diff --git a/library/psa_its_file.c b/library/psa_its_file.c
index ac1561c..ee11cb3 100644
--- a/library/psa_its_file.c
+++ b/library/psa_its_file.c
@@ -197,14 +197,8 @@
     size_t n;
 
     memcpy( header.magic, PSA_ITS_MAGIC_STRING, PSA_ITS_MAGIC_LENGTH );
-    header.size[0] = MBEDTLS_BYTE_0( data_length );
-    header.size[1] = MBEDTLS_BYTE_1( data_length );
-    header.size[2] = MBEDTLS_BYTE_2( data_length );
-    header.size[3] = MBEDTLS_BYTE_3( data_length );
-    header.flags[0] = MBEDTLS_BYTE_0( create_flags );
-    header.flags[1] = MBEDTLS_BYTE_1( create_flags );
-    header.flags[2] = MBEDTLS_BYTE_2( create_flags );
-    header.flags[3] = MBEDTLS_BYTE_3( create_flags );
+    MBEDTLS_PUT_UINT32_LE( data_length, header.size, 0 );
+    MBEDTLS_PUT_UINT32_LE( create_flags, header.flags, 0 );
 
     psa_its_fill_filename( uid, filename );
     stream = fopen( PSA_ITS_STORAGE_TEMP, "wb" );