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I recently purchased a used Moto X. I know the person I bought it from so I'm not terribly worried but here is my idea to shore up the security on a used device.

  1. Don't put in your SIM card or log into any accounts
  2. Factory reset
  3. Encrypt device
  4. Factory reset
  5. Encrypt device
  6. Install SIM, log into accounts
  7. Enjoy

This seems like a solid way to make sure the device is completely wiped out... except for the boot loader. I'm not sure how the comes into play, or if this method even makes that big of a difference. The process I outlined isn't difficult, it just takes a few hours.

  • Is it worth it?
  • Can it be improved?
  • What about the boot loader?
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    If you're that worried - to the point that you're concerned about the bootloader being malicious - then a factory reset seems insufficient. It wouldn't have any effect on the boot partition or the system partition, both of which are modified when installing a custom ROM. If its feasible with the Moto X, reflashing the phone seems like the better course.
    – eldarerathis
    Commented Sep 3, 2014 at 3:27

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Factory-resetting twice is pointless. The first factory reset will already wipe all user data: the second won't delete anything that wasn't already deleted by the first. Encrypting the device is also a waste of time, because you've already factory-reset the device.

It might make some sense if you were the seller of the phone, and you wanted to overwrite the internal storage several times to make sure the buyer couldn't recover your data, but that's the opposite of what you want. A single factory reset will delete all the user data, giving you a clean slate.

If you're worried about the previous owner having left a backdoor in the OS, then ensure the bootloader is locked and flash the stock ROM yourself. This way you're sure to have a clean OS.

If you suspect the buyer has enough skill to backdoor the bootloader so that it will also make changes to the new OS as you flash it, and also disable signature verification so that it will still boot but looks like the bootloader is locked, then just don't buy a phone from them. If you think they're that good, and that desperate to get hold of your data, then they'll find a way around whatever you do. You just can't trust the device at all in that case.

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  • Is this answer still applicable i.e. there aren't better ways? Also, if I'm planning on putting a custom ROM in my phone, would that as well be sufficient for having suspicions that the previous owner left a backdoor in the OS (if they aren't as skilled as you mention in your 4th paragraph)?
    – Honinbo Shusaku
    Commented Feb 26, 2016 at 20:37
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    If you're installing a custom ROM anyway, there's no need to worry about the seller having done anything to the OS. Your only fears then are the bootloader (as in my 4th paragraph) and of course the hardware itself.
    – Dan Hulme
    Commented Feb 27, 2016 at 12:00
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Another option to secure delete info on your phone is to get the app IShredder 3 fom the Playstore to securely overwrite any data on empty space.

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