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It’s pretty straightforward to use PGP keys on a YubiKey in a Chromebook for SSH:
Acquire a YubiKey and create the necessary PGP keys. Here’s an example procedure.
Go to the Chrome Web Store and install the Smart Card Connector app from Google.
Go to the Chrome Web Store and install the Secure Shell Extension
Launch the Smart Card Connector app and verify your YubiKey shows up in the “Smart Card readers” list.
Launch the Secure Shell app. You should immediately see a prompt that says “The app ‘Secure Shell App’ is trying to acces the Smart Card Connector”. When you do, click “Allow”.
In the Secure Shell app configure the connection to your target
machine as you normally would, with one addition: set “SSH relay
server options” to --ssh-agent=gsc
Start the connection. You’ll be prompted to “Enter PIN to unlock key”; this is the PIN you set up previously for the OpenPGP app on your YubiKey.
If you changed the setting to require a physical press of your YubiKey, tap the YubiKey.
You should be logged in to your target machine.