Secure Element Key Management#

psa_key_creation_method_t#

psa_key_creation_method_t

An enumeration indicating how a key is created.

psa_drv_se_allocate_key_t#

typedef psa_status_t(* psa_drv_se_allocate_key_t) (psa_drv_se_context_t *drv_context, void *persistent_data, const psa_key_attributes_t *attributes, psa_key_creation_method_t method, psa_key_slot_number_t *key_slot) )(psa_drv_se_context_t *drv_context, void *persistent_data, const psa_key_attributes_t *attributes, psa_key_creation_method_t method, psa_key_slot_number_t *key_slot)

A function that allocates a slot for a key.

To create a key in a specific slot in a secure element, the core first calls this function to determine a valid slot number, then calls a function to create the key material in that slot. In nominal conditions (that is, if no error occurs), the effect of a call to a key creation function in the PSA Cryptography API with a lifetime that places the key in a secure element is the following:

1. The core calls psa_drv_se_key_management_t::p_allocate (or in some implementations psa_drv_se_key_management_t::p_validate_slot_number). The driver selects (or validates) a suitable slot number given the key attributes and the state of the secure element.

2. The core calls a key creation function in the driver.

The key creation functions in the PSA Cryptography API are:

• psa_import_key(), which causes a call to p_allocate with method = PSA_KEY_CREATION_IMPORT then a call to psa_drv_se_key_management_t::p_import.

• psa_generate_key(), which causes a call to p_allocate with method = PSA_KEY_CREATION_GENERATE then a call to psa_drv_se_key_management_t::p_import.

• psa_key_derivation_output_key(), which causes a call to p_allocate with method = PSA_KEY_CREATION_DERIVE then a call to psa_drv_se_key_derivation_t::p_derive.

• psa_copy_key(), which causes a call to p_allocate with method = PSA_KEY_CREATION_COPY then a call to psa_drv_se_key_management_t::p_export.

In case of errors, other behaviors are possible.

• If the PSA Cryptography subsystem dies after the first step, for example because the device has lost power abruptly, the second step may never happen, or may happen after a reset and re-initialization. Alternatively, after a reset and re-initialization, the core may call psa_drv_se_key_management_t::p_destroy on the slot number that was allocated (or validated) instead of calling a key creation function.

• If an error occurs, the core may call psa_drv_se_key_management_t::p_destroy on the slot number that was allocated (or validated) instead of calling a key creation function.

Errors and system resets also have an impact on the driver's persistent data. If a reset happens before the overall key creation process is completed (before or after the second step above), it is unspecified whether the persistent data after the reset is identical to what it was before or after the call to p_allocate (or p_validate_slot_number).

psa_drv_se_validate_slot_number_t#

typedef psa_status_t(* psa_drv_se_validate_slot_number_t) (psa_drv_se_context_t *drv_context, void *persistent_data, const psa_key_attributes_t *attributes, psa_key_creation_method_t method, psa_key_slot_number_t key_slot) )(psa_drv_se_context_t *drv_context, void *persistent_data, const psa_key_attributes_t *attributes, psa_key_creation_method_t method, psa_key_slot_number_t key_slot)

A function that determines whether a slot number is valid for a key.

To create a key in a specific slot in a secure element, the core first calls this function to validate the choice of slot number, then calls a function to create the key material in that slot. See the documentation of psa_drv_se_allocate_key_t for more details.

As of the PSA Cryptography API specification version 1.0, there is no way for applications to trigger a call to this function. However some implementations offer the capability to create or declare a key in a specific slot via implementation-specific means, generally for the sake of initial device provisioning or onboarding. Such a mechanism may be added to a future version of the PSA Cryptography API specification.

This function may update the driver's persistent data through persistent_data. The core will save the updated persistent data at the end of the key creation process. See the description of psa_drv_se_allocate_key_t for more information.

psa_drv_se_import_key_t#

typedef psa_status_t(* psa_drv_se_import_key_t) (psa_drv_se_context_t *drv_context, psa_key_slot_number_t key_slot, const psa_key_attributes_t *attributes, const uint8_t *data, size_t data_length, size_t *bits) )(psa_drv_se_context_t *drv_context, psa_key_slot_number_t key_slot, const psa_key_attributes_t *attributes, const uint8_t *data, size_t data_length, size_t *bits)

A function that imports a key into a secure element in binary format.

This function can support any output from psa_export_key(). Refer to the documentation of psa_export_key() for the format for each key type.

psa_drv_se_destroy_key_t#

typedef psa_status_t(* psa_drv_se_destroy_key_t) (psa_drv_se_context_t *drv_context, void *persistent_data, psa_key_slot_number_t key_slot) )(psa_drv_se_context_t *drv_context, void *persistent_data, psa_key_slot_number_t key_slot)

A function that destroys a secure element key and restore the slot to its default state.

This function destroys the content of the key from a secure element. Implementations shall make a best effort to ensure that any previous content of the slot is unrecoverable.

This function returns the specified slot to its default state.

psa_drv_se_export_key_t#

typedef psa_status_t(* psa_drv_se_export_key_t) (psa_drv_se_context_t *drv_context, psa_key_slot_number_t key, uint8_t *p_data, size_t data_size, size_t *p_data_length) )(psa_drv_se_context_t *drv_context, psa_key_slot_number_t key, uint8_t *p_data, size_t data_size, size_t *p_data_length)

A function that exports a secure element key in binary format.

The output of this function can be passed to psa_import_key() to create an equivalent object.

If a key is created with psa_import_key() and then exported with this function, it is not guaranteed that the resulting data is identical: the implementation may choose a different representation of the same key if the format permits it.

This function should generate output in the same format that psa_export_key() does. Refer to the documentation of psa_export_key() for the format for each key type.

psa_drv_se_generate_key_t#

typedef psa_status_t(* psa_drv_se_generate_key_t) (psa_drv_se_context_t *drv_context, psa_key_slot_number_t key_slot, const psa_key_attributes_t *attributes, uint8_t *pubkey, size_t pubkey_size, size_t *pubkey_length) )(psa_drv_se_context_t *drv_context, psa_key_slot_number_t key_slot, const psa_key_attributes_t *attributes, uint8_t *pubkey, size_t pubkey_size, size_t *pubkey_length)

A function that generates a symmetric or asymmetric key on a secure element.

If the key type type recorded in attributes is asymmetric (PSA_KEY_TYPE_IS_ASYMMETRIC(type) = 1), the driver may export the public key at the time of generation, in the format documented for psa_export_public_key() by writing it to the pubkey buffer. This is optional, intended for secure elements that output the public key at generation time and that cannot export the public key later. Drivers that do not need this feature should leave *pubkey_length set to 0 and should implement the psa_drv_key_management_t::p_export_public function. Some implementations do not support this feature, in which case pubkey is NULL and pubkey_size is 0.