I'd say they're on-topic by virtue of them certainly not being off-topic. Just because something is on-topic on another site doesn't mean it's automatically off-topic here.
That said, we still have standards. It needs to be answerable, ideally in a way where the top two answers cover 99% of use cases. Questions that solicit infinite lists of answers (eg "Please recommend any application") will fall straight into the "too broad" close reason.
I don't think we need to be as declarative as Software Recs are, but that meta post is certainly a good pointer for improving bad software recommendations.
I think the first half of ThomasW's comment was a poor choice of words but ultimately I don't disagree with the decision to close this questionthis question. The OP has thrown a bunch of buzzwords at the screen, it's hard to divine what they'd consider sufficient encryption.
I don't think it asking for a cross platform solution is a bad thing, it's just one more limit.