To store my Bitcoin safely, I have an offline transaction setup. Computer A is o

Question

To store my Bitcoin safely, I have an offline transaction setup. Computer A is online with a copy of the blockchain, and computer B is offline with the private keys. To make a transaction, I craft it on A, transfer it to B via USB, sign it on B, transfer it back to A via USB, and broadcast it on A.

It is possible the USB key gets infected on computer A, and compromises computer B. To mitigate that, I want to use a tiny USB stick (say, 1kB) which can only fit a few small transactions. That way, there's less chance a virus can physically fit on the USB in the first place.

Is using a tiny USB stick a good protection? Is this trick used for other purposes? Where I can buy those tiny "security" sticks (the smallest I could find was 512MB)?

Explanation / Answer

Your setup is called air gap. 512 MB (even 2) is still a lot for a virus payload that only needs to steal a private key. I think what you want is to use QR codes for both parties. If you print the QR codes, be sure to use separate printers, just like with all peripheral devices (even a mouse chip could contain an exploit that got there through your online computer). But you can also display the QR codes on the device's screens. This would provide a communications channel for viruses, but for that to work the isolated computer would need to get infected, or exploit a vulnerability in the QR code reader application.

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