PSA key storage: systematically store the bit-size

[gilles-peskine-arm]

Currently the bit-size is stored for dynamic-driver secure element keys and calculated for the data for transparent keys. To simplify the design of storage for opaque drivers that store the key as a wrapped blob (#3289), store the bit-size systematically with the other attributes.

The storage format currently has a 32-bit field for the key type, but key types are now 16-bit, as are key sizes. So I propose to split these 32-bits into type and size.

• Update psa_format_key_data_for_storage and psa_parse_key_data_from_storage to store the bit-size with the other attributes.
• Update the dynamic-driver secure element code to not store the bit-size again.
• Update test data in test_suite_psa_crypto_persistent_key.
• Update the storage format documentation.

[torstenes]

I have started working on this ticket and can have a PR ready fairly soon.

Should a solution to this ticket include updates of the storage format documentation? It seems like a separate ticket was created for this #3737

[gilles-peskine-arm]

#3737 is to update historical documentation. If you change the format, please add a new section to https://github.com/ARMmbed/mbedtls/blob/development/docs/architecture/mbed-crypto-storage-specification.md that describes the new format.