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6 \begin{document} |
4 \begin{document} |
7 |
5 |
8 \section*{Homework 5} |
6 \section*{Homework 5} |
9 |
7 |
10 \begin{enumerate} |
8 \begin{enumerate} |
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9 \item What can attacker that controls the network do to a communication |
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10 between a client and a server? |
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11 |
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12 \item Before starting a TCP connection, client and servers |
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13 perform a three-way handshake. Describe how can this three-way |
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14 handshake can be abused by an attacker? |
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15 |
11 \item Consider the following simple mutual authentication protocol: |
16 \item Consider the following simple mutual authentication protocol: |
12 |
17 |
13 \begin{center} |
18 \begin{center} |
14 \begin{tabular}{ll} |
19 \begin{tabular}{ll} |
15 $A \rightarrow B$: & $N_a$\\ |
20 $A \to B$: & $N_a$\\ |
16 $B \rightarrow A$: & $\{N_a, N_b\}_{K_{ab}}$\\ |
21 $B \to A$: & $\{N_a, N_b\}_{K_{ab}}$\\ |
17 $A \rightarrow B$: & $N_b$\\ |
22 $A \to B$: & $N_b$\\ |
18 \end{tabular} |
23 \end{tabular} |
19 \end{center} |
24 \end{center} |
20 |
25 |
21 Explain how an attacker $B'$ can launch an impersonation attack by |
26 Explain how an attacker $B'$ can launch an impersonation attack by |
22 intercepting all messages for $B$ and make $A$ decrypt her own challenges. |
27 intercepting all messages for $B$ and make $A$ decrypt her own challenges. |
23 |
28 |
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29 \item What is the main problem with the following |
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30 authentication protocol where $A$ sends $B$ mutually |
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31 shared key? |
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32 |
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33 \begin{center} |
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34 $A \to B: K_{AB}$ |
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35 \end{center} |
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36 |
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37 \item Nonces are unpredicatble random numbers used in protocols? |
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38 Consider the following protocol |
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39 |
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40 \begin{center} |
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41 \begin{tabular}{ll} |
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42 $A \to B$: & $N$\\ |
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43 $B \to A$: & $\{N + 1\}_{K_{ab}}$\\ |
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44 \end{tabular} |
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45 \end{center} |
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46 |
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47 Write down three facts that $A$ can infer after this protocol has been |
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48 successfully completed? |
24 |
49 |
25 \item Before starting a TCP connection, client and servers |
50 \item Before starting a TCP connection, client and servers |
26 perform a three-way handshake: |
51 perform a three-way handshake: |
27 |
52 |
28 \begin{center} |
53 \begin{center} |
33 \end{tabular} |
58 \end{tabular} |
34 \end{center} |
59 \end{center} |
35 |
60 |
36 How can this protocol be abused causing trouble on the server? |
61 How can this protocol be abused causing trouble on the server? |
37 |
62 |
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63 \item Write down a protocol which establishes a secret key |
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64 between $A$ and $B$ using a mutually trusted third party $S$. |
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65 You can assume $A$ and $S$, respectfully $B$ and $S$, share |
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66 secret keys. |
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67 |
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68 \item Consider the following protocol between a car and a |
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69 key transponder: |
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70 |
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71 \begin{enumerate} |
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72 \item $C$ generates a random number $r$ |
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73 \item $C$ calculates $(F,G) = \{r\}_K$ |
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74 \item $C \to T$: $r, F$ |
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75 \item $T$ calculates $(F',G') = \{r\}_K$ |
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76 \item $T$ checks that $F = F'$ |
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77 \item $T \to C$: $r, G'$ |
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78 \item $C$ checks that $G = G'$ |
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79 \end{enumerate} |
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80 |
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81 In Step 2 and 4 a message is split into two halves. Explain |
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82 what the purpose of this split is? |
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83 |
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84 |
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85 |
38 \end{enumerate} |
86 \end{enumerate} |
39 \end{document} |
87 \end{document} |
40 |
88 |
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