

   CWT = SUIT_Authentication_Block

   $$SUIT_severable-members-extensions //=
       (suit-dependency-resolution => bstr .cbor SUIT_Command_Sequence)

   SUIT_Integrated_Dependency = (
       suit-integrated-dependency-key => bstr .cbor SUIT_Envelope)
   suit-integrated-dependency-key = tstr

   $$severable-manifest-members-choice-extensions //= (
       suit-dependency-resolution => \
           bstr .cbor SUIT_Command_Sequence / SUIT_Digest)

   $$SUIT_Common-extensions //= (
           suit-dependencies => SUIT_Dependencies
   )

   SUIT_Dependencies         = [ + SUIT_Dependency ]

   SUIT_Dependency = {



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       suit-dependency-digest => SUIT_Digest,
       ? suit-dependency-prefix => SUIT_Component_Identifier,
       * $$SUIT_Dependency-extensions,
   }

   SUIT_Directive //= (
       suit-directive-set-dependency-index, IndexArg)
   SUIT_Directive //= (
       suit-directive-process-dependency, SUIT_Rep_Policy)
   SUIT_Directive //= (suit-directive-set-parameters,
       {+ SUIT_Parameters})
   SUIT_Directive //= (
       suit-directive-unlink, SUIT_Rep_Policy)

   suit-delegation = 1
   suit-dependency-resolution = 7

   suit-dependencies = 1

   suit-dependency-digest = 1
   suit-dependency-prefix = 2

   suit-directive-set-dependency-index     = 13
   suit-directive-process-dependency       = 18
   suit-directive-set-parameters           = 19
   suit-directive-unlink                   = 33

Author's Address

   Brendan Moran
   Arm Limited

   Email: Brendan.Moran@arm.com


















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