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How secure is an offline android phone?

Some background. I want to use an app called Bither on android, which allows me to create a cold wallet on 1 phone and a hot wallet on another phone.

Cold wallet is a wallet which is created on a computer which never connects to the outside world.

Hot wallet is a wallet which is created on a computer which does connect to the outside world.

Assuming that I keep the offline phone in a secure safe, switched off, and only take it out of the safe and switch it on when I want to sign a transaction.

Even if I remove the sim card, is it possible for someone to get access to the data in the offline phone without physically having access to the phone, even if the phone is switched off and the battery is removed?

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    When you use the phone, does it need to connect to the outside world? E.g. do you re-insert the SIM to allow for data transfer of any kind? Clearly no-one can access data on a switched off, battery-less phone in a safe, but existing malware on the phone may do something the moment connectivity is possible. Nov 12, 2014 at 13:14
  • A SIM card will never be inserted into the phone when the phone is used. To use the software in the phone, it will be SIMless, and the app will simply generate a qr code for another phone to scan in to complete a transaction. Nov 12, 2014 at 13:58
  • Question/doubt on usability: how are you going to sign cold transactions without sporadic access to the network? The cold phone needs some connection, sometimes! Jul 17, 2020 at 13:15

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Removing the SIM alone wouldn't prevent a remote hack if the phone was powered up as WiFi would still be available and therefore an attack vector. However, a phone that is off with no source of power is not remotely hackable. At least there's no demonstrated techniques or even published research that I know of which would make it possible. Without a power source there's no CPU or antenna operation, it's just a sophisticated brick.

The thing is that in order for this cold wallet to be useful it will need to talk to something - as some point you will need to turn this on and connect and then it's just as vulnerable as anything else. Android is designed as a vehicle to get your personal information to the outside world so is chatty by default, however there are some Android ROMs which are designed with security in mind, it would be best if you installed one of those and limited the operation of your cold wallet phone to the bare essentials.

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  • When I do switch the phone on, it will be simless, and it doesn't need any wireless connections. The software in the phone will generate a qr code which will be scanned in by a different phone to complete the transaction. Unless ofcourse switching the phone on automatically starts broadcasting some sort of signal which I am unaware of? Nov 12, 2014 at 14:00
  • It shouldn't if you have airplane mode turned on.
    – GdD
    Nov 12, 2014 at 14:03
  • Is it possible to switch android on with airplane mode enabled by default? Nov 12, 2014 at 14:39
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    I don't know about by default, however if it was shut down in airplane mode it will start in airplane mode, that's by design.
    – GdD
    Nov 12, 2014 at 14:50
  • however there are some Android ROMs which are designed with security in mind could you name any?
    – Suncatcher
    Nov 30, 2019 at 21:34
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When the goal is to build an airgap system, I think that there are better choices than mobile phones.

Here is a youtube recording of CIA executive Gus Hunt at a presentation to journalists in 2013:

(https://www.youtube.com/watch?v=edP95iJWVBI)

Somewhere around minute 11:15 he claims that 'mobile is not secure', and that 'somebody can know where you are all the time because you wear a mobile device ... even if that mobile device is turned off ... you know this, I hope'.

Well, now this is 7 years ago, during a time when most phones had removable batteries (which might have been some line of defence), but this is not the case any more today. So there is no reliable way to disconnect the phone circuits from power any more. Is there a guarantee that the built-in Wifi does what you expect it to do?
Let's put two and two together and conclude that there might be an attack surface to contemporary phones even if they are switched off, and especially when they're switched on, which is what you have to do when you use your cold wallet.

In my opinion, using a traditional PC without built-in Wifi card is the safer option, or a Raspberry PI where you have physical control over the network connection devices of any kind (Wifi, UMTS, Bluetooth).

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  • is there an easy way to display a QR code on a raspberry PI, or run an app? asking because i havent looked a them for years
    – daniel
    Jul 13, 2020 at 11:29
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    sudo apt install qrencode should get you qrencode which you can use in the Raspbian shell. Also, there is qtqr which offers a GUI to create QR codes.
    – lab9
    Jul 13, 2020 at 13:17

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