With this extension to the initramfs-tools, you can unlock a LUKS encrypted disk using your YubiKey as a second factor.

The challenge-response mechanism of the YubiKey is used to generate a response based on a PIN/password you have to enter.

Only the combination of the correct password and the matching YubiKey will generate a response, that is a valid key of the LUKS disk. Alternatively you can use any other LUKS passphrase when the YubiKey is not present.