Hello,

RSA key objects include a mutex. `mbedtls_rsa_private` locks the mutex because it caches some auxiliary values used for blinding in the key object. (`mbedtls_rsa_public` also locks the mutex but it seems pointless.) This allows applications to create a key (this must be done in a single-threaded way), then use that key concurrently.

This feature has a number of downsides.  From a high-level architectural perspective, the RSA module is a low-level part of the code dedicated to peforming calculations; managing concurrency is outside its scope. The presence of the mutex complicates the lifecycle of RSA contexts, leading to unmet expectations (https://github.com/ARMmbed/mbedtls/issues/2621) and bugs on certain platforms (https://github.com/ARMmbed/mbedtls/pull/4104). ECC contexts do not have a mutex, even though they would need one, so a multithreaded application that works with RSA keys can't easily be changed to ECC keys.

As a consequence, I propose to remove mutexes from RSA keys in Mbed TLS 3.0. Applications that currently rely on the mutex should either migrate to the PSA API or wrap an RSA object (or a pk object, which would allow algorithm agility) in a mutex.

This proposal is also recorded with more details at https://github.com/ARMmbed/mbedtls/issues/4124 .