65528 bits is more than any reasonable key until we start supporting
post-quantum cryptography.
This limit is chosen to allow bit-sizes to be stored in 16 bits, with
65535 left to indicate an invalid value. It's a whole number of bytes,
which facilitates some calculations, in particular allowing a key of
exactly PSA_CRYPTO_MAX_STORAGE_SIZE to be created but not one bit
more.
As a resource usage limit, this is arguably too large, but that's out
of scope of the current commit.
Test that key import, generation and derivation reject overly large
sizes.
Stored keys must contain lifetime information. The lifetime used to be
implied by the location of the key, back when applications supplied
the lifetime value when opening the key. Now that all keys' metadata
are stored in a central location, this location needs to store the
lifetime explicitly.
Update persistent_key_load_key_from_storage to the new attribute-based
key creation interface. I tweaked the code a little to make it simpler
and more robust without changing the core logic.
Remove pkcs-1 and rsaEncryption front matter from RSA public keys. Move
code that was shared between RSA and other key types (like EC keys) to
be used only with non-RSA keys.
Switch from the direct use of slot numbers to handles allocated by
psa_allocate_key.
The general principle for each function is:
* Change `psa_key_slot_t slot` to `psa_key_handle_t handle` or
`psa_key_id_t key_id` depending on whether it's used as a handle to
an open slot or as a persistent name for a key.
* Call psa_create_key() before using a slot, instead of calling
psa_set_key_lifetime to make a slot persistent.
Remove the unit test persistent_key_is_configurable which is no longer
relevant.
Allow use of persistent keys, including configuring them, importing and
exporting them, and destroying them.
When getting a slot using psa_get_key_slot, there are 3 scenarios that
can occur if the keys lifetime is persistent:
1. Key type is PSA_KEY_TYPE_NONE, no persistent storage entry:
- The key slot is treated as a standard empty key slot
2. Key type is PSA_KEY_TYPE_NONE, persistent storage entry exists:
- Attempt to load the key from persistent storage
3. Key type is not PSA_KEY_TYPE_NONE:
- As checking persistent storage on every use of the key could
be expensive, the persistent key is assumed to be saved in
persistent storage, the in-memory key is continued to be used.
