Uptane is a standard, and does not have an official distribution or implementation. We do provide a reference implementation in Python, and there are a number of open source projects such as aktualizr, rust-tuf, Notary, and OTA Community Edition implementing all or part of the Standard. In addition, commercial Uptane offerings are available in the marketplace from HERE Technologies and Airbiquity.
However, in any serious Uptane installation, a number of deployment decisions will need to be made, and policies and practices for software signing and key management will need to be implemented. Additionally, some OEMs may wish to develop their own Uptane implementation. Here, we provide a set of best practices for how to set up, operate, integrate, and adapt Uptane to work in a variety of situations. We also discuss the human operations required, and describe Uptane-compatible ways to implement some specific features that OEMs have requested guidance or clarification on in the past.
All of these guidelines should be viewed as complementary to the official Uptane Standard: they should be taken as advice, not gospel.
In addition, these guidelines may be used in the creation of POUFs. POUFs contain the Protocols, Operations, Usage, and Formats of an Uptane implementation. These details can be used to design interoperable Uptane implementations.
At the highest level, the basic requirement for an ECU to be capable of supporting Uptane is that it be able to perform either full or partial verification, and access a secure source of time. (See the Uptane Standard for official requirements.)
To bootstrap an Uptane-capable ECU, a few things need to be provisioned into the unit:
There are three big decisions to make about each Uptane ECU: first, whether it will perform full or partial verification, second, whether it will use an asymmetric or symmetric ECU key, and third, whether it will use encrypted or unencrypted update images. Here, we offer some advice on making those choices.
Uptane is designed with automotive requirements in mind, and one of the difficulties in that space is that ECUs requiring OTA updates might have very slow and or memory-limited microcontrollers. To accommodate those ECUs, Uptane includes the option of partial verification. So, how do you choose between full and partial verification for a particular ECU?
Firstly, if the ECU is a Primary ECU, partial verification is not an option. Primaries need to perform full verification. For other ECUs, full verification is preferable when possible, for at least two reasons:
Partial verification ECUs are expected to have the Root and Targets metadata present at the time of manufacturing or installation in the vehicle. To update the Root metadata, the ECU SHOULD install a new image containing the metadata. To update the Targets metadata, the ECU SHOULD follow the steps described in the Uptane Standard. Partial verification Secondaries MAY additionally fetch and check metadata from other roles or the Image repository if the ECU is powerful enough to process them, and the implementer wishes to take advantage of their respective security benefits.
Figure 1. An arrangement that an OEM SHOULD use when using symmetric ECU keys.
ECUs are permitted to use either symmetric or asymmetric keys. This choice is effectively a performance vs. security trade-off. Symmetric keys allow for faster cryptographic operations, but expose a larger attack surface because the Director will need online access to the key. Asymmetric ECU keys are not affected by this problem, because the Director only needs access to the ECU’s public key.
Basically, choosing symmetric keys increases the performance of the common case (checking signatures and decrypting images), but makes disaster recovery harder, because a compromised key server could require updating ECU keys on every vehicle.
If you choose to use symmetric ECU keys, it would be a good idea to store the keys on an isolated, separate key server, rather than in the inventory database. This separate key server can then expose only two very simple operations to the Director:
Unencrypted images should be loaded onto the symmetric key server by some out-of-band physical channel (for example, via USB stick).
The Director repository may encrypt images if required (see Section 5.3.2 of the Uptane Standard). However, no Uptane implementation should support interactive requests from an ECU for encryption. Allowing the Target ECU to explicitly request an encrypted image at download time would not only increase the attack surface, but could also be used to turn off encryption. This would make it easy for attackers to reverse engineer unencrypted firmware and steal intellectual property. Only the OEM and its suppliers should determine policy on encrypting particular binaries, and this policy should be configured for use by the Director repository, rather than being toggled by the Target ECU.
Currently, implementation instructions are written with the implicit assumptions that: (1) ECUs are able to parse the string filenames of metadata and images, and that (2) ECUs may have filesystems to read and write these files. However, not all ECUs, especially partial verification Secondaries, may fit these assumptions. There are two important observations:
First, filenames need not be strings. Even if there is no explicit notion of “files” on an ECU, it is important for distinct pieces of metadata and images to have distinct names. This is needed for Primaries to perform full verification on behalf of Secondaries, which entails comparing the metadata for different images for different Secondaries. Either strings or numbers may be used to refer to distinct metadata and images, as long as different files have different file names or numbers. The Image and Director repositories can continue to use filesystems, and may also use either strings or numbers to represent file names.
Second, ECUs need not have a filesystem in order to use Uptane. It is only important that ECUs are able to recognize distinct metadata and images by using either strings or numbers as file names or numbers, and that they can allocate different parts of storage to different files.
As described in the Standard, all Secondaries MUST store some metadata objects. For partial verification Secondaries, this MAY include only the Targets metadata from the Director repository. If an ECU does not have any or enough secondary storage to store even just that one object, then it cannot be considered an Uptane Secondary. However, such a non-Uptane ECU can still be updated within an Uptane system, albeit with weaker guarantees than for fully-supported ECUs. In this case, the Director and Primary would treat a non-Uptane ECU like any other Secondary, but the ECU will not be able to perform any verification for itself. It must rely entirely on the Primary’s verification of the metadata and images.
This section outlines recommended procedures for the one-time operations that an OEM and its suppliers SHOULD perform when they set up Uptane for the first time. In particular, they SHOULD correctly configure the Director and Image repositories, and make sure they have access to a secure source of time, so that the impact of a repository/server compromise is as limited as possible.
Without access to a secure source of time, ECUs may be prevented from receiving the most recent updates. If the ECU’s time is set too far ahead, it will determine that the current valid metadata is expired and thus be unable to perform an update. If the ECU’s time is set too far behind, an attacker can freeze or replay old metadata to the ECU. (ECUs in Uptane will not accept an earlier time than what has previously been seen and signed with the same key.)
If a Primary ECU does not have a secure clock, then that Primary ECU SHALL use some other secure external means to acquire accurate time. If a Secondary ECU does not have a secure clock, then the ECU SHALL use the time messages from its Primary ECU to acquire accurate time. The following subsection describes how time servers can be used in an Uptane implementation.
The IETF Network Time Protocol v4 NTPv4, RFC 5905 with IETF Network Time Security for the Network Time Protocol NTS for NTP, RFC 8915 SHOULD be used by an ECU to acquire accurate time. If IETF NTPv4 (or a higher version) is used, then that ECU SHALL conform to IETF Network Time Protocol Best Current Practices BCP 223 / RFC 8633. If IETF NTPv4 (or higher version) is used, then that ECU SHALL discard any received NTP mode 6 and mode 7 packets to prevent a DDOS attack caused by an old (1989) NTP implementation bug described here and here.
The work-in-progress IETF Roughtime protocol and the IETF Roughtime Ecosystem MAY be used by an ECU to acquire sufficiently accurate time to verify certificates (i.e., expiration) and signatures (i.e., freshness). Note that these are a revision and enhancement of the original Google Roughtime. See also the Cloudflare implementation.
The US Global Positioning System (GPS), originally Navstar GPS, SHOULD NOT be used as a secure time source by any Uptane ECU, because spoofing attacks against the unsecured, civilian GPS signals are common, as described here and here. . #### Changes to the Director repository
If an external time source is in use, a representation of its public key is CONDITIONALLY REQUIRED in Director repository Root metadata.
If an external time source is implemented AND partial verification Secondaries are used, the following metadata is CONDITIONALLY REQUIRED in the Director repository’s Targets metadata:
Listing the public key of the external time source in Director Targets metadata is necessary to allow partial verification Secondaries to perform key rotation.
If an external time source is implemented, the Primary SHOULD follow its stipulated procedure for verifying the time. This procedure occurs after the vehicle version manifest is sent and will fulfill the Download and check current time step of the Uptane Standard.
If the response of the external time source meets verification criteria, update the Primary ECU’s clock and retain the time source response for distribution to Secondary ECUs. If it fails to meet this criteria, discard the response and continue the procedure without an updated time.
The ECU version report from each Secondary may contain a token to be sent to the external time source in whatever manner the implementer chooses.
After the vehicle has been assembled, ECUs MAY receive an attestation of the current time as downloaded from the external time source.
As the first step to verifying metadata, described in the Standard for both the Primary and Secondaries, the ECU SHOULD load and verify the most recent time from the designated time source by following its designated procedure. This will likely include verifying that the signatures on the downloaded time are valid
If all steps are completed without error, the ECU SHOULD overwrite its current attested time with the time it has just downloaded.
If any check fails, the ECU SHOULD NOT overwrite its current attested time, but SHOULD jump to the last step (Create and send version report)to report the error.
In order to prevent a new time source from accidentally causing a rollback warning, the clock will be reset as folllows: check the time source key after updating to the most recent Root metadata file. If the key is listed in the Root metadata has been rotated, reset the clock used to set the expiration of metadata to a minimal value (e.g., zero, or any time that is guaranteed to not be in the future based on other evidence). It will be updated in the next cycle.
As partial verification Secondaries only check the Targets metadata from the Director repository, the time source keys on these ECUs will be checked when verifying the Targets metadata. To do this, check the key after verifying the most recent Targets metadata file. If the external time source key is listed in the Targets metadata and has been rotated, reset the clock used to determine the expiration of metadata to a minimal value as described above.
In the event of a key compromise for an external time source, an attacker would be able to return a time attestation that contains an arbitrary time. The attacker could then either:
Make all metadata appear to be expired. If the returned time is far in the future, the vehicle would interpret all Uptane metadata as expired. The vehicle would be unable to verify the metadata, thus creating a denial of service.
Make expired metadata appear to be current. If the returned time is in the past, Uptane metadata that was valid at that point in the past will seem current to the vehicle, thus allowing for a freeze attack. This cannot be used for a rollback attack as the ECU will not accept a time earlier than the time of their previous update.
All of these attacks can be mitigated by rotating any key associated with an external time in Root metadata, as described in Managing signing keys and metadata expiration.
Figure 1. Diagram showing supplier signing arrangements. Suppliers are free to ask the OEM to sign images on its behalf (supplier A), or can sign them itself (supplier B). In the latter case, it MAY also delegate some or all of this responsibility to others (supplier C).
Either the OEM or a tier-1 supplier SHOULD sign for images for any ECUs produced by that supplier, so unsigned images are never installed. This provides security against arbitrary software attacks. An OEM will decide whether or not a tier-1 supplier SHOULD sign its own images. Otherwise, the OEM will sign images on behalf of the supplier, and the supplier SHOULD only deliver update images to the OEM as outlined under the [Normal Operating Guidelines] (https://uptane.github.io/deployment-considerations/normal_operation.html) section of this document. If the supplier signs its own images, it MUST first set up roles and metadata using the following steps:
A tier-1 supplier and its delegatees MAY use the Uptane repository and supplier tools to produce these signed metadata.
The OEM sets up and configures the Director and Image repositories. To host these backend services, the OEM MAY use its own private infrastructure, or cloud computing.
Note that all information about setting up signing keys for this repository can be found in the Managing signing keys and metadata expiration section of this document.
In order to provide on-demand customization of vehicles, the OEM SHALL also set up the Director repository following the guidance in the Uptane Standard. In addition, an OEM must keep in mind the following factors. Unlike the Image repository, the Director repository:
Steps to initialize the repository
In order to initialize the repository, an OEM SHOULD perform the following steps:
The automated process MAY use the repository tools from our Reference Implementation to generate new metadata.
Figure 2. A proposed configuration of roles on the Director repository.
Unlike the Image repository, the Director repository does not delegate images. Therefore, the Director repository SHOULD contain only the Root, Timestamp, Snapshot, and Targets roles, as illustrated in Figure 2. In the following subsections, we will discuss how metadata for each of these roles is produced.
An OEM SHOULD define a private API for the Director repository that is able to: (1) upload images, and (2) update the inventory database. This API is private in the sense that only the OEM should be able to perform these actions.
This API SHOULD require authentication, so that each user is allowed to access only certain information. The OEM is free to use any authentication method as long as it is suitably strong. Examples include client certificates, a password, or an API key encrypted over TLS. For additional security, the OEM may use multi-factor authentication that utilizes more than one authentication method.
In order to allow automated processes on the Director repository to perform their respective functions, without also allowing any attackers who might compromise the repository to tamper with the inventory database, it is strongly RECOMMENDED that these processes should have some boundaries. That is, the automated processes SHOULD be able to read any record in the database and write new records, but SHOULD NOT be able to update or delete existing records.
Figure 3. How Primaries would interact with the Director repository.
An OEM SHOULD define a public API to the Director repository so that it is able to send updates to vehicles. This API can be designed to the wishes of the OEM, and can use either a push or pull model to send updates to Primaries. The difference between the models lies in whether or not a running vehicle can be told to immediately download an update (via a push), or can wait until a pull occurs.
Either way, the OEM can control how often updates are released to vehicles. In the push model, the OEM can send an update to a vehicle whenever it likes, as long as the vehicle is online. In the pull model, the OEM can configure the frequency at which Primaries pull updates. In most realistic cases, there will be little practical difference between the two models.
There is also no significant difference between these methods when it comes to resistance to denial-of-service (DoS) attacks or flash crowds. In the push model, a vehicle can control how often updates are pushed to it, so that vehicles can withstand DoS attacks, even if the repository has been compromised. In the pull model, the repository can similarly stipulate when vehicles SHOULD download updates, and how often.
Regardless of what model is used to send updates, as illustrated in Figure 4, the API SHOULD allow a Primary to: * send a vehicle version manifest (step 1) * download associated metadata and image files (step 4).
The API MAY require authentication, depending on the OEM’s requirements.
Sending an update from the Director repository to a Primary requires the following five steps, as shown in Figure 3.
Since the automated process is continually producing new metadata files (and, possibly, encrypted images), these files SHOULD be deleted as soon as Primaries have consumed them, so that storage space can be reclaimed. This MAY be done by simply tracking whether Primaries have successfully downloaded these files within a reasonable amount of time.
Note that all information about setting up signing keys for this repository can be found in the Managing signing keys and metadata expiration section of this document.
Finally, in order to provide compromise resilience, the OEM MUST set up the Image repository following the guidance in the Uptane Standard. The Image repository differs from the Director repository in a number of ways. First, it is managed by human administrators who use offline keys to sign Targets metadata. It also MAY delegate images to suppliers, and it provides the same metadata to all Primaries. Lastly, it does not encrypt images per ECU, and it updates its metadata and images relatively infrequently (e.g., every two weeks or monthly).
Steps to initialize the repository
In order to initialize the repository, an OEM SHOULD perform the following steps. Note that, as with the Director repository, all users are expected to follow basic set up instructions, as well as the specific set up instructions mandated by the users’ choices of storage mechanisms and protocols. 1. Set up the storage mechanism. 2. Set up the transport protocol. 3. Set up the private and public APIs to interact over the chosen transport protocol. 4. Set up the Root, Timestamp, Snapshot, and Targets roles. 5. Sign delegations from the Targets role to all tier-1 supplier roles. The public keys of tier-1 suppliers SHOULD be verified using some out-of-band mechanism (e.g., telephone calls or certified mail), so that the OEM can double-check their authenticity and integrity. 6. Upload metadata and images from all delegated Targets roles (including tier-1 suppliers). Verify the metadata and images, and add them to the storage mechanism.
An OEM and its suppliers MAY use the repository and supplier tools from the Reference Implementation to produce new metadata.
Figure 4. A proposed configuration of roles on the Image repository.
Using delegations allows the OEM to: (1) control which roles sign for which images, (2) control precisely which Targets metadata vehicles need to download, and (3) distribute, revoke, and replace public keys used to verify Targets metadata, and hence, images. In order to set up delegations, an OEM and its suppliers MAY use the configuration of roles illustrated in Figure 4. There are two important points.
The metadata for each tier-1 supplier MAY be signed by the OEM (e.g., supplier A), or the supplier itself (e.g., suppliers B and C). In turn, a tier-1 supplier MAY delegate images to members of its organization, such as supplier C who has delegated a subset of its images to one of its developers, or to its tier-2 suppliers who MAY delegate further to tier-3 suppliers.
Every delegation SHOULD be prefixed with the unique name of a tier-1 supplier, so that the filenames of images do not conflict with each other. Other than this constraint, a tier-1 supplier is free to name its images however it likes. For example, it MAY use the convention “supplier-X-ECU-Y-version-Z.img” to denote an image produced by supplier X, for ECU model Y, and with a version number Z.
An OEM SHOULD define a public API for Primaries to use when downloading metadata and images to the Image repository. This API can be defined in whatever manner the OEM wishes.
Depending on the OEM’s requirements, this API MAY require authentication before Primaries are allowed to download updates. Such a choice affects only how certain the OEM can be that it is communicating with authentic Primaries, and not how resilient ECUs are to a repository compromise. The OEM is free to use any authentication method.
Uptane implementations may sometimes need to accommodate update systems where existing software comes from several different locations. Implementers may assume that this would mandate the use of multiple different Image repositories in any equivalent Uptane implementation. However, this is rarely necessary, and using multiple Image repositories (implemented via repository mapping metadata as described in TAP-4) would require a significantly larger effort.
In almost all cases, it is preferable to have a single Image repository containing all of the Uptane metadata, and redirect clients to download the actual images from other locations. This can be implemented via an API on the Image repository, or via a custom field in the Targets metadata directing the clients to one or more alternate URL where the images are available.
An API solution could be as simple as an HTTP 3xx redirect to the appropriate download location. More complex schemes, e.g., cases where existing legacy repositories have a custom authentication scheme, can usually be implemented by adding custom metadata. See the relevant section of the Standard for more information on how custom metadata can be added.
In setting up an Uptane system, an implementer will need to specify how information, such as metadata files and vehicle version manifests, should be encoded. As a guiding principle of the Uptane framework is to give each implementer as much design flexibility as possible, the Uptane Standard does not specify particular data binding formats. Instead, OEMs and suppliers can continue to use the protocols and formats of existing update systems, or they can select formats that best ensure interoperability with other essential technologies.
To facilitate coordination between implementations, an Uptane adopter can choose to write a POUF, an added layer to the Standard in which an implementer can specify choices of Protocols, Operations, Usage and Formats. A POUF provides an easy way for an implementer to specify the elements that can ensure interoperability. It can also be customized for the special needs of fleet owners in a particular industry, such as taxis, car sharing networks, police forces, or the military.
Information on writing a POUF can be found on the POUF Purpose and Guidelines page of the Uptane website. A sample POUF, written for the Uptane Reference Implementation, offers sample metadata written in ASN.1/DER.
There is no short answer to the question of what it costs to implement Uptane because such a decision involves a number of variables, starting with how to plan an implementation. An OEM with an existing OTA system has two methods to implement Uptane: either buy an off-the-shelf solution and integrate with it, or build a custom solution, with some greater or smaller degree of reliance on available open-source client and server components. If a project is starting from scratch, there is a third option: Uptane could be integrated from the start, beginning with the design stage. In that case, the cost of implementing Uptane itself would be minimal, and may even represent a cost reduction, as it provides a framework for organizing the analysis and development that must be undertaken regardless.
If a user is contemplating options one or two, the relative costs are admittedly more difficult to estimate. When considering off-the-shelf products, an Uptane solution may be more or less expensive than other OTA solutions, but Uptane itself may not be the primary cost driver. There is also the possibility that the goals of the provider and the OEM could be misaligned, which can make integration difficult and likely expensive. Lastly, there is always a risk profile associated with outsourcing an OTA system to a third party, as the supplier could go out of business or decide to phase out the product in the wake of a shift in corporate priority.
A variant of the custom in-house option may be a safer choice for larger OEMs, but it can also come with a high price tag and require substantial effort to integrate. In any case, when upgrading from a legacy solution, a risk assessment of the current solution and evaluation of the current weaknesses and opportunities for improvement will be necessary. Uptane provides an organized framework for this assessment, which could help to reduce costs compared to an assessment without such a framework. The costs of the upgrade itself are dependent upon the individual needs, security concerns, and existing infrastructure of the implementer.
Ultimately, the issue of cost cannot be decided without also estimating the value received for that expense. Value in this case is enhanced security, so when discussing costs, the trade-off between taking shortcuts and sacrificing security, or doing it right and spending more time and money, must be considered. For example, robust key rotation is one of the most important pillars of TUF and Uptane, but it is also something that users may be prepared to ignore if they think it will save money. Therefore, before thinking about cost, potential implementers should do a risk assessment of the current solution, evaluate its weaknesses, and identify the major opportunities for improvement. Only by knowing the risks they are defending against can implementers legitimately assess if the cost is an investment in the security of their project and their company, rather than just another expense eating away at profits.
This section addresses both setup and maintenance issues for the signing keys used by Uptane. These include understanding the function of online vs. offline keys, the use of signing thresholds to improve security, and the management of metadata expiration dates.
The secure management of cryptographic key material has been well-documented in previous studies. Implementers of Uptane SHOULD follow best practices outlined in IETF RFC 4107 / BCP 107 - Guidelines for Cryptographic Key Management.
On both the Director and the Image repository, the OEM maintains the keys to the Root, Timestamp, Snapshot, and Targets roles. However, for any delegated Targets roles on the Image repository, the corresponding keys are expected to be maintained by the supplier to which the corresponding images have been delegated. For example, if a tier-1 supplier signs its own images, then the supplier would maintain its own (ideally offline) keys.
Repository administrators SHOULD use offline keys to sign the Root metadata on the Director repository, so attackers cannot tamper with this file after a repository compromise. The Timestamp, Snapshot, and Targets metadata SHOULD be signed using online keys, so that an automated process can instantly generate fresh metadata.
On the Image repository, there are two options for signing the Timestamp and Snapshot metadata, each with the opposite trade-off from the other. In the first option, the OEM uses online keys, meaning automated processes for renewing the Timestamp and Snapshot metadata when new Targets metadata and/or images are available. With this option, fresh metadata can be instantly generated by the automated process. On the other hand, if attackers compromise a supplier’s key as well as the Image repository, they could instantly publish malicious images. If these attackers also compromise the Director repository, then they can execute arbitrary software attacks by selecting these malicious images on the Image repository for installation. Such an attack could also facilitate mix-and-match attacks.
In the second option, the OEM uses offline keys to sign Timestamp and Snapshot metadata, which reduces the risk of attackers immediately publishing malicious images. Here again, though, there is a trade-off, in this case related to the metadata expiration dates. If the Timestamp and Snapshot metadata expire relatively quickly, then it may be cumbersome to use offline keys to renew their signatures. Yet, if a longer expiration time is used, it would give a man-in-the-middle attacker more time to execute freeze attacks, hence defeating the purpose of the Timestamp role.
For most use cases, the online option may be best, but if stronger security guarantees are desired, consider using the offline option instead for the Timestamp and Snapshot roles.
The keys to all other roles (Root, Targets, and all delegations, which includes suppliers’ keys) on the Image repository SHOULD be kept offline to prevent a repository compromise from immediately affecting full verification ECUs. It is also a practical decision as these metadata are infrequently updated. It does not matter where an offline key is stored (e.g., in a Hardware Security Module, YubiKey, or a USB stick in a safe deposit box), as long as the key is not accessible from the repository. Each key SHOULD be kept separate from the others, so that a compromise of one does not affect them all.
Since a compromise of the Root role keys would have the greatest impact on the Director repository, it SHOULD use a sufficiently large threshold number of keys. This ensures that a single key compromise does not allow that corrupted key to sign the Root metadata file on its own. Each key within the threshold MAY belong to a different repository administrator. For example, if there are 8 administrators, then at least 5 keys SHOULD be required to sign the Root metadata file, so that a quorum is required to trust the metadata.
The Timestamp, Snapshot, and Targets roles MAY each use a single key, because for these roles using more keys does not provide any additional security. As long as these keys are online, attackers who compromise the repository can use them, regardless of how many are present.
Since the Root role keys on the Director repository are not expected to be revoked and replaced often, its metadata file MAY expire after a relatively long time, such as one year.
The Timestamp, Snapshot, and Targets metadata files SHOULD expire relatively quickly, such as in a day, because they are used to indicate whether updated images are available.
Table 1 lists an example of expiration times for metadata files on the Director repository.
Table 1. An example of the duration of time until the metadata for a role expires.
For the Image repository, each role MAY use as many keys as is desired, though the greater the impact of key compromise for a given role, then the greater the number of keys that it SHOULD use. Also, a threshold number of keys SHOULD be required, so that a single key compromise is generally insufficient to sign new metadata. To further increase compromise resilience, each key SHOULD be unique across all roles.
Since the Root role has the highest impact when its keys are compromised, it SHOULD use a sufficiently large threshold number of keys. Each key MAY belong to a different repository administrator. For example, if there are 8 administrators, then at least 5 keys SHOULD be required to sign the Root metadata file, so that a quorum is required to trust the metadata.
Since the Targets role also has a high impact when its keys are compromised, it SHOULD also use a sufficiently large threshold number of keys. For example, 3 out of 4 keys MAY be required to sign the Targets metadata file.
Since the Timestamp and Snapshot roles have a relatively low impact when its keys are compromised, each role MAY use a small threshold number of keys. For example, each role MAY use 1 out of 2 keys to sign its metadata file.
Finally, each delegated Targets role SHOULD use at least 1 out of 2 keys to sign its metadata file, so that one key is available in case the other is lost. It is RECOMMENDED that the higher the number of ECUs that can be compromised if a delegated Targets role is compromised, then the higher the threshold number of keys that SHOULD be used to sign the role metadata.
The Uptane Standard requires all metadata files to have expiration times in order to prevent or limit freeze attacks. If ECUs know the time, then attackers cannot indefinitely replay outdated metadata, and hence, images. In general, the expiration date for a metadata file depends on how often it is updated. The more frequently it is updated, then the faster it SHOULD expire, so that man-in-the-middle attackers are unable to execute freeze attacks for too long. Even if it is not updated frequently, it SHOULD expire after a bounded period of time, so that stolen or lost keys can be revoked and replaced.
Since the Root role keys are expected to be revoked and replaced relatively rarely, its metadata file MAY expire after a relatively long time, such as one year.
Table 2 lists an example of expiration times for metadata files on the Image repository.
Table 2. An example number of keys that MAY be used by each role. Each role uses a threshold of (n, m) keys, where n out of m signatures are required to trust the signed metadata.
An OEM and its suppliers SHOULD be prepared to handle a key compromise. If the recommended number and type of keys are used, this should be a rare event. Nevertheless, when it happens OEMs and suppliers could use the following recovery procedures.
Since the Director repository SHALL keep at least some software signing keys online, a compromise of this repository can lead to some security threats, such as mix-and-match attacks. Thus, the OEM SHOULD take great care to protect this repository and reduce its attack surface as much as possible. This MAY be done, in part, by using a firewall. However, if the repository has been compromised, then the following procedure SHOULD be performed in order to recover ECUs from the compromise. Following the type and placement of keys prescribed for the Director repository, we assume that attackers have compromised the online keys to the Timestamp, Snapshot, and Targets roles, but not the offline keys to the Root role.
First, the OEM SHOULD use the Root role to revoke and replace the keys to the Timestamp, Snapshot, and Targets roles, because only the Root role can replace these keys.
Second, the OEM SHOULD consider a manual update of all vehicles in order to replace these keys, particularly if the vehicle has partial verification Secondaries. This update MAY be done by requiring vehicle owners to visit the nearest dealership. Although an OEM could replace these keys on a full verification ECU by using over-the-air broadcasts, a manual update is recommended because: 1. the OEM SHOULD perform a safety inspection of the vehicles, in case of security attacks, and 2. partial verification Secondaries are not designed to handle key revocation and replacement over-the-air. In order to update keys for partial verification Secondaries, the OEM SHOULD overwrite their copies of the Root metadata file, perhaps using new images.
After inspecting the vehicle, the OEM SHOULD replace and update metadata and images on all ECUs to ensure that the images are known to be safe and that partial verification Secondaries have replaced the keys for the Director repository.
If the recommendations for the type and placement of keys described above for the Image repository are followed, then a key compromise of this repository should be an unlikely event. However, should one occur, it is a much more serious affair. A compromise of the Image repository would allow attackers to tamper with images without being detected, and thus execute arbitrary software attacks. There are two cases for handling a key compromise, depending on whether the key is managed by a delegated supplier or by the OEM.
In the first case, where a tier-1 supplier or one of its delegatees has had one or more of its keys compromised, the supplier and its affected delegatees (if any) SHOULD revoke and replace keys. They SHOULD update metadata, including delegations and images, and send them to the OEM.
The OEM SHOULD then manually update only affected vehicles that run software maintained by this supplier in order to replace metadata and images. This MAY be done by requiring vehicle owners to visit the nearest dealership. A manual update SHOULD be done because, without trusted hardware (such as a TPM), it is difficult to ensure that compromised ECUs can be remotely and securely updated. After inspecting the vehicle, the OEM SHOULD replace and update metadata and images on all ECUs so that these images are known to be safe.
The second case, where the OEM has had a key compromised, can be far more serious than the first case. An attacker in such a position may be able to execute attacks on all vehicles, depending on which keys have been compromised. If the keys are for the Timestamp and Snapshot roles, or the Targets or Root roles, then the OEM SHOULD use the following recovery procedure.
First, the OEM SHOULD use the Root role to revoke and replace keys for all affected roles. Second, it SHOULD restore all metadata and images on the Image repository to a known good state using an offline backup. Third, the OEM SHOULD manually update all vehicles in order to replace metadata and images. A manual update SHOULD be done because, without trusted hardware (such as a TPM), it is difficult to ensure that compromised ECUs can be remotely and securely updated.
If ECU keys are compromised, then the OEM SHOULD manually update vehicles to replace these keys. This is the safest course of action because, after a key compromise, an OEM cannot be sure whether it is remotely replacing keys controlled by attackers or the intended ECUs.
An OEM MAY use the Director repository and its inventory database to infer whether ECU keys have been compromised. This database is used to record vehicle version manifests that list what images an ECU has installed over time. Therefore, an OEM MAY check for any abnormal patterns of installation that could have been caused by an ECU key compromise. Note, however, that this method is not perfect, because if attackers control ECU keys, then they can also use these keys to send fraudulent ECU version reports.
In this section, we discuss how to perform regular maintenance operations. Since these operations are carried out on a regular basis, it is important to ensure they are performed in a systematic manner so that software updates are delivered securely to ECUs.
An OEM SHOULD perform the following steps whenever a new update is delivered. First, the OEM verifies the authenticity and integrity of new images delivered by its suppliers. Second, the OEM tests whether the images work as intended, before releasing them to end-user vehicles.
In order to prevent updates from being tampered with by man-in-the-middle attackers, images SHOULD be delivered from the tier-1 supplier to the OEM in a manner that supports an extremely high degree of confidence in their timeliness and authenticity. This may entail any manner of technical, physical, and/or personnel controls.
An OEM and its suppliers MAY use any transport mechanism to deliver these files. For example, an OEM MAY maintain a private web portal where metadata and/or images from suppliers can be uploaded. This private server MAY be managed by either the OEM or the tier-1 supplier, and SHOULD require authentication to restrict which users are allowed to read and/or write certain files. Alternatively, the OEM and its suppliers MAY use email or courier mail.
If the supplier signs its own images, then it delivers all of its metadata, including delegations, and associated images. Otherwise, if the OEM signs images on behalf of the supplier, then the supplier needs to update only images, leaving the OEM responsible for producing signed metadata. Regardless of which party produces signed metadata, the release counters associated with images SHOULD be incremented, so that attackers who may compromise the Director repository can not rollback to obsolete images (see the Enhanced Security Practices section of this document for more on this attack.)
Regardless of the transport mechanism used to deliver them, the OEM needs to ensure that the images received are authentic and have not been altered. The OEM SHOULD double-check the authenticity and integrity of these images by using some out-of-band mechanism for verification. For example, to obtain a higher degree of assurance, and for additional validation, the OEM MAY also require the supplier’s update team to send a PGP/GPG signed email to the OEM’s security team listing the cryptographic hashes of the new files.
Alternatively, the OEM MAY require that updates be transmitted via a digital medium that is delivered by a bonded and insured courier. To validate the provided files, the OEM and a known contact at the supplier MAY have a video call in which the supplier provides the cryptographic hashes of the metadata and/or images, and the OEM confirms that the hashes match.
An OEM SHOULD perform this verification even if a trusted transport mechanism is used to ensure the mechanism has not been compromised. If the suppliers have signed metadata, then the OEM SHOULD verify metadata and images by checking version numbers, expiration timestamps, delegations, signatures, and hashes, so that it can be sure that the metadata matches the images.
After the OEM has somehow verified the authenticity and integrity of new metadata and images received from the tier-1 supplier, the OEM SHOULD test both before releasing them to ensure that the images work as intended on end-user vehicles. To do so, It SHOULD use the following steps.
First, the OEM SHOULD add these metadata and images to the Image repository. It SHOULD also add information about these images to the inventory database, including any dependencies and conflicts between images for different ECUs. Both of these steps are done to make the new metadata and images available to vehicles.
Optionally, if images are encrypted on demand per ECU, then the OEM SHOULD ensure that the Director repository has access to the original, unencrypted images, so that automated processes running the Director repository are able to encrypt them in the first place. It does not matter how the original, unencrypted images are stored on the Director repository. For example, they MAY be stored unencrypted, or they MAY be encrypted using a master key that is known by the automated processes. See the Preparing an ECU for Uptane section of this document for more details.
Second, the OEM SHOULD test the updated metadata and images on reserved vehicles before releasing them to all vehicles in circulation. This step is done to verify whether these images work as intended. If testing is done, the OEM MAY instruct the Director repository to first install the updated images on these reserved vehicles.
Finally, the OEM SHOULD update the inventory database, so that the Director repository is able to instruct appropriate ECUs on all affected vehicles on how to install these updated images.
The OEM SHOULD regularly perform backup and garbage collection of the metadata and images on the Image repository. This is done to ensure the OEM is able to safely recover from a repository compromise, and that the repository continues to have sufficient storage space. To do so, an OEM MAY use either the following steps, or its own corporate backup and garbage collection policy.
First, an automated process SHOULD store every file on the Image repository, as well as its cryptographic hash on a separate, offline system. A copy of the inventory database from the Director repository SHOULD also be stored on this offline system. This allows administrators to detect and recover from a repository compromise.
Second, the automated process SHOULD remove expired metadata from the Image repository to reclaim storage space. If the OEM is interested in supporting delta updates for vehicles that have not been updated for a long time, then the automated process SHOULD NOT remove images associated with expired metadata, because these images MAY be needed in order to compute delta images. (See the Delta update strategies subsection of the Customizing Uptane section of this document).
In this section, we discuss operations that are generally performed only in exceptional cases. As performing these operations may have security implications for software updates, they should be carried out with great care.
Sometimes an OEM may determine that the latest updates are less reliable than previous ones. In that case, it may be necessary to roll back to a previous update.
By default, Uptane does not allow updates to be rolled back and enforces this action with two mechanisms. First, Uptane rejects any new metadata file with a version number lower than the one contained in the previous metadata file. Second, Uptane will reject any new image associated with a release counter that is lower than the release counter of the previous image in the previous Targets metadata file. The first mechanism prevents an attacker from replaying an old metadata file. The second mechanism prevents an attacker who compromises the Director repository from being able to choose old versions of images, despite being able to sign new metadata. See Figure 1 for an example.
Figure 1. Uptane prevents rollback attacks by rejecting older: (1) metadata files, and/or (2) images.
There are at least two ways to allow rollbacks, each with different advantages and disadvantages.
In the first option, an OEM MAY choose to never increment the release counters of images (see Figure 2). Uptane will accept any new image associated with a release counter, as long as it is equal to the release counter of the previous image in the previous Targets metadata file. If release counters are never incremented, then all images would have the same release counters. In this situation, an ECU would accept the installation of any compatible image referred to in the new Targets metadata. (See the Enhanced Security Practices section of this document for more details.)