On space-constrained platforms, it is a useful configuration to be able
to import/export and perform RSA key pair operations, but to exclude RSA
key generation, potentially saving flash space. It is not possible to
express this with the PSA_WANT_ configuration system at the present
time. However, in previous versions of Mbed TLS (v2.24.0 and earlier) it
was possible to configure a software PSA implementation which was
capable of making RSA signatures but not capable of generating RSA keys.
To do this, one unset MBEDTLS_GENPRIME.
Since the addition of MBEDTLS_PSA_BUILTIN_KEY_TYPE_RSA_KEY_PAIR, this
expressivity was lost. Expressing that you wanted to work with RSA key
pairs forced you to include the ability to generate key pairs as well.
Change psa_crypto_rsa.c to only call mbedtls_rsa_gen_key() if
MBEDTLS_GENPRIME is also set. This restores the configuration behavior
present in Mbed TLS v2.24.0 and earlier versions.
It left as a future exercise to add the ability to PSA to be able to
express a desire for a software or accelerator configuration that
includes RSA key pair operations, like signature, but excludes key pair
generation.
Without this change, linker errors will occur when attempts to call,
which doesn't exist when MBEDTLS_GENPRIME is unset.
psa_crypto_rsa.c.obj: in function `rsa_generate_key':
psa_crypto_rsa.c:320: undefined reference to `mbedtls_rsa_gen_key'
Fixes#4512
Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>
