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So if your private keys are air gapped and if even using a usb to sign transactions isn't 100% safe, what is the alternative to signing transactions from an offline computer?
#1 QR Code, e.g. the bither app uses them
#2 sound [1]
#3 type the TX by hand, which is IMHO the safest way, unless your brain was hacked [3]. That level of security is almost never needed. On the other hand its not that hard unless you create a TX with 100 outputs/inputs. The nasty part would be the signed TX, which is just hex code and typos can happen easily.
#4 floppy [2]
#5 CD/DVD which is costly and has the similar problems USB has
Edit: In case this wasnt clear, I think for the majority of bitcoin users an USB stick is fine to transport data from cold to hot wallet. Not everyone needs/has a tinfoil hat.
[1]
https://bitcointalk.org/index.php?topic=735111.0[2]
https://bitcointalk.org/index.php?topic=813295.0[3]
