問79
頻出度★★★
送信する電子メールの本文と添付ファイルを暗号化し, 宛先に指定した受信者だけが内容を読むことができるようにしたい。このとき使用する技術として, 最も適切なものはどれか。
平成26年度 春期 ITパスポート試験 問79
This question's explanation and your answers so far.
解説
問 79
Incorrectア
APOP is an authentication method used when receiving mail with POP3; its purpose is to send the password using a challenge-response scheme instead of transmitting it as-is. It protects only the password used at authentication time, and the mail body and attachments are not encrypted. Therefore it cannot be used to make the content readable only by the recipient.
Incorrectイ
IMAP is a retrieval protocol for viewing mail while managing it on the mail server. It only provides a means to read mail and has no encryption function. On the contrary, the mail is left on the server in plaintext, so the content cannot be restricted to the specified recipient alone.
- ウ
正解 The correct answer is ウ, S/MIME (Secure/Multipurpose Internet Mail Extensions). S/MIME is a standard that uses public-key cryptography to encrypt and digitally sign the MIME-format body and the attachments of an email themselves. The sender encrypts the content with the public key of the intended recipient, so only that recipient, who holds the matching private key, can decrypt it. The content stays as ciphertext while the mail is relayed through mail servers and while it is stored on them, so it exactly meets the requirement that «only the recipient specified in the destination can read the content» (end-to-end encryption). At the same time, the signature also prevents impersonation of the sender and detects tampering.
Incorrectエ
SSL (and its successor TLS) encrypts the «communication path», such as the path between a client and a server. It can prevent eavesdropping along the path, but once the mail reaches the mail server the body is decrypted and is stored and relayed in plaintext. Therefore it does not achieve a state where only the final recipient can read the mail, and it is insufficient for this question's requirement of protecting the body and attachments themselves.
ここがポイント
- The technology that encrypts the mail body and attachments themselves so that only the recipient can read them (end-to-end encryption) is S/MIME. It encrypts with public-key cryptography, and its signature also provides tamper detection and sender authentication.
- Distinguish between «encryption of the path (SSL/TLS)» and «encryption of the message itself (S/MIME)». APOP is authentication and IMAP is a retrieval protocol; neither has anything to do with encrypting the body.
用語