How to successfully make phone device storage to be clean completely

[_act_]

I have a friend that used cheap Android phone to store his bitcoin, he is not using the phone for anything, he password the phone and keep it somewhere. He only on the phone when necessary.

I told him about how phones can be vulnerable to hack if stolen or sold, we have been discussing this on this forum since some weeks now. He told me that he is not ready to buy a hardware wallet, but that if there is something he can do to delete all the data on the phone completely as he wants to make use of paper backup instead.

But that is not the main solution, he wants to know how he can make sure he deletes all the data stored on the phone. I know by deleting the data from the phone, the data has not been deleted on the UFS or the eMMC of the phone.

He needs how he can permanently delete all the data on his phone.

[Cookdata]

Why not do factory reset, that's enough to wipe everything on the phone.

[Satofan44]

Why not do factory reset, that's enough to wipe everything on the phone.

These days I think encryption is on by default, but in case that for some reason it is not then this is not enough. Anyway, there is no reason to do it just once. Furthermore, the phone can be encrypt again several times. While this may take some time it is usually passive waiting and therefore time is not a concern. This is the loop basically that you can repeat a few times:

1. Enable encryption.
2. Factor reset.

They can also do stuff with ADB if they really want to go all the way. Both a factory reset from ADB is possible and overwriting empty data too.

https://stackoverflow.com/questions/10817721/wipe-data-factory-reset-through-adb

Even running the loop once should be enough but I always ask why not more? Don't be lazy.

[LoyceV]

he wants to make use of paper backup instead.

A backup is only a backup if you still have an original, otherwise it becomes your only version.

He needs how he can permanently delete all the data on his phone.

I don't trust phones and for that reason keep all old phones forever.

Why not do factory reset, that's enough to wipe everything on the phone.

I've seen this fail, and wouldn't rely on it.

[Pieq]

Assuming the phone is encrypted, most if not all are today.
You can do the following:

1. Factory reset.
2. Run this script:

Code:

cd /data; for i in 0 1 2 3 4 5 6 7 8 9; do cat /dev/urandom > $i; rm $i; done

Found here on Reddit.

Technically, the first step should be enough to securely erase the device as it clears encryption keys.
If you want extra peace of mind, you can repeat these steps as many times as you like.

However, it's not generally recommended to hold funds on phones.
If he has access to a computer he could create an offline wallet using Electrum, write down his new seed phrase then transfer the funds from the first wallet to the new one, after making a test transaction.

[BitMaxz]

Why not do factory reset, that's enough to wipe everything on the phone.

A factory reset still had some data that could be recoverable even if it was deleted, and it was cleaned out by a factory reset.
Some phone tools like JTAG kits (like Z3X, Medusa, etc.) that can directly access eMMC can still be able to recover those data.
If you sell that phone and the buyer has the knowledge and tools to try recovering data from eMMC, they will be able to access those wallet files. That's why wiping out or doing a factory reset isn't enough to completely remove your wallet or recovery data from that phone.

If you have Easy JTAG, you can permanently delete those data by overwriting the entire storage plus wiping them. However, this tool isn't cheap, so if you are looking for a cheap solution, then destroying eMMC is the best option, but it can kill the phone.

[Findingnemo]

Android phones are not a great option for an airgapped wallet. Most of the cheap Android devices come with preinstalled apps that are nothing but spywares, which can't be uninstalled or even disabled. So if your friend is not interested in buying a HW then the best option would be a cheap PC that can be bought for the same price and is much more customizable compared to smartphones.

[ABCbits]

Why not do factory reset, that's enough to wipe everything on the phone.

I've seen this fail, and wouldn't rely on it.

Are you talking about malware/backdoor that can survive factory reset such as xHelper[1]? or about factory reset actually doesn't wipe all partition[2]?

Why not do factory reset, that's enough to wipe everything on the phone.

A factory reset still had some data that could be recoverable even if it was deleted, and it was cleaned out by a factory reset.
Some phone tools like JTAG kits (like Z3X, Medusa, etc.) that can directly access eMMC can still be able to recover those data.
--snip--

FYI, Android require encryption since Android 10[3] which makes data recovery and decryption more difficult.

[1] https://arstechnica.com/information-technology/2020/04/solved-how-android-backdoor-called-xhelper-survives-factory-resets/
[2] https://android.stackexchange.com/a/26326
[3] https://security.stackexchange.com/a/264645

[_act_]

Are you talking about malware/backdoor that can survive factory reset such as xHelper[1]? or about factory reset actually doesn't wipe all partition[2]?

LoyceV is not talking about malware but how the data might still be extracted with the use of some tools.

FYI, Android require encryption since Android 10[3] which makes data recovery and decryption more difficult.

I do not know if this is true, I have heard of police case that someone's phone has been damaged but was used to extract information. I mean information were extracted from the phone in the UFS.

What I read online was that if the phone is encrypted, that data extraction will be very difficult, maybe it was possible because the phone was not factory reset yet. I am referring to a completely damaged phone.

I have heard about warnings about how people should be careful of their old phones, that data can be extracted from it if damaged and dumped somewhere.

[LoyceV]

Are you talking about malware/backdoor that can survive factory reset such as xHelper[1]? or about factory reset actually doesn't wipe all partition[2]?

Years ago, I used the factory reset on my Android phone several times, and every time I found back old personal data. It may or may not have improved since then, but I can't verify that. Better safe than sorry.

[dkbit98]

It depends what is the brand and model of his phone, but doing a reset should return phone close to factory state.
There could be some leftovers on phone drive that could be restored, so I would choose Pixel phones that have solid encryption and can be used with graphene os.
If you heard latest news you will see that budget Samsung phones have hidden spyware in them for tracking, and that is the case with mnay smartphones these days. 
https://www.malwarebytes.com/blog/news/2025/11/budget-samsung-phones-shipped-with-unremovable-spyware-say-researchers

[bitmover]

Are you talking about malware/backdoor that can survive factory reset such as xHelper[1]? or about factory reset actually doesn't wipe all partition[2]?

Years ago, I used the factory reset on my Android phone several times, and every time I found back old personal data. It may or may not have improved since then, but I can't verify that. Better safe than sorry.

Was this a long time ago?

You found your data by just connecting your phone to USD and checking the folders and files?

That is really annoying... I would do the same in your situation and become paranoid about it. I never checked if my phone really deleted everything after factory reset.

Personally, I just use bad smartphones from samsung, so they are never worth anything. I just keep them or throw in the trash.

Why not do factory reset, that's enough to wipe everything on the phone.

I would just do that, no need to overthink. But, after LoyceV comment, I would check if data is really gone lol

[LoyceV]

Was this a long time ago?

Years.

You found your data by just connecting your phone to USD and checking the folders and files?

I found data back on the phone itself.

I would do the same in your situation and become paranoid about it. I never checked if my phone really deleted everything after factory reset.

Being paranoid doesn't mean the risk can't be real

Personally, I just use bad smartphones from samsung, so they are never worth anything. I just keep them or throw in the trash.

I wouldn't trust the trash either, so they slowly pile up. That pile includes phones with actual number keys from before fragile touch screens (and before Bitcoin).

I would check if data is really gone lol

I don't trust phones enough to even know what to check. With hard drives, I know for sure my data is overwritten if I format it, and fill it with random data. I realize there might be some remapped sectors, but that's not my main concern.
It's also the reason I never claim warranty when a disk stops working.

[ABCbits]

FYI, Android require encryption since Android 10[3] which makes data recovery and decryption more difficult.

I do not know if this is true, I have heard of police case that someone's phone has been damaged but was used to extract information. I mean information were extracted from the phone in the UFS.

What I read online was that if the phone is encrypted, that data extraction will be very difficult, maybe it was possible because the phone was not factory reset yet. I am referring to a completely damaged phone.

I have heard about warnings about how people should be careful of their old phones, that data can be extracted from it if damaged and dumped somewhere.

It should be true, since it's stated on official android website.

All devices launching with Android 10 and higher are required to use file-based encryption.

I would guess what you read online is device that originally shipped with Android version lower than 10, where the phone company decide not to use encryption.

Are you talking about malware/backdoor that can survive factory reset such as xHelper[1]? or about factory reset actually doesn't wipe all partition[2]?

Years ago, I used the factory reset on my Android phone several times, and every time I found back old personal data. It may or may not have improved since then, but I can't verify that. Better safe than sorry.

I see, it's rather concerning. But from quick google search, most people who complaint similar things happened more than 5 years ago.

[LoyceV]

I see, it's rather concerning. But from quick google search, most people who complaint similar things happened more than 5 years ago.

Maybe they've improved it, or maybe they're better at hiding the files that are still on there I still can't know for sure.

[BitMaxz]

FYI, Android require encryption since Android 10[3] which makes data recovery and decryption more difficult.

[1] https://arstechnica.com/information-technology/2020/04/solved-how-android-backdoor-called-xhelper-survives-factory-resets/
[2] https://android.stackexchange.com/a/26326
[3] https://security.stackexchange.com/a/264645

Yeah, I agree, but I'm operating some JTAG decrypting data that we dump from eMMC. It is supported by the software called Easy JTAG, but not all units are supported; it sometimes fails to decrypt, but old security patches are easy to decrypt.
Easy JTAG always has an update to support new units and new security patches, so if the eMMC data from Android 10 is encrypted and can't able to decrypt today then in the future they are going to release a new solution for decrypting new security patches.

[Smartprofit]

As far as I know, specialized equipment capable of recovering data from modern smartphones currently exists in Israel. Israelis actively export this equipment to other countries (they're likely not the only ones producing such equipment). 🙋

In my country, such equipment is widely used at airports and customs. Suspects' smartphones are confiscated for 10 minutes, taken to a special room, and hacked. All information from the smartphone is recovered (even if it was deleted) and copied to a special database for subsequent analysis. Unfortunately, a factory reset doesn't help.

Cybersecurity experts suggest the following solution: delete all data from the smartphone, then completely refill the memory with new data. Then repeat the process. In this case, there's a chance the original data will become completely inaccessible.

However, as far as I understand, the most reliable method of data deletion remains disassembling the phone and then smashing its contents with a heavy hammer. If your phone is seriously physically damaged, data recovery is impossible.  💃