diff --git a/library/psa_crypto_storage.c b/library/psa_crypto_storage.c
index 0a5805b..687269b 100644
--- a/library/psa_crypto_storage.c
+++ b/library/psa_crypto_storage.c
@@ -147,7 +147,7 @@
     return( PSA_SUCCESS );
 }
 
-psa_status_t psa_save_persistent_key( const psa_key_slot_t key,
+psa_status_t psa_save_persistent_key( const psa_key_id_t key,
                                       const psa_key_type_t type,
                                       const psa_key_policy_t *policy,
                                       const uint8_t *data,
@@ -185,7 +185,7 @@
     mbedtls_free( key_data );
 }
 
-psa_status_t psa_load_persistent_key( psa_key_slot_t key,
+psa_status_t psa_load_persistent_key( psa_key_id_t key,
                                       psa_key_type_t *type,
                                       psa_key_policy_t *policy,
                                       uint8_t **data,
diff --git a/library/psa_crypto_storage.h b/library/psa_crypto_storage.h
index 167b0db..478daef 100644
--- a/library/psa_crypto_storage.h
+++ b/library/psa_crypto_storage.h
@@ -56,20 +56,20 @@
  * already occupied non-persistent key, as well as validating the key data.
  *
  *
- * \param key           Slot number of the key to be stored. This must be a
- *                      valid slot for a key of the chosen type. This should be
- *                      an occupied key slot with an unoccupied corresponding
- *                      storage location.
+ * \param key           Persistent identifier of the key to be stored. This
+ *                      should be an unoccupied storage location.
  * \param type          Key type (a \c PSA_KEY_TYPE_XXX value).
  * \param[in] policy    The key policy to save.
  * \param[in] data      Buffer containing the key data.
  * \param data_length   The number of bytes that make up the key data.
  *
  * \retval PSA_SUCCESS
+ * \retval PSA_ERROR_INSUFFICIENT_MEMORY
  * \retval PSA_ERROR_INSUFFICIENT_STORAGE
  * \retval PSA_ERROR_STORAGE_FAILURE
+ * \retval PSA_ERROR_OCCUPIED_SLOT
  */
-psa_status_t psa_save_persistent_key( const psa_key_slot_t key,
+psa_status_t psa_save_persistent_key( const psa_key_id_t key,
                                       const psa_key_type_t type,
                                       const psa_key_policy_t *policy,
                                       const uint8_t *data,
@@ -87,10 +87,8 @@
  * this function to zeroize and free this buffer, regardless of whether this
  * function succeeds or fails.
  *
- * \param key               Slot number whose content is to be loaded. This
- *                          must be an unoccupied key slot with an occupied
- *                          corresponding storage location. The key slot
- *                          lifetime must be set to persistent.
+ * \param key               Persistent identifier of the key to be loaded. This
+ *                          should be an occupied storage location.
  * \param[out] type         On success, the key type (a \c PSA_KEY_TYPE_XXX
  *                          value).
  * \param[out] policy       On success, the key's policy.
@@ -100,8 +98,9 @@
  * \retval PSA_SUCCESS
  * \retval PSA_ERROR_INSUFFICIENT_MEMORY
  * \retval PSA_ERROR_STORAGE_FAILURE
+ * \retval PSA_ERROR_EMPTY_SLOT
  */
-psa_status_t psa_load_persistent_key( psa_key_slot_t key,
+psa_status_t psa_load_persistent_key( psa_key_id_t key,
                                       psa_key_type_t *type,
                                       psa_key_policy_t *policy,
                                       uint8_t **data,
@@ -110,16 +109,18 @@
 /**
  * \brief Remove persistent data for the given key slot number.
  *
- * \param key           Slot number whose content is to be removed
+ * \param key           Persistent identifier of the key to remove
  *                      from persistent storage.
  *
  * \retval PSA_SUCCESS
+ *         The key was successfully removed,
+ *         or the key did not exist.
  * \retval PSA_ERROR_STORAGE_FAILURE
  */
-psa_status_t psa_destroy_persistent_key( const psa_key_slot_t key );
+psa_status_t psa_destroy_persistent_key( const psa_key_id_t key );
 
 /**
- * \brief Zeroizes and frees the given buffer.
+ * \brief Free the temporary buffer allocated by psa_load_persistent_key().
  *
  * This function must be called at some point after psa_load_persistent_key()
  * to zeroize and free the memory allocated to the buffer in that function.
diff --git a/library/psa_crypto_storage_backend.h b/library/psa_crypto_storage_backend.h
index 3ca9a1d..47896b8 100644
--- a/library/psa_crypto_storage_backend.h
+++ b/library/psa_crypto_storage_backend.h
@@ -47,15 +47,16 @@
  * This function reads data from a storage backend and returns the data in a
  * buffer.
  *
- * \param key              Slot number whose content is to be loaded. This must
- *                         be a key slot whose lifetime is set to persistent.
- * \param[out] data        Buffer where the data is to be written.
- * \param data_size        Size of the \c data buffer in bytes.
+ * \param key               Persistent identifier of the key to be loaded. This
+ *                          should be an occupied storage location.
+ * \param[out] data         Buffer where the data is to be written.
+ * \param data_size         Size of the \c data buffer in bytes.
  *
  * \retval PSA_SUCCESS
  * \retval PSA_ERROR_STORAGE_FAILURE
+ * \retval PSA_ERROR_EMPTY_SLOT
  */
-psa_status_t psa_crypto_storage_load( const psa_key_slot_t key, uint8_t *data,
+psa_status_t psa_crypto_storage_load( const psa_key_id_t key, uint8_t *data,
                                       size_t data_size );
 
 /**
@@ -63,7 +64,8 @@
  *
  * This function stores the given data buffer to a persistent storage.
  *
- * \param key           Slot number whose content is to be stored.
+ * \param key           Persistent identifier of the key to be stored. This
+ *                      should be an unoccupied storage location.
  * \param[in] data      Buffer containing the data to be stored.
  * \param data_length   The number of bytes
  *                      that make up the data.
@@ -71,8 +73,9 @@
  * \retval PSA_SUCCESS
  * \retval PSA_ERROR_INSUFFICIENT_STORAGE
  * \retval PSA_ERROR_STORAGE_FAILURE
+ * \retval PSA_ERROR_OCCUPIED_SLOT
  */
-psa_status_t psa_crypto_storage_store( const psa_key_slot_t key,
+psa_status_t psa_crypto_storage_store( const psa_key_id_t key,
                                        const uint8_t *data,
                                        size_t data_length );
 
@@ -82,26 +85,26 @@
  * This function checks if any key data or metadata exists for the key slot in
  * the persistent storage.
  *
- * \param key           Slot number whose content is to be checked.
+ * \param key           Persistent identifier to check.
  *
  * \retval 0
  *         No persistent data present for slot number
  * \retval 1
  *         Persistent data present for slot number
  */
-int psa_is_key_present_in_storage( const psa_key_slot_t key );
+int psa_is_key_present_in_storage( const psa_key_id_t key );
 
 /**
  * \brief Get data length for given key slot number.
  *
- * \param key              Slot number whose stored data length is to be obtained.
- * \param[out] data_length The number of bytes
- *                         that make up the data.
+ * \param key               Persistent identifier whose stored data length
+ *                          is to be obtained.
+ * \param[out] data_length  The number of bytes that make up the data.
  *
  * \retval PSA_SUCCESS
  * \retval PSA_ERROR_STORAGE_FAILURE
  */
-psa_status_t psa_crypto_storage_get_data_length( const psa_key_slot_t key,
+psa_status_t psa_crypto_storage_get_data_length( const psa_key_id_t key,
                                                  size_t *data_length );
 
 
diff --git a/library/psa_crypto_storage_file.c b/library/psa_crypto_storage_file.c
index 03c711a..95857fa 100644
--- a/library/psa_crypto_storage_file.c
+++ b/library/psa_crypto_storage_file.c
@@ -48,15 +48,15 @@
 
 enum { MAX_LOCATION_LEN = sizeof(CRYPTO_STORAGE_FILE_LOCATION) + 40 };
 
-static void key_slot_to_location( const psa_key_slot_t key,
-                                  char *location,
-                                  size_t location_size )
+static void key_id_to_location( const psa_key_id_t key,
+                                char *location,
+                                size_t location_size )
 {
     mbedtls_snprintf( location, location_size,
                       CRYPTO_STORAGE_FILE_LOCATION "psa_key_slot_%d", key );
 }
 
-psa_status_t psa_crypto_storage_load( const psa_key_slot_t key, uint8_t *data,
+psa_status_t psa_crypto_storage_load( const psa_key_id_t key, uint8_t *data,
                                       size_t data_size )
 {
     psa_status_t status = PSA_SUCCESS;
@@ -64,7 +64,7 @@
     size_t num_read;
     char slot_location[MAX_LOCATION_LEN];
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
     file = fopen( slot_location, "rb" );
     if( file == NULL )
     {
@@ -81,12 +81,12 @@
     return( status );
 }
 
-int psa_is_key_present_in_storage( const psa_key_slot_t key )
+int psa_is_key_present_in_storage( const psa_key_id_t key )
 {
     char slot_location[MAX_LOCATION_LEN];
     FILE *file;
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
 
     file = fopen( slot_location, "r" );
     if( file == NULL )
@@ -99,7 +99,7 @@
     return( 1 );
 }
 
-psa_status_t psa_crypto_storage_store( const psa_key_slot_t key,
+psa_status_t psa_crypto_storage_store( const psa_key_id_t key,
                                        const uint8_t *data,
                                        size_t data_length )
 {
@@ -114,7 +114,7 @@
      * affect actual keys. */
     const char *temp_location = CRYPTO_STORAGE_FILE_LOCATION "psa_key_slot_0";
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
 
     if( psa_is_key_present_in_storage( key ) == 1 )
         return( PSA_ERROR_OCCUPIED_SLOT );
@@ -154,12 +154,12 @@
     return( status );
 }
 
-psa_status_t psa_destroy_persistent_key( const psa_key_slot_t key )
+psa_status_t psa_destroy_persistent_key( const psa_key_id_t key )
 {
     FILE *file;
     char slot_location[MAX_LOCATION_LEN];
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
 
     /* Only try remove the file if it exists */
     file = fopen( slot_location, "rb" );
@@ -173,7 +173,7 @@
     return( PSA_SUCCESS );
 }
 
-psa_status_t psa_crypto_storage_get_data_length( const psa_key_slot_t key,
+psa_status_t psa_crypto_storage_get_data_length( const psa_key_id_t key,
                                                  size_t *data_length )
 {
     psa_status_t status = PSA_SUCCESS;
@@ -181,7 +181,7 @@
     long file_size;
     char slot_location[MAX_LOCATION_LEN];
 
-    key_slot_to_location( key, slot_location, MAX_LOCATION_LEN );
+    key_id_to_location( key, slot_location, MAX_LOCATION_LEN );
 
     file = fopen( slot_location, "rb" );
     if( file == NULL )
diff --git a/library/psa_crypto_storage_its.c b/library/psa_crypto_storage_its.c
index 29394b5..86e0e89 100644
--- a/library/psa_crypto_storage_its.c
+++ b/library/psa_crypto_storage_its.c
@@ -68,12 +68,12 @@
     }
 }
 
-static uint32_t psa_its_identifier_of_slot( psa_key_slot_t key )
+static uint32_t psa_its_identifier_of_slot( psa_key_id_t key )
 {
     return( key );
 }
 
-psa_status_t psa_crypto_storage_load( const psa_key_slot_t key, uint8_t *data,
+psa_status_t psa_crypto_storage_load( const psa_key_id_t key, uint8_t *data,
                                       size_t data_size )
 {
     psa_its_status_t ret;
@@ -92,7 +92,7 @@
     return( status );
 }
 
-int psa_is_key_present_in_storage( const psa_key_slot_t key )
+int psa_is_key_present_in_storage( const psa_key_id_t key )
 {
     psa_its_status_t ret;
     uint32_t data_identifier = psa_its_identifier_of_slot( key );
@@ -105,7 +105,7 @@
     return( 1 );
 }
 
-psa_status_t psa_crypto_storage_store( const psa_key_slot_t key,
+psa_status_t psa_crypto_storage_store( const psa_key_id_t key,
                                        const uint8_t *data,
                                        size_t data_length )
 {
@@ -143,7 +143,7 @@
     return( status );
 }
 
-psa_status_t psa_destroy_persistent_key( const psa_key_slot_t key )
+psa_status_t psa_destroy_persistent_key( const psa_key_id_t key )
 {
     psa_its_status_t ret;
     uint32_t data_identifier = psa_its_identifier_of_slot( key );
@@ -163,7 +163,7 @@
     return( PSA_SUCCESS );
 }
 
-psa_status_t psa_crypto_storage_get_data_length( const psa_key_slot_t key,
+psa_status_t psa_crypto_storage_get_data_length( const psa_key_id_t key,
                                                  size_t *data_length )
 {
     psa_its_status_t ret;