問60
頻出度★★★
受信した電子メールに PKI (公開鍵基盤) を利用したディジタル署名が付与されている場合に判断できることだけを全て挙げたものはどれか。 a 電子メールの添付ファイルはウイルスに感染していない。 b 電子メールの内容は通信途中において, 他の誰にも盗み見られていない。 c 電子メールの発信者は, なりすましされていない。 d 電子メールは通信途中で改ざんされていない。
平成23年度 秋期 ITパスポート試験 問60
This question's explanation and your answers so far.
解説
問 60
Incorrectア
Incorrect. Neither a nor b can be determined from a signature. Item a, “the attached file is not infected with a virus”, is something that should be checked by scanning with anti-virus software; a signature only shows that the body or the attachment has not been tampered with, and guarantees nothing at all about whether that content is safe. On the contrary, a sender may not even notice their own infection and may correctly sign an email that carries a virus. Item b, preventing eavesdropping, is likewise not the role of a signature, as explained below.
Incorrectイ
Incorrect. Item c (the sender has not been impersonated) is right, but item a (the attached file is not infected with a virus) is wrong. Verifying a signature tells you only “who signed it” and “whether the content has changed since it was signed”; it cannot decide whether that content contains malware. It is confusing because it includes one correct item, but it does not satisfy the condition of listing all and only the things that can be determined.
Incorrectウ
Incorrect. Item d (not tampered with) is right, but item b (not seen by anyone else during transmission) is wrong. Preventing eavesdropping is a matter of confidentiality, a property you obtain only by encrypting the message body. A digital signature sends the body as plain text and merely attaches a signature to it, so a third party on the communication path can read the content as it is. If confidentiality is required, encryption must be used together with the signature but separately from it (for example, encrypting the body with S/MIME).
- エ
正解 The correct answer is エ (c, d). In a digital signature that uses PKI, the sender encrypts the hash value (message digest) of the message with “their own private key” and attaches it, and the receiver decrypts it with “the sender's public key” (which a digital certificate issued by a certificate authority confirms really belongs to that person) and checks whether it matches the hash value the receiver recomputes from the message body. Two things follow from this at the same time. First, the fact that it decrypts correctly means that only the holder of the matching private key could have created the signature, so you can judge that the sender really is the person they claim to be and has not been impersonated (c). Second, the fact that the hash values match means the body was not tampered with during transmission, because the value would change if even one bit of the body were rewritten (d). In other words, what a digital signature guarantees is the two properties of “authentication (identity of the sender)” and “integrity (detection of tampering)”, and エ, which lists this combination, is the correct answer.
ここがポイント
- What a digital signature (using PKI) guarantees is two things: authentication, meaning the sender is the genuine person and has not been impersonated, and integrity, meaning the content has not been tampered with. Because the sender encrypts a hash value with their private key and the receiver verifies it with the sender's public key, both points can be confirmed at the same time (and this also leads to non-repudiation of sending).
- The key point is to distinguish what a signature cannot provide. Confidentiality (preventing eavesdropping) is the role of encrypting the message body, and guaranteeing that there is no virus infection is the role of anti-virus software; neither is an effect of a digital signature.
用語