After having read about the various "side-channel" attacks that will happen with anything that is "connected" I have come to the conclusion that the best way that you can "secure" cryptographic keys is by using a smart phone that is permanently surrounded by a Faraday cage.
I have discussed the problem of charging such a phone and from what I understand shielding all but extremely low frequencies will prevent side-channel attacks but would still allow for DC charging and also if the material is transparent enough for QR codes then charging using solar should be possible (although am not sure how practical that would be with current technology).
Why use a (smart) mobile phone?
Simply because it is the cheapest device that will do QR codes, has a camera and has enough processing power to do the cryptographic things like signing offline txs.
Such a device could also be used for doing QR code "logins" such as has been developed here:
https://bitcointalk.org/index.php?topic=310282.0There was a crowd-funded project that created a "Faraday cage pouch" for mobile funds a few years back (sorry can't find the link now) so perhaps this idea could be done in a similar way (I am not wanting to have any "ownership" of anything for this idea).
My understanding is that the same tech that is used to create "tinted windows" can actually be applied to creating "see through Faraday cages" which would then allow QR codes to be used for data transfer (the safest way I think transfers can be done).
As far as doing Bitcoin txs "offline" via QR codes I developed the CIYAM Safe (
https://susestudio.com/a/kp8B3G/ciyam-safe) some years back - I'm not sure if that would be suitable for mobile phones but I would be happy to help a savvy developer to work out how to do the same thing on Android and/or iOS.
