diff --git a/include/psa/crypto_platform.h b/include/psa/crypto_platform.h
index 50ca546..0f3ede8 100644
--- a/include/psa/crypto_platform.h
+++ b/include/psa/crypto_platform.h
@@ -49,4 +49,27 @@
 /* Integral type representing a key handle. */
 typedef uint16_t psa_key_handle_t;
 
+/* This implementation distinguishes *application key identifiers*, which
+ * are the key identifiers specified by the application, from
+ * *key file identifiers*, which are the key identifiers that the library
+ * sees internally. The two types can be different if there is a remote
+ * call layer between the application and the library which supports
+ * multiple client applications that do not have access to each others'
+ * keys. The point of having different types is that the key file
+ * identifier may encode not only the key identifier specified by the
+ * application, but also the the identity of the application.
+ *
+ * Note that this is an internal concept of the library and the remote
+ * call layer. The application itself never sees anything other than
+ * #psa_app_key_id_t with its standard definition.
+ */
+
+/* The application key identifier is always what the application sees as
+ * #psa_key_id_t. */
+typedef uint32_t psa_app_key_id_t;
+
+/* By default, a key file identifier is just the application key identifier. */
+typedef psa_app_key_id_t psa_key_file_id_t;
+#define PSA_KEY_FILE_GET_KEY_ID( id ) ( id )
+
 #endif /* PSA_CRYPTO_PLATFORM_H */