問97
頻出度★★★
公開鍵暗号方式では, 暗号化のための鍵と復号のための鍵が必要となる。4人が相互に通信内容を暗号化して送りたい場合は, 全部で8個の鍵が必要である。このうち, 非公開にする鍵は何個か。
令和2年度 ITパスポート試験 問97
This question's explanation and your answers so far.
解説
問 97
Incorrectア
“1” is wrong. The private key is not a single key shared by everyone; each person holds their own, one per person. With 4 people, 4 private keys must be kept private, so 1 is not enough.
Incorrectイ
“2” is wrong. This number comes from confusing it with the number of keys in one person's pair (public 1 + private 1 = 2), or from mistakenly thinking that only some people hold a private key. What must be kept private are the private keys of all 4 people, so the answer is 4.
- ウ
正解 In public-key cryptography, each person holds one pair consisting of a public key (for encryption) and a private key (for decryption). For 4 people to communicate with one another using encryption, 4 people × 2 = 8 keys are needed, which matches the “total of 8” stated in the question. Of these, the key that is never handed to others and is kept secret by its owner alone is the private key used for decryption, and there is 1 per person. Therefore the number of keys to keep private is 4 people × 1 = 4, so the answer is ウ (4). The public key may be disclosed to anyone, so only the private key needs to be kept private — that is the key point.
Incorrectエ
“6” is wrong. This equals the number of keys in symmetric-key (shared-key) cryptography when 4 people are paired two at a time, 4 × 3 ÷ 2 = 6; it is not the number of private keys in public-key cryptography. In public-key cryptography each person has 1 private key, so the answer is 4.
ここがポイント
- In public-key cryptography, each user holds one pair of a public key (for encryption, may be disclosed to anyone) and a private key (for decryption, kept secret by the owner alone). Only the private key must be kept private.
- When n people communicate with one another using encryption, 2n keys are needed (n public + n private), and n private keys must be kept private. In this question n = 4, so there are 4 private keys.
用語