問86
頻出度★★★
AさんはBさんの公開鍵をもっている。Bさんの公開鍵を使ってAさんができることはどれか。
平成29年度 春期 ITパスポート試験 問86
This question's explanation and your answers so far.
解説
問 86
Incorrectア
A digital signature is created with the signer's own private key. To create Person A's own signature, Person A needs Person A's own private key; it cannot be created with Person B's public key, which is what Person A holds.
Incorrectイ
Having a notification automatically sent back upon receipt is a function of email delivery/read receipts, which has nothing to do with encryption or public keys. It is not something that can be achieved with Person B's public key.
- ウ
正解 In public-key cryptography, ciphertext produced by encrypting with someone's public key can be decrypted only with the private key that only that person holds. Person A has Person B's public key, so encrypting the plaintext with it yields ciphertext that only Person B, who holds the corresponding private key, can decrypt. Therefore choice ウ, “create ciphertext that only Person B can decrypt,” is correct. This is the basic use of a public key for secret communication (ensuring confidentiality).
Incorrectエ
Detecting whether the communication content has been tampered with is done by the sender, Person A, attaching a digital signature with Person A's own private key, which the recipient then verifies. With Person B's public key that Person A holds, no mechanism can be built for Person A to detect tampering of Person A's own sent content.
ここがポイント
- Public-key cryptography: encrypt with the recipient's public key → decryptable only with the recipient's private key = the basic use for achieving confidentiality (secret communication).
- A digital signature is created with your own private key and verified with your own public key. The key point is not to confuse the uses of keys (encryption = the recipient's public key / signing = your own private key).
用語