pam_usb is a PAM module which allows authentication through a USB storage device such as USB memory sticks/key/whatever you want to call them. It used DSA private/public keys for authentication. Once your USB storage device is setup and your pam.d directory is updated, you just plug in your usb storage device when you login, and the module checks against the public key on the host and your private key on the USB storage device. Nice. 🙂