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67 |
67 |
68 \item Consider the following protocol between a car and a |
68 \item Consider the following protocol between a car and a |
69 key transponder: |
69 key transponder: |
70 |
70 |
71 \begin{enumerate} |
71 \begin{enumerate} |
72 \item $C$ generates a random number $r$ |
72 \item $C$ generates a random number $N$ |
73 \item $C$ calculates $(F,G) = \{r\}_K$ |
73 \item $C$ calculates $(F,G) = \{N\}_K$ |
74 \item $C \to T$: $r, F$ |
74 \item $C \to T$: $N, F$ |
75 \item $T$ calculates $(F',G') = \{r\}_K$ |
75 \item $T$ calculates $(F',G') = \{N\}_K$ |
76 \item $T$ checks that $F = F'$ |
76 \item $T$ checks that $F = F'$ |
77 \item $T \to C$: $r, G'$ |
77 \item $T \to C$: $N, G'$ |
78 \item $C$ checks that $G = G'$ |
78 \item $C$ checks that $G = G'$ |
79 \end{enumerate} |
79 \end{enumerate} |
80 |
80 |
81 In Step 2 and 4 a message is split into two halves. Explain |
81 In Step 2 and 4 a message is split into two halves. Explain |
82 what the purpose of this split is? Assume the key $K$ is shared |
82 what the purpose of this split is? Assume the key $K$ is shared |