Can you use a phone for signing Bitcoin transactions offline?

[Abiky]

I always use an air-gapped computer to sign Bitcoin transactions offline using the Electrum wallet software. Everything is seamless, and secure with little to no technical knowledge required to perform said task. Now on the go, it would great to be able to sign Bitcoin transactions offline using your mobile device. I have an Android smartphone which never connects to the Internet, as I only use it to play games when I'm bored. I was wondering if I could somehow use my phone as a sort of cold storage, while at the same time being able to sign any Bitcoin transaction offline?

If so, would you recommend me of a good application that would allow me to perform this task? I believe that it would be more convenient to sign transactions using a phone instead of a computer which is often bulky to carry around. Any help would be greatly appreciated with this. Thanks in advance.

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[pooya87]

it should technically be possible since a phone is just a small computer that you carry around and any wallet could add the offline signing feature with minimal changes. in fact i think Electrum may support it already for their android release. but i don't think you can ever call it "air-gapped" or even "offline" just because you don't turn your phone network on.
with a PC all you have to do is to disconnect its network cable and it won't have any way of transmitting data unless you choose to add extra equipment to it such as Bluetooth, connect to LAN,...
but with a phone, even if you disconnects its network it still has other ways of transmitting data and it is always connected to a network.

[Yaunfitda]

If so, would you recommend me of a good application that would allow me to perform this task? I believe that it would be more convenient to sign transactions using a phone instead of a computer which is often bulky to carry around. Any help would be greatly appreciated with this. Thanks in advance.

The old but reliable Electrum apps.

And this thread can help you out: Electrum on Android: How to create a SIGNED raw transaction please?.

[cryp24x]

I agree to them about the offline settings of the computer versus a smartphone. Computer can be really isolated from a network especially when it has no LAN connection or Wifi dongle. It is impossible for the internet to be connected to a computer that have no wires, no bluetooth and no wifi dongle connection. While on a smartphone, one touch or click on the wifi connection or data, it will somehow be connected to the internet. So what is my take? It depends on how you handle your gadget. It is really good to utilize the power of mobile and its applications but in terms of security, we need to be very careful.

[TimDavis]

Using an air-gapped computer or mobile phone as cold storage is a very unsafe practice. In his very exhaustive research, Dr. Mordechai Guri, clearly explains how your private keys can be extracted from them. The research paper can be found here: https://arxiv.org/pdf/1804.08714.pdf

Put briefly, when you’re signing and broadcasting your transaction, you would probably need to introduce removable media such as an SD card or USB cable to your air-gapped computer. A virus can then infiltrate your system via the USB, after which it can control and send instructions to a specific component in your computer to export your private keys. One of the surprising ways it can do that is by taking control of the computer’s fan to extract information from the sound it makes! The output from your computer, be it in the form of light, sound, or radio signal emissions can be picked up to extract your private keys. Android devices are not safe either. They rely on TrustZone, which is susceptible to side-channel attacks, and are hence unsafe to be used as cold storage. The iPhone uses a secure enclave, but it can only be used for Apple functions, requiring you to download a third-party app if use your mobile as a cold wallet.

Check out this interesting article that clearly explains how security can be compromised if you plan on using an air-gapped computer or mobile phone for your crypto: https://medium.com/cobo-vault/air-gapped-computers-and-phones-vs-hardware-wallets-whats-the-difference-f06790316f03

[Abiky]

it should technically be possible since a phone is just a small computer that you carry around and any wallet could add the offline signing feature with minimal changes. in fact i think Electrum may support it already for their android release. but i don't think you can ever call it "air-gapped" or even "offline" just because you don't turn your phone network on.
with a PC all you have to do is to disconnect its network cable and it won't have any way of transmitting data unless you choose to add extra equipment to it such as Bluetooth, connect to LAN,...
but with a phone, even if you disconnects its network it still has other ways of transmitting data and it is always connected to a network.

That's certainly true, mate. The phone's network (3G, 4G, etc.) might still be connected without the person noticing it. This is not like a PC where you can simply disconnect the LAN cable or remove the internal wireless card. Maybe removing the phone's SIM card will help. But just to be safe, it's advised to activate "Airplane Mode" whenever it's possible. For Android devices, the best way to achieve complete privacy and security would be to "root" them. Gaining "root" access would allow you to "tamper" with the device to your heart's content. An experienced developer could easily deactivate the phone's radio transmitter, along with Bluetooth and even Wi-Fi at a software level. Only then, one will be able to use the phone "off the grid".

I think Android tablets would work better than smartphones as mobile Bitcoin wallets where you can sign transactions offline. There are many tablets out there which only have Wi-Fi capabilities, unlike smartphones of today. As long as you're able to import a raw Bitcoin transaction from the PC to your Android device to sign it offline, you might not need to spend a fortune on "Hardware Wallets" after all.

The old but reliable Electrum apps.

And this thread can help you out: Electrum on Android: How to create a SIGNED raw transaction please?.

Thank you very for much for sharing this. I've managed to sign a raw transaction on my Android phone in an offline manner via the use of Electrum Bitcoin wallet on the PC. The process is simple as you'd import the data from the PC wallet's QR code into the site provided on the link above to convert it into a readable format for the mobile version of Electrum. While this works, I would've preferred to do everything within my phone without having to use the PC at all. If someone achieves this in the future, it could allow anyone to have a truly mobile "Hardware Wallet" that's completely air-gapped, and safe from external attacks at a fraction of the cost.

Nonetheless, I'll be locking this thread since my problem has been solved. If anyone else has other ideas, feel free to contact me via PM anytime.