How to safely decrypt a private key on a computer that has been online

[21XO]

I erased and reformatted my Mac hard drive and then encrypted my Trezor backup private key with a 20+ alphanumeric password before connecting to the internet for the first time. Now my Trezor just broke and I need to decrypt this file...problem is now my MacBook has been online...
So how can I decrypt the file safely without having to erase and reformat my drive again

Thank you 

[craZyLovE0916]

I erased and reformatted my Mac hard drive and then encrypted my Trezor backup private key with a 20+ alphanumeric password before connecting to the internet for the first time. Now my Trezor just broke and I need to decrypt this file...problem is now my MacBook has been online...
So how can I decrypt the file safely without having to erase and reformat my drive again

Thank you 

You could do it anyways given that the odds of you getting hacked just for logging online are slim-to-none.

Otherwise I suggest buying a small Chromebook, using it for whatever you need and then return it when you are finished.

[HCP]

If you're worried, try using a Linux Live CD... with no persistent storage and no networking functionality built in/switched on.

If you boot into that OS and do the decryption and then reboot, all traces should be gone... and the viable attack vectors or chances for data leakage are minimised.

[jackg]

I erased and reformatted my Mac hard drive and then encrypted my Trezor backup private key with a 20+ alphanumeric password before connecting to the internet for the first time. Now my Trezor just broke and I need to decrypt this file...problem is now my MacBook has been online...
So how can I decrypt the file safely without having to erase and reformat my drive again

Thank you 

You should be able to just take your computer offline if it's just the private keys you want to decrypt.

Disconnect your internet or even turn off your router, then, once private keys are decrypted load them into a secure encrypted client.
Presuming the only things you use to do this is the clipboard, it's unlikely any data will be stolen.

[GetFreeCoins]

If you do not have anything important on this disk - better buy a new one.