問75
頻出度★★★
A さんは B さんだけに伝えたい内容を書いた電子メールを, 公開鍵暗号方式を用いて B さんの鍵で暗号化して B さんに送った。この電子メールを復号するために必要な鍵はどれか。
平成31年度 春期 ITパスポート試験 問75
This question's explanation and your answers so far.
解説
問 75
Incorrectア
A's public key is public information that anyone can obtain, and it is not paired with text encrypted using B's public key, so it cannot decrypt it. Sender A's keys are irrelevant to decrypting this confidential communication.
Incorrectイ
A's private key is the key that A uses for purposes such as digital signatures; it is not paired with the key that decrypts text encrypted with B's public key, so it cannot decrypt it.
Incorrectウ
B's public key is the very key used for encryption this time. In public-key cryptography you cannot decrypt with the key used to encrypt (decrypting with the same key is symmetric-key cryptography); decryption requires the paired private key, so this is wrong.
- エ
正解 In public-key cryptography, the public key and the private key form a mathematical pair, and text encrypted with one can only be decrypted with the other of the pair. For confidential communication, the sender A encrypts with the recipient B's public key. The only thing that can undo this ciphertext is B's private key, which only B keeps secret. Therefore the key needed for decryption is エ, B's private key. Since no one other than B holds B's private key, no third party can decrypt the ciphertext, so the content can be conveyed to B alone.
ここがポイント
- In communication that protects confidentiality with public-key cryptography, the sender encrypts with the recipient's public key, and the recipient decrypts with their own private key.
- The public key and the private key are a pair; text encrypted with one can only be decrypted with the other. The key used for encryption is not the same as the key used for decryption — this is the difference from symmetric-key cryptography.
用語