encryption is not needed, RFC6234, combined with a sequence number and a replay window, may be used.  When using an encryption mode that depends on a nonce, such as AES- CCM, repeated use of the same nonce under the same key causes the cipher to fail catastrophically.

This is some advice on how big the replay window needs to be.
doc> a 64 bit tag is recommended, combined with a sequence number and a doc> replay window. Where encryption is not needed, HMAC-SHA-256, doc> combined with a sequence number and a replay window, may be used. [[TODO]]
An absolute timestamp is probably not necessary, as the key should be newly generated on a reboot. A boot-relative timestamp may be enough. The window pretty much becomes the "patience" value -- how long does the client wait for getting its response?

encryption is not needed, RFC6234, combined with a sequence number and a replay window, may be used. <ins> This size of the replay window depends upon the number of outstanding requests that need to be outstanding.  This can be determined from the rate at which new ones are made, and the expected duration in which responses are expected. For instance, given a CoAP ACK_TIMEOUT of 2s, and a request rate of 10 requests/second, any request that is not answered within 2s will be considered to have failed.  Thus at most 20 request can be outstanding at a time, and any convenient replay window larger than 20 will work.  As replay windows are often implemented with a sliding window and a bit, the use of a 32-bit window would be sufficient. For use cases where requests are being relayed from another node, the request rate may be estimated by the total link capacity allocated for that kind of traffic.  An alternate view would consider how many IPv6 Neighbor Cache Entries (NCEs) the system can afford to allocate for this use. </ins> When using an encryption mode that depends on a nonce, such as AES- CCM, repeated use of the same nonce under the same key causes the cipher to fail catastrophically.


Firmware Encryption with SUIT Manifests <del> draft-ietf-suit-firmware-encryption-03 </del>  Abstract manifest with key establishment provided by the hybrid public-key encryption (HPKE) scheme and the AES Key Wrap (AES-KW) with a pre- shared key-encryption key.  Encryption of the firmware image is <del> encrypted using AES-GCM or AES-CCM. </del>  1. D.ietf-suit-manifest, the SUIT information model RFC9124 and the SUIT architecture RFC9019. <del> The terms sender and recipient are defined in I-D.irtf-cfrg-hpke and have the following meaning: </del>  Sender: Role of entity which sends an encrypted message. Content-encryption key (CEK), a term defined in RFC 2630 RFC2630. <del> Hybrid Public Key Encryption (HPKE), defined in I-D.irtf-cfrg- hpke. </del>  3. 6. <del> Hybrid public-key encryption (HPKE) I-D.irtf-cfrg-hpke is a scheme that provides public key encryption of arbitrary-sized plaintexts given a recipient's public key. </del>  For use with firmware encryption the scheme works as follows: HPKE, which internally utilizes a non-interactive ephemeral-static Diffie- An example of the COSE_Encrypt structure using the HPKE scheme is shown in hpke-example.  It uses the following algorithm combination: <del> AES-GCM-128 for encryption of the (detached) firmware image (at layer 0). </del>  <del> AES-GCM-128 for encryption of the CEK in layer 1 as well as ECDH with NIST P-256 and HKDF-SHA256 as a Key Encapsulation Mechanism (KEM). </del>  7. The suit-cek-verification parameter contains a byte string resulting <del> from the encryption of 8 bytes of 0xA5 using the CEK. </del>  <del> [[Editor's Note: Guidance about the selection of an IV needs to be added here.]] </del>  8.

Based on discussions with Russ, I have updated the text in Section 7 about CEK Verification. Text about the parameters used for encryption were missing.

Firmware Encryption with SUIT Manifests  <ins> draft-ietf-suit-firmware-encryption-04 </ins> Abstract manifest with key establishment provided by the hybrid public-key encryption (HPKE) scheme and the AES Key Wrap (AES-KW) with a pre- shared key-encryption key.  Encryption of the firmware image is  <ins> accomplished using the established content encryption key and a mutually agreed symmetric encryption algorithm, such as AES-GCM or AES-CCM. </ins> 1. D.ietf-suit-manifest, the SUIT information model RFC9124 and the SUIT architecture RFC9019.  <ins> The terms sender and recipient are defined in RFC9180 and have the following meaning: </ins> Sender: Role of entity which sends an encrypted message. Content-encryption key (CEK), a term defined in RFC 2630 RFC2630.  <ins> Hybrid Public Key Encryption (HPKE), defined in RFC9180. The main use case of this document is to encrypt firmware.  However, SUIT manifests may require other payloads than firmware images to experience confidentiality protection using encryption.  While the term firmware is used throughout the document, plaintext other than firmware images may get encrypted using the described mechanism. Hence, the terms firmware (image) and plaintext are used interchangably. </ins> 3. 6.  <ins> Hybrid public-key encryption (HPKE) RFC9180 is a scheme that provides public key encryption of arbitrary-sized plaintexts given a recipient's public key. </ins> For use with firmware encryption the scheme works as follows: HPKE, which internally utilizes a non-interactive ephemeral-static Diffie- An example of the COSE_Encrypt structure using the HPKE scheme is shown in hpke-example.  It uses the following algorithm combination:  <ins> AES-GCM-128 for encryption of the (detached) firmware image. </ins>  <ins> AES-GCM-128 for encryption of the CEK as well as ECDH with NIST P-256 and HKDF-SHA256 as a Key Encapsulation Mechanism (KEM). </ins> 7. The suit-cek-verification parameter contains a byte string resulting  <ins> from the encryption of 8 bytes of 0xA5 using the CEK with a nonce of all zeros and empty additional data using the cipher algorithm and mode also used to encrypt the plaintext. </ins>  <ins> As explained in arch, the suit-cek-verification parameter is optional to implement and optional to use.  When used, it reduces the risk of an battery exhaustion attack against the IoT device. </ins> 8.


A Firmware Update Architecture for Internet of Things <del> draft-ietf-suit-architecture-12 </del>  Abstract Vulnerabilities with Internet of Things (IoT) devices have raised the <del> need for a solid and secure firmware update mechanism that is also suitable for constrained devices.  Incorporating such update </del>  mechanism to fix vulnerabilities, to update configuration settings as well as adding new functionality is recommended by security experts. <del> This document lists requirements and describes an architecture for a firmware update mechanism suitable for IoT devices.  The architecture is agnostic to the transport of the firmware images and associated meta-data. </del>  1. <del> When developing Internet of Things (IoT) devices, one of the most difficult problems to solve is how to update firmware on the device. Once the device is deployed, firmware updates play a critical part in its lifetime, particularly when devices have a long lifetime, are deployed in remote or inaccessible areas where manual intervention is cost prohibitive or otherwise difficult.  Updates to the firmware of an IoT device are done to fix bugs in software, to add new functionality, and to re-configure the device to work in new </del>  environments or to behave differently in an already deployed context. <del> The firmware update process, among other goals, has to ensure that </del>  The firmware image is authenticated and integrity protected. <del> Attempts to flash a modified firmware image or an image from an unknown source are prevented. </del>  The firmware image can be confidentiality protected so that attempts by an adversary to recover the plaintext binary can be <del> prevented.  Obtaining the firmware is often one of the first steps to mount an attack since it gives the adversary valuable insights into used software libraries, configuration settings and generic functionality (even though reverse engineering the binary can be a tedious process). This version of the document assumes asymmetric cryptography and a public key infrastructure.  Future versions may also describe a symmetric key approach for very constrained devices. </del>  While the standardization work has been informed by and optimised for firmware update use cases of Class 1 devices (according to the device the architecture that restricts its use to only these constrained IoT devices.  Moreover, this architecture is not limited to managing software updates, but can also be applied to managing the delivery of <del> arbitrary data, such as configuration information and keys. More details about the security goals are discussed in architecture and requirements are described in requirements. </del>  2.  This document uses the following terms: Manifest: The manifest contains meta-data about the firmware contains more than one microcontroller.  Often it is also a compressed archive that contains code, configuration data, and even the entire file system.  The image may consist of a <del> differential update for performance reasons.  Firmware is the more universal term.  The terms, firmware image, firmware, and image, are used in this document and are interchangeable. </del>  <del> Software: The terms "software" and "firmware" are used interchangeably. Bootloader: A bootloader is a piece of software that is executed once a microcontroller has been reset.  It is responsible for deciding whether to boot a firmware image that is present or whether to obtain and verify a new firmware image.  Since the bootloader is a security critical component its functionality may be split into separate stages.  Such a multi-stage bootloader may offer very basic functionality in the first stage and resides in ROM whereas the second stage may implement more complex functionality and resides in flash memory so that it can be updated in the future (in case bugs have been found).  The exact split of components into the different stages, the number of firmware images stored by an IoT device, and the detailed functionality varies throughout different implementations.  A more detailed discussion is provided in bootloader. </del>  Microcontroller (MCU for microcontroller unit): An MCU is a compact integrated circuit designed for use in embedded systems. A typical microcontroller includes a processor, memory (RAM and flash), input/output (I/O) ports and other features connected via some bus on a single chip.  The term 'system on chip (SoC)' is <del> often used for these types of devices. </del>  System on Chip (SoC): An SoC is an integrated circuit that <del> integrates all components of a computer, such as CPU, memory, input/output ports, secondary storage, etc. </del>  Homogeneous Storage Architecture (HoSA): A device that stores all firmware components in the same way, for example in a file system Heterogeneous Storage Architecture (HeSA): A device that stores at least one firmware component differently from the rest, for <del> example a device with an external, updatable radio, or a device </del>  with internal and external flash memory. Trusted Execution Environments (TEEs): An execution environment <del> that runs alongside of, but is isolated from, an REE. </del>  Rich Execution Environment (REE): An environment that is provided and governed by a typical OS (e.g., Linux, Windows, Android, iOS), Trusted applications (TAs): An application component that runs in a TEE. <del> For more information about TEEs see I-D.ietf-teep-architecture.  TEEP requires the use of SUIT for delivering TAs. </del>  <del> The following entities are used: </del>  Author: The author is the entity that creates the firmware image. <del> There may be multiple authors in a system either when a device consists of multiple micro-controllers or when the the final firmware image consists of software components from multiple companies. </del>  Firmware Consumer: The firmware consumer is the recipient of the firmware image and the manifest.  It is responsible for parsing firmware image.  The firmware consumer plays the role of the update component on the IoT device typically running in the application firmware.  It interacts with the firmware server and <del> with the status tracker, if present. (IoT) Device: A device refers to the entire IoT product, which consists of one or many MCUs, sensors and/or actuators.  Many IoT devices sold today contain multiple MCUs and therefore a single device may need to obtain more than one firmware image and manifest to succesfully perform an update.  The terms device and firmware consumer are used interchangably since the firmware consumer is one software component running on an MCU on the device. Status Tracker: The status tracker offers device management functionality to retrieve information about the installed firmware on a device and other device characteristics (including free memory and hardware components), to obtain the state of the firmware update cycle the device is currently in, and to trigger the update process.  The deployment of status trackers is flexible and they may be used as cloud-based servers, on-premise servers, embedded in edge computing device (such as Internet access gateways or protocol translation gateways), or even in smart phones and tablets.  IoT devices that self-initiate updates may run a status tracker.  Similarly, IoT devices that act as a proxy for other IoT devices in a protocol translation or edge computing device node may also run a status tracker.  However, if the device contains multiple MCUs, the main MCU may act as a limited status tracker towards the other MCUs if updates are to be synchronized across MCUs.  How much functionality a status tracker includes depends on the selected configuration of the device management functionality and the communication environment it is used in.  In a generic networking environment the protocol used between the client and the server-side of the status tracker need to deal with Internet communication challenges involving firewall and NAT traversal.  In other cases, the communication interaction may be rather simple.  This architecture document does not impose requirements on the status tracker. </del>  Firmware Server: The firmware server stores firmware images and manifests and distributes them to IoT devices.  Some deployments may require a store-and-forward concept, which requires storing the firmware images/manifests on more than one entity before they reach the device.  There is typically some interaction <del> between the firmware server and the status tracker but those </del>  entities are often physically separated on different devices for scalability reasons. <del> Device Operator: The actor responsible for the day-to-day operation of a fleet of IoT devices. Network Operator: The actor responsible for the operation of a network to which IoT devices connect. Claim: A piece of information asserted about a recipient or payload. In addition to the entities in the list above there is an orthogonal infrastructure with a Trust Provisioning Authority (TPA) distributing trust anchors and authorization permissions to various entities in the system.  The TPA may also delegate rights to install, update, enhance, or delete trust anchors and authorization permissions to other parties in the system.  This infrastructure overlaps the communication architecture and different deployments may empower certain entities while other deployments may not.  For example, in some cases, the Original Design Manufacturer (ODM), which is a company that designs and manufactures a product, may act as a TPA and may decide to remain in full control over the firmware update process of their products. The terms 'trust anchor' and 'trust anchor store' are defined in RFC6024: "A trust anchor represents an authoritative entity via a public key and associated data.  The public key is used to verify digital signatures, and the associated data is used to constrain the types of information for which the trust anchor is authoritative." "A trust anchor store is a set of one or more trust anchors stored in a device.  A device may have more than one trust anchor store, each of which may be used by one or more applications."  A trust anchor store must resist modification against unauthorized insertion, deletion, and modification. </del>  3. <del> The firmware update mechanism described in this specification was designed with the following requirements in mind: Agnostic to how firmware images are distributed Friendly to broadcast delivery Use state-of-the-art security mechanisms Rollback attacks must be prevented High reliability Operate with a small bootloader Small Parsers Minimal impact on existing firmware formats Robust permissions </del>  <del> Diverse modes of operation </del>  <del> Suitability to software and personalization data </del>  <del> 3.1. </del>  Firmware images can be conveyed to devices in a variety of ways, <del> including USB, UART, WiFi, BLE, low-power WAN technologies, etc.  and use different protocols (e.g., CoAP, HTTP).  The specified mechanism needs to be agnostic to the distribution of the firmware images and manifests. 3.2. This architecture does not specify any specific broadcast protocol. However, given that broadcast may be desirable for some networks, updates must cause the least disruption possible both in metadata and firmware transmission. </del>  For an update to be broadcast friendly, it cannot rely on link layer, network layer, or transport layer security.  A solution has to rely on security protection applied to the manifest and firmware image use of third-party content distribution networks for payload distribution. <del> 3.3. End-to-end security between the author and the device is shown in architecture. Authentication ensures that the device can cryptographically identify the author(s) creating firmware images and manifests.  Authenticated identities may be used as input to the authorization process. Integrity protection ensures that no third party can modify the manifest or the firmware image. For confidentiality protection of the firmware image, it must be done in such a way that every intended recipient can decrypt it.  The information that is encrypted individually for each device must maintain friendliness to Content Distribution Networks, bulk storage, and broadcast protocols. A manifest specification must support different cryptographic algorithms and algorithm extensibility.  Due of the nature of unchangeable code in ROM for use with bootloaders the use of post- quantum secure signature mechanisms, such as hash-based signatures RFC8778, are attractive.  These algorithms maintain security in presence of quantum computers. A mandatory-to-implement set of algorithms will be specified in the manifest specification I-D.ietf-suit-manifest}. 3.4. A device presented with an old, but valid manifest and firmware must not be tricked into installing such firmware since a vulnerability in the old firmware image may allow an attacker to gain control of the device. 3.5. A power failure at any time must not cause a failure of the device. Equally, adverse network conditions during an update must not cause the failure of the device.  A failure to validate any part of an update must not cause a failure of the device.  One way to achieve this functionality is to provide a minimum of two storage locations for firmware and one bootable location for firmware.  An alternative approach is to use a 2nd stage bootloader with build-in full featured firmware update functionality such that it is possible to return to the update process after power down. Note: This is an implementation requirement rather than a requirement on the manifest format. </del>  <del> 3.6. Throughout this document we assume that the bootloader itself is distinct from the role of the firmware consumer and therefore does not manage the firmware update process.  This may give the impression that the bootloader itself is a completely separate component, which is mainly responsible for selecting a firmware image to boot. </del>  <del> The overlap between the firmware update process and the bootloader functionality comes in two forms, namely First, a bootloader must verify the firmware image it boots as part of the secure boot process.  Doing so requires meta-data to be stored alongside the firmware image so that the bootloader can cryptographically verify the firmware image before booting it to ensure it has not been tampered with or replaced.  This meta-data used by the bootloader may well be the same manifest obtained with the firmware image during the update process (with the severable fields stripped off). Second, an IoT device needs a recovery strategy in case the firmware update / boot process fails.  The recovery strategy may include storing two or more firmware images on the device or offering the ability to have a second stage bootloader perform the firmware update process again using firmware updates over serial, USB or even wireless connectivity like a limited version of Bluetooth Smart.  In the latter case the firmware consumer functionality is contained in the second stage bootloader and requires the necessary functionality for executing the firmware update process, including manifest parsing. In general, it is assumed that the bootloader itself, or a minimal part of it, will not be updated since a failed update of the bootloader poses a risk in reliability. All information necessary for a device to make a decision about the installation of a firmware update must fit into the available RAM of a constrained IoT device.  This prevents flash write exhaustion. This is typically not a difficult requirement to accomplish because there are not other task/processing running while the bootloader is active (unlike it may be the case when running the application firmware). Note: This is an implementation requirement. 3.7. Since parsers are known sources of bugs, any parsers used to process the manifest must be minimal.  Additionally, it must be easy to parse only those fields that are required to validate at least one signature or MAC with minimal exposure. 3.8. The design of the firmware update mechanism must not require changes to existing firmware formats. 3.9. When a device obtains a monolithic firmware image from a single author without any additional approval steps then the authorization flow is relatively simple.  There are, however, other cases where more complex policy decisions need to be made before updating a device. In this architecture the authorization policy is separated from the underlying communication architecture.  This is accomplished by separating the entities from their permissions.  For example, an author may not have the authority to install a firmware image on a device in critical infrastructure without the authorization of a device operator.  In this case, the device may be programmed to reject firmware updates unless they are signed both by the firmware author and by the device operator. Alternatively, a device may trust precisely one entity, which does all permission management and coordination.  This entity allows the device to offload complex permissions calculations for the device. 3.10. There are three broad classifications of update operating modes. Client-initiated Update Server-initiated Update Hybrid Update Client-initiated updates take the form of a firmware consumer on a device proactively checking (polling) for new firmware images. Server-initiated updates are important to consider because timing of updates may need to be tightly controlled in some high- reliability environments.  In this case the status tracker determines what devices qualify for a firmware update.  Once those devices have been selected the firmware server distributes updates to the firmware consumers. Note: This assumes that the status tracker is able to reach the device, which may require devices to keep reachability information at the status tracker up-to-date.  This may also require keeping state at NATs and stateful packet filtering firewalls alive. Hybrid updates are those that require an interaction between the firmware consumer and the status tracker.  The status tracker pushes notifications of availability of an update to the firmware consumer, and it then downloads the image from a firmware server as soon as possible. While these broad classifications encompass the majority of operating modes, some may not be covered in these classifications.  By reinterpreting these modes as a set of operations performed by the system as a whole, all operating modes can be represented. The steps performed in the course of an update by the system containing an updatable device are: Notification Pre-authorisation Dependency resolution Download Installation This is a coarse-grained high level view of steps required to install a new firmware.  By considering where in the system each of these steps is performed, each operating mode can be represented.  Each of these steps is broken down into smaller constituent parts. architecture defines the steps taken from the perspective of the communication between actors in the system. bootloader describes some additional steps that a bootloader takes in addition to those described here. example shows an example of the steps undertaken by each party in the course of an update. The notification step consists of the status tracker informing the firmware consumer that an update is available.  This can be accomplished via polling (client-initiated), push notifications (server-initiated), or more complex mechanisms. </del>  The pre-authorisation step involves verifying whether the entity signing the manifest is indeed authorized to perform an update.  The initiates the download process.  When a download is server-initiated, this means that the status tracker tells the device when to download or that it initiates the transfer directly to the firmware consumer. <del> For example, a download from an HTTP-based firmware server is client- initiated.  Pushing a manifest and firmware image to the transfer to the Package resource of the LwM2M Firmware Update object LwM2M is server-initiated. </del>  If the firmware consumer has downloaded a new firmware image and is <del> ready to install it, it may need to wait for a trigger from the status tracker to initiate the installation, may trigger the update automatically, or may go through a more complex decision making process to determine the appropriate timing for an update (such as delaying the update process to a later time when end users are less impacted by the update process). </del>  Installation is the act of processing the payload into a format that the IoT device can recognise and the bootloader is responsible for then booting from the newly installed firmware image. <del> Each of these steps may require different permissions. 3.11. The work on a standardized manifest format initially focused on the most constrained IoT devices and those devices contain code put together by a single author (although that author may obtain code from other developers, some of it only in binary form). Later it turns out that other use cases may benefit from a standardized manifest format also for conveying software and even personalization data alongside software.  Trusted Execution Environments (TEEs), for example, greatly benefit from a protocol for managing the lifecycle of trusted applications (TAs) running inside a TEE.  TEEs may obtain TAs from different authors and those TAs may require personalization data, such as payment information, to be securely conveyed to the TEE. To support this wider range of use cases the manifest format should therefore be extensible to convey other forms of payloads as well. </del> 4. <del> The information conveyed from an Author to a Firmware Consumer can be considered to be Claims as described in RFC7519 and RFC8392.  The same security considerations apply to the Claims expressed in the manifest.  The chief difference between manifest Claims and CWT or JWT claims is that a manifest has multiple subjects.  The manifest contains: Claims about the Firmware, including its dependencies Claims about the Firmware Consumer's physical or software properties </del>  <del> Claims about the Author, or the Author's delegate </del>  <del> The credential used to authenticate these Claims must be directly or indirectly related to the trust anchor installed at the device by the Trust Provisioning Authority. The baseline claims for all manifests are described in I-D.ietf-suit- information-model. 5. arch-figure shows the communication architecture where a firmware image is created by an author, and uploaded to a firmware server. The firmware image/manifest is distributed to the device either in a push or pull manner using the firmware consumer residing on the device.  The device operator keeps track of the process using the status tracker.  This allows the device operator to know and control what devices have received an update and which of them are still pending an update. End-to-end security mechanisms are used to protect the firmware image and the manifest although e2e-figure does not show the manifest itself since it may be distributed independently. Whether the firmware image and the manifest is pushed to the device or fetched by the device is a deployment specific decision. The following assumptions are made to allow the firmware consumer to verify the received firmware image and manifest before updating software: To accept an update, a device needs to verify the signature covering the manifest.  There may be one or multiple manifests that need to be validated, potentially signed by different parties.  The device needs to be in possession of the trust anchors to verify those signatures.  Installing trust anchors to devices via the Trust Provisioning Authority happens in an out-of- band fashion prior to the firmware update process. Not all entities creating and signing manifests have the same permissions.  A device needs to determine whether the requested action is indeed covered by the permission of the party that signed the manifest.  Informing the device about the permissions of the different parties also happens in an out-of-band fashion and is also a duty of the Trust Provisioning Authority. For confidentiality protection of firmware images the author needs to be in possession of the certificate/public key or a pre-shared key of a device.  The use of confidentiality protection of firmware images is deployment specific. There are different types of delivery modes, which are illustrated based on examples below. There is an option for embedding a firmware image into a manifest. This is a useful approach for deployments where devices are not connected to the Internet and cannot contact a dedicated firmware server for the firmware download.  It is also applicable when the firmware update happens via a USB stick or via Bluetooth Smart. attached-firmware-figure shows this delivery mode graphically. online-firmware-figure shows an option for remotely updating a device where the device fetches the firmware image from some file server. The manifest itself is delivered independently and provides information about the firmware image(s) to download. This architecture does not mandate a specific delivery mode but a solution must support both types. </del>  <del> 6. </del>  <del> In order for a device to apply an update, it has to make several decisions about the update: </del>  <del> Does it trust the author of the update? </del>  <del> Has the firmware been corrupted? </del>  <del> Does the firmware update apply to this device? </del>  <del> Is the update older than the active firmware? </del>  <del> When should the device apply the update? </del>  <del> How should the device apply the update? </del>  <del> What kind of firmware binary is it? </del>  <del> Where should the update be obtained? </del>  <del> Where should the firmware be stored? </del>  <del> The manifest encodes the information that devices need in order to make these decisions.  It is a data structure that contains the following information: </del>  <del> information about the device(s) the firmware image is intended to be applied to, </del>  <del> information about when the firmware update has to be applied, </del>  <del> information about when the manifest was created, </del>  <del> dependencies on other manifests, </del>  <del> pointers to the firmware image and information about the format, </del>  <del> information about where to store the firmware image, </del>  <del> cryptographic information, such as digital signatures or message authentication codes (MACs). </del>  <del> The manifest information model is described in I-D.ietf-suit- information-model. </del>  <del> 7. </del>  <del> Although these documents attempt to define a firmware update architecture that is applicable to both existing systems, as well as yet-to-be-conceived systems; it is still helpful to consider existing architectures. </del>  <del> 7.1. </del>  The simplest, and currently most common, architecture consists of a single MCU along with its own peripherals.  These SoCs generally contain some amount of flash memory for code and fixed data, as well <del> as RAM for working storage.  These systems either have a single firmware image, or an immutable bootloader that runs a single image. A notable characteristic of these SoCs is that the primary code is generally execute in place (XIP).  Combined with the non-relocatable nature of the code, firmware updates need to be done in place. </del>  <del> 7.2. </del>  Another configuration consists of a similar architecture to the previous, with a single CPU.  However, this CPU supports a security either image independently, as well as to update them together atomically, as specified in the associated manifests. <del> 7.3. </del>  In more complex SoCs with symmetric multi-processing support, advanced operating systems, such as Linux, are often used.  These <del> SoCs frequently use an external storage medium such as raw NAND flash or eMMC.  Due to the higher quantity of resources, these devices are often capable of storing multiple copies of their firmware images and selecting the most appropriate one to boot.  Many SoCs also support bootloaders that are capable of updating the firmware image, however this is typically a last resort because it requires the device to be held in the bootloader while the new firmware is downloaded and installed, which results in down-time for the device.  Firmware updates in this class of device are typically not done in-place. 7.4. </del>  This configuration has two or more heterogeneous CPUs in a single SoC <del> that share memory (flash and RAM).  Generally, they will be a protection mechanism to prevent one CPU from accessing the other's memory.  Upgrades in this case will typically be done by one of the CPUs, and is similar to the single CPU with secure mode. </del>  <del> 7.5. </del>  This configuration has two or more heterogeneous CPUs, each having their own memory.  There will be a communication channel between happening on multiple CPUs, upgrading the two images atomically is challenging. <del> 8. </del>  <del> More devices today than ever before are being connected to the Internet, which drives the need for firmware updates to be provided over the Internet rather than through traditional interfaces, such as USB or RS232.  Updating a device over the Internet requires the device to fetch not only the firmware image but also the manifest. Hence, the following building blocks are necessary for a firmware update solution: </del>  <del> the Internet protocol stack for firmware downloads (*), </del>  <del> the capability to write the received firmware image to persistent storage (most likely flash memory) prior to performing the update, the ability to unpack, decompress or otherwise process the received firmware image, the features to verify an image and a manifest, including digital signature verification or checking a message authentication code, a manifest parsing library, and integration of the device into a device management server to perform automatic firmware updates and to track their progress. (*) Because firmware images are often multiple kilobytes, sometimes exceeding one hundred kilobytes, in size for low end IoT devices and even several megabytes large for IoT devices running full-fledged operating systems like Linux, the protocol mechanism for retrieving these images needs to offer features like congestion control, flow control, fragmentation and reassembly, and mechanisms to resume interrupted or corrupted transfers. All these features are most likely offered by the application, i.e. firmware consumer, running on the device (except for basic security algorithms that may run either on a trusted execution environment or on a separate hardware security MCU/module) rather than by the bootloader itself. Once manifests have been processed and firmware images successfully downloaded and verified the device needs to hand control over to the bootloader.  In most cases this requires the MCU to restart.  Once the MCU has initiated a restart, the bootloader takes over control and determines whether the newly downloaded firmware image should be executed. The boot process is security sensitive because the firmware images may, for example, be stored in off-chip flash memory giving attackers easy access to the image for reverse engineering and potentially also for modifying the binary.  The bootloader will therefore have to perform security checks on the firmware image before it can be booted.  These security checks by the bootloader happen in addition to the security checks that happened when the firmware image and the manifest were downloaded. The manifest may have been stored alongside the firmware image to allow re-verification of the firmware image during every boot attempt.  Alternatively, secure boot-specific meta-data may have been created by the application after a successful firmware download and verification process.  Whether to re-use the standardized manifest format that was used during the initial firmware retrieval process or whether it is better to use a different format for the secure boot- specific meta-data depends on the system design.  The manifest format does, however, have the capability to serve also as a building block for secure boot with its severable elements that allow shrinking the size of the manifest by stripping elements that are no longer needed. In order to satisfy the reliability requirements defined in reliability, devices must always be able to return to a working firmware image.  This has implications for the design of the bootloader: If the firmware image contains the firmware consumer functionality, as described above, then the bootloader must be able to roll back to a working firmware image.  Alternatively, the bootloader may have enough functionality to fetch a firmware image plus manifest from a firmware server over the Internet.  A multi- stage bootloader may soften this requirement at the expense of a more sophisticated boot process. For a bootloader to offer a secure boot mechanism it needs to provide the following features: ability to access security algorithms, such as SHA-256 to compute a fingerprint over the firmware image and a digital signature algorithm. access keying material directly or indirectly to utilize the digital signature.  The device needs to have a trust anchor store. ability to expose boot process-related data to the application firmware (such as to the device management software).  This allows a device management server to determine whether the firmware update has been successful and, if not, what errors occurred. to (optionally) offer attestation information (such as measurements). While the software architecture of the bootloader and its security mechanisms are implementation-specific, the manifest can be used to control the firmware download from the Internet in addition to augmenting secure boot process.  These building blocks are highly relevant for the design of the manifest. </del>  <del> 9. </del>  <del> firmware-update illustrates an example message flow for distributing a firmware image to a device starting with an author uploading the new firmware to firmware server and creating a manifest.  The firmware and manifest are stored on the same firmware server.  This setup does not use a status tracker and the firmware consumer component is therefore responsible for periodically checking whether a new firmware image is available for download. firmware-update2 shows an example follow with the device using a status tracker.  For editorial reasons the author publishing the manifest at the status tracker and the firmware image at the firmware server is not shown.  Also omitted is the secure boot process following the successful firmware update process. The exchange starts with the device interacting with the status tracker; the details of such exchange will vary with the different device management systems being used.  In any case, the status tracker learns about the firmware version of the devices it manages. In our example, the device under management is using firmware version A.B.C.  At a later point in time the author uploads a new firmware along with the manifest to the firmware server and the status tracker, respectively.  While there is no need to store the manifest and the firmware on different servers this example shows a common pattern used in the industry.  The status tracker may then automatically, based on human intervention or based on a more complex policy decide to inform the device about the newly available firmware image.  In our example, it does so by pushing the manifest to the firmware consumer.  The firmware consumer downloads the firmware image with the newer version X.Y.Z after successful validation of the manifest.  Subsequently, a reboot is initiated and the secure boot process starts. </del>  <del> 10. </del>  <del> This document does not require any actions by IANA. </del>  <del> 11. </del>  <del> Firmware updates fix security vulnerabilities and are considered to be an important building block in securing IoT devices.  Due to the importance of firmware updates for IoT devices the Internet Architecture Board (IAB) organized a 'Workshop on Internet of Things (IoT) Software Update (IOTSU)', which took place at Trinity College Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at the big picture.  A report about this workshop can be found at RFC8240.  A standardized firmware manifest format providing end-to- end security from the author to the device will be specified in a separate document. There are, however, many other considerations raised during the workshop.  Many of them are outside the scope of standardization organizations since they fall into the realm of product engineering, regulatory frameworks, and business models.  The following considerations are outside the scope of this document, namely installing firmware updates in a robust fashion so that the update does not break the device functionality of the environment this device operates in. </del>  <del> installing firmware updates in a timely fashion considering the complexity of the decision making process of updating devices, potential re-certification requirements, and the need for user consent to install updates. </del>  <del> the distribution of the actual firmware update, potentially in an efficient manner to a large number of devices without human involvement. </del>  <del> energy efficiency and battery lifetime considerations. </del>  <del> key management required for verifying the digital signature protecting the manifest. </del>  <del> incentives for manufacturers to offer a firmware update mechanism as part of their IoT products. </del>

First chunk of changes.
This is a really great set of edits. I think that it really improves the document as a whole! I have suggested a couple of minor edits.Quick partial language review

A Firmware Update Architecture for Internet of Things  <ins> draft-ietf-suit-architecture-13 </ins> Abstract Vulnerabilities with Internet of Things (IoT) devices have raised the  <ins> need for a reliable and secure firmware update mechanism suitable for devices with resource constraints.  Incorporating such update </ins> mechanism to fix vulnerabilities, to update configuration settings as well as adding new functionality is recommended by security experts.  <ins> In addition to the definition of terminology and an architecture this document motivates the standardization of a manifest format as transport-agnostic means for describing and protecting firmware updates. </ins> 1.  <ins> Firmware updates can help to fix security vulnerabilities and are considered to be an important building block in securing IoT devices. Due to rising concerns about insecurre IoT devices the Internet Architecture Board (IAB) organized a 'Workshop on Internet of Things (IoT) Software Update (IOTSU)', which took place at Trinity College Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at the bigger picture.  A report about this workshop can be found at RFC8240.  The workshop revealed a number of challenges for developers and led to the formation of the IETF Software Updates for Internet of Things (SUIT) working group. Developing secure Internet of Things (IoT) devices is not an easy task and supporting a firmware update solution requires skillful engineers.  Once devices are deployed, firmware updates play a critical part in their lifecycle management, particularly when devices have a long lifetime, are deployed in remote or inaccessible areas where manual intervention is cost prohibitive or otherwise difficult.  Firmware updates for IoT devices are expected to work automatic, i.e. without user involvement.  Automatic updates that do not require human intervention are key to a scalable solution for fixing software vulnerabilities. Firmware updates are not only done to fix bugs, but they can also add new functionality, and to re-configure the device to work in new </ins> environments or to behave differently in an already deployed context.  <ins> The firmware update process has to ensure that </ins> The firmware image is authenticated and integrity protected.  <ins> Attempts to flash a maliciously modified firmware image or an image from an unknown, untrusted source must be prevented.  In examples this document uses asymmetric cryptography because it is the preferred approach by many IoT deployments.  The use of symmetric credentials is also supported and can be used by very constrained IoT devices. </ins> The firmware image can be confidentiality protected so that attempts by an adversary to recover the plaintext binary can be  <ins> mitigated or at least made more difficult.  Obtaining the firmware is often one of the first steps to mount an attack since it gives the adversary valuable insights into used software libraries, configuration settings and generic functionality.  Even though reverse engineering the binary can be a tedious process modern reverse engineering frameworks have made this task a lot easier. </ins> While the standardization work has been informed by and optimised for firmware update use cases of Class 1 devices (according to the device the architecture that restricts its use to only these constrained IoT devices.  Moreover, this architecture is not limited to managing software updates, but can also be applied to managing the delivery of  <ins> arbitrary data, such as configuration information and keys.  Unlike higher end devices, like laptops and desktop PCs, many IoT devices do not have user interfaces and support for unattended updates are therefore essential for the design of a practical solution. Constrained IoT devices often use a software engineering model where a developer is responsible for creating and compiling all software running on the device into a single, monolithic firmware image.  On higher end devices application software is, on the other hand, often downloaded separately and even obtained from developers different to the developers of the lower level software.  The details for how to obtain those application layer software binaries then depends heavily on the platform, programming language uses and the sandbox the software is executed in. While the IETF standardization work has been focused on the manifest format, a fully interoperable solution needs more than a standardized manifest.  For example, protocols for transferring firmware images and manifests to the device need to be available as well as the status tracker functionality.  These building blocks have been developed by various organizations under the umbrella of an IoT device management solution.  The LwM2M protocol is one IoT device management protocol. There are, however, several areas that (partially) fall outside the scope of the IETF and other standards organizations but need to be considered by firmware authors, as well as device and network operators.  Here are some of them, as highlighted during the IOTSU workshop: Installing firmware updates in a robust fashion so that the update does not break the device functionality of the environment this device operates in.  This requires proper testing and offering recovery strategies when a firmware update is unsuccessful. Making firmware updates available in a timely fashion considering the complexity of the decision making process for updating devices, potential re-certification requirements, the length of a supply chain an update needs to go through before it reaches the end customer, and the need for user consent to install updates. Ensuring an energy efficient design of a battery-powered IoT devices because a firmware update, particularly writing the firmware image to flash, is a heavy task for a device. Creating incentives for device operators to use a firmware update mechanism and to demand the integration of it from IoT device vendors. This document starts with a terminology followed by the description of the architecture.  We then explain the bootloader and how it integrates with the firmware update mechanism.  Subsequently, we offer a categorization of IoT devices in terms of their hardware capabilities relevant for firmware updates.  Next, we talk about the manifest structure and how to use it to secure firmware updates.  We conclude with a more detailed example. </ins> 2. <ins> 2.1. </ins> This document uses the following terms: Manifest: The manifest contains meta-data about the firmware contains more than one microcontroller.  Often it is also a compressed archive that contains code, configuration data, and even the entire file system.  The image may consist of a  <ins> differential update for performance reasons. </ins>  <ins> The terms, firmware image, firmware, and image, are used in this document and are interchangeable.  We use the term application firmware image to differentiate it from a firmware image that contains the bootloader.  An application firmware image, as the name indicates, contains the application program often including all the necessary code to run it (such as protocol stacks, and embedded operating system). </ins> Microcontroller (MCU for microcontroller unit): An MCU is a compact integrated circuit designed for use in embedded systems. A typical microcontroller includes a processor, memory (RAM and flash), input/output (I/O) ports and other features connected via some bus on a single chip.  The term 'system on chip (SoC)' is  <ins> often used interchangeably with MCU, but MCU tends to imply more limited peripheral functions. </ins> System on Chip (SoC): An SoC is an integrated circuit that  <ins> contains all components of a computer, such as CPU, memory, input/ output ports, secondary storage, a bus to connect the components, and other hardware blocks of logic. </ins> Homogeneous Storage Architecture (HoSA): A device that stores all firmware components in the same way, for example in a file system Heterogeneous Storage Architecture (HeSA): A device that stores at least one firmware component differently from the rest, for  <ins> example a device with an external, updateable radio, or a device </ins> with internal and external flash memory. Trusted Execution Environments (TEEs): An execution environment  <ins> that runs alongside of, but is isolated from, an REE.  For more information about TEEs see I-D.ietf-teep-architecture. </ins> Rich Execution Environment (REE): An environment that is provided and governed by a typical OS (e.g., Linux, Windows, Android, iOS), Trusted applications (TAs): An application component that runs in a TEE.  <ins> Trust Anchor: A trust anchor, as defined in RFC6024, represents an authoritative entity via a public key and associated data.  The public key is used to verify digital signatures, and the associated data is used to constrain the types of information for which the trust anchor is authoritative." </ins>  <ins> Trust Anchor Store: A trust anchor store, as defined in RFC6024, is a set of one or more trust anchors stored in a device.  A device may have more than one trust anchor store, each of which may be used by one or more applications.  A trust anchor store must resist modification against unauthorized insertion, deletion, and modification. 2.2. The following stakeholders are used in this document: </ins> Author: The author is the entity that creates the firmware image.  <ins> There may be multiple authors involved in producing firmware running on an IoT device. device talks about those IoT device deployment cases. Device Operator: The device operator is responsible for the day- to-day operation of a fleet of IoT devices.  Customers of IoT devices, as the owners of IoT devices - such as enterprise customers or end users, interact with their IoT devices indirectly through the device operator via web or smart phone apps. Network Operator: The network operator is responsible for the operation of a network to which IoT devices connect. Trust Provisioning Authority (TPA): The TPA distributes trust anchors and authorization policies to various stakeholders.  The TPA may also delegate rights to stakeholders.  For example, in some cases, the Original Design Manufacturer (ODM), which is a company that designs and manufactures a product, may act as a TPA and may decide to remain in full control over the firmware update process of their products. 2.3. (IoT) Device: A device refers to the entire IoT product, which consists of one or many MCUs, sensors and/or actuators.  Many IoT devices sold today contain multiple MCUs and therefore a single device may need to obtain more than one firmware image and manifest to succesfully perform an update. Status Tracker: The status tracker has a client and a server component and performs three tasks: 1) It communicates the availability of a new firmware version.  This information will flow from the server to the client. 2) It conveys information about software and hardware characteristics of the device.  The information flow is from the client to the server. 3) It can remotely trigger the firmware update process.  The information flow is from the server to the client. For example, a device operator may want to read the installed firmware version number running on the device and information about available flash memory.  Once an update has been triggered, the device operator may want to obtain information about the state of the firmware update.  If errors occurred, the device operator may want to troubleshoot problems by first obtaining diagnostic information (typically using a device management protocol). We make no assumptions about where the server-side component is deployed.  The deployment of status trackers is flexible and may be found at cloud-based servers, on-premise servers, or may be embedded in edge computing device.  A status tracker server component may even be deployed on an IoT device.  For example, if the IoT device contains multiple MCUs, then the main MCU may act as a status tracker towards the other MCUs.  Such deployment is useful when updates have to be synchronized across MCUs. </ins> Firmware Consumer: The firmware consumer is the recipient of the firmware image and the manifest.  It is responsible for parsing firmware image.  The firmware consumer plays the role of the update component on the IoT device typically running in the application firmware.  It interacts with the firmware server and  <ins> with the status tracker client (locally). </ins> Firmware Server: The firmware server stores firmware images and manifests and distributes them to IoT devices.  Some deployments may require a store-and-forward concept, which requires storing the firmware images/manifests on more than one entity before they reach the device.  There is typically some interaction  <ins> between the firmware server and the status tracker and these two </ins> entities are often physically separated on different devices for scalability reasons.  <ins> Bootloader: A bootloader is a piece of software that is executed once a microcontroller has been reset.  It is responsible for deciding what code to execute. </ins> 3.  <ins> More devices today than ever before are connected to the Internet, which drives the need for firmware updates to be provided over the Internet rather than through traditional interfaces, such as USB or RS-232.  Updating updates over the Internet requires the device to fetch the new firmware image as well as the manifest. </ins>  <ins> Hence, the following components are necessary on a device for a firmware update solution: </ins>  <ins> the Internet protocol stack for firmware downloads.  Because firmware images are often multiple kilobytes, sometimes exceeding one hundred kilobytes, in size for low end IoT devices and even several megabytes large for IoT devices running full-fledged operating systems like Linux, the protocol mechanism for retrieving these images needs to offer features like congestion control, flow control, fragmentation and reassembly, and mechanisms to resume interrupted or corrupted transfers. </ins>  <ins> the capability to write the received firmware image to persistent storage (most likely flash memory). a manifest parser with code to verify a digital signature or a message authentication code. the ability to unpack, to decompress and/or to decrypt the received firmware image. (optionally) a status tracker. The features listed above are most likely offered by code in the application firmware image running on the device rather than by the bootloader itself.  Note that cryptographic algorithms will likely run in a trusted execution environment, on a separate MCU, in a hardware security module, or in a secure element rather than in the same context with the application code. arch-figure shows the architecture where a firmware image is created by an author, and made available to a firmware server.  For security reasons, the author will not have the permissions to upload firmware images to the firmware server and to initiate an update him- or herself.  Instead, authors will make firmware images available to the device operators.  Note that there may be a longer supply chain involved to pass software updates from the author all the way to the party that can then finally make a decision to deploy it with IoT devices. As a first step in the firmware update process, the the status tracker client need to be made aware of the availability of a new firmware update by the status tracker server.  This can be accomplished via polling (client-initiated), push notifications (server-initiated), or more complex mechanisms (such as a hybrid approach): Client-initiated updates take the form of a status tracker client proactively checking (polling) for updates. With Server-initiated updates the server-side component of the status tracker learns about a new firmware version and determines what devices qualify for a firmware update.  Once the relevant devices have been selected, the status tracker informs these devices and the firmware consumers obtain those images and manifests.  Server-initiated updates are important because they allow a quick response time.  Note that the client-side status tracker needs to be reachable by the server-side component.  This may require devices to keep reachability information on the server-side up-to-date and state at NATs and stateful packet filtering firewalls alive. Using a hybrid approach the server-side of the status tracker pushes notifications of availability of an update to the client side and requests the firmware consumer to pull the manifest and the firmware image from the firmware server. Once the device operator triggers update via the status tracker, it will keep track of the update process on the device.  This allows the device operator to know what devices have received an update and which of them are still pending an update. </ins> Firmware images can be conveyed to devices in a variety of ways,  <ins> including USB, UART, WiFi, BLE, low-power WAN technologies, mesh networks and many more.  At the application layer a variety of protocols are also available: MQTT, CoAP, and HTTP are the most popular application layer protocols used by IoT devices.  This architecture does not make assumptions about how the firmware images are distributed to the devices and therefore aims to support all these technologies. In some cases it may be desireable to distribute firmware images using a multicast or broadcast protocol.  This architecture does not make recommendations for any such protocol.  However, given that broadcast may be desirable for some networks, updates must cause the least disruption possible both in metadata and firmware transmission. </ins> For an update to be broadcast friendly, it cannot rely on link layer, network layer, or transport layer security.  A solution has to rely on security protection applied to the manifest and firmware image use of third-party content distribution networks for payload distribution.  <ins> Firmware images and manifests may be conveyed as a bundle or detached.  The manifest must support both approaches. </ins>  <ins> For distribution as a bundle, the firmware image is embedded into the manifest.  This is a useful approach for deployments where devices are not connected to the Internet and cannot contact a dedicated firmware server for the firmware download.  It is also applicable when the firmware update happens via a USB sticks or short range radio technologies (such as Bluetooth Smart). </ins>  <ins> Alternatively, the manifest is distributed detached from the firmware image.  Using this approach, the firmware consumer is presented with the manifest first and then needs to obtain one or more firmware images as dictated in the manifest. </ins> The pre-authorisation step involves verifying whether the entity signing the manifest is indeed authorized to perform an update.  The initiates the download process.  When a download is server-initiated, this means that the status tracker tells the device when to download or that it initiates the transfer directly to the firmware consumer.  <ins> For example, a download from an HTTP/1.1-based firmware server is client-initiated.  Pushing a manifest and firmware image to the Package resource of the LwM2M Firmware Update object LwM2M is server- initiated update. </ins> If the firmware consumer has downloaded a new firmware image and is  <ins> ready to install it, to initiate the installation, it may - either need to wait for a trigger from the status tracker, - or trigger the update automatically, - or go through a more complex decision making process to determine the appropriate timing for an update.  Sometimes the final decision may require confirmation of the user of the device for safety reasons. </ins> Installation is the act of processing the payload into a format that the IoT device can recognise and the bootloader is responsible for then booting from the newly installed firmware image.  4.  <ins> architecture describes the steps for getting the firmware image and the manifest from the author to the firmware consumer on the IoT device.  Once the firmware consumer has retrieved and successfully processed the manifest and the firmware image it needs to hand control over to the bootloader. </ins>  <ins> In most cases this requires the MCU to restart.  Once the MCU has initiated a restart, the bootloader determines whether a newly available firmware image should be executed. </ins>  <ins> A power failure at any time during a firmware update must not cause a failure of the device.  Equally, adverse network conditions during an update must not cause the failure of the device. </ins>  <ins> One way to achieve this functionality is to provide a minimum of two storage locations for firmware.  An alternative approach is to use a second stage bootloader with build-in full featured firmware update functionality such that it is possible to return to the update process after power down. </ins>  <ins> Assuming the first approach, there are (at least) three firmware images available on the device: </ins>  <ins> First, the bootloader is also firmware.  If a bootloader is updatable then its firmware image is treated like any other application firmware image. </ins>  <ins> Second, the firmware image that has to be replaced is still available on the device as a backup in case the freshly downloaded firmware image does not boot correctly. </ins>  <ins> Third, there is the newly downloaded firmware image. </ins>  <ins> Since many low end IoT devices use non-relocatable code, the bootloader needs to copy the newly downloaded application firmware image into the location of the old application firmware image and vice versa. </ins>  <ins> The boot process is security sensitive.  An attacker will typically try to retrieve a firmware image from the device for reverse engineering or will try to get the bootloader to excute an attacker- modified firmware image.  The bootloader will therefore have to perform security checks on the firmware image before it can be booted.  These security checks by the bootloader happen in addition to the security checks that took place when the firmware image and the manifest were downloaded by the firmware consumer. </ins>  <ins> Throughout this document we assume that the bootloader itself is distinct from the role of the firmware consumer and therefore does not manage the firmware update process.  This may give the impression that the bootloader itself is a completely separate component, which is mainly responsible for selecting a firmware image to boot. </ins>  <ins> The overlap between the firmware update process and the bootloader functionality comes in two forms, namely </ins>  <ins> A bootloader must verify the firmware image it boots as part of the secure boot process.  Doing so requires meta-data to be stored alongside the firmware image so that the bootloader can cryptographically verify the firmware image before booting it to ensure it has not been tampered with or replaced.  This meta-data used by the bootloader may well be the same manifest obtained with the firmware image during the update process (with the severable fields stripped off). </ins>  <ins> An IoT device needs a recovery strategy in case the firmware update / boot process fails.  As mentioned earlier, the recovery strategy may include storing two or more application firmware images on the device or offering the ability to have a second stage bootloader perform the firmware update process again using firmware updates over serial, USB or even wireless connectivity like Bluetooth Smart.  In the latter case the firmware consumer functionality is contained in the second stage bootloader and requires the necessary functionality for executing the firmware update process, including manifest parsing. </ins>  <ins> In general, it is assumed that the bootloader itself, or a minimal part of it, will not be updated since a failed update of the bootloader poses a reliability risk. </ins>  <ins> For a bootloader to offer a secure boot functionality it needs to implement the following functionality: </ins>  <ins> The bootloader needs to fetch the manifest (or manifest-alike headers) stored in flash memory alongside the firmware image and parse their content for subsequent cryptographic verification. </ins>  <ins> Cryptographic libraries with hash functions, digital signatures (for asymmetric crypto), keyed message digests (for symmetric crypto) need to be accessible. </ins>  <ins> The device needs to have a trust anchor store to verify the digital signature.  (Alternatively, access to a key store for use with the keyed message digest.) </ins>  <ins> Ability to expose boot process-related data to the application firmware (such as to the status tracker).  This allows to share information about the current firmware version, and the status of the firmware update process and whether errors have occurred. </ins>  <ins> Produce boot measurements as part of an attestation solution.  See I-D.ietf-rats-architecture for more information. (optional) </ins>  <ins> Ability to decrypt firmware images, in case confidentiality protection was applied).  This requires a solution for key management. (optional) </ins>  <ins> 5. </ins>  <ins> There are billions of MCUs used in devices today produced by a large number of silicon manufacturers.  While MCUs can vary significantly in their characteristics, there are a number of similiaries allowing us to categorize in groups. </ins>  <ins> The firmware update architecture, and the manifest format in particular, needs to offer enough flexibility to cover these common deployment cases. </ins>  <ins> 5.1. </ins> The simplest, and currently most common, architecture consists of a single MCU along with its own peripherals.  These SoCs generally contain some amount of flash memory for code and fixed data, as well  <ins> as RAM for working storage.  A notable characteristic of these SoCs is that the primary code is generally execute in place (XIP).  Due to the non-relocatable nature of the code, the firmware image needs to be placed in a specific location in flash since the code cannot be executed from an arbitrary location in flash.  Hence, then the firmware image is updated it is necessary to swap the old and the new image. </ins>  <ins> 5.2. </ins> Another configuration consists of a similar architecture to the previous, with a single CPU.  However, this CPU supports a security either image independently, as well as to update them together atomically, as specified in the associated manifests.  <ins> 5.3. </ins> In more complex SoCs with symmetric multi-processing support, advanced operating systems, such as Linux, are often used.  These  <ins> SoCs frequently use an external storage medium, such as raw NAND flash or eMMC.  Due to the higher quantity of resources, these devices are often capable of storing multiple copies of their firmware images and selecting the most appropriate one to boot.  Many SoCs also support bootloaders that are capable of updating the firmware image, however this is typically a last resort because it requires the device to be held in the bootloader while the new firmware is downloaded and installed, which results in down-time for the device.  Firmware updates in this class of device are typically not done in-place. 5.4. </ins> This configuration has two or more heterogeneous CPUs in a single SoC  <ins> that share memory (flash and RAM).  Generally, there will be a mechanism to prevent one CPU from unintentionally accessing memory currently allocated to the other.  Upgrades in this case will typically be done by one of the CPUs, and is similar to the single CPU with secure mode. </ins>  <ins> 5.5. </ins> This configuration has two or more heterogeneous CPUs, each having their own memory.  There will be a communication channel between happening on multiple CPUs, upgrading the two images atomically is challenging.  <ins> 6. </ins>  <ins> In order for a firmware consumer to apply an update, it has to make several decisions using manifest-provided information and data available on the device itself.  For more detailed information and a longer list of information elements in the manifest consult the information model specification I-D.ietf-suit-information-model, which offers justifications for each element, and the manifest, see I-D.ietf-suit-manifest}, for details about how this information is included in the manifest. manifest-info provides examples of decisions to be made. (1): A device presented with an old, but valid manifest and firmware must not be tricked into installing such firmware since a vulnerability in the old firmware image may allow an attacker to gain control of the device. Keeping the code size and complexity of a manifest parsers small is important for constrained IoT devices.  Since the manifest parsing code may also be used by the bootloader it is part of the trusted computing base. A manifest may not only be used to protect firmware images but also personalization data related to firmware or software.  Trusted Execution Environments (TEEs), for example, benefit from a protocol for managing the lifecycle of trusted applications (TAs) running inside a TEE.  TEEs may obtain TAs from different authors and those TAs may require personalization data, such as payment information, to be securely conveyed to the TEE. </ins>  <ins> 7. </ins>  <ins> Using firmware updates to fix vulnerabilities in devices is important but securing this update mechanism is equally important since security problems are exacerbated.  Failure to secure the firmware update process will help attackers to take control over devices. </ins>  <ins> End-to-end security mechanisms are used to protect the firmware image and the manifest.  The following assumptions are made to allow the firmware consumer to verify the received firmware image and manifest before updating software: Authentication ensures that the device can cryptographically identify the author(s) creating firmware images and manifests. Authenticated identities may be used as input to the authorization process.  Not all entities creating and signing manifests have the same permissions.  A device needs to determine whether the requested action is indeed covered by the permission of the party that signed the manifest.  Informing the device about the permissions of the different parties also happens in an out-of- band fashion and is also a duty of the Trust Provisioning Authority. Integrity protection ensures that no third party can modify the manifest or the firmware image.  To accept an update, a device needs to verify the signature covering the manifest.  There may be one or multiple manifests that need to be validated, potentially signed by different parties.  The device needs to be in possession of the trust anchors to verify those signatures.  Installing trust anchors to devices via the Trust Provisioning Authority happens in an out-of-band fashion prior to the firmware update process. For confidentiality protection of the firmware image, it must be done in such a way that the intended firmware consumer(s) can decrypt it.  The information that is encrypted individually for each device must maintain friendliness to Content Distribution Networks, bulk storage, and broadcast protocols.  For confidentiality protection of firmware images the author needs to be in possession of the certificate/public key or a pre-shared key of a device.  The use of confidentiality protection of firmware images is optional. </ins>  <ins> A manifest specification must support different cryptographic algorithms and algorithm extensibility.  Due of the nature of unchangeable code in ROM for bootloaders the use of post-quantum secure signature mechanisms, such as hash-based signatures RFC8778, are attractive.  These algorithms maintain security in presence of quantum computers. </ins>  <ins> When a device obtains a monolithic firmware image from a single author without any additional approval steps then the authorization flow is relatively simple.  There are, however, other cases where more complex policy decisions need to be made before updating a device. </ins>  <ins> In this architecture the authorization policy is separated from the underlying communication architecture.  This is accomplished by separating the entities from their permissions.  For example, an author may not have the authority to install a firmware image on a device in critical infrastructure without the authorization of a device operator.  In this case, the device may be programmed to reject firmware updates unless they are signed both by the firmware author and by the device operator. </ins>  <ins> Alternatively, a device may trust precisely one entity, which does all permission management and coordination.  This entity allows the device to offload complex permissions calculations for the device. </ins>  <ins> 8. </ins>  <ins> firmware-update illustrates an example message flow for distributing a firmware image to a device.  The firmware and manifest are stored on the same firmware server and distributed in a detached manner. firmware-update2 shows an exchange that starts with the status tracker querying the device for its current firmware version.  Later, a new firmware version becomes available and since this device is running an older version the status tracker server interacts with the device to initiate an update. The manifest and the firmware are stored on different servers in this example.  When the device processes the manifest it learns where to download the new firmware version.  The firmware consumer downloads the firmware image with the newer version X.Y.Z after successful validation of the manifest.  Subsequently, a reboot is initiated and the secure boot process starts.  Finally, the device reports the successful boot of the new firmware version. </ins>  <ins> 9. </ins>  <ins> This document does not require any actions by IANA. </ins>  <ins> 10. </ins>  <ins> This document describes terminology, requirements and an architecture for firmware updates of IoT devices.  The content of the document is thereby focused on improving security of IoT devices via firmware update mechanisms and informs the standardization of a manifest format. </ins>