問64
頻出度★★★
電子メールの宛先入力欄におけるBccに関する記述として, 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64
テクノロジ系ネットワーク › ネットワーク応用
This question's explanation and your answers so far.
解説
問 64
Incorrectア
Bcc has no encryption function at all. Mail addressed to Bcc recipients is sent in plain text, just like mail to To and Cc. Email encryption is provided by separate mechanisms such as S/MIME or PGP (which use digital certificates and public-key cryptography), or by SSL/TLS protecting the communication path; it has nothing to do with which recipient field is used. The belief that putting someone in Bcc makes the message safe to send is a typical misunderstanding — what Bcc protects is not the secrecy of the content but the secrecy of the recipient address.
Incorrectイ
Bcc recipients also receive a message with exactly the same content, including the subject, body, and attachments. The body is never removed to leave only the subject. What Bcc conceals is only the fact that the recipient does not appear in the recipient list; it is not a mechanism that alters the content of the message. Therefore this statement is wrong as a description of how Bcc works.
- ウ
正解 The correct answer is ウ. An email has three kinds of recipient input fields: To (the main recipient), Cc (carbon copy = recipients who receive a copy), and Bcc (blind carbon copy = recipients who receive a copy but whose presence is hidden). Addresses written in To and Cc appear as-is in the mail header, so everyone who receives the message can see each other's addresses. In contrast, addresses specified in Bcc are stripped from the header at delivery time, and are shown neither to the To and Cc recipients nor to the other Bcc recipients. In other words, ウ — the email addresses of the recipients specified in Bcc are not disclosed to the other recipients — is exactly the definition of Bcc, so it is appropriate. Because of this property, when sending one message at once to many people who do not know each other, putting everyone in Bcc (= preventing mutual disclosure of addresses) is basic etiquette and an information security measure. Conversely, mistakenly putting into To or Cc a recipient who should have been in Bcc causes an incident in which email addresses — personal information — leak to all recipients. Note as well that the body, subject, and attachments of a message sent via Bcc are the same as those sent to To and Cc; the only thing hidden is the recipient address.
Incorrectエ
The destination for delivery errors (the error message returned when mail cannot be delivered, i.e. a bounce message) is not the Bcc recipient but the envelope sender address (Return-Path, normally the sender's From address). Bcc is merely a recipient who secretly receives a copy; it is not a field for specifying where error notifications go. This statement confuses Bcc with the specification of a reply or error-return address (Return-Path or Reply-To).
ここがポイント
- The difference between To / Cc / Bcc: To is the main recipient; Cc sends a copy and the address is also made public; Bcc sends a copy but its address is not shown to the other recipients at all. Bcc stands for blind carbon copy, and as blind (invisible) suggests, its function is to hide the recipient address.
- Practical usage and cautions: when sending one message at once to many people who do not know each other, put everyone in Bcc to prevent mutual disclosure of addresses. Bcc has nothing to do with encrypting the body or specifying where error notifications go — encryption is handled by S/MIME, PGP, or SSL/TLS, and the return address for delivery errors is handled by the Return-Path (the sender's address).
用語