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What is the smartest way, before selling a mobile phone, to be sure that no one will be able to access my data?

In my case we are talking about an LG G4.

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    @MarkStewart It indeed is a duplicate, however, I don't think that that answer is correct. A factory reset is not enough if you want to keep the data secure. With the correct tools, it is very easy to restore the data. – Thomas Vos Jun 1 '16 at 18:28
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The first thing you'd want to do is simply remove the SD card if any in use. If you intend on providing an SD card, purchase a new SD Card.

As suggested, the first step would be to encrypt your data (you can read up on this here), then perform a factory reset of the phone - I'd suggest performing this approximately 3 times. (1 should be sufficient, but 3 will determinately just make it a little harder to recover the data). You can find a pretty neat guide/explanation on this website.

Just to add onto Jay's answer, here is a basic guide on using ADB to encrypt your device. Note that the commands are noted to be:

user@laptop # adb root
user@laptop # adb shell
root@phone:/ #
root@phone:/ # /system/bin/vdc cryptfs enablecrypto inplace PASSWORD

If you have rooted your phone (and intend to sell as is) you can install other applications that can encrypt certain directories, but I doubt you'll want to do this.

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  • Why do you want to use ADB? Why not on the phone itself? And wiping the phone multiple times will not make a difference, the data is overwritten by the encryption. – Thomas Vos Jun 2 '16 at 5:47
  • ADB just as another option for the user. You're correct regarding the multiple rounds of encryption though. – DankyNanky Jun 2 '16 at 9:12
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A factory reset is not enough. It just "disables" your data and doesn't really delete it. Some data restore tools can still restore the data after a normal reset.

The best way I know of is to encrypt the phone and then do a factory reset. The encryption will overwrite the unencrypted data, so that will be gone forever. Then after a reset, if the new user tries to restore the data, it just gets the encrypted data. (=useless)

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