This question's explanation and your answers so far.
ハイブリッド暗号方式を用いてメッセージを送信したい。メッセージと復号用の鍵の暗号化手順を表した図において，メッセージの暗号化に使用する鍵を (1) とし，(1) の暗号化に使用する鍵を (2) としたとき，図の a，b に入れる字句の適切な組合せはどれか。 - (1)：「a 鍵」→「暗号化」→「暗号化された a 鍵」 - (2)：「受信者の b 鍵」→ 上記の「暗号化」に使用 - 「メッセージ」→「暗号化」→「暗号化されたメッセージ」 - 「暗号化された a 鍵」と「暗号化されたメッセージ」をまとめて「送信」する。
正解 Hybrid cryptography encrypts the data itself with fast symmetric-key cryptography, and delivers only that symmetric key safely to the other party using public-key cryptography. In the figure, (1) is the key used to encrypt the message, so key a is the symmetric key. Next, (2), which encrypts that symmetric key (key a), is the recipient's key b; so that only the recipient can extract it with their matching private key, it is encrypted with the recipient's public key. Therefore b is public. So a: symmetric key, b: public key, and the correct answer is ア.
a: symmetric key is correct, but b: private key is wrong. In the figure, (2) encrypts the symmetric key with the recipient's key b, but the sender does not hold the recipient's private key. If it were encrypted with the recipient's private key, anyone could decrypt it with the public key, so it would not be delivered safely. The correct approach is to encrypt the symmetric key with the recipient's public key.
a: public key is wrong. Encrypting the message itself (the part encrypted with key a) using public-key cryptography is slow, which goes against the idea of hybrid cryptography that the message itself should be encrypted with a fast symmetric key. Key a should be the symmetric key; b: symmetric key is also wrong.
a: public key is wrong (the message itself should be encrypted with a symmetric key). b: private key is also wrong (the symmetric key should be encrypted with the recipient's public key). Both a and b are inappropriate.