As of Mbed TLS 3.3.0, the PSA Driver Interface has only been partially implemented. As a result, the deliverables for writing a driver and the method for integrating a driver with Mbed TLS will vary depending on the operation being accelerated. This document describes how to write and integrate cryptoprocessor drivers depending on which operation or driver type is being implemented.
The `docs/proposed/` directory contains three documents which pertain to the proposed, work-in-progress driver system. The [PSA Driver Interface](https://github.com/Mbed-TLS/mbedtls/blob/development/docs/proposed/psa-driver-interface.md) describes how drivers will interface with Mbed TLS in the future, as well as driver types, operation types, and entry points. As many key terms and concepts used in the examples in this document are defined in the PSA Driver Interface, it is recommended that developers read it prior to starting work on implementing drivers.
The PSA Driver [Developer](https://github.com/Mbed-TLS/mbedtls/blob/development/docs/proposed/psa-driver-developer-guide.md) Guide describes the deliverables for writing a driver that can be used with Mbed TLS, and the PSA Driver [Integration](https://github.com/Mbed-TLS/mbedtls/blob/development/docs/proposed/psa-driver-integration-guide.md) Guide describes how a driver can be built alongside Mbed TLS.
The PSA Driver Interface specification specifies which cryptographic operations can be accelerated by third-party drivers. Operations that are completed within one step (one function call), such as verifying a signature, are called *Single-Part Operations*. On the other hand, operations that consist of multiple steps implemented by different functions called sequentially are called *Multi-Part Operations*. Single-part operations implemented by a driver will have one entry point, while multi-part operations will have multiple: one for each step.
There are two types of drivers: *transparent* or *opaque*. See below an excerpt from the PSA Driver Interface specification defining them:
* **Transparent** drivers implement cryptographic operations on keys that are provided in cleartext at the beginning of each operation. They are typically used for hardware **accelerators**. When a transparent driver is available for a particular combination of parameters (cryptographic algorithm, key type and size, etc.), it is used instead of the default software implementation. Transparent drivers can also be pure software implementations that are distributed as plug-ins to a PSA Cryptography implementation (for example, an alternative implementation with different performance characteristics, or a certified implementation).
* **Opaque** drivers implement cryptographic operations on keys that can only be used inside a protected environment such as a **secure element**, a hardware security module, a smartcard, a secure enclave, etc. An opaque driver is invoked for the specific [key location](https://github.com/Mbed-TLS/mbedtls/blob/development/docs/proposed/psa-driver-interface.md#lifetimes-and-locations) that the driver is registered for: the dispatch is based on the key's lifetime.
Mbed TLS contains a **driver dispatch layer** (also called a driver wrapper layer). For each cryptographic operation that supports driver acceleration (or sub-part of a multi-part operation), the library calls the corresponding function in the driver wrapper. Using flags set at compile time, the driver wrapper ascertains whether any present drivers support the operation. When no such driver is present, the built-in library implementation is called as a fallback (if allowed). When a compatible driver is present, the driver wrapper calls the driver entry point function provided by the driver author.
The long-term goal is for the driver dispatch layer to be auto-generated using a JSON driver description file provided by the driver author.
For some cryptographic operations, this auto-generation logic has already been implemented. When accelerating these operations, the instructions in the above documents can be followed. For the remaining operations which do not yet support auto-generation of the driver wrapper, developers will have to manually edit the driver dispatch layer and call their driver's entry point functions from there.
Auto-generation of the driver wrapper is supported for the operation entry points specified in the table below. Certain operations are only permitted for opaque drivers. All other operation entry points do not support auto-generation of the driver wrapper.
| Transparent Driver | Opaque Driver |
|---------------------|---------------------|
| `import_key` | `import_key` |
| `export_key` | `export_key` |
| `export_public_key` | `export_public_key` |
| | `copy_key` |
| | `get_builtin_key` |
### Process for Entry Points where auto-generation is implemented
If the driver is accelerating operations whose entry points are in the above table, the instructions in the driver [developer](https://github.com/Mbed-TLS/mbedtls/blob/development/docs/proposed/psa-driver-developer-guide.md) and [integration](https://github.com/Mbed-TLS/mbedtls/blob/development/docs/proposed/psa-driver-integration-guide.md) guides should be followed.
**TODO: Provide brief summary of the method using the Mbed TLS test driver as an example**
### Process for Entry Points where auto-generation is not implemented
If the driver is accelerating operations whose entry points are not present in the table, a different process is followed where the developer manually edits the driver dispatch layer. In general, the following steps must be taken **for each single-part operation** or **for each sub-part of a multi-part operation**:
**1. Choose a driver prefix and a macro name that indicates whether the driver is enabled** \
A driver prefix is simply a word (often the name of the driver) that all functions/macros associated with the driver should begin with. This is similar to how most functions/macros in Mbed TLS begin with `PSA_XXX/psa_xx` or `MBEDTLS_XXX/mbedtls_xxx`. The macro name can follow the form `DRIVER_PREFIX_ENABLED` or something similar; it will be used to indicate the driver is available to be called. When building with the driver present, define this macro at compile time. For example, when using `make`, this is done using the `-D` flag.
**2. Locate the function in the driver dispatch layer that corresponds to the entry point of the operation being accelerated.** \
The file `psa_crypto_driver_wrappers.c.jinja` contains the driver wrapper functions. For the entry points that have driver wrapper auto-generation implemented, the functions have been replaced with `jinja` templating logic. While the file has a `.jinja` extension, the driver wrapper functions for the remaining entry points are simple C functions. The names of these functions are of the form `psa_driver_wrapper` followed by the entry point name. So, for example, the function `psa_driver_wrapper_sign_hash()` corresponds to the `sign_hash` entry point.
**3. If a driver entry point function has been provided then ensure it has the same signature as the driver wrapper function.** \
If one has not been provided then write one. Its name should begin with the driver prefix, followed by transparent/opaque (depending on driver type), and end with the entry point name. It should have the same signature as the driver wrapper function. The purpose of the entry point function is to take arguments in PSA format for the implemented operation and return outputs/status codes in PSA format. \
**5. Conditionally include header files required by the driver**
Include any header files required by the driver in `psa_crypto_driver_wrappers.h`, placing the `#include` statements within an `#if defined` block which checks if the driver is available:
```
#if defined(DRIVER_PREFIX_ENABLED)
#include ...
#endif
```
**6. Modify the driver wrapper function** \
Each driver wrapper function contains a `switch` statement which checks the location of the key. If the key is stored in local storage, then operations are performed by a transparent driver. If it is stored elsewhere, then operations are performed by an opaque driver.
* **Transparent drivers:** Calls to driver entry points go under `case PSA_KEY_LOCATION_LOCAL_STORAGE`.
* **Opaque Drivers** Calls to driver entry points go in a separate `case` block corresponding to the key location.
The diagram below shows the layout of a driver wrapper function which can dispatch to two transparent drivers `Foo` and `Bar`, and one opaque driver `Baz`.
All code related to driver calls within each `case` must be contained between `#if defined(PSA_CRYPTO_ACCELERATOR_DRIVER_PRESENT)` and a corresponding `#endif`. Within this block, each individual driver's compatibility checks and call to the entry point must be contained between `#if defined(DRIVER_PREFIX_ENABLED)` and a corresponding `#endif`. Checks that involve accessing key material using PSA macros, such as determining the key type or number of bits, must be done in the driver wrapper.
This guide assumes you are building Mbed TLS from source alongside your project. If building with a driver present, the chosen driver macro (`DRIVER_PREFIX_ENABLED`) must be defined. This can be done in two ways:
* *At compile time via flags.* This is the preferred option when your project uses Mbed TLS mostly out-of-the-box without significantly modifying the configuration. When building with Make this can be done by passing the macro name to Make with the `-D` flag. When building with CMake this can be done by modifying `CMakeLists.txt`.
* *Providing a user config file.* This is the preferred option when your project requires a custom configuration that is significantly different to the default. Define the macro for the driver, along with any other custom configurations in a separate header file, then use `config.py`, to set `MBEDTLS_USER_CONFIG_FILE`, providing the path to the defined header file. This will include your custom config file after the default. If you wish to completely replace the default config file, set `MBEDTLS_CONFIG_FILE` instead.
### Example: Manually integrating a software accelerator alongside Mbed TLS
[p256-m](https://github.com/mpg/p256-m) is a minimalistic implementation of ECDH and ECDSA on NIST P-256 curves, specifically optimized for use in constrained 32-bit environments. As such, it serves as a software accelerator. This section demonstrates the integration of `p256-m` as a transparent driver alongside Mbed TLS, serving as a guide for implementation.
The code for p256-m can be found in `3rdparty/p256-m/p256m`. In this demonstration, p256-m is built from source alongside Mbed TLS.
**NOTE:** p256-m also implements key generation. However, it's RNG is based on `stdlib`, making this feature **unsuitable for production builds**. It is included with Mbed TLS purely to be used as an example.
The driver prefix for p256-m is `P256`/`p256`. The driver macro is `MBEDTLS_P256M_EXAMPLE_DRIVER_ENABLED`. To build with and use p256-m, set this macro by running the following command from the root of the `mbedtls/` directory:
python3 scripts/config.py set MBEDTLS_P256M_EXAMPLE_DRIVER_ENABLED
p256-m implements four entry points: `generate_key`, `key_agreement`, `sign_hash`, `verify_hash`. The `sign/verify_hash` entry points are used instead of `sign/verify_message` as messages must be hashed prior to any operation, and p256-m does not implement this. The driver entry point functions can be found in `p256m_driver_entrypoints.[hc]`. These functions act as an interface between Mbed TLS and p256-m; converting between PSA and p256-m argument formats and performing sanity checks. If the driver's status codes differ from PSA's, it is recommended to implement a status code translation function. The function `p256_to_psa_error()` converts error codes returned by p256-m into PSA error codes.
The driver wrapper functions in `psa_crypto_driver_wrappers.c.jinja` for all four entry points have also been modified. The code block below shows the additions made to `psa_driver_wrapper_sign_hash()`. In adherence to the defined process, all code related to the driver call is placed within a check for `MBEDTLS_P256M_EXAMPLE_DRIVER_ENABLED`. p256-m only supports non-deterministic ECDSA using keys based on NIST P256; these constraints are enforced through checks (see the `if` statement). Checks that involve accessing key material, (e.g. checking key type or bits) **must** be performed in the driver wrapper. This is because this information is marked private and may not be accessed outside the library. Other checks can be performed here or in the entry point function. The status returned by the driver is propagated up the call hierarchy **unless** the driver does not support the operation (i.e. return `PSA_ERROR_NOT_SUPPORTED`). In that case the next available driver/built-in implementation is called.
```
#if defined (MBEDTLS_P256M_EXAMPLE_DRIVER_ENABLED)
Following this, p256-m is now ready to use alongside Mbed TLS as a software accelerator. If `MBEDTLS_P256M_EXAMPLE_DRIVER_ENABLED` is set in the config, p256-m's implementations of key generation, ECDH, and ECDSA will be used where applicable.
