I have heard stories of more aggressive interrogation of electronic devices at US border crossings. I know a lot about securing computers, but very little about securing phones.

Are there easy ways to delete data—files, photos, etc.—on phones so it can’t be recovered? Does resetting a phone to factory defaults erase data, or is it still recoverable? That is, does the reset erase the old encryption key, or just sever the password that access that key? When the phone is rebooted, are deleted files still available?

We need answers for both iPhones and Android phones. And it’s not just the US; the world is going to become a more dangerous place to oppose state power.