Yes! By default, your Fission account has a file system where files are encrypted and private only to you. You can see what this looks like by signing up for Drive: https://drive.fission.codes
Only files placed in the Public folder are accessible on the web.
The in progress version we have up now at https://tiddlywiki.fission.app requires you to put a TW index.html in your public folder. You can do this by uploading through Drive. We’ll post a walkthrough of this.
We will support private only TiddlyWikis, where you need to be signed into your account, but this will work across devices including mobile.