Implement custom RSA public exponent for keys in secure elements

[gilles-peskine-arm]

Description

In psa_get_key_attributes, reporting the domain parameters for a key in a secure element is not implemented. Specifically, we're supposed to report the public exponent of an RSA key, so that psa_get_key_attributes on a key followed by psa_generate_key creates a key with the same public exponent as the first key.

Issue request type

[ ] Question
[x] Enhancement
[ ] Bug

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/IOTCRYPT-864

[gilles-peskine-arm]

The secure element driver interface enabled by MBEDTLS_PSA_CRYPTO_SE_C is deprecated. We will add new features to it.

The same issue applies to the new “unified” driver interface, but also to the PSA API, since we have removed the ability to randomly generate an RSA key with a custom public exponent. Since the driver interface now follows the API, if we add this possibility to the API, we will also add it to the driver interface.