問60
頻出度★★★
公開鍵暗号方式で使用する鍵に関する次の記述中の a, b に入れる字句の適切な組合せはどれか。 それぞれ公開鍵と秘密鍵をもつ A 社と B 社で情報を送受信するとき, 他者に通信を傍受されても内容を知られないように, 情報を暗号化して送信することにした。 A 社から B 社に情報を送信する場合, A 社は [ a ] を使って暗号化した情報を B 社に送信する。B 社は A 社から受信した情報を [ b ] で復号して情報を取り出す。
令和4年度 ITパスポート試験 問60
This question's explanation and your answers so far.
解説
問 60
Incorrectア
Both a and b are “Company A’s public key.” The recipient is Company B, yet Company A’s key is used, which is wrong. In addition, information encrypted with a public key cannot be decrypted with the same public key (encryption and decryption are done with the paired public key and private key), so Company B cannot retrieve the information.
Incorrectイ
If a encrypts with “Company A’s public key,” it can only be decrypted with the paired Company A’s private key. The b of “Company B’s private key” does not match that key pair and cannot decrypt. This also violates the principle that encryption should use the recipient Company B’s key.
Incorrectウ
The a of “Company B’s public key” is correct, but b is “Company A’s public key,” which is wrong. Information encrypted with Company B’s public key can only be decrypted with the paired Company B’s private key. A public key cannot be used to decrypt, and the key’s owner is not Company A but Company B.
- エ
正解 When protecting the confidentiality of communication content with public-key cryptography, the sender encrypts with “the recipient’s public key” and the recipient decrypts with “their own private key.” Here Company A sends to Company B, so the recipient is Company B. Therefore a is “Company B’s public key” and b is “Company B’s private key,” making the combination エ. Information encrypted with Company B’s public key can only be decrypted with the paired Company B’s private key, and only Company B holds that private key. A third party who intercepts the communication has no private key and so cannot learn the content.
ここがポイント
- In communication that protects confidentiality with public-key cryptography, the rule is “encryption = recipient’s public key, decryption = recipient’s private key.” Choose the key based on who you are sending to (the recipient).
- A public key and a private key form a pair; information encrypted with one can only be decrypted with the other. A private key is held only by its owner, so the content stays protected even if it is intercepted.
用語