kukai VS airgap-vault

good question, doesn't seem like there's an option for exporting your seed. I would recommend creating a new issue on the kukai github issue tracker and ask the developers about whether such a feature exists.

I did get a pretty quick reply by posting this issue on the airgap github. Their implementation of BIP-39 passphrases is at the address level. This is unfortunate because a BIP39 passphrase generates a unique private key with its own set of addresses. I'm going to let this percolate for a bit...

Fast: https://github.com/airgap-it/airgap-vault (Get old android, flash lineageOS for good measure then airgap)

You might get a kick out of the airgap vault project: https://github.com/airgap-it/airgap-vault

The project has apps for mobile, which make turning an old phone into an hardware wallet pretty easy, as long as your phone can support it (more on that in a bit). If you're technically minded you can build the project yourself and run it as a web app: https://github.com/airgap-it/airgap-vault (Note, it's a JS project. I had issues building with npm but no problems building with yarn which the dockerfile also uses). This will allow you to turn an old desktop or laptop into hardware wallet. I wanted to try on a raspberry pi, but I don't have a camera and have read about some issues getting chromium to recognize pi cameras.

The project is completely open source and free. Our app is also reproducible, however, due to some recent issues where builds were randomly not reproducible, the status on walletscrutiny is currently "not reproducible" (if you read the analysis, it will state that the latest version is in fact reproducible, but until mentioned issue is addressed, it will stay "not reproducible"). We're working with walletscrutiny to resolve this and it looks like we have resolved the issue with the latest release, but it will take some time to verify it. (see https://github.com/airgap-it/airgap-vault/issues/127).

Thanks for giving our wallet a shoutout. A few months ago someone asked us a similar question regarding the "popularity" of our wallet, you can read my response here: https://github.com/airgap-it/airgap-vault/issues/74. In summary, I think it's a combination between people assuming that a HW device must be more secure, and a lack of marketing on our side.

Are you planning to use AirGap as a co-signer in a Bitcoin Multisig setup? This is currently not possible, but we are working on bringing our Bitcoin implementation up to speed and support the latest standards. You can read more about it here: https://github.com/airgap-it/airgap-vault/issues/73