So factory reset doesn't safely remove your data, what app

>>58123341
Firstly, I use GNU/Soldering Iron.
It safely removes flash memory chips from PCB.
Next I proceed to Nitric Acid+® under BSD licence. It provides military-grade data erase on most physical media produced by comsumer electronics market. I put these memory chips in Nitric Acid+® until they are fully dissolved into AcidFlash™ boullion.
At this state, all sensitive data is securely erased and can't be read even by top NSA™ cryptologists.

>>58123341
sudo dd if=/dev/zero of=/dev/sdX (locate your device with lsblk). I highly doubt you could erase data on an iPhone through an app on the iPhone App Store.

>>58123583
* dd if=/dev/zero of=/dev/sdX bs=1M, my mistake.

>>58123583
I'm talking about Android
Wouldn't that remove the Android OS from the phone?

>>58123651
Sure it would

>>58123583
I doesn't work on flash memory.

>>58123694
I don't understand the problem.

>>58123341
On Nexus, reinstall stock firmware through fastboot.
On any other Android device format from recovery (assuming you don't want to install some shit phone management soft on your PC).
On iPhones you're screwed and you have to install iTunes and reflash the latest iOS.