How to completely destroyed all data from an iPhone before reselling it?

Question

I would wish to know more about how to erase 100% data of a used iPhone.

Basically, technician from Apple Store only said “Erase & Clear” within the iPhone setting could do all the work perfectly and all data of an iPhone are encrypted, so there is no way anyone could steal any traces of the data.

I am not sure about that answer as I believe the hard drive in a iPhone works the same as computers which means old data could only be completely erased unless they have been overwritten by new data. My speculation was confirmed by a iPhone data eraser tool called “iShredder”, as it claimed it would help to erase everything up to security agency standard.

So my question is if “iShredder” is a legitimate tool for going forward about erasing iPhone’s data in the most professional way? What do you think?

If the iPhone had used some APPs in connection with entertainment like music and movies, sometimes they offer you an off-line viewing by caching the content, does that mean it is not different from downloading those data on your hard drive of the iPhone, which means you would have to overwrite them completely in order to erase them, and “Clearing cache” option in those Apps only marked them as deleted but not exactly erasing them?

Also, when you are trying to delete photos when the iCloud is full, it says they would be deleted immediately and you would not be able to recover them from “Recently Deleted”, does that those photos were also completely gone or again were just marked as “Free Space” until new data overwrites them in the hard drive?

Sorry for a series of questions and looking forward to your advices on this! Many thanks!

Answer 1

Like many modern smartphones, the iPhone uses flash storage. A flash memory cell has a limited number of rewrite cycles. Two things make data shredding on flash storage difficult:

1. Wear balancing: the storage controller will automatically remap physical storage blocks to ensure that all storage cells are used equally. This also means that overwriting a file in place will actually rewrite the file in a different physical location before the storage controller remaps it to same virtual location.

2. Overcapacity: a flash memory device always has more physical storage than its advertised capacity. This extra space is used by the storage controller to replace bad blocks that have died so the end user won't see decreased capacity over the expected lifetime of the drive.

These things combined mean that overwriting the drive until it runs out of space is not guaranteed to wipe out all traces of your data from the drive. So it has become standard practice to use full disk encryption on flash storage, so that deleting flash storage can be done by simply removing the encryption key.

Assuming the encryption is solid, this should be a much more reliable way to delete all data in a flash storage device. Some users still overwrite as much data as possible by creating files until the disk is full, in case there's a vulnerability in the encryption or their encryption key was compromised. While this isn't necessary, you should do so if it eases your worry.

Answer 2

If it's a recent Iphone, a factory reset should cover it; the storage is encrypted with a device specific key and a 'session' key. randomizing that key renders the information stored there inaccessible, irrespective of wear leveling concerns.

https://www.howtogeek.com/339705/what-is-apples-secure-enclave-and-how-does-it-protect-my-iphone-or-mac/