Application IDs (AppID), Credentials, and Process Loading

Working Group: Kernel
Type: Documentary
Status: Draft
Author: Philip Levis, Johnathan Van Why
Draft-Created: 2021/09/01
Draft-Modified: 2022/10/14
Draft-Version: 10


This document describes the design and implementation of application identifiers (AppIDs) in the Tock operating system. AppIDs provide a mechanism to identify the application contained in a userspace binary that is distinct from a process identifier. AppIDs allow the kernel to apply security policies to applications as their code evolves and their binaries change. A board defines how the kernel verifies AppIDs and which AppIDs the kernel will load. This document describes the Rust traits and software architecture for AppIDs as well as the reasoning behind them. This document is in full compliance with TRD1.

1 Introduction

The Tock kernel needs to be able to manage and restrict what userspace applications can do. Examples include:

  • making sure other applications cannot access an application's sensitive data stored in non-volatile memory,
  • restricting certain system calls to be used only by trusted applications,
  • run and load only applications that a trusted third party has signed.

In order to accomplish this, the kernel needs a way to identify an application and know whether a particular userspace binary belongs to an application. Multiple binaries can be associated with a single application. For example, software updates may cause a system to have more than one version of an application, such that it can roll back to the old version if there is a problem with the new one. In this case, there are two different userspace binaries, both associated with the same application.

To remain flexible and support many use cases, the Tock kernel makes minimal assumptions on the structure and form of application credentials and corresponding application identifiers. Application credentials are arbitrary k-byte sequences that are stored in a userspace binary's Tock binary format (TBF) footers. Before a process is eligible to execute, a Tock board uses an AppID (application identifier) checker to determine the AppIDs of each userspace binary available on the board and decide whether to load the binary into a process.

The Tock kernel ensures that each running process has a unique application identifier; if two userspace binaries have the same AppID, the kernel will only permit one of them to run at any time.

Most of the complications in AppIDs stem from the fact that they are a general mechanism used for many different use cases. Therefore, the exact structure and semantics of application credentials can vary widely. Tock's TBF footer formats, kernel interfaces and mechanisms must accommodate this wide range.

The interfaces and standard implementations for AppIDs and AppID checkers are in the kernel crate, in the module process_checker. There are three main traits:

  • kernel::process_checker::AppCredentialsPolicy is responsible for defining which types of application credentials the kernel accepts and whether it accepts a particular application credential for a specific application binary. The kernel only loads userspace programs that the AppCredentialsPolicy accepts.

  • kernel::process_checker::AppUniqueness compares the application identifiers of two processes and reports whether they differ. The kernel uses this trait to ensure that each running process has a unique application identifier.

  • kernel::process_checker::Compress compresses application identifiers into short, 32-bit identifiers called ShortIds. ShortIds provide a mechanism for fast comparison, e.g., for an application identifier against an access control list.

Example implementations can be found in kernel::process_checker::basic.

In normal use of Tock, a software tool running on a host copies TBF Objects into an application flash region. When the Tock kernel boots, it scans this application flash region for TBF Objects. After inspecting the Userspace Binary, TBF headers, and TBF Footers in a TBF Object, the kernel assigns it an Application Identifier and decides whether to run it.

2 Terminology

This document uses several terms in precise ways. Because these terms overlap somewhat with general terminology in the Tock kernel, this section defines them for clarity. The Tock kernel often uses the term "application" to refer to what this document calls an "Application Binary."

Userspace Binary: a code image compiled to run in a Tock process, consisting of text, data, read-only data, and other segments.

TBF Object: a Tock binary format object stored on a Tock device, containing TBF headers, a Userspace Binary, and TBF footers. TBF Objects are typically generated from ELF files using the elf2tab tool and are the standard binary format for Tock userspace processes.

Application: userspace software developed and maintained by an individual, group, corporation, or organization that meets the requirements of a Tock device use case. An Application can have multiple Userspace Binaries, e.g., to support versioning.

Application Identifier: a numerical identifier for an application. Each loaded process has a single Application Identifier. Application Identifiers are not unique across loaded processes: multiple loaded processes can share the same application identifier. Application Identifiers, however, are unique across running processes. If multiple loaded processes share the same Application Identifier, at most one of them can run at any time. An Application Identifier can be persistent across boots or restarts of a userspace binary. The Tock kernel assigns Application Identifiers to processes using a Identifier Policy.

Application Credentials: metadata that establish integrity of a Userspace Binary. Application Credentials are usually stored in Tock Binary Format footers. A TBF object can have multiple Application Credentials.

Process Checker: the component of the Tock kernel which is responsible for validating Application Credentials and determining which Application Credential (if any) the kernel should apply to a process.

Identifier Policy: the algorithm that the Process Checker uses to assign Application Identifiers to processes. An Identifier Policy defines an Application Identifier space.

Credentials Checking Policy: the algorithm that the Process Checker uses to decide how Tock responds to particular Application Credentials. The boot sequence typically passes the Credentials Checking Policy to the Process Checker to use when loading processes.

Global Application Identifier: an Application Identifier which, given an expected combination of Credentials Checking Policy and Identifier Policy, is both globally consistent across all TBF objects for a particular Application and unique to that Application. All instances of the Application loaded with this combination of policies have this Application Identifier. No instances of other Applications loaded with this Credentials Checking Policy have this Application Identifier. One example of a Global Application Identifier is a public key used to verify the digital signature of every TBF Object of a single Application. Another example of a Global Application Identifier is a string name stored in a TBF Object header; in this case the party installing TBF Objects needs to make sure there are no unintended collisions between these string names.

Locally Unique Application Identifier: a special kind of Application Identifier that is by definition unique from all other Application Identifiers. Locally Unique Application Identifiers do not have a concrete value that can be examined or stored. All tests for equality with a Locally Unique Application Identifier return false. Locally Unique Application Identifiers exist in part to be an easy way to indicate that a process has no special privileges and its identity is irrelevant from a security standpoint.

Short ID: a 32-bit compressed representation of an Application Identifier. Application Identifiers can be large (e.g., an RSA key) or expensive to compare (a string name); Short IDs exist as a way for an Identifier Policy to map Application Identifiers to a small identifier space in order to improve both the space and time costs of checking identity.

3 Application Identifiers and Application Credentials

Application Identifiers and Application Credentials are related but they are not the same thing. An Application Identifier is a numerical representation of the Application's identity. Application Credentials are data that, combined with an Identifier Policy, can cryptographically bind an Application Identifier to a process.

For example, suppose there are two versions (v1.1 and v1.2) of the same Application. They have different Userspace Binaries. Each version has an Application Credentials consisting of a signature over the TBF headers and Userspace Binary, signed by a known public key. The Identifier Policy is that the public key defines the Application Identifier: all versions of this Application have Application Credentials signed by this key. The two versions have different Application Credentials, because their hashes differ, but they have the same Application Identifier.

3.1 Application Identifiers

The key restriction Application Identifiers impose is that the kernel MUST NOT simultaneously run two processes that have the same Application Identifier. This restriction is because an Application Identifier provides an identity for a Userspace Binary. Two processes with the same Application Identifier are two copies or versions of the same Application. As Application Identifiers are used to control access to resources such as storage, this restriction ensures there is at most one process accessing resources or data belonging to an Application Identifier, which precludes the need for consistency mechanisms for concurrent access.

Application Identifiers can be used for security policy decisions in the rest of the kernel. For example, a kernel may allow only Applications whose Application Credentials use a particular trusted public key to access restricted functionality, but restrict other applications to use a subset of available system calls. By defining the Application Identifier of a process to be the public key, the system can map this key to a Short ID (described below) that gives access to restricted functionality.

The Tock kernel assigns each Tock process a unique process identifier, which can be re-used over time (like POSIX process identifiers). These process identifiers are separate from and unrelated to Application Identifiers. An Application Identifier identifies an Application, while a process identifier identifies a particular execution of a binary. For example, if a Userspace Binary exits and runs a second time, the second execution will have the same Application Identifier but may have a different process identifier.

3.1.1 Global Application Identifiers

Global Application Identifiers are a class of Application Identifiers that have properties which make them useful for security policies. For Applications that use Global Application Identifiers, the combination of the Application Credentials put in TBF Objects, Credentials Checking Policy, and Identifier Policy establish a one-to-one mapping between Applications and Global Application Identifiers. If an Application has a Global Application Identifier, then every process running that Application has that Global Application Identifier. Conversely, that Global Application Identifier is unique to that Application; two Applications do not share a Global Application Identifier.

One important implication of this mapping is that Global Application Identifiers MUST persist across process restarts or reloads.

Poor management of Global Application Identifiers can lead to unintended collisions. For example, an Identifier Policy might define the Global Application Identifier of processes to be the public key of a key pair to sign an Application Credential. If a developer accidentally uses the wrong key to sign a Userspace Binary, the Tock kernel will think that Userspace Binary is a different Application. Similarly, if the Identifier Policy uses a string name in a TBF Object header as the Global Application Identifier, then incorrectly giving two different programs the same name could lead them to sharing data.

3.1.2 The "Locally Unique" Identifier

Some Tock use cases do not require a real notion of Application identity. In many research or prototype systems, for example, every Userspace Binary has complete access to the system and there is no need for persistent storage or identity. Running processes need an Application Identifier, but in these cases it is not necessary for a Tock kernel and Application build system to manage Global Application Identifiers.

In such use cases, the Identifier Policy can assign a special Application Identifier called the "Locally Unique Identifier". This identifier does not have a concrete value: it is simply a value that is by definition different from all other Application Identifiers. Because it does not have a concrete value, one cannot test for equality with Locally Unique Application Identifier. All comparisons with a Locally Unique Application Identifier return false.

3.2 Application Credentials

Application Credentials are information stored in TBF Footers. The exact format and information of Application Credentials are described in the next section. They typically store cryptographic information that establishes the Application a Userspace Binary belongs to as well as provide integrity.

Application Identifiers can, but do not have to be, be derived from Application Credentials. For example, a Tock system with a permissive Credentials Checking Policy may allow processes with no Application Credentials to run, and have an Identifier Policy that defines Application Identifiers to be the ASCII name stored in a TBF header.

In cases when a TBF Object does not have any Application Credentials, the Identifier Policy MAY assign it a Global Application Identifier. This identifier must follow all of the requirements in Section 3.1.1.

3.3 Example Use Cases

The following five use cases demonstrate different ways in which Application Policies can assign Application Identifiers, some of which use Application Credentials:

  1. A research system that (memory permitting) runs every Userspace Binary loaded on it. The Identifier Policy assigns every Userspace Binary a Locally Unique Application Identifier and the Credentials Checking Policy approves TBF Objects independently of their credentials.

  2. A system which runs only a small number of pre-defined Applications and an Application is defined by a particular public RSA key. The Credentials Checking Policy only accepts TBF Objects with an Application Credentials containing an RSA signature from a small number of pre-approved keys. The Identifier Policies defines that the Global Application Identifier of a process is the public key used to generate the accepted Application Credentials for the TBF Object. Before verifying a signature in a TBF footer, the Process Checker decides whether to it accepts the associated public key using the Credentials Checking Policy. The Identifier Policy assigns a Global Application Identifier as the public key in the TBF footer.

  3. A system which runs any number of Applications but all Applications must be signed by a particular RSA key. The Credentials Checking Policy only accepts TBF Objects with a Credentials of an RSA signature from the approved key. The Identifier Policy defines the Application Identifier as the UTF-8 encoded package name stored in the TBF Header (or "" if none is stored). Two Userspace Binaries with the same package name will not run concurrently.

  4. A system that loads the same Userspace Binary in multiple different processes at the same time. The Identifier Policy assigns a Userspace Binary a Locally Unique Identifier. If the Userspace Binary needs integrity or authenticity then the Credentials Checking Policy can require signatures. This differs from the first example in that a single Userspace Binary can be loaded into multiple processes, instead of loading each Userspace Binary once. The use cases are different but can (in terms of identifiers and credentials) implemented the same way.

As the above examples illustrate, Application Credentials can vary in size and content. The credentials that a kernel's Credentials Checking Policy will accept depends on its use case. Certain devices might only accept Application Credentials which include a particular public key, while others will accept many. Furthermore, the internal format of these credentials can vary. Finally, the cryptography used in credentials can vary, either due to security policies or certification requirements.

Because the Identifier Policy is responsible for assigning Application Identifiers to processes, it is possible for the same Userspace Binary to have different Application Identifiers on different Tock systems. For example, suppose a TBF Object has two Application Credentials TBF footers: one signs with a key A, and the other with key B. Tock systems using a Credentials Checking Policy that accepts key A may use A as the Global Application Identifier, while Tock systems using a different policy that accepts key B may use B as the Global Application Identifier.

4 Process Loading

Tock defines its process loading algorithm in order to provide deterministic behavior in the presence of colliding Application Identifiers. This algorithm is designed to protect against downgrade attacks and misconfiguration.

The process loading operation consists of three stages:

  1. When it boots, the Tock kernel scans for a TBF Object stored in its application flash region. While parsing the TBF Object, the kernel checks that the TBF Object is valid and can run on the system (e.g., do not require a newer kernel version).

  2. After finding a valid and suitable TBF Object, the kernel checks the credentials of the TBF Object. Using the provided Credentials Checking Policy (described in Section 6), it decides whether the process has permission to run. If the TBF Object is allowed to run, the kernel loads the process binary into a slot in the process binaries array.

  3. Each process in the process binaries array is runnable in terms of its credentials. However, at any given time it might not be allowed to run because its Application Identifier or Short ID conflicts with another process. The kernel scans the array of process binaries and determines whether to run the process based on its Application Identifier, Short ID, and the Application Binary version number (stored in the Program Header, described in Section 5.1). At boot, the kernel starts a process if either of:

    • The process has a unique Application Identifier and Short ID,
    • The process has a higher Application Binary version number than all processes it shares its Application Identifier or Short ID with,

    If two processes which share a Short ID or Application ID have the same version number, the kernel starts one of them. The one which starts is the first one discovered in the process binaries array.

    Once a process is determined to be runnable based on credentials and uniqueness, the process is loaded into a slot in the processes array. At this point the process will be run.

Once a Tock system is running, management interfaces may change the set of running processes from those which the boot sequence selected. E.g., the process console might terminate a process so that it can run a different process with the same Short ID and a lower Userspace Binary version number (rollback). The kernel maintains that a running process has a unique Application Identifier and a unique Short ID among running processes.

5 Credentials and Version in Tock Binary Format Objects

This section describes the format and semantics of Program Headers and Credentials Footers.

Application Credentials are usually stored in a TBF Object, along with the Userspace Binary they are associated with. They are usually stored as footers (after the TBF header and Userspace Binary) to simplify computing integrity values such as checksums or hashes. This requires that TBF Objects have a TBF header that specifies where the application binary ends and the footers begin, information which the TbfHeaderV2Main header (the Main Header) does not include. Including Application Credentials in a TBF Object therefore requires using an alternative TbfHeaderV2Program header (the Program Header), which specifics where footers begin.

The Tock process loading algorithm uses version numbers when deciding the which processes with the same Application Identifier to run. Version numbers are stored in a TBF Object in the Version field of a TBF Program Header.

5.1 Program Header

The Program Header is similar to the Main Header, in that it specifies the offset of the entry function of the executable and memory parameters. It adds one field, binary_end_offset, which indicates the offset at which the Userspace Binary ends within the TBF object. The space between this offset and the end of the TBF object is reserved for footers.

This is the format of a Program Header:

0             2             4             6             8
| Type (9)    | Length (16) | init_fn_offset            |
| protected_size            | min_ram_size              |
| binary_end_offset         | version                   |

It is represented in the Tock kernel with this Rust structure:

fn main() {
pub struct TbfHeaderV2Program {
    init_fn_offset: u32,
    protected_size: u32,
    minimum_ram_size: u32,
    binary_end_offset: u32,
    version: u32,

A TBF object MUST NOT have more than one Program Header. If a TBF Object has both a Program Header and a Main Header, the kernel's policy decides which is used. For example, older kernels that do not understand a Program Header may use the Main Header, while newer kernels may choose the Program Header.

To support credentials, the Tock Binary Format has a TbfFooterV2Credentials TLV. This TLV is variable length and has two fields, a 32-bit value specifying the format of the credentials and a variable length data field. The format field defines the format and size of the data field. Each value of the format field except Reserved MUST have a fixed data size and format. This is the format of a Credentials Footer:

0             2             4                           8
| Type (128)  | Length      | format                    |
| data                      |

It is represented in the Tock kernel with this structure:

fn main() {
pub struct TbfFooterV2Credentials {
    format: TbfFooterV2CredentialsType,
    data: &[u8],

Which types of credentials a Credentials Checking Policy supports are kernel-specific. For example, an application that only accepts TBF Objects signed with a particular 4096-bit RSA key can support only those credentials, while an open research system might support no credentials. Because the length field specifies the length of a given credentials, not understanding a particular credentials type does not prevent parsing others.

5.3 Integrity Region

TbfFooterV2Credentials follow the compiled app binary in a TBF object. If a TbfFooterV2Credentials footer includes a cryptographic hash, signature, or other value to check the integrity of a process binary, this value MUST be computed over the TBF Header and Userspace Binary, from the start of the TBF object until binary_end_offset. This region is called the integrity region. Computing an integrity value in a Credentials Footer MUST NOT include the contents of Footers. If new metadata associated with an application binary needs to be covered by integrity, it MUST be a Header. If new metadata associated with an application binary needs to not be covered by integrity, it MUST be a Footer.

The integrity region is from the end of the TBF Header to the location indicated by the binary_end_offset field in the Program Header. The size of the integrity region slice is therefore equal to binary_end_offset.

6 Credentials Checking Policy: the AppCredentialsPolicy trait

The AppCredentialsPolicy trait defines the interface that implements the Credentials Checking Policy of the Process Checker: it accepts, passes on, or rejects Application Credentials. When a Tock board asks the kernel to load processes, it passes a reference to a AppCredentialsPolicy, which the kernel uses to check credentials. An implementer of AppCredentialsPolicy sets the security policy of Userspace Binary loading by deciding which types of credentials, and which credentials, are acceptable and which are rejected.

fn main() {
pub enum CheckResult {

pub trait Client<'a> {
    fn check_done(&self,
                  result: Result<CheckResult, ErrorCode>,
                  credentials: TbfFooterV2Credentials,
                  integrity_region: &'a [u8]);

pub trait AppCredentialsPolicy<'a> {
    fn set_client(&self, client: &'a dyn Client<'a>);
    fn require_credentials(&self) -> bool;
    fn check_credentials(&self,
                         credentials: TbfFooterV2Credentials,
                         integrity_region: &'a [u8]) ->
        Result<(), (ErrorCode, TbfFooterV2Credentials, &'a [u8])>;

If the kernel has been instructed to check credentials of Userspace Binaries, after it successfully parses a Userspace Binary it checks the credentials of the process binary.

To check the integrity of a process, the kernel scans the footers in order, starting at the beginning of that process's footer region. At each TbfFooterV2Credentials footer it encounters, the kernel calls check_credentials on the provided AppCredentialsPolicy. If check_credentials returns CheckResult::Accept, the kernel stops processing credentials and stores the process binary in the process binaries array. If the AppCredentialsPolicy returns CheckResult::Reject, the kernel stops processing credentials and does not load the process binary.

If the AppCredentialsPolicy returns CheckResult::Pass, the kernel tries the next TbfFooterV2Credentials, if there is one. If the kernel reaches the end of the TBF Footers (or if there is a Main Header and so no Footers) without encountering a Reject or Accept result, it calls require_credentials to ask the AppCredentialsPolicy what the default behavior is. If require_credentials returns true, the kernel does not load the process binary. If require_credentials returns false, the kernel loads the process binary into the process binaries array. If a process binary has no TbfFooterV2Credentials footers then there will be no Accept or Reject results and require_credentials defines whether the Userspace Binary is runnable.

The binary argument to check_credentials is a reference to the integrity region of the process binary.

7 Identifier Policy: the AppUniqueness trait

The AppUniqueness trait defines the API the Process Checker provides to decide whether two processes have the same Application Identifier or Short ID. An implementer of AppUniqueness implements the different_identifier method, which performs a pairwise comparison of two processes.

fn main() {
trait AppUniqueness {
  // Returns true if the two processes have different application
  // identifiers.
  fn different_identifier(&self,
                          processA: &ProcessBinary,
                          processB: &ProcessBinary) -> bool;

  fn different_identifier_process(&self,
                                  processA: &ProcessBinary,
                                  processB: &dyn Process) -> bool;

  fn different_identifier_processes(&self,
                                    processA: &dyn Process,
                                    processB: &dyn Process) -> bool;

This interfaces encapsulate the methods by which a module assigns or calculates application identifiers. As process binaries must be compared to both other process binaries and already loaded processes, there are two version of the different_identifier method to support both cases.

8 Short IDs and the Compress trait

While TbfFooterV2Credentials often define the identity and credentials of an application, they are large data structures that are too large to store in RAM. When parts of the kernel wish to apply security or access policies based on Application Identifiers, they need a concise way to represent these identifiers. Requiring policies to be encoded in terms of raw Application Identifiers can be extremely costly: a table, for example, that says that only Applications signed with a particular 4096-bit RSA key can access certain system calls requires storing the whole 4096-bit key. If there are multiple such security policies through the kernel, they must each store this information.

The Compress trait provides a mechanism to map an Application Identifier to a small (32-bit) integer called a Short ID. Short IDs can be used throughout the kernel as an identifier of an Application.

For example, suppose that a device wants to grant access to all Userspace Binaries signed by a certain 3072-bit RSA key K and has no other security policies. The Credentials Checking Policy only accepts 3072-bit RSA credentials with key K. The Compress trait implementation assigns a Short ID based on a string match with the process package name, with certain names receiving particular Short IDs. Access control systems within the kernel can define their policies in terms of these identifiers, such that they can check access by comparing 32-bit integers rather than 384-byte keys.

Short IDs support the concept of a "Locally Unique" identifier by having a special LocallyUnique value. All tests for equality with ShortId::LocallyUnique return false.

8.1 Short ID Properties and Examples

Given a particular combination of deterministic Identifier Policy and Credentials Checking Policy, Short IDs have two requirements. They

  1. MUST be unique across running processes,
  2. MUST be consistent across all running instances of an Application on Tock systems.

Short IDs are locally unique for three reasons. First, it simplifies process management and naming: a particular Short ID uniquely identifies a running process. Second, it ensures that resources bound to an application identifier (such as non-volatile storage) do not have to handle concurrent accesses from multiple processes. Finally, generally one does not want two copies of the same Application running: they can create conflicting responses and behaviors.

These two requirements restrict the set of possible combinations of Credentials Checking Policy and Identifier Policy. For example, a Short ID cannot be an incrementing counter; it must be deterministically derived from the Application Identifier.

A basic challenge that arises with Short IDs is that they are a form of compression. In the ideal case, Short IDs would have two additional properties:

  • Different Application Identifiers map to different Short IDs, and
  • All Application Identifiers have a concrete Short ID that identifies the Application.

Unfortunately, it is not possible to satisfy both of these properties simultaneously. This is because Short IDs potentially compress Application Identifiers. Consider, for example, a system where the Application Identifier is the public key in an 4096-bit RSA credential. Short IDs are 32 bits, but there are more than 2^32 4096-bit RSA keys. If every RSA key receives a different Short ID, and that Short ID is always the same, after 2^32 keys the Short ID space is exhausted.

Every algorithm to map Application Identifiers to Short IDs therefore sacrifices one of these two properties:

  • Different Application Identifiers can map to the same Short ID: An Identifier Policy with this property is one that uses string names as Global Application Identifiers and calculates the Short ID of process to be the checksum (or hash) of the string name. Two different names can checksum or hash to the same value. These collisions, however, can be acceptable if a developer is willing to pick string names that do not collide or change them when they do. A research or prototyping system might use this Identifier Policy.
  • Some Application Identifiers do not receive concrete Short IDs: An Identifier Policy with this property is one that uses public keys in signature credentials as Application Identifiers and has a set of public keys it knows and trust. It maps these known keys to a small set of Short IDs (e.g., 1 through N). The system may run Userspace Binaries signed by other keys, but assigns them a Locally Unique Application identifier, which results in a Locally Unique Short ID.

8.2 Example Short ID use cases

Here are three example use cases of Short IDs.

8.2.1 Use Case 1: Anonymous Applications

There are many Tock systems that do not particularly care about the identity of Applications. They do not have security policies, or track Application Identifiers. A prototyping system whose Credentials Checking Policy accepts all TBF Objects regardless of Application Credentials is an example of such a system. At boot, it scans the set of TBF Objects in application flash, trying to load and run each one until it runs out of resources (RAM, process slots). Applications cannot store data they expect to persist across reboots. Because the Tock kernel does not care about the identity of Applications, it has no security policies for limiting access to functionality or resources (e.g., system call filters).

In this use case, the Credentials Checking Policy accepts all correctly formatted TBF Objects and the Identifier Policy assigns every process a Locally Unique Identifier and a Locally Unique Short ID.

8.2.2 Use Case 2: U2F Application

In this use case, Tock needs to run a Universal 2nd Factor Authentication (U2F) application. This Application needs to store a private key in flash. No other Application should be able to access this key. The Tock kernel also restricts certain system calls to only the U2F Application, such as invoking cryptographic accelerators. Finally, the U2F Application needs a consistent identity over reboots of its Userspace Binary, the kernel, and upgrades of the Application with new versions (and Userspace Binaries).

In this use case, the Application Identifier is a Global Identifier. To establish the authenticity and integrity of the U2F Application, the Credential Checking Policy requires that an Application has a valid 4096-bit RSA credential. The system assumes that each Application has its own public-private key pair. While the system will load and run any process whose Userspace Binary has a valid 4096-bit RSA credential, it only gives special permissions and access to the U2F Application.

The Identifier Policy defines the Application Identifier of a process to depend on the public key of its 4096-bit RSA credential. If it is the key known to belong to the U2F Application, the Application Identifier is the key. If the key is not recognized, the Application Identifier is a Locally Unique Identifier. The Short ID of the U2F Application is 1 and the Short ID of all other Applications is Locally Unique.

8.2.3 Use Case 3: Application Isolation

In this use case, Tock needs to support multiple Applications that can read and write local flash. Each Application has its own flash storage, and Tock isolates their flash storage from one another. An Application cannot access the flash of another Application. However, this is a development system or a system which does not require confidentiality. While there is storage isolation between Applications, this is for debuggability, easy of composition, and simplicity and not to meet security requirements. The Credentials Checking Policy is permissive and tries to run every properly formatted TBF Objects.

In this use case, the Application Identifier is a Global Identifier. It is the string name of the TBF Object as encoded in a TBF Header. The Short ID is a one's complement checksum of the string name.

If a developer installs two TBF Objects with the same string name, the Tock kernel thinks they are the same Application and only runs one of them. If a developer accidentally uses two different string names that have the same checksum (e.g. both "dog" and "mal" checksum to 0x13a), the Tock kernel also only runs one of them. Some local modifications to tockloader check for these collisions and prevent the developer from accidentally installing colliding Applications.

Note that in this case it is possible that the "mal" application could read data stored by the "dog" application.

8.3 Short ID Format

The 32-bit value MUST be non-zero. ShortId uses core::num::NonZeroU32 so that an ShortId can be 32 bits in size, with 0 reserved for LocallyUnique.

fn main() {
#[derive(Clone, Copy)]
enum ShortId {

pub trait Compress {
    fn to_short_id(process: &ProcessBinary) -> ShortId;

Generally, the Process Checker that implements AppUniqueness also implements Compress. This allows it to share copies of public keys or other credentials that it uses to make decisions, reducing flash space dedicated to these constants. Doing so also makes it less likely that the two are inconsistent.

8.4 Short ID Considerations

It is RECOMMENDED that the Fixed field of ShortId be completely hidden and unknown to modules that use ShortId to manage security policies. They should depend on obtaining ShortId values based on known names or methods. For example, the implementation of an Identifier Policy can define a method, privileged_id, which returns the Short ID associated with special privileges. Kernel modules which want to give these processes extra permissions can check whether the ShortId associated with a process matches the ShortId returned from privileged_id. Alternatively, when they are initialized, they can be passed a slice or array of ShortIds which are allowed; system initialization generates this set once and passes it into the module so it does not need to maintain a reference to the structure implementing Compress.

The exact Fixed values used is an internal implementation decision for the implementer of Compress and the Identifier Policy. Doing so cleanly decouples modules through APIs and does not leak internal state.

9 The AppIdPolicy Trait

The AppIdPolicy trait is a composite trait that combines AppUniqueness and Compress into a single trait so it can be passed as a single reference.

fn main() {
pub trait AppIdPolicy: AppUniqueness + Compress {}
impl<T: AppUniqueness + Compress> AppIdPolicy for T {}

10 Capsules

Capsules can use AppID to restrict access to only certain processes or to partition a resource based among processes. By using AppID, this assignment is persistent across reboots and application updates.

For example, consider a display that is divided such that different applications are given access to different regions of the display. These assignments should be persistent to main continuity for the user looking at the display, even if applications are added or removed.

This is a very incomplete example but it shows the general use of ShortId within a capsule. Note that accessing a ShortId is done using ProcessId.

fn main() {
pub struct AppScreenRegion {
    app_id: kernel::process::ShortId,
    frame: Frame,

pub struct ScreenShared<'a, S: hil::screen::Screen<'a>> {
    screen: &'a S,
    apps: Grant<App, UpcallCount<1>, AllowRoCount<{ ro_allow::COUNT }>, AllowRwCount<0>>,
    apps_regions: &'a [AppScreenRegion],

impl<'a, S: hil::screen::Screen<'a>> ScreenShared<'a, S> {
    fn get_app_screen_region_frame(&self, process_id: ProcessId) -> Option<Frame> {
        // Check if a process with that short ID has an allocated frame.
        let short_id = process_id.short_app_id();

        for app_screen_region in self.apps_regions {
            if short_id == app_screen_region.app_id {
                return Some(app_screen_region.frame);

    fn write_screen(&self, process_id: ProcessId) {
      let screen_region = self.get_app_screen_region_frame(process_id);

impl<'a, S: hil::screen::Screen<'a>> SyscallDriver for ScreenShared<'a, S> {
    fn command(&self, command_num: usize, _: usize, _: usize, process_id: ProcessId) -> CommandReturn {
        match command_num {
            // Driver existence check
            0 => CommandReturn::success(),

            // Write
            1 => {

            _ => CommandReturn::failure(ErrorCode::NOSUPPORT),

11 Implementation Considerations

Notes about requirements for application identifier generation/calculation (must be synchronous).

12 Authors' Addresses

Philip Levis
409 Gates Hall
Stanford University
Stanford, CA 94305

Johnathan Van Why <>

Brad Campbell <>