Bitcoin Forum

Hello, I am getting a new phone and need to make sure that my private keys and passwords are completely off my old phone before I sell it. Is a factory reset enough or would I need to go further to be completely sure?

A factory reset often does not wipe the flash storage completely. It will format it, but the data is still there and could be theoretically retrieved. The only way to be sure that your private keys do not exist on the device are to delete the file and overwrite it with 0's or random data. Unfortunately that is a bit hard to do on a phone.

For most use cases, unless you think the person you are selling the phone to both a) has the expertise to perform data recovery on a phone and b) knows you had coins on the phone, it should be safe. Additionally, if your wallet has a passphrase on it and is encrypted, then your private keys should be safe.

You probably won't be able to do this in a simple manner. The process would definitively be very risky as it would probably brick your phone if you wanted to do it the right way.

Probably your best option is to fill your phones memory by installing random apps and then uninstalling them when it is full. Do this a couple of times and then fill your memory with media files like music and videos a couple of times as well. Don't forget to fill the memory all the way up, as much as you can.
Hopefully this would overwrite your private keys.

Thanks guys, I appreciate the quick responses.

Probably the easiest solution is to move your coins to a new address, after you have your new phone.
Make a new seed too, so that your old wallet would be useless. No coins in there.

The only drawback is that you will have to pay the small transfer fee.

Overwrite /data partition with random bytes. I'm joking, this will brick your phone.

Safest (paranoid) way when you sell off it.
1: Generate new seed on new phone.
2: Send all your BTC from old wallet to new one. Only drawback is that you pay a small fee.
3: Get Secure Wipe from Google Play.
4: Get SD Maid.
5: Make external (Secure Wipe App) APK file in SD Maid.
6: Transfer this APK to SD card or to computer over USB.
7: Turn off phone.
8: Pull off SIM card.
9: Turn on phone (in normal mode).
10: Wipe it from settings menu. Use the code you use to unlocking screen (some people don't know about it).
11: When it boots on fully, wait one minute. Then turn off the phone.
12: Turn on phone (in recovery mode).
13: Wipe it and reboot.
14: You will see configuration. Avoid logging into any accounts. We copied the APK first to SD Card or USB to do not do it.
15: (USB copy only) Get Secure Wipe APK from computer.
16: Install it from SD card and run.
17: Wipe the storage with this tool.
18: Repeat steps 7-13 except 8.
19: Turn off phone when it sees configuration.
20: Sell it knowing that no one will ever hack your BTC.

Easiest way would be to just send the coins to a new wallet, which can of course be annoying if you have your funds on different labled adresses and don´t want to just put everything in one address.

Otherwise you should uninstall your wallet, fill the whole storage up with apps or other files, factory reset your phone. Then the guy which buys your phone would have to: know you had coins, be somehow able to restore partial files, break your encryption (~0% chance)

This thread is being locked because the OP has not responded in a while and it is just becoming full of spam.