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\fbox{ \parbox{5.2in}{ {\bf NOTA}: Obviamente, en este cap\'{\i}tulo no hablaremos del tratamiento de ficheros (creaci\'on, borrado, modificaci\'on, jerarqu\'{\i}a de directorios\ldots), sino de temas referentes a la seguridad de los archivos y el sistema de ficheros. Para informaci\'on sobre la gesti\'on de ficheros se puede consultar cualquier obra que estudie Unix desde una perspectiva general, como \cite{kn:tho82}, \cite{kn:chr94} o \cite{kn:man91}. Para un conocimiento m\'as profundo sobre los ficheros y los sistemas de archivos se puede consultar \cite{kn:tan91}, \cite{kn:bac86} ({\sc bsd}), \cite{kn:goo94} ({\it System V}) o, en el caso de Linux, \cite{kn:car97} o \cite{kn:bec96}. }}% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{section} \stepcounter{section} \stepcounter{section} {\newpage\clearpage \lthtmlfigureA{figure2383}% \begin{figure}\vspace{0.5cm} \setlength{\unitlength}{0.00083300in}% \begingroup\makeatletter\ifx% \reset@font\fontsize{\undefined}{\fipt}% \fontfamily{\endgroup}\fontseries{\begin}\fontshape{picture}% \selectfont (4524,1614)(589,-1363) \thicklines \put(601,-286){\framebox (1200,525){}} \put(2101,-286){\framebox (1200,525){}} \put(3601,-286){\framebox (1200,525){}} \put(4201,-286){\line( 0,-1){375}} \put(4201,-661){\vector( 1, 0){900}} \put(2701,-286){\line( 0,-1){675}} \put(2701,-961){\vector( 1, 0){1725}} \put(1201,-286){\line( 0,-1){975}} \put(1201,-1261){\vector( 1, 0){1725}} \put(751,-136){\makebox(0,0)[lb]{\smash{% \reset@font\fontsize{17}{20.4pt}% \fontfamily{\ttdefault}\fontseries{\mddefault}\fontshape{\updefault}% \selectfont r w x}}} \put(2251,-136){\makebox(0,0)[lb]{\smash{% \reset@font\fontsize{17}{20.4pt}% \fontfamily{\ttdefault}\fontseries{\mddefault}\fontshape{\updefault}% \selectfont r - 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Se trata de un sistema Unix con unos 1300 usuarios, dedicado a c\'alculo cient\'{\i}fico (obviamente, no vamos a decir el nombre del servidor). A pesar de utilizar un mecanismo que no permite que los usuarios elijan claves d\'ebiles, en menos de dos horas de ejecuci\'on sobre un Pentium MMX a 233 MHz el programa Crack corriendo sobre Solaris ha encontrado seis claves de usuario utilizando exclusivamente diccionarios de demostraci\'on que acompa\~nan al programa (seguramente si utiliz\'aramos diccionarios en castellano o relacionados con temas como el deporte o la m\'usica nacionales -- que los hay-- habr\'{\i}amos encontrado alguna clave m\'as\ldots). Se puede pensar que s\'olo seis usuarios de entre 1300 es algo bastante aceptable, pero no es as\'{\i}: cualquier combinaci\'on v\'alida de {\it login} y {\it password} es una puerta abierta en nuestro sistema; si un intruso consigue entrar por esta puerta, tiene m\'as del 70\% del camino recorrido para obtener el control total de la m\'aquina. Si queremos conseguir un sistema m\'{\i}nimamente fiable, no podemos permitir ni una sola clave d\'ebil.\\ Sin embargo, tampoco hay que pensar que programas como {\tt Passwd+} no desempe\~nan bien su labor: en 1994, cuando en el sistema con el que hemos realizado la prueba anterior no dispon\'{\i}a de estos mecanismos de seguridad, en menos de 12 horas de ejecuci\'on de un programa adivinador sobre un 486DX a 33 MHz utilizando Linux, se consiguieron extraer m\'as de cien claves, entre ellas algunas de usuarios con cierto nivel de privilegio dentro del sistema. }}% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{subsubsection} \stepcounter{subsubsection} \stepcounter{subsubsection} \stepcounter{subsubsection} \stepcounter{section} \stepcounter{part} \stepcounter{chapter} \stepcounter{section} \stepcounter{section} \stepcounter{section} \stepcounter{section} \stepcounter{section} \stepcounter{section} \stepcounter{subsection} \stepcounter{subsection} \stepcounter{subsection} 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{\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline24561}% $K_{CT}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline24563}% $N$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline24565}% $K_{C}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{subsection} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline24569}% $C\,\rightarrow\, T:\,\{auth(C)\}_{K_{CT}}\, ,\, \{ticket(C,T)\}_{K_{T}}\, ,\, S\, ,\, N$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline24571}% $T\,\rightarrow\, C:\,\{K_{CS},N\}_{K_{CT}}\, ,\, \{ticket(C,S)\}_{K_{S}}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{subsection} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline24577}% $C\,\rightarrow\, S:\,\{auth(C)\}_{K_{CS}}\, ,\, \{ticket(C,T)\}_{K_{S}}\, ,\, peticion\, ,\,N$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline24579}% $S\,\rightarrow\, C:\,\{N\}_{K_{CS}}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline24723}% \fbox{ \parbox{\textwidth}{ \begin{center} \includegraphics[width=\textwidth]{kerby.png} \end{center} }}% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{section} \stepcounter{part} \stepcounter{chapter} \stepcounter{section} \stepcounter{section} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25343}% $\cal A,K,E,D$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25345}% $\cal A$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25347}% $q$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25349}% ${\cal Z}_{q}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25351}% $q\in{\cal N}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25353}% $\cal K$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25355}% $\cal E:A\rightarrow A$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25357}% ${\cal E}(k,a)=c$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25359}% $k\in{\cal K}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25361}% $a\in{\cal A}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25363}% $c\in{\cal A}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage 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{\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25389}% ${\cal K}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25391}% ${\cal K}^{\prime}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25395}% $\frac{N(N-1)}{2}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{subsection} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25397}% $\cal E$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25399}% $\cal D$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25401}% ${\cal D}(k,{\cal E}(k^{\prime},M))=M$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{section} \stepcounter{section} \stepcounter{subsection} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25409}% ${\cal A}={\cal Z}_{m}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25411}% $m$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25413}% $a$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25415}% $b$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25417}% $f(x)=ax+b$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25419}% $a\neq 0$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25421}% $(a,b)$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25423}% ${\cal Z}_{26}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25425}% $(a..z)$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25427}% $(1,b)$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25429}% $a=1$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25431}% $f(x)=x+b$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25435}% $f(x)$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25439}% $f^{-1}(x)=x-b$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25441}% $b=4$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25443}% $(1,4)$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25445}% $f(x)=x+4$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25451}% $f^{-1}(x)=x-4$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25453}% $f(x)>25$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25455}% $f(x)>m-1$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25463}% $f(x)=26$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25465}% $mod(26,26)=0$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25467}% $b=0$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{subsection} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25469}% $26\times26$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25473}% $k\geq 1$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25475}% $Z_{26}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25477}% $Z_{26}\times Z_{26}\times...\times Z_{26}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25483}% $(l,p)$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25485}% $(p,l)$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{section} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25489}% $K_{1}, K_{2},...,K_{16}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25491}% $K_{16},..., K_{1}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25493}% $K_{1}=K_{16}, K_{2}=K_{15},..., K_{8}=K_{9}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25495}% $2^{16}$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} \stepcounter{section} \stepcounter{subsection} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25497}% $\theta(2n)$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25499}% $p$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25507}% $p\times q$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25509}% $d$% \lthtmlinlinemathZ \lthtmlcheckvsize\clearpage} {\newpage\clearpage \lthtmlinlinemathA{tex2html_wrap_inline25511}% $mcd(d,(p-1)\times(q-1))=1,\: d