問93
頻出度★★★
公開鍵暗号方式を利用した処理と，その処理に使用する公開鍵の組合せ a～c のうち，適切なものだけを全て挙げたものはどれか。
| 処理 | 使用する公開鍵 | |
|---|---|---|
| a | 作成した電子メールに対するディジタル署名の付与 | 電子メール作成者の公開鍵 |
| b | 受信した電子メールに付与されているディジタル署名の検証 | 電子メール作成者の公開鍵 |
| c | 使用しているブラウザからWebサーバへの暗号化通信 | Webサーバの公開鍵 |
平成30年度 秋期 ITパスポート試験 問93
This question's explanation and your answers so far.
解説
問 93
Incorrectア
It is wrong because it includes a. Attaching a digital signature (creating it) is a process the creator performs with their own private key, using the creator's private key rather than a public key. In the table, a states that the 'public key of the email creator' is used, which is inappropriate, so ア, which includes a, is wrong.
Incorrectイ
This is a combination of a and c, but even though c is correct, a is inappropriate. Attaching a signature is a process done with the creator's private key, so a, which uses a public key, is wrong. The items that work correctly are b and c, so イ is not an appropriate combination.
Incorrectウ
b alone is not enough. b is indeed appropriate (verifying the signature with the creator's public key), but c is also appropriate (encrypted communication with the Web server's public key). Because the question asks to list all and only the appropriate items, ウ, which omits c, is incomplete and wrong.
- エ
正解 In public-key cryptography, the basic relationship is that data encrypted with a public key can only be decrypted with the paired private key, and data encrypted (signed) with a private key can only be verified with the paired public key. The appropriate items are b and c, so the correct answer is エ, 'b, c'. For b, verifying a received digital signature: because the signature is created by the sender (the email creator) with their own private key, the receiver verifies it with that creator's public key. So the key used, the 'public key of the email creator', is correct. For c, when encrypting data sent from the browser to the Web server, it is encrypted with the Web server's public key so that only the Web server, which holds the corresponding private key, can decrypt it. So the 'public key of the Web server' is correct.
ここがポイント
- Basics of public-key cryptography: encrypt with the other party's public key → the other party decrypts with their private key (encrypted communication); the sender signs with their private key → the receiver verifies with the sender's public key (digital signature).
- Do not confuse which key is used: 'attaching a signature' uses the private key, while 'verifying a signature' uses the other party's public key.
用語