The simple truth is: there are no “100% safe” apps.
You can maybe get pretty close but any complex app, even when open source, can be found unsafe years later when someone discovers a nasty exploit or even a very well hidden backdoor.
Best to be realistic and to determine a threat model who/what you really want to defend against and to not give in to paranoia.
The PG guide (and others) will help you with that. 