This app was mentioned in 7 comments, with an average of 3.43 upvotes
Just search calculator photo vault in the app store of your choice, however I would recommend this app, as it has a decoy passcode that opens a different set of folders to fool people.
I have an old Android phone with the application Private photo vault installed (link here). I can't for the life of me recall my password. I tried searching in all the folders for a hidden folder, or hidden photo files with the .jpeg renamed to another extension to hide the files, but that doesn't work. Any ideas ? Thanks.
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https://play.google.com/store/apps/details?id=com.enchantedcloud.photovault&hl=en_CA&gl=US
I use Between as well. Also, there are various apps for creating a "private" picture gallery. I've used Private Photo Vault on Android. https://play.google.com/store/apps/details?id=com.enchantedcloud.photovault&hl=en
and on iOS I use Photo Vault.
Finally, I also "hide" the apps (in plain sight). On Android and iOS I have "Work" folders and I stash these apps deep inside those app folders. The kids first see a laundry list of work apps, and should miss the "Special" apps. And the apps are password protected if the kids do find them.
Private Photo Vault is good