case bitcoin hardware wallet

Part of the reason why I pre-ordered a case hardware wallet, was probably that there is no good wallet software on ubuntu phone. But the case is way more secure than a software wallet on a phone. Roughly the same size of the smallest feature phone you can buy, it contains an eInk display, a camera, a GSM chip, a fingerprint reader and wireless charging. For improved security it makes use of Bitcoin’s multisig feature. One key is on the device and one on the case servers. The third key is used only to restore funds in case the device is lost or stolen. You can either leave it with a third party or manage it yourself. Of course I chose to manage it myself, after all BitCoin is about empowering people instead of depending on third parties. The server part allows to implement spending limits and maybe other validity checks in the future. The fingerprint is used to authenticate against the server.

When the presale started in early May, the projected shipping date was end of summer. It was later fixed to September 21st. Because they found a problem with the uptate mechanism, they postponed the delivery. Communication suffered during this period. Everybody understands that this kind of problem can happen with such an early limited batch of a new class of product. But weekly updates would be very valuable in this situation to keep customers happy.

November 20th the box was finally delivered. The packaging is not as nice as with the trezor, but I don’t mind. The contents are much more important than the box. Build quality looks good. When I read that it is charged wirelessly and has no connector, I assumed it would be water proof. That is definitely not the case, but wouldn’t be too hard to achieve as it looks. It has tiny gaps around the fingerprint reader and the camera where water and dust can enter. The buttons are from the same film that covers the screen.

The setup process looks simple. You scan a qrcode from the website and your xpub for the 3rd key. Then you register your fingerprint by swiping it ten times. I was warned that it had to be at the right speed and angle. My first two tries were misreads, but after that I had only two more in total. It is easy to get right. The bigger problems were the server error messages that I got ten times in a row. Maybe the server had too much load. When I tried it again after waiting for an hour, it succeeded the first time.

Updating the firmware is a breeze, and worked like a charm the first time I tried.

Sending and receiving BitCoins couldn’t be easier and works very well. The connection can take a moment sometimes, but I think they are working on improving this aspect. Although I can’t tell for sure, but I had the impression it improved already with the first firmware upgrade. Specifically, I could now see the bars of the GSM signal already while scanning the qrcode.

What I miss is the current balance of the account.

But what I miss even more is an option to set the transaction fees. MultiSig transactions are bigger than standard transactoin, and thus the transaction fee is naturally higher. But a standard transaction in electrum with the dynamic fees slider in the middle costs usually mBTC 0.05  ($ 0.02) @ 223 bytes. The three outgoing transactions I did so far with the case were not that much bigger at 372 bytes but carried a transaction fee of mBTC 4.43751 ($ 1.43). This is more than an order of magnitude too high, and there needs to be a way to adjust.
Update: The fourth transaction with my case which I performed just an hour after writing this post, and the fifth the day after, had a regular transaction fee of mBTC 0.1 ($ 0.03).

Of course the three keys of the multisig are not just three ordinary BitCoin addresses. On the transaction level they are, but they are part of three hierarchical deterministic wallets that are linked together just like multisig wallets in electrum. Thus I was releaved to see that it asked me for an xpub as my 3rd key, and I could happily use one from the Trezor. But so far I couldn’t figure out how to retrieve the xpub’s of the other two components. Once I get them I can setup a readonly multisig wallet in electrum to track the transactions and the current ballance. But even more importantly, I can then assign descriptions to the transaction in a familiar environment for my personal accounting.

After the important issues above will be fixed, we can move to some wishes I would also have:

It would be nice if I could use the case for a custom multisig scheme. With the xpub of a Trezor, a HW1 and the Case, I could set up a multisig wallet in electrum. With electrum I would construct the transaction, and partially sign it with the Trezor. Then I would display the partially signed tx with electrum on my computer screen. Case could scan it from there and add it’s signature to it. Then it would either display a qrcode with the signed transaction on its screen, or transmit it to the BitCoin network over GSM. Once this works, I could set up an electrum wallet with the same xpubs as used in normal case operation. This would give me peace of mind because it would allow me to access my funds even if case went out of business or decided to censor my payments for whatever reason.

To improve privacy, it would be cool if I could run the server part on my own web server. This would require it to be open sourced, and a configuration option would have to be added to the firmware.

Last but not least, an option to sweep paper wallets by scanning the private key would also be useful.



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1 Comment

  1. Definitely great post about bitcoin wallet, you can always find other hardware or software it’s just that they are not available most of the time on the malls or computer shop most of them is via authorize distribution channel only.

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