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% Version           : $Revision: 1.1 $
% Last-modified     : $Date: 2005/01/04 11:45:32 $
% Last-modifier     : FJ

\documentclass{splib}

\begin{document}
% main environment
\begin{protocol}%
% name
{Amended Needham Schroeder Symmetric Key}%
% authors
{Roger Needham and Michael Schroeder}%
% date of the first publication
{01/1987}%
% editor name
{}%
% date of submission
{}%
% date of last change, empty or = date of submission
{08/11/2002}

\begin{abstract}
This is an amended version of \crossref{nssk}, by the same authors.
Distribution of a shared symmetric key by a trusted server
and mutual authentification.
Symmetric key cryptography with server.
\end{abstract}

\begin{core}
\begin{declarations}
\dcl{A, B, S}{principal}
\dcl{Na, Nb}{number}
\dcl{Kas, Kbs, Kab}{key}
\dcl{dec}{number -> number}
\end{declarations}

\begin{messages}
\msg{1}{A}{B}{A}
\msg{2}{B}{A}{\crypt{A, Nb}{Kbs}}
\msg{3}{A}{S}{A, B, Na, \crypt{A, Nb}{Kbs}}
\msg{4}{S}{A}{\crypt{Na, B, Kab, \crypt{Kab, Nb, A}{Kbs}}{Kas}}
\msg{5}{A}{B}{\crypt{Kab, Nb, A}{Kbs}}
\msg{6}{B}{A}{\crypt{Nb}{Kab}}
\msg{7}{A}{B}{\crypt{dec(Nb)}{Kab}}
\end{messages}
\end{core}

\begin{explanation}
See \crossref{nssk}.
The extra exchange of the nonce \code{Nb} prevents
the Denning Sacco freshness attack described in \crossref{nssk}.
\end{explanation}

\begin{property}
See \crossref{nssk}.
\end{property}

\begin{references}
\cite{NeedhamSchroeder87}.
\end{references}

\begin{see} 
\crossref{nssk},
\crossref{kerberos}.
\end{see}

\end{protocol}
\end{document}

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