問71
頻出度★★★
クライアントPCのブラウザからの接続要求に対し, Webサーバが電子証明書などを送信し, クライアントPC側でWebサーバを認証するために用いられるものはどれか。
平成22年度 春期 ITパスポート試験 問71
This question's explanation and your answers so far.
解説
問 71
Incorrectア
ISP (Internet Service Provider) refers to the access provider that connects users to the Internet; it is neither a protocol nor an authentication mechanism. It has nothing to do with sending or receiving digital certificates, or with verifying that a Web server is genuine.
Incorrectイ
PNG (Portable Network Graphics) is a lossless-compression image file format. It is a data format used to display images on Web pages, and it plays no part at all in encrypting communication or authenticating the other party.
Incorrectウ
S/MIME also uses digital certificates, but what it targets is email: it is used to encrypt the body of a message and to prove the sender's identity with a digital signature. The question asks about server authentication in the connection between a browser and a Web server, so S/MIME does not fit. Do not jump to the conclusion that anything using certificates must be S/MIME — the point is to tell them apart by what they apply to, email or the Web.
- エ
正解 The correct answer is エ, SSL. SSL (Secure Sockets Layer; its successor standard is TLS) is a protocol that both encrypts the communication between a browser and a Web server and checks that the other party is genuine. When the browser requests a connection, the Web server first sends back its own server certificate (digital certificate). The browser verifies whether the certificate carries the signature of a trusted certificate authority (CA), whether the server name in the certificate matches the domain being connected to, and whether the certificate has expired; in this way it authenticates whether the party it is now connected to is the real Web server. Once authentication is finished, the public key contained in the certificate is used to share a symmetric key safely, and the communication from then on is encrypted. The flow described in the question — the server sends a digital certificate and the client side authenticates the server — is exactly how the SSL/TLS handshake works, and the padlock icon in the browser address bar with a URL beginning with https:// is evidence that this mechanism is working.
ここがポイント
- SSL/TLS is a protocol that protects the communication path of the Web, and it has two roles: (1) server authentication by means of a server certificate, and (2) encryption of the content of the communication. https:// and the padlock icon show that this mechanism is in effect.
- Distinguish the technologies that use digital certificates by their purpose: SSL/TLS for Web server authentication, S/MIME for encrypting and signing email. Learn similar terms together with what each one protects.
用語