IPA IP 2020 Question 97
A question IPA Japan published in 令和2年度 10月 ITパスポート試験 公開問題 問97, translated into English. ITPEC is based on the same exams.
Same question on ITPEC: Nov 2021 Q45
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.
Security · AI-written, may contain errors
This question comes from the past questions IPA Japan publishes, translated into English by ITPEC Practice. ITPEC Practice is an independent study tool and is not affiliated with IPA or ITPEC. See the original paper (PDF) or Report an issue.