We use challenge/response devices from the Internet: \begin{figure} \begin{quote} \logsize \begin{verbatim} $ telnet guard.research.att.com Trying... Connected to guard.research.att.com. Escape character is '^]'. This is the new inet. Authorized use only. Authentication Server. Id? ches challenge: 348201 response: d2c3f97d TCP host name? cetus rlogin cetus -l ches IRIX Release 4.0.5C System V cetus.research.att.com Copyright 1987-1992 Silicon Graphics, Inc. All Rights Reserved. cetus=; exit Connection closed.Connection closed by foreign host. $ \end{verbatim} \end{quote} \caption{\label{fig:hha-connect} The full text of an actual terminal session using our challenge/response-based guard.} \end{figure}