Everyone in a group of N people wants to communicate secretly with the N − 1 others using a symmetric key system, with no pair's traffic decodable by the others. The number of keys required in the system as a whole is

1. AN(N − 1)
2. BN(N − 1)/2
3. C2N
4. D(N − 1)²

Pick an option to check yourself, or open the answer below.

Show answer & explanation

Correct answer: B. N(N − 1)/2

Explanation

Each pair needs its own shared key: C(N, 2) = N(N − 1)/2.