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Depends on how thick your tinfoil hat is. No security, I mean NO security is 100.000000% bulletproof. And since that is true, there is an infinate amount of OPSEC that can be imagined to get you closer to 99.99999999%
In most cases if something is really hard to hack, it’s safer than other „soft“ targets. So…
1. Hard wallets are always better than soft wallets.
2. Hard-air-gapped are better than hard-networked wallets
3. Hard-air-gapped-multisig wallets are better than Hard-air-gapped
4. Hard-air-gapped-multisig geographically isolated wallets are better than Hard-air-gapped-multisig wallets.
Of course you can keep going by putting them in a faraday cage and doing the signing ops in a cleanroom. But still, your just at 99.9999999…..99999% secure.
> For example , use one spare mobile(flight mode /no internet forever)to create a software wallet.
What you turn off, [they can turn on](https://www.wired.com/2014/06/nsa-bug-iphone/). Networking has been found while in „airplane mode“
> Then it is even better than Trezor or other HW wallet who could simply extract our seed with a firmware whenever they want.
Despite what you’ve read, this is not true.
> But even if the software we installs could be able extract seed , the seed could not go to the internet still if we set our first mobile flight mode or just not provide it with internet forever.
Many hardware wallets do not have networking hardware for this very reason.