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Period tracking app refuses to disclose data to American authorities
(www.newsweek.com)
This is a most excellent place for technology news and articles.
Probably because they want to be able to maintain users during device switches. Given much of the world is on an annual or bi-annual cycle it'd suck to lose your users each time.
They could just do the password manager approach where the data is encrypted on your phone but stored in the cloud. App retains users, sensitive data remains private.
I wonder how many average users would be bothered to export their period database and transfer to a new phone every time they get a new phone. I do that when I get a new phone (not often, I use my phones till they break/are literally unusable and unfixable), but I've had real trouble getting other people to do these kinds of things.