問87
頻出度★★★
公開鍵暗号方式を用いた機密データの受渡しに関する記述中の a, b に入れる字句の適切な組合せはどれか。 データの受信者Aは, 自分の [ a ] 鍵と [ b ] 鍵を用意して [ a ] 鍵を送信者Bに送付する。データの送信者Bは, 受信者Aから送付された [ a ] 鍵を用いてデータを暗号化して受信者Aに送信する。受信者Aは, 送信者Bから受信した暗号化されたデータを自分の [ b ] 鍵を使って復号する。
平成28年度 春期 ITパスポート試験 問87
This question's explanation and your answers so far.
解説
問 87
Incorrectア
a = public is correct, but b = common is wrong. A common key is a term from common-key (symmetric) cryptography, in which the sender and receiver share the same single key; it does not appear in public-key cryptography. What A uses for decryption is the private key that pairs with the public key.
- イ
正解 In public-key cryptography, receiver A prepares a pair consisting of a public key and a private key. Of these, the one that may be handed out to others is the public key, so A sends the public key to sender B (a = public). B encrypts the data with A's public key that it received and sends it back. Data encrypted with a public key can only be decrypted with A's matching private key, so A decrypts it with the private key kept in his/her own hands (b = private). Therefore the correct combination is a = public, b = private, which is choice イ.
Incorrectウ
a and b are swapped. A private key must be kept strictly by its owner alone and must never be handed to anyone else, so treating a, the key sent to B, as the private key is wrong; the key to send is the public key. Also, treating b (the key used for decryption) as the public key is wrong, because a public key cannot decrypt; the private key is required.
Incorrectエ
a = private is wrong. What is sent to B is the public key, which may be made public; the private key must not be sent. Furthermore, b = common also does not fit the framework of public-key cryptography (a common key is a term from symmetric cryptography).
ここがポイント
- In public-key cryptography, the receiver holds a pair of a public key and a private key, distributes the public key to the other party for encryption, and decrypts with his/her own matching private key.
- Data encrypted with a public key can only be decrypted with the corresponding private key (the key-pair relationship). A common key is a term from symmetric cryptography, in which the sender and receiver share the same single key, and it does not apply to this question.
用語