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% beamer stuff 

\newcommand{\bl}[1]{\textcolor{blue}{#1}}  

\renewcommand{\slidecaption}{APP 05, King's College London}

\begin{document}

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\begin{frame}[t]

\frametitle{%

  \begin{tabular}{@ {}c@ {}}

  \\

  \LARGE Access Control and \\[-3mm] 

  \LARGE Privacy Policies (5)\\[-6mm] 

  \end{tabular}}\bigskip\bigskip\bigskip

  \normalsize

  \begin{center}

  \begin{tabular}{ll}

  Email:  & christian.urban at kcl.ac.uk\\

  Office: & S1.27 (1st floor Strand Building)\\

  Slides: & KEATS (also homework is there)\\

  \end{tabular}

  \end{center}

\end{frame}

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\begin{frame}[c]

\frametitle{Protocols}

Some examples where ``over-the-air'' protocols are used:

\begin{itemize}

\item wifi

\item card readers (you cannot trust the terminals)

\item RFID (passports)

\item car transponders

\end{itemize}\medskip\pause

\end{frame}

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\begin{frame}[c]

\frametitle{Keyless Car Transponders}

\begin{center}

\includegraphics[scale=0.1]{../pics/keyfob.jpg}

\quad

\includegraphics[scale=0.27]{../pics/startstop.jpg}

\end{center}

\begin{itemize}

\item There are two security mechanisms: one remote central 

locking system and one passive RFID tag (engine immobiliser).

\item How can I get in? How can thieves be kept out? 

How to avoid MITM attacks?

\end{itemize}\medskip

\footnotesize

\hfill Papers: Gone in 360 Seconds: Hijacking with Hitag2,\\

\hfill Dismantling Megamos Crypto: Wirelessly Lockpicking\\

\hfill a Vehicle Immobilizer

\end{frame}

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\begin{frame}[c]

\frametitle{HTTPS / GSM}

\begin{center}

\includegraphics[scale=0.25]{../pics/barclays.jpg}

\quad

\includegraphics[scale=0.25]{../pics/phone-signal.jpg}

\end{center}

\begin{itemize}

\end{itemize}

\end{frame}

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\begin{frame}[c]

\frametitle{Handshakes}

\begin{itemize}

\item starting a TCP connection between a client and a server

initiates the following three-way handshake protocol:

\end{itemize}

\begin{columns}[t]

\begin{column}{5cm}

\begin{minipage}[t]{4cm}

\begin{center}

\raisebox{-2cm}{\includegraphics[scale=0.5]{../pics/handshake.png}}

\end{center}

\end{minipage}

\end{column}

\begin{column}{5cm}

\begin{tabular}[t]{rl}

Alice:  & Hello server!\\

Server: & I heard you\\

Alice:  & Thanks

\end{tabular}

\end{column}

\begin{center}

\begin{tabular}{rl}

\bl{$A \rightarrow S$}: & \bl{SYN}\\

\bl{$S \rightarrow A$}: & \bl{SYN-ACK}\\

\bl{$A \rightarrow S$}: & \bl{ACK}\\

\end{tabular}

\begin{textblock}{3}(11,5)

\begin{bubble}[3.2cm]

SYNflood attacks:\medskip\\

\includegraphics[scale=0.4]{../pics/synflood.png}

\end{bubble}

\end{textblock}}

\end{frame}

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\begin{frame}[c]

\frametitle{Authentication}

\begin{columns} 

\begin{column}{8cm}

\begin{minipage}[t]{7.5cm}

\begin{center}

\raisebox{-2cm}{\includegraphics[scale=0.4]{../pics/dogs.jpg}}

\end{center}

\end{minipage}

\end{column}

\begin{column}{5cm}

\begin{minipage}[t]{4.5cm}

\begin{tabular}{l}

Knock Knock!\\

Who's there?\\

Alice.\\

Alice who?

\end{tabular}

\end{minipage}

\end{column}

\end{columns}

\end{frame}

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\begin{frame}[c]

\frametitle{Authentication Protocols}

Alice (\bl{$A$}) and Bob (\bl{$B$}) share a secret key

\bl{$K_{AB}$}\bigskip

\begin{center}

\bl{$A \rightarrow B: K_{AB}$} 

\end{center}\pause\bigskip

Problems: Eavesdropper can capture the secret and replay it;

\bl{$B$} cannot confirm the identity of \bl{$A$}  

\end{frame}

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\begin{frame}[c]

\frametitle{Authentication Protocols}

Alice (\bl{$A$}) and Bob (\bl{$B$}) share a secret key

\bl{$K_{AB}$}\bigskip

\begin{center}

\begin{tabular}{lll}

\bl{$A \rightarrow B:$} & \bl{Hi I am A}\\

\bl{$B \rightarrow A:$} & \bl{$N$} & (challenge)\\

\bl{$A \rightarrow B:$} & \bl{$\{N\}_{K_{AB}}$}\\

\end{tabular} 

\end{center}\pause

\begin{itemize}

\end{itemize}

\end{frame}

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\begin{frame}[c]

\frametitle{Authentication Protocols}

Alice (\bl{$A$}) and Bob (\bl{$B$}) share a secret key \bl{$K_{AB}$}\bigskip

Mutual Challenge Response:

\begin{center}

\begin{tabular}{ll}

\bl{$A \rightarrow B:$} & \bl{$N_A$}\\

\bl{$B \rightarrow A:$} & \bl{$\{N_A, N_B\}_{K_{AB}}$}\\

\bl{$A \rightarrow B:$} & \bl{$N_B$}\\

\end{tabular} 

\end{frame}

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\begin{frame}[c]

\frametitle{Nonces}

\begin{enumerate}

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

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

it back to me

\end{enumerate}

I can infer:

\begin{itemize}

\item you must have received my message

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

message

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

\end{itemize}

\end{frame}

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\mode<presentation>{

\begin{frame}[c]

\begin{center}

\begin{tabular}{ll}

\bl{$A \rightarrow B$:} & \bl{$N_A$}\\  

\bl{$B \rightarrow A$:} & \bl{$\{N_A, N_B\}_{K_{ab}}$}\\

\bl{$A \rightarrow B$:} & \bl{$N_B$}\\

\end{tabular}

\end{center}

The attack (let $A$ decrypt her own messages):

\begin{center}

\begin{tabular}{ll}

\bl{$A \rightarrow E$:} & \bl{$N_A$}\\ 

\textcolor{gray}{$E \rightarrow A$:} & \textcolor{gray}{$N_A$}\\ 

\textcolor{gray}{$A \rightarrow E$:} & \textcolor{gray}{$\{N_A, N_A'\}_{K_{AB}}$}\\

\bl{$E \rightarrow A$:} & \bl{$\{N_A, N_A'\}_{K_{AB}}$}\\

\bl{$A \rightarrow E$:} & \bl{$N_A' \;\;(= N_B)$}\\

\end{tabular}

\end{center}\pause

\small Solutions: \bl{$K_{AB} \not= K_{BA}$} or include an id in the second message

\end{frame}}

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  \mode<presentation>{

  \begin{frame}[c]

  \frametitle{Encryption to the Rescue?}

 \begin{itemize}

 \item \bl{$A \,\rightarrow\, B :  \{A, N_A\}_{K_{AB}}$}\hspace{1cm} encrypted\bigskip 

 \item \bl{$B\,\rightarrow\, A : \{N_A, K'_{AB}\}_{K_{AB}}$}\bigskip

 \item \bl{$A \,\rightarrow\, B : \{N_A\}_{K'_{AB}}$}\bigskip

 \end{itemize}\pause

means you need to send separate ``Hello'' signals (bad), or worse 

share a single key between many entities

\end{frame}}

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\mode<presentation>{

\begin{frame}[c]

\frametitle{Trusted Third Party}

Simple protocol for establishing a secure connection via a mutually

trusted 3rd party (server):

\begin{center}

\begin{tabular}{r@ {\hspace{1mm}}l}

\bl{$A \rightarrow S :$} & \bl{$A, B$}\\

\bl{$S \rightarrow A :$} & \bl{$\{K_{AB}\}_{K_{AS}}$} and \bl{$\{\{K_{AB}\}_{K_{BS}} \}_{K_{AS}}$}\\

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

\bl{$A \rightarrow B :$} & \bl{$\{m\}_{K_{AB}}$}\\

\end{tabular}

\end{center}

\end{frame}}

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\mode<presentation>{

\begin{frame}[c]

\frametitle{Public-Key Infrastructure}

\begin{itemize}

\item the idea is to have a certificate authority (CA)

\item you go to the CA to identify yourself

\item CA: ``I, the CA, have verified that public key 

  \bl{$P^{pub}_{Bob}$} belongs to Bob''\bigskip

\item CA must be trusted by everybody

\item What happens if CA issues a false certificate? Who pays in case of loss? (VeriSign 

explicitly limits liability to \$100.)

\end{itemize}

\end{frame}}

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\begin{frame}[c]

\frametitle{Person-in-the-Middle}

``Normal'' protocol run:\bigskip

\begin{itemize}

\item \bl{$A$} sends public key  to \bl{$B$}

\item \bl{$B$} sends public key  to \bl{$A$}

\item \bl{$A$} sends message encrypted with \bl{$B$}'s public key, \bl{$B$} decrypts it

with its private key

\item \bl{$B$} sends message encrypted with \bl{$A$}'s public key, \bl{$A$} decrypts it

with its private key

\end{itemize}

\end{frame}

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\mode<presentation>{

\begin{frame}[c]

\frametitle{Person-in-the-Middle}

Attack:

\begin{itemize}

\item \bl{$A$} sends message encrypted with \bl{$C$}'s public key, \bl{$C$} decrypts it

with its private key, re-encrypts with \bl{$B$}'s public key 

\end{itemize}

\end{frame}}

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\begin{frame}[c]

\frametitle{Person-in-the-Middle}

Prevention:

\begin{itemize}

\item \bl{$A$} sends public key  to \bl{$B$}

\item \bl{$B$} sends public key  to \bl{$A$}

\end{itemize}\pause

\bl{$C$} would have to invent a totally new message

\end{frame}

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\begin{frame}[c]

\frametitle{Car Transponder (HiTag2)}

\begin{enumerate}

\item \bl{$C$} generates a random number \bl{$r$}

\item \bl{$C$} calculates \bl{$(F,G) = \{r\}_K$}

\item \bl{$C \to T$}: \bl{$r, F$}

\item \bl{$T$} calculates \bl{$(F',G') = \{r\}_K$}

\item \bl{$T$} checks that \bl{$F = F'$}

\item \bl{$T \to C$}: \bl{$r, G'$}

\item \bl{$C$} checks that \bl{$G = G'$}

\end{enumerate}\pause

\small

This process means that the transponder believes the car knows

the key \bl{$K$}, and the car believes the transponder knows

to each other.

\end{frame}

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\begin{frame}[c]

\frametitle{Person-in-the-Middle}

\begin{itemize}

\item Border Gateway Protocol (BGP) --- routers believe 

  their neighbours

\item it is possible to advertise bad routes

\item can be done over continents\bigskip 

\end{itemize}

\hfill\footnotesize\url{http://www.renesys.com/2013/11/mitm-internet-hijacking/}

\end{frame}

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\mode<presentation>{

\begin{frame}[c]

\frametitle{Best Practices}

{\bf Principle 1:} Every message should say what it means: the interpretation of 

a message should not depend on the context.\bigskip\pause

{\bf Principle 2:} If the identity of a principal is essential to the meaning of a message, it is prudent 

to mention the principal’s name explicitly in the message (though difficult).\bigskip