問55
頻出度★★★
PKI において, ディジタル署名をした電子メールに関する記述として, 適切なものだけを全て挙げたものはどれか。 a 送信者が本人であるかを受信者が確認できる。 b 電子メールが途中で盗み見られることを防止できる。 c 電子メールの内容が改ざんされていないことを受信者が確認できる。
平成28年度 秋期 ITパスポート試験 問55
This question's explanation and your answers so far.
解説
問 55
Incorrectア
The combination a, b. a (confirming the sender's identity) is correct, but b (preventing eavesdropping = confidentiality) is wrong. A digital signature does not encrypt the message body, so it cannot prevent the body from being read as-is (eavesdropped) on the communication channel. To prevent eavesdropping, the body itself must be encrypted.
- イ
正解 A digital signature is a mechanism that attaches to an email “the hash value of the message body, encrypted with the sender's private key”. The recipient verifies the signature with the sender's public key (whose authenticity is guaranteed by a PKI certificate), so they can confirm a “the sender is genuine” (authentication). Furthermore, by checking whether the hash value recomputed from the received body matches the hash value inside the signature, they can also confirm c “the content has not been tampered with” (integrity). Only a and c are appropriate, so the correct answer is イ (a, c).
Incorrectウ
The combination b, c. c (tamper detection = integrity) is correct, but it is wrong because it includes b (preventing eavesdropping). A signature does not provide confidentiality, so b does not hold; it is also inappropriate in that it omits the correct identity-verification item a.
Incorrectエ
The combination of all three, a, b, and c. a and c are correct, but it becomes wrong the moment it includes b (preventing eavesdropping). A digital signature guarantees authentication and integrity, but does not provide confidentiality (preventing eavesdropping through encryption).
ここがポイント
- A digital signature guarantees two things: “authentication (whether the sender is genuine)” and “integrity (whether the content has been tampered with)”. Because it does not encrypt the message body, it does not guarantee “confidentiality (preventing eavesdropping)”.
- PKI (Public Key Infrastructure) is a mechanism that uses certificates (issued by a certificate authority) to back up the fact that the public key used to verify a signature truly belongs to the sender, thereby underpinning the reliability of identity verification.
用語