318 \item 

   319 The system policy automatically prevents the flow of information from high-level objects to lower levels. A process that reads top secret data becomes tagged as top secret by the operating system, as will be all files into which it writes afterwards. 

   320 %Each user has a maximum allowed confidentiality level specified and cannot receive data beyond that level. A selected set of trusted subjects is allowed to bypass the restrictions, in order to permit the declassification of information.

   321 \end{itemize}

   322 

   323 \end{frame}

   324 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%   

   325 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   326   \mode<presentation>{

   327   \begin{frame}[c]

   328   \frametitle{Bell-LaPadula}

   329   \small

   330   

   331   \begin{itemize}

   332   \item \alert{Read Rule}: A principal \bl{$P$} can read an object \bl{$O$} if and only if

   333   \bl{$P$}'s security level is at least as high as \bl{$O$}'s.

   334   \item \alert{Write Rule}: A principal \bl{$P$} can write an object \bl{$O$} if and only if

   335   \bl{$O$}'s security level is at least as high as \bl{$P$}'s.\medskip

   336 

   337   \item Meta-Rule: All principals in a system should have a sufficiently high security level

   338   in order to access an object.

   339   \end{itemize}\bigskip

   340 

   341   This restricts information flow $\Rightarrow$ military\bigskip\bigskip\pause

   342 

   343   Bell-LaPadula: {\bf `no read up'} - {\bf `no write down'}

   344 

   345   \end{frame}}

   346   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   347 %

   348 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   349   \mode<presentation>{

   350   \begin{frame}[c]

   351   \frametitle{\begin{tabular}{c}Principle of\\[-2mm] Least Privilege\end{tabular}}

   352   

   353   \begin{tikzpicture}

   354   \draw (0,0) node[inner sep=2mm,fill=cream, ultra thick, draw=red, rounded corners=2mm] 

   355   {\normalsize\color{darkgray}

   356   \begin{minipage}{10cm}\raggedright

   357   A principal should have as few privileges as possible to access a resource.

   358   \end{minipage}};

   359   \end{tikzpicture}\bigskip\bigskip

   360   \small

   361 

   362   \begin{itemize}

   363   \item Bob ($T\!S$) and Alice ($S$) want to communicate

   364   \item[] $\Rightarrow$ Bob should lower his security level

   365   \end{itemize}

   366 

   367   \end{frame}}

   368   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   369 %

   370 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   371   \mode<presentation>{

   372   \begin{frame}[c]

   373   \frametitle{Biba Policy}

   374   \small

   375   

   376   Data Integrity (rather than data confidentiality)

   377 

   378   \begin{itemize}

   379   \item Biba: {\bf `no read down'} - {\bf `no write up'}

   380   \item \alert{Read Rule}: A principal \bl{$P$} can read an object \bl{$O$} if and only if

   381   \bl{$P$}'s security level is lower or equal than \bl{$O$}'s.

   382   \item \alert{Write Rule}: A principal \bl{$P$} can write an object \bl{$O$} if and only if

   383   \bl{$O$}'s security level is lower or equal than \bl{$P$}'s.

   384   \end{itemize}\bigskip\bigskip\pause

   385 

   386   E.g.~Generals write orders to officers; officers write oders to solidiers\\

   387   Firewall: you can read from inside the firewall, but not from outside\\

   388   Phishing: you can look at an approved PDF, but not one from a random email\\

   389 

   390   \end{frame}}

   391   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   392 %

   393 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   394 \mode<presentation>{

   395 \begin{frame}[c]

   396 \frametitle{Shared Access Control}

   397 

   398 \begin{center}

   399 \includegraphics[scale=0.7]{pics/pointsplane.jpg}

   400 \end{center}

   401 

   402 \begin{textblock}{11}(10.5,10.5)

   403 \small

   404 To take an action you\\[-1mm] 

   405 need at least either:

   406 \begin{itemize}

   407 \item 1 CEO\\[-5mm]

   408 \item 2 MDs, or\\[-5mm]

   409 \item 3 Ds

   410 \end{itemize}

   411 \end{textblock}

   412 

   413 \end{frame}}

   414 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%   

   415 

   416 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   417 \mode<presentation>{

   418 \begin{frame}[c]

   419 \frametitle{Lessons from Access Control}

   420 

   421 Not just restricted to Unix:

   422 

   423 \begin{itemize}

   424 \item if you have too many roles (i.e.~too finegrained AC), then 

   425 	hierarchy is too complex\\

   426 	\textcolor{gray}{you invite situations like\ldots let's be root}\bigskip

   427 

   428 \item you can still abuse the system\ldots

   429 

   430 \end{itemize}

   431 

   432 \end{frame}}

   433 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%   

   434 

   435  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   436   \mode<presentation>{

   437   \begin{frame}[t]

   438   \frametitle{Protocols}

   439  

   440 \mbox{} 

   441   

   442 \begin{tabular}{l}

   443 {\Large \bl{$A\;\text{sends}\; B : \ldots$}}\\

   444 \onslide<2->{\Large \bl{$B\;\text{sends}\; A : \ldots$}}\\

   445 \onslide<2->{\Large \;\;\;\;\;\bl{$:$}}\bigskip

   446 \end{tabular}  

   447   

   448  \begin{itemize}

   449  \item by convention \bl{$A$}, \bl{$B$} are named principals \bl{Alice\ldots}\\

   450  but most likely they are programs, which just follow some instructions (they are more like roles)\bigskip

   451 \item<2-> indicates one ``protocol run'', or session,  which specifies some 

   452 order in the communication

   453 \item<2-> there can be several sessions in parallel (think of wifi routers) 

   454 \end{itemize} 

   455   

   456   \end{frame}}

   457   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%   

   458   

   459 

   460 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   461 \mode<presentation>{

   462 \begin{frame}[c]

   463 \frametitle{A mutual authentication protocol}

   464 

   465 \begin{center}

   466 \begin{tabular}{ll}

   467 \bl{$A \rightarrow B$:} & \bl{$N_a$}\\  

   468 \bl{$B \rightarrow A$:} & \bl{$\{N_a, N_b\}_{K_{ab}}$}\\

   469 \bl{$A \rightarrow B$:} & \bl{$N_b$}\\

   470 \end{tabular}

   471 \end{center}

   472 

   473 Explain how an attacker \bl{$B'$} can launch an impersonation attack by 

   474 intercepting all messages for \bl{$B$} and make \bl{$A$} decrypt her own challenges.

   475 

   479 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   480 \mode<presentation>{

   481 \begin{frame}[c]

   482 \frametitle{Nonces}

   483 

   484 \begin{enumerate}

   485 \item I generate a nonce (random number) and send it to you encrypted with a key we share

   486 \item you increase it by one, encrypt it under a key I know and send

   487 it back to me

   488 \end{enumerate}

   489 

   490 

   491 I can infer:

   492 

   493 \begin{itemize}

   494 \item you must have received my message

   495 \item you could only have generated your answer after I send you my initial

   496 message

   497 \item if only you and me know the key, the message must have come from you

   498 \end{itemize}

   499 

   500 \end{frame}}

   501 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%   

   502 

   503 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   504 \mode<presentation>{

   505 \begin{frame}[c]

   506 

   507 \begin{center}

   508 \begin{tabular}{l}

   509 \bl{$A \rightarrow S :$} \bl{$A, B, N_A$}\\

   510 \bl{$S \rightarrow A :$} \bl{$\{N_A, B, K_{AB},\{K_{AB}, A\}_{K_{BS}} \}_{K_{AS}}$}\\

   511 \bl{$A \rightarrow B :$} \bl{$\{K_{AB}, A\}_{K_{BS}} $}\\

   512 \bl{$B \rightarrow A :$} \bl{$\{N_B\}_{K_{AB}}$}\\

   513 \bl{$A \rightarrow B :$} \bl{$\{N_B-1\}_{K_{AB}}$}\pause\\

   514 \hspace{5cm}compromise \bl{$K_{AB}$}\pause\\

   515 \bl{$A \rightarrow S :$} \bl{$A, B, N'_A$}\\

   516 \bl{$S \rightarrow A :$} \bl{$\{N'_A, B, K'_{AB},\{K'_{AB}, A\}_{K_{BS}} \}_{K_{AS}}$}\pause\\

   517 \bl{$I(A) \rightarrow B :$} \bl{$\{K_{AB}, A\}_{K_{BS}} $}\hspace{0.5cm} replay of older run\pause\\

   518 \bl{$B \rightarrow I(A) :$} \bl{$\{N'_B\}_{K_{AB}}$}\\

   519 \bl{$I(A) \rightarrow B :$} \bl{$\{N'_B-1\}_{K_{AB}}$}\

   520 \end{tabular}

   521 \end{center}\pause

   522 

   523 \bl{$B$} believes it is following the correct protocol,

   524 intruder \bl{$I$} can form the correct response because it knows \bl{$K_{AB}$} and

   525 talks to \bl{$B$} masquerading as \bl{$A$}

   526 \end{frame}}

   527 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%