4. Sample architectures

This section describes some example architectures that can be used for implementations of the interface described in this specification. This list is not exhaustive and the section is entirely non-normative.

4.1. Single-partition architecture

In the single-partition architecture, there is no security boundary inside the system. The application code can access all the system memory, including the memory used by the cryptographic services described in this specification. Thus, the architecture provides no isolation.

This architecture does not conform to the Arm Platform Security Architecture Security Model. However, it is useful for providing cryptographic services that use the same interface, even on devices that cannot support any security boundary. So, while this architecture is not the primary design goal of the API defined in the present specification, it is supported.

The functions in this specification simply execute the underlying algorithmic code. Security checks can be kept to a minimum, since the cryptoprocessor cannot defend against a malicious application. Key import and export copy data inside the same memory space.

This architecture also describes a subset of some larger systems, where the cryptographic services are implemented inside a high-security partition, separate from the code of the main application, though it shares this high-security partition with other platform security services.

4.2. Cryptographic token and single-application processor

This system is composed of two partitions: one is a cryptoprocessor and the other partition runs an application. There is a security boundary between the two partitions, so that the application cannot access the cryptoprocessor, except through its public interface. Thus, the architecture provides cryptoprocessor isolation. The cryptoprocessor has some non-volatile storage, a TRNG, and possibly, some cryptographic accelerators.

There are a number of potential physical realizations: the cryptoprocessor might be a separate chip, a separate processor on the same chip, or a logical partition using a combination of hardware and software to provide the isolation. These realizations are functionally equivalent in terms of the offered software interface, but they would typically offer different levels of security guarantees.

The PSA crypto API in the application processor consists of a thin layer of code that translates function calls to remote procedure calls in the cryptoprocessor. All cryptographic computations are, therefore, performed inside the cryptoprocessor. Non-volatile keys are stored inside the cryptoprocessor.

4.3. Cryptoprocessor with no key storage

As in the Cryptographic token and single-application processor architecture, this system is also composed of two partitions separated by a security boundary and also provides cryptoprocessor isolation. However, unlike the previous architecture, in this system, the cryptoprocessor does not have any secure, persistent storage that could be used to store application keys.

If the cryptoprocessor is not capable of storing cryptographic material, then there is little use for a separate cryptoprocessor, since all data would have to be imported by the application.

The cryptoprocessor can provide useful services if it is able to store at least one key. This might be a hardware unique key that is burnt to one-time programmable memory during the manufacturing of the device. This key can be used for one or more purposes:

  • Encrypt and authenticate data stored in the application processor.
  • Communicate with a paired device.
  • Allow the application to perform operations with keys that are derived from the hardware unique key.

4.4. Multi-client cryptoprocessor

This is an expanded variant of Cryptographic token and single-application processor. In this variant, the cryptoprocessor serves multiple applications that are mutually untrustworthy. This architecture provides caller isolation.

In this architecture, API calls are translated to remote procedure calls, which encode the identity of the client application. The cryptoprocessor carefully segments its internal storage to ensure that a client’s data is never leaked to another client.

4.5. Multi-cryptoprocessor architecture

This system includes multiple cryptoprocessors. There are several reasons to have multiple cryptoprocessors:

  • Different compromises between security and performance for different keys. Typically, this means a cryptoprocessor that runs on the same hardware as the main application and processes short-term secrets, a secure element or a similar separate chip that retains long-term secrets.
  • Independent provisioning of certain secrets.
  • A combination of a non-removable cryptoprocessor and removable ones, for example, a smartcard or HSM.
  • Cryptoprocessors managed by different stakeholders who do not trust each other.

The keystore implementation needs to dispatch each request to the correct processor. For example:

  • All requests involving a non-extractable key must be processed in the cryptoprocessor that holds that key.
  • Requests involving a persistent key must be processed in the cryptoprocessor that corresponds to the key’s lifetime value.
  • Requests involving a volatile key might target a cryptoprocessor based on parameters supplied by the application, or based on considerations such as performance inside the implementation.