How to back up your encryption keys

Without really noticing it I’ve gradually acquired more and more encryption keys without understanding how to back them up properly. Until fairly recently I lazily assumed that remembering the passphrases would be enough in case of my laptop catching on fire, but this is not the case.

I use GPG keys both for authenticity over email, and encryption when sending people passwords for stuff I’m setting up for them. The Ubuntu launchpad also uses GPG for signing packages for which I use a different key. I also run a bunch of servers, for which I use ssh keys to prove my identity, then there is the Android play store, that requires binaries to be signed, using yet another key, which is also shared for OUYA packages too.

The main algorithm in use for authentication in all these cases is called RSA. RSA and similar algorithms generate a pair of keys, one public and one private. The private key data is encrypted using yet another algorithm (usually AES) which is what your passphrase is used for. When it’s needed, you type your passphrase in, it decrypts the RSA private key and uses that to identify you. So it’s vitally important that this key data is backed up as it can’t be recreated from your passphrase. There doesn’t seem very much information online on the practicalities of all this, so I’m documenting the process with links to where I got info here, partly in order to get feedback if it’s wrong!

With ssh it’s just a matter of copying the contents of your .ssh directory – which contain the public and encrypted private key. The android keys are in a file called .keystore, in your home directory.

When it comes to GPG the best way seems to be to list and export them individually with:

gpg --list-secret-keys
gpg --export-secret-keys your-id-number > secret-key.asc

The id number is the part after the slash for each keypair. At the same time, it’s important to back up a revocation key for each key – this allows you to tell the GPG trust network if your identity becomes compromised by someone finding out your key (or losing/forgetting your passphrase, which is perhaps more likely). You can do this with:

gpg --gen-revoke your-id-number

And paste the result into a text file.

So you can take all these files and store them somewhere safe on a usb stick for example. It all needs to be encrypted so it doesn’t matter if it’s found and accessed. I almost made a fundamental mistake and encrypted it with GPG, which would have been like locking my house keys inside the house. Instead you can encrypt the file using AES independently using this command:

openssl aes-256-cbc -in your-key-file.tar.gz -out your-key-file.tar.gz.enc

I’m assuming once this is done, the best practice is to put it in various places to reduce the chances of it getting lost, as it doesn’t matter if it’s accessible. Make sure to use a long passphrase you won’t forget! The advice given here is to use a long randomly generated string and write it on a piece of paper, which is stored in a safety deposit box – this is the approach to take if you are in charge of really important stuff, I’m not sure that I’m at that point yet :)

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