Security of the files

Hi, I have a question about the security of the files I upload. I would like only logged in users to be able to download the files. I have created a category with 2 files to test. The category is only visible to logged in users, but the download of the files, if I get the URL via the unmask it is possible to download them even if I am not logged in. Is there any possibility to avoid this? I would like the files to be completely private. Should I use the URL masking option? Is this completely safe ? or can Google index these files somehow ?