問89
頻出度★★★
Aさんは, Bさんから次の4種類のメッセージを受け取った。Aさんが, 受け取ったメッセージを復号して読むことができるものだけを全て挙げたものはどれか。 a AさんとBさんとの共通鍵で暗号化したメッセージ b Aさんの公開鍵で暗号化したメッセージ c Bさんの公開鍵で暗号化したメッセージ d Bさんの秘密鍵で暗号化したメッセージ
令和7年度 ITパスポート試験 問89
This question's explanation and your answers so far.
解説
問 89
- ア
正解 The correct answer is ア (a, b, d). Keys follow the principle that you decrypt with the key paired to the one used for encryption. a is encrypted with the shared key common to A and B, and A also holds that shared key, so A can decrypt it. b is a message encrypted with A's public key, and only A holds the paired A's private key, so A can decrypt it. d is encrypted with B's private key, and the paired B's public key is published so A can obtain it too, so A can decrypt it (this is also the digital-signature mechanism for confirming the sender). Therefore the messages A can decrypt and read are a, b, and d — three of them — and ア is correct.
Incorrectイ
It lists a, c, d, but c is wrong. c is a message encrypted with B's public key, and decrypting it requires the paired B's private key, which only B holds and A does not, so A cannot decrypt it. It also omits b, which A can decrypt.
Incorrectウ
It lists only b, d, but omits a. a is encrypted with the shared key common to A and B, and A also holds that shared key, so A can decrypt it. Therefore ウ, which does not include a, is incomplete.
Incorrectエ
It lists c, d, but c cannot be decrypted, so it is wrong (c requires B's private key, which A does not have). It also omits a and b, which A can decrypt.
ここがポイント
- In public-key cryptography, you decrypt with the paired key: 'encrypt with the public key → decrypt with the private key' and 'encrypt with the private key → decrypt with the public key.' Receiver A can decrypt when decryption uses A's private key (i.e., something encrypted with A's public key), or when it uses a public key that anyone can obtain (i.e., something encrypted with the sender's private key).
- In symmetric-key cryptography, the same shared key is used for both encryption and decryption, so A, who shares that key, can decrypt. On the other hand, A cannot decrypt anything that requires the other party B's private key.
用語