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I have recently decided to sell my Android device (MotoG3). I know that before I sell it I am supposed to Encrypt the device before I perform a factory reset as it is my understanding that the Encryption keys will also be deleted, therefore making any recoverable data unreadable.

However I am confused as to whether this also applies to data previously stored on the device BEFORE the device was Encrypted.

E.g. I have had many photos (and other files) on my device in the past which I have decided to delete to free up space, however at the time of deletion my device was NOT Encrypted.

So what happens to these files after I Encrypt my device and perform a factory reset, are they recoverable or is the hidden space which the files previously occupied also encrypted?

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I believe modern android versions support "fast encryption". To quote the documentation -

fast encryption, which only encrypts used blocks on the data partition to avoid first boot taking a long time. Only ext4 and f2fs filesystems currently support fast encryption.

This to me suggests it will not overwrite currently unused storage. If you are concerned I would suggest filling the disk as completely as feasible (either before encrypting or while encrypted).

It is worth noting recovering deleted files from flash media is considerably more difficult than from spinning disk media. As such the risk here is extremely low.

  • Thank you for the prompt reply. The device is currently using Android 6.0.1. I always thought that anything below Android 7 used Full Disk Encryption? It begs the question, what is the point of going through the process of Encrypting a device and performing a factory reset if any data that was previously stored on the device but had since been deleted (before Encryption) is recoverable? – William Hyll Oct 18 '17 at 8:44
  • It will protect all data currently in the filesystem. I expect the argument is that if you were willing to store it unencrypted it can't be that confidential - i.e. what would have happened if you'd lost your phone before encrypting it?. I imagine this advice is mostly aimed at those with already encrypted devices and the advice is to encrypt AFTER wiping - there have been issues with factory resets not deleting encryption keys due to poor driver implementations. Either that or its outdated advice from before fast encryption was a thing. – Hector Oct 18 '17 at 9:02
  • The data was indeed confidential but this was before I learned of the dangers of selling phones which have only had a factory reset performed. I was essentially like 99% of people who think that once something has been deleted, it is gone forever. I always used to think using a password/pin/pattern on the phone was enough to prevent access should I have lost my phone. – William Hyll Oct 18 '17 at 9:17

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