Check out this video with a very early version of an air-gapped device we made.
Basically you have a completely offline wallet which you use to generate a private key. You can actually create your own key by using converting coin flips to hexidecimal numbers to make sure you have a completely randomly generated private key. Then you use the wallet to use the private key to create a public key. (BTW you don’t have to use coin flips to make the key we have a true random number generator on there but if you want to be the safest flip the coins) More on that concept here:
Once you have that set up you generate QR codes that you use to send transactions to the blockchain. So you need a wallet app on the air-gapped device that can generate transactions and receive send to wallet addresses in QR code form. These QR codes are signed by your private key so the blockchain knows they are legit. On the otherside you need an app on a PC or phone that can read the QR code generated by the air-gapped device and broadcast the transaction to the blockchain.
At this point there are a number of air-gapped hardware type of devices on the market but none that I know of work with Cardano yet. Trezor and Ledger are solid products as well but air-gapping gives one greater control in my opinion. Especially if you have the option to build it yourself from scratch. I think it would be a very useful option for people in the developing world as as well. The device we are looking at can be used to sign transactions of all kinds with the private key. So you could message people with it among other things.
Here is more on airgapping:
And some other air-gapped wallet products:
BTW we figured out how to completely encrypt the micro SD card in the Pi as well so you can use it to store all kinds of private information and make multiple copies.
Also I personally don’t like the idea of using a phone as the air-gapped device because I think there are to many ways to hack them. I like the Raspberry Pi 2b because I know it has no wireless capability at all.
Let me know if you have more questions.