問55
頻出度★★★
PKI(公開鍵基盤)における電子証明書に関する記述のうち, 適切なものはどれか。
平成26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
Tampering is detected by the recipient, who verifies the digital signature (comparing message digests) on their own side. A certificate authority only manages the issuance and revocation of certificates; it neither receives nor monitors the actual communication content exchanged between users, so tampering cannot be detected at the certificate authority.
Incorrectイ
A digital certificate exists to guarantee who owns a public key; it is not a mechanism for tracking the delivery status of email. If proof of a delivery record is needed, a separate service such as certified electronic content mail is used. A certificate authority does not keep delivery records, so it cannot be queried for them.
- ウ
正解 The correct answer is ウ. A digital certificate in PKI (Public Key Infrastructure) is an electronic ID document in which a certificate authority (CA) certifies that “this public key really belongs to this applicant (owner)”, attaching the CA's own digital signature. By verifying the digital certificate received from the sender with the CA's public key, the recipient can confirm that the binding between the public key and its owner is genuine; then, by using that public key to verify the digital signature on the email, the recipient can confirm that the signer really is the person named in the certificate. In other words, an attempt by a third party to impersonate someone else and forge a signature can be detected. Choice ウ states exactly this core function of a digital certificate — the CA guarantees the owner of the public key, thereby making impersonation detectable — so it is appropriate.
Incorrectエ
A digital signature is created by the sender with their own private key and is simply sent to the recipient together with the email; it does not pass through the certificate authority. A certificate authority does not keep copies (the body) of individual emails, and if it did, that would also be inappropriate from the standpoint of the secrecy of communications.
ここがポイント
- A digital certificate is a certificate authority's (CA) signed guarantee of the correspondence between a public key and its owner. It lets you confirm that a public key is legitimate and lets you detect impersonation.
- Distinguish the roles: tamper detection and identity confirmation come from the recipient verifying the digital signature, while guaranteeing the legitimacy of a public key is the certificate authority's job. A certificate authority does not relay or store email bodies, nor does it manage delivery records.
用語