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Expand and improve psa-crypto-implementation-structure.md

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Signed-off-by: Ronald Cron <ronald.cron@arm.com>
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Ronald Cron 2021-01-18 18:11:19 +01:00
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@ -47,4 +47,27 @@ psa_status_t psa_api( ... )
* clean-up in case of error ... */
}
```
Obviously, for some PSA APIs the implementation is more complicated with several potential conditional calls to different driver entry points but most of PSA API code aims to be organized along those lines. The implementation of the `psa_driver_wrapper_<entry_point>` function is generated by the build system based on the JSON driver description files of the various PSA drivers making up the Mbed TLS PSA Cryptography API implementation.
The code of most PSA APIs is expected to match precisely the above layout. However, it is likely that the code structure of some APIs will be more complicated with several calls to the driver interface, mainly to encompass a larger variety of hardware designs. For example, to encompass hardware accelerators that are capable of verifying a MAC and those that are only capable of computing a MAC, the psa_mac_verify() API could call first psa_driver_wrapper_mac_verify() and then fallback to psa_driver_wrapper_mac_compute().
The implementations of `psa_driver_wrapper_<entry_point>` functions are generated by the build system based on the JSON driver description files of the various PSA drivers making up the Mbed TLS PSA Cryptography API implementation. The implementations are generated in a psa_crypto_driver_wrappers.c C file and the function prototypes declared in a psa_crypto_driver_wrappers.h header file.
The psa_driver_wrapper_<entry_point>() functions dispatch cryptographic operations to accelerator drivers, secure element drivers as well as to the software implementations of cryptographic operations.
Note that the implementation allows to build the library with only a C compiler by shipping a generated file corresponding to a pure software implementation. The driver entry points and their code in this generated file are guarded by pre-processor directives based on PSA_WANT_xyz macros (see [Conditional inclusion of cryptographic mechanism through the PSA API in Mbed TLS](psa-conditional-inclusion-c.html). That way, it is possible to compile and include in the library only the desired cryptographic operations.
### Key creation
Key creation implementation in Mbed TLS PSA core is articulated around three internal functions: psa_start_key_creation(), psa_finish_key_creation() and psa_fail_key_creation(). Implementations of key creation PSA APIs, namely psa_import_key(), psa_generate_key(), psa_key_derivation_output_key() and psa_copy_key() go by the following sequence:
1. Check the input parameters.
2. Call psa_start_key_creation() that allocates a key slot, prepares it with the specified key attributes, and in case of a volatile key assign it a volatile key identifier.
3. Generate or copy the key material into the key slot. This entails the allocation of the buffer to store the key material.
4. Call psa_finish_key_creation() that mostly saves persistent keys into persistent storage.
In case of any error occuring at step 3 or 4, psa_fail_key_creation() is called. It wipes and cleans the slot especially the key material: reset to zero of the RAM memory that contained the key material, free the allocated buffer.
## Mbed TLS PSA Cryptography API implementation drivers
A driver of the Mbed TLS PSA Cryptography API implementation (Mbed TLS PSA driver in the following) is a driver in the sense that it is compliant with the PSA driver interface specification. But it is not an actual driver that drives some hardware. It implements cryptographic operations purely in software.
An Mbed TLS PSA driver C file is named psa_crypto_<driver_name>.c and its associated header file psa_crypto_<driver_name>.h. The functions implementing a driver entry point as defined in the PSA driver interface specification are named as mbedtls_psa_<driver name>_<entry point>(). As an example, the psa_crypto_rsa.c and psa_crypto_rsa.h are the files containing the Mbed TLS PSA driver implementing RSA cryptographic operations. This RSA driver implements among other entry points the "import_key" entry point. The function implementing this entry point is named mbedtls_psa_rsa_import_key().