Add comment on exiting early from mbedtls_gcm_update().
@@ -454,7 +454,9 @@ int mbedtls_gcm_update( mbedtls_gcm_context *ctx,
*output_length = input_length;
/* Exit early if input_length==0 so that we don't do any pointer arithmetic
- * on a potentially null pointer. */
+ * on a potentially null pointer.
+ * Returning early also means that the last partial block of AD remains
+ * untouched for mbedtls_gcm_finish */
if( input_length == 0 )
return( 0 );
