PSA interface to retrieve constants used by the implementation

[gilles-peskine-arm]

With certain cryptographic mechanisms, there are constants that an implementation must have internally, and that most applications don't need but some do need because those constants need to be embedded as metadata in some network protocol or file format. I'm specifically thinking of:

• Diffie-Hellman prime and generator corresponding to a psa_dh_family_t value and bit-size.
• ECC curve parameters corresponding to a psa_ecc_family_t value and bit-size.

Obviously the application can embed the constants it needs. That requires the application to know all of the groups/curves it supports, but it presumably needs this anyway because it needs to translate between the PSA representation and the protocol/format representation of the choice of group/curve. That requires the application to duplicate the numbers, which may be a concern on resource-constrained devices.

Whatever the interface is, it has to allow for the constant to be copied into application memory, since:

• the crypto implementation may be running in its own memory space;
• even if the crypto implementation is running in the same memory space, it may have a custom internal representation (e.g. different endianness, or perhaps no representation as such for ECC implementations with curve-specific optimized arithmetic).

I'm not sure that there is real demand for such an API. The Mbed TLS team currently has no plan to work on this. Please let us know if you need it and why.