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I want to wipe the phone with android 4.3.

https://www.cnet.com/how-to/the-best-way-to-completely-wipe-your-android-device/

says that I should encrypt the phone before factory reset because factory reset doesn't actually clean the phone (https://www.androidpit.com/why-a-factory-reset-doesn-t-wipe-all-your-data).

I want to know if it is a good idea to delete the accounts (gmail etc) on the phone before encryption and factory reset?

By the way, I don't think I can update android version... It is a huawei mate 2.

update: Found description of factory reset: How does an android factory reset work?

Not sure about how encryption work though.

  • In a layman's words, encrypting means taking a key and replacing the data on the device with garbage, derived from a series of procedures based on that key. To decrypt, you provide that same key, and the garbage passes through a series of procedures in order to rebuild the old data. – Death Mask Salesman May 13 '17 at 15:23
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    but, exactly what is encrypted? what files/partition? and how.. Factory reset is not trustworthy... I am a bit paranoid for this reason. Does encryption only works on files, but not deleted files? Or, encryption works on the whole partition? – hamster on wheels May 13 '17 at 15:27
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    I think it is full disk encryption for android 4.3. nelenkov.blogspot.com/2012/08/… – hamster on wheels May 13 '17 at 19:25
  • Unlike iPhone, this phone stores the encryption key with software in qualcomm chips and people can easily get that out. So, it is better to shred the data with other ways first. arstechnica.com/security/2016/07/… – hamster on wheels May 13 '17 at 19:26
  • Delete accounts, encrypt, factory reset, install and use data shredder (so nothing for the data shredder to steal), then encrypt and factory reset again. – hamster on wheels May 13 '17 at 19:29
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I don't see how it is necessary to delete any accounts (Step 1) using the steps you outlined in your comments. However, if you're using Android 5.1.1+, you likely want to avoid Factory Reset Protection. In that case, it's just necessary to delete the main Google account tied to the device. (I added parenthesis around two of your steps that I think are unnecessary):

  1. (Delete accounts) Just delete the main Google account tied to the device if your device is on Android 5.1.1+
  2. (Encrypt) This is the default in Android 6+
  3. Factory reset
  4. Install and use data shredder
  5. (then encrypt and factory reset again)

The account tokens are encrypted in Step 2 and are therefore garbled for any bad actor trying to recover your information.
When you overwrite the free space in Step 4, the original master encryption key may be overwritten (and any space that is not 'free space' was already overwritten by the factory reset). Android may store the key in an area that isn't reached by an eraser app that overwrites free space; however, the free space that originally held the encrypted data is overwritten by the eraser and thus even a bad actor who has access to the master key won't be able to decrypt anything. Therefore, any subsequent encrypt/resets beyond the original Steps 2 and 3 are pointless.

Updated the answer with Robert's comment below... and for a second time.

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    The delete accounts step is important for the main Google account. Because otherwise you are running into the Google Account Verification (AKA Factory Reset Protection) and after the factory reset you have to enter your Google credentials to unlock the phone. – Robert May 16 at 17:51
  • Thanks for the heads up. I just factory reset and older Android phone and that's why I didn't know about Factory Reset Protection. I updated the answer. – Wuschelbeutel Kartoffelhuhn May 16 at 17:59
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    A second hint: On flash ROM chips overwriting the free disk space will not necessary overwrite the original master encryption key as flash media usually keep some spare blocks that are not accessible via file-system. – Robert May 16 at 18:06
  • @Robert How can one can overwrite these blocks to make the key irrecoverable? – Wuschelbeutel Kartoffelhuhn May 16 at 18:11
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    The common way to make a key irrecoverable that is stored on a flash media is to physically destroy the media. All software based ways to delete it may or may not work depending on the flash chip where and how the wear leveling is implemented and how careful the key change was implemented (if implemented correctly changing the key should overwrite the flash block the old key was stored in). Of course overwriting the free space overwrites large parts of the data that could be decrypted using the old key. Therefore it reduces the risk that anybody may be able to recover anything. – Robert May 16 at 18:19

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