What is the best way to backup an SSH key in case my hard drive fails, etc?

What is the format of the key you have?

If it is a using the OpenSSH format (and not putty) then it is probably a plain text file. Just copy the text into a secure note in bitwarden, lastpass or whatever password manager you use. If key is in some binary format, you could just base64 encode it and store it somewhere.

Heck, you could print it out. If you wanted to make it easy to type, you could make a QR encoded version of it. ED25519 keys are both secure, and pretty short, and should easily fit on a card you can carry in a wallet.

If you do choose to store a copy on the cloud, you should be certain that you have set a strong pass-phrase on the key so that it is encrypted.

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