psa: Add PSA_KEY_ID_INIT

A macro useful for initializing psa_key_id_t, whether
MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER is set or not. Without this
macro, it is necessary to know if
MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER as with it the key ID is
non-scalar and needs to be initialized with {0, 0}, and 0 otherwise when
key ID is scalar.

include/psa/crypto_platform.h

@@ -89,6 +89,7 @@ typedef struct
  * `psa_key_file_id_t` argument. As a workaround, make `psa_key_id_t` an
  * alias for `psa_key_file_id_t` when building for a multi-client service. */
 typedef psa_key_file_id_t psa_key_id_t;
+#define PSA_KEY_ID_INIT {0, 0}
 
 #else /* !MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER */
 

include/psa/crypto_struct.h

@@ -379,7 +379,7 @@ typedef struct
     psa_key_attributes_flag_t flags;
 } psa_core_key_attributes_t;
 
-#define PSA_CORE_KEY_ATTRIBUTES_INIT {0, 0, 0, PSA_KEY_POLICY_INIT, 0, 0}
+#define PSA_CORE_KEY_ATTRIBUTES_INIT {0, 0, PSA_KEY_ID_INIT, PSA_KEY_POLICY_INIT, 0, 0}
 
 struct psa_key_attributes_s
 {

include/psa/crypto_types.h

@@ -120,6 +120,7 @@ typedef uint32_t psa_key_lifetime_t;
  * psa_key_id_t in crypto_platform.h instead of here. */
 #if !defined(MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER)
 typedef uint32_t psa_key_id_t;
+#define PSA_KEY_ID_INIT 0
 #endif
 
 /**@}*/