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Suppose that everyone in a group on $N$ people wants to communicate secretly with
the $(\text{N - 1})$ others using symmetric Key cryptographic system.
The communication between any two person should not be decodable by the others
in the group. The numbers of keys required in the system as a whole to satisfy
the confidentiality requirement is

1. $2N$
2. $N(N-1)$
3. $\dfrac{N(N-1)}{2}$
4. $(N-1)^{2}$