問85
頻出度★★★
公開鍵基盤（PKI）において認証局（CA）が果たす役割はどれか。
平成21年度 秋期 ITパスポート試験 問85
This question's explanation and your answers so far.
解説
問 85
Incorrectア
Incorrect. Distributing the software or programs used for SSL (TLS) encrypted communication is the job of browser and server product vendors, not the role of a CA. What a CA provides is not a program but a digital certificate that guarantees who owns a public key.
Incorrectイ
Incorrect. A server certificate guarantees the identity that the server's public key belongs to the organization or domain written in the certificate; it does not guarantee that the server contains no fraudulent mechanism (malware or unauthorized programs). Checking safety is handled by other mechanisms such as vulnerability assessment.
- ウ
正解 The correct answer is ウ. In a public key infrastructure (PKI), anyone can distribute a public key, so a mechanism is needed in which a third party guarantees whether that public key really belongs to the person it claims to belong to (the authenticated subject). This trusted third party is the certificate authority (CA). After verifying the applicant's identity, the CA issues a digital certificate that binds the public key to the owner's information, and attaches the CA's own signature so that it cannot be tampered with. By verifying the CA's signature, a user can confirm who owns the public key they obtained. In other words, the role of the CA is to issue a certificate showing that a public key belongs to the authenticated subject, so ウ is correct.
Incorrectエ
Incorrect. Creating a digital signature and sending it to the other party is done by the signer, that is, the authenticated subject themselves (with their private key); the CA does not send the signature on their behalf. The CA's position is to back up, in the form of a certificate, the legitimacy of the public key needed to verify the signature.
ここがポイント
- A certificate authority (CA) is the trusted third party in a PKI. Its core role is to issue and manage (including revoking) digital certificates that bind a public key to its owner.
- A digital certificate guarantees the identity of who owns a public key; it does not guarantee the safety or harmlessness of a server or software.
用語