\begin{thebibliography}{1}

\bibitem{rfc2543}
M.~Handley, H.~Schulzrinne, E.~Schooler, and J.~Rosenberg, ``{SIP:} session
  initiation protocol,'' Request for Comments 2543, Internet Engineering Task
  Force, Mar. 1999.

\bibitem{rfc2828}
R.~Shirey, ``Internet security glossary,'' Request for Comments 2828, Internet
  Engineering Task Force, May 2000.

\bibitem{Schn96:Applied}
B.~Schneier, {\em Applied Cryptography}.
\newblock New York: Wiley, 1996.

\bibitem{draft-undery-sip-digest}
J.~Undery, ``Sip authentication: Sip digest access authentication,'' {Internet
  Draft}, Internet Engineering Task Force, July 2000.
\newblock Work in progress.

\bibitem{rfc2633}
B.~Ramsdell and Ed, ``{S/MIME} version 3 message specification,'' Request for
  Comments 2633, Internet Engineering Task Force, June 1999.

\bibitem{rfc2630}
R.~Housley, ``Cryptographic message syntax,'' Request for Comments 2630,
  Internet Engineering Task Force, June 1999.

\bibitem{rfc2104}
H.~Krawczyk, M.~Bellare, and R.~Canetti, ``{HMAC:} keyed-hashing for message
  authentication,'' Request for Comments 2104, Internet Engineering Task Force,
  Feb. 1997.

\end{thebibliography}
