# Guidelines for porting Cryptocell to Mbed OS

Cryptocell (CC) 310 is a hardware accelerator you can port to boards that have CC 310 embedded in their hardware.

The CC 310 driver consists of three libraries:

* A common core library (`libcc_core.a`).
* A platform-specific TRNG library containing TRNG-related information for sampling sufficient entropy on the specific platform (`libcc_trng.a`).
* A platform-specific library containing extra information, such as the CC register's base addresses on the specific board (`libcc_ext.a`).

Library version information:

* The CC 310 libraries were built from version `arm_sw-cc310-1.1.0.1285`.
* The `IAR` libraries were built using `IAR ANSI C/C++ Compiler V7.80.1.11864/W32 for ARM` with `--cpu Cortex-M4f`.
* The `ARM` libraries were built using `ARM Compiler 5.06 update 4 (build 422)` with `--cpu cortex-m4`.
* The `GCC_ARM` libraries were built using `arm-none-eabi-gcc 6.3.1 20170620 (release)` with `-mcpu=cortex-m4`.

To port your CC 310 driver to Mbed OS on your specific target:

1. In `targets.json`, add the following  to your target:
 	* `MBEDTLS_CONFIG_HW_SUPPORT` to `macros_add` key. This instructs Mbed TLS to look for an alternative cryptographic implementation.
 	* `CRYPTOCELL310` to `feature`. Use this in your common code that you need to remove from compilation in case CC exists in your board. Use `#if !defined(FEATURE_CRYPTOCELL310)` and  `#if defined(FEATURE_CRYPTOCELL310)`.
1. In `objects.h`, include `objects_cryptocell.h`. You can use the `FEATURE_CRYPTOCELL310` precompilation check as defined above.
1. In `features/cryptocell/FEATURE_CRYPTOCELL310/TARGET_<target name>`, add your platform-specific libraries for all toolchains in `TOOLCHAIN_ARM`, `TOOLCHAIN_GCC_ARM` and `TOOLCHAIN_IAR` respectively.
1. Add your CC setup code:
	* Implement `crypto_platform_setup()` and `crypto_platform_terminate()` to enable CC on your platform, in case you have board-specific setup functionality, required for CC setup. You MUST call `SaSi_LibInit()` and `SaSi_LibFini()` in these functions.
	* Define `crypto_platform_ctx` in `crypto_device_platform.h` in a way that suits your implementation.

## Enabling optional alternative drivers

Three additional modules that are not enabled by default have alternative implementation support. This allows backward compatibility because these modules don't have full functionality and returns `MBEDTLS_ERR_PLATFORM_FEATURE_UNSUPPORTED` for some features. The modules are:

* `AES`, which only supports 128 bit key size, as opposed to previous support for all key sizes.
* `CMAC`, which only supports AES 128 bit key size, as opposed to previous support for DES and all key sizes.
* `SHA512`, which only supports SHA512, as opposed to previous support for SHA384, as well.

To enable these modules, define `MBEDTLS_AES_ALT`, `MBEDTLS_CMAC_ALT` and `MBEDTLS_SHA512_ALT`, either in `mbed_app.json` or in your `MBEDTLS_USER_CONFIG_FILE`, to have hardware accelerated module with reduced RAM size, on the expense of full functionality.