Study: PSA random generation drivers for 4.0 (replace entropy.h)

[daverodgman]

As per #5156 (comment) - investigate what is required to support PSA entropy drivers. The outcome of this task is to define an epic with the work needed for 4.0.

https://github.com/Mbed-TLS/mbedtls/blob/development/docs/proposed/psa-driver-interface.md#random-generation-entry-points

This includes an overhaul of support for a stored seed, replacing MBEDTLS_ENTROPY_NV_SEED and mbedtls_psa_inject_entropy().

The functionality of MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG and mbedtls_psa_external_get_random will be subsumed. But we may want to keep them as a simpler interface, especially until we have driver generation scripts.

[gilles-peskine-arm]

Mbed TLS has a number of ways to configure the entropy and DRBG modules, which we may or may want to offer in the new random module.

• MBEDTLS_ENTROPY_FORCE_SHA256: choice of primitive to combine and whiten entropy sources.
• MBEDTLS_ENTROPY_MIN_HARDWARE: cryptographic strength of the entropy collection.
• MBEDTLS_ENTROPY_NV_SEED: support for an entropy store in addition to or in combination with entropy peripherals. This needs to stay, but not necessarily in its present form. On PSA platforms, the NV seed should be in the PSA storage. But on hosted platforms such as Linux, users generally don't use the PSA storage but use a named file instead.