ITPEC IP October 2021 Question 45
In public key cryptography, a key for encryption and a key for decryption are required. If four (4) people want to encrypt communication and send it to each other, a total of eight (8) keys are required. Of these, how many of these keys are not made public?
Explanation
4 — each of the four people has one key pair, and only the private key of each pair is kept secret.
In public key cryptography every participant owns a pair: a public key, published so that anyone can encrypt messages to its owner, and a private key, kept by the owner alone to decrypt them. Four people need four pairs, 4 × 2 = 8 keys: the 4 public keys are made public, the 4 private keys are not.
Answer (c)
Why not others:
- •(a) 1 and (b) 2 — fewer secret keys than people would leave someone without a private key, so messages to that person could be decrypted by others
- •(d) 6 — the number of keys for common key (symmetric) cryptography, one secret key per pair of people: 4 × 3 ÷ 2 = 6
Key rule: For n people, public key cryptography needs 2n keys, n of them secret; common key cryptography needs n(n − 1) ÷ 2 keys, all secret.
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