Authenticate using SSH keys

 This pluggable authentication module (PAM) provides single sign-on
 using secure shell (SSH) keys:
 - during authentication, the user types a SSH passphrase and is authenticated
   if the passphrase successfully decrypts the user's SSH private keys;
 - in session phase, a ssh-agent process is started and decrypted keys are
   added, and thus the user can SSH to other hosts that accept key
   authentication without typing more passwords for the entire session.