問99
頻出度★★★
文書を A さんから B さんに送るとき, 公開鍵暗号方式を用いた暗号化とディジタル署名によって, セキュリティを確保したい。このとき, A さんの公開鍵が使われる場面はどれか。
平成29年度 春期 ITパスポート試験 問99
This question's explanation and your answers so far.
解説
問 99
Incorrectア
Encrypting the document is an operation that protects confidentiality, and sender A encrypts it with the public key of the receiver B. The key used is B's public key, not A's public key.
Incorrectイ
Adding the digital signature is done by sender A themselves, using A's private key (the signing key). The key used is the private key, not the public key, so this is wrong.
- ウ
正解 In a digital signature, sender A creates the signature with A's own private key, and receiver B verifies that signature with A's public key. In public-key cryptography the keys work as a pair, and a signature made with a private key can be correctly verified only with the matching public key. Therefore A's public key is used in the scene of verifying the digital signature attached to the document that B received. This confirms that the document truly comes from A (authenticity) and that it has not been tampered with. Hence choice 'ウ' is correct.
Incorrectエ
Decrypting the received document is done by receiver B, using B's own private key. A's public key is not used.
ここがポイント
- In public-key cryptography, keys work as a pair. Encryption for confidentiality is done with the receiver's public key, and decryption with the receiver's private key.
- A digital signature is added with the sender's private key and verified by the receiver with the sender's public key. Therefore sender A's public key is used on B's side to verify A's signature.
用語