being proven in some way to be secure and/or private. proving it could entail auditing the software, looking at the design and technologies used etc
ok and can i see if the app is audited and proven that is secure and private?
Steve Ballmer, is that you?
Being open sourced is not equal to being trustworthy but it’s a prerequisite to that.
I fully agree with that, but would like to add a precision (in my personal opinion): non-free software can also be trustworthy.
Notably, it depends on how far you push the open source requirement. For example, Proton’s software is all open source, except their server software. Does this make them untrustworthy? No.
Additionally, it’s possible for non-free applications to be audited, and to be trustworthy. Usually though, there isn’t a reason for a free application to be private, but not open source. The servers make sense, as without them being non-free, it wouldn’t be as easy to make money from them.
Paid applications can be non-free and still trustworthy. Most paid applications are non-free, since, as stated previously, it’s not as easy to make money from paid applications that are open source.
Tl;dr: my philosophy is: you should always use open source software when possible, for both privacy and freedom. But, you should always make sure this software is trustworthy first. And, if you need non-free software for something, you should, provided it’s trustworthy, not write it off due to it not being open source.