問84
頻出度★★★
メッセージダイジェストを利用した送信者のデジタル署名が付与された電子メールに関する記述のうち，適切なものはどれか。
令和5年度 ITパスポート試験 問84
This question's explanation and your answers so far.
解説
問 84
Incorrectア
A digital signature verifies the legitimacy of “the sender who signed (and their key)”, but it cannot determine which mail server the email passed through or was sent from. It cannot be used to tell whether the email was sent from an unauthorized mail server, so it is wrong.
Incorrectイ
The conclusion that sender impersonation can be detected is correct, but the means is wrong. An email's digital signature is verified with “the sender's (individual) public key certificate”. A server certificate is meant to prove a server's authenticity in SSL/TLS and is not used to verify the signature of the email body, so it is wrong.
Incorrectウ
A digital signature is a mechanism for tamper detection and sender authentication (integrity and authenticity); it has no function to encrypt the body. Even with a signature attached, the body remains plaintext, so it cannot prevent leakage of the content (cannot ensure confidentiality), and is therefore wrong.
- エ
正解 The correct answer is エ. A digital signature using a message digest works as follows: the sender computes a hash value (message digest) of the email body, encrypts it with the sender's own private key, and attaches it. The recipient decrypts the signature with the sender's public key to obtain that digest, and compares it with a digest recomputed from the received body. If the two do not match, it shows that the body was altered in transit; therefore, while tampering itself cannot be “prevented”, the fact that tampering occurred can be “detected”. エ correctly states this property, so it is appropriate.
ここがポイント
- A digital signature (message digest + private key) provides integrity (tamper detection) and authenticity (sender authentication and non-repudiation), but does not provide confidentiality (encryption).
- Verification of an email signature is done with the sender's individual public key certificate, whose role differs from that of a server certificate (used for TLS).
用語