問74
頻出度★★★
図の構成で，HTTPSプロトコルを用いてブラウザとWebサーバで通信を行うとき，通信内容が暗号化される区間はどれか。 [ブラウザ] ― [プロキシサーバ] ― [ルータ] ― (インターネット) ― [Webサーバ]
平成25年度 春期 ITパスポート試験 問74
This question's explanation and your answers so far.
解説
問 74
Incorrectア
“Only between the browser and the proxy server” is wrong. The proxy server is not the party that decrypts HTTPS, so the encryption does not end (terminate) at the proxy server. The segments beyond the proxy server - toward the router, the Internet, and the Web server - also remain encrypted, so restricting the answer with “only” already makes it wrong.
- イ
正解 HTTPS is a protocol that encrypts HTTP with SSL/TLS, and encryption and decryption are handled by the two ends of the communication - that is, by the “Browser” at the left of the figure and the “Web server” at the right themselves. The browser and the Web server exchange keys with each other to establish an encrypted communication path, so the “Proxy server”, “Router”, and “Internet” that sit in between are nothing more than relays that pass the still-encrypted data on to the next hop (with HTTPS, the proxy server does not decrypt the content; it works as a tunnel that simply lets the data through). The segment whose communication content is encrypted is therefore not broken anywhere along the way: it is the entire segment between the browser and the Web server, so the correct answer is イ. A scheme like this, in which only the two ends encrypt and decrypt, is called end-to-end encryption.
Incorrectウ
“Only between the proxy server and the router” is wrong. This segment is merely one part of the internal LAN. Encryption by SSL/TLS is a function that the browser and the Web server provide; it is not something the proxy server or the router re-applies segment by segment. In reality, the browser-to-proxy-server segment and the Internet segment are encrypted as well.
Incorrectエ
“Only between the proxy server and the Web server” is wrong. Encryption starts at the browser. If the segment between the browser and the proxy server were plaintext (not encrypted), passwords and the like could be eavesdropped on within the internal LAN, and there would be no point in using HTTPS. The starting point of encryption is always the browser.
ここがポイント
- HTTPS is a protocol that encrypts HTTP with SSL/TLS, and encryption and decryption are performed at the two ends of the communication (the browser and the Web server) = end-to-end encryption.
- Relay devices such as proxy servers and routers only forward the encrypted data as it is; they do not redo encryption and decryption segment by segment.
用語