問68
頻出度★★★
迷惑メールの対策に関して，次の記述中の a，b に入れる字句の適切な組合せはどれか。 迷惑メールに困っていた A さんは，その対策として，自分の受信したいメールアドレスを a に登録し，電子メールの受信は，a に登録しているメールアドレスから送信されたものだけを b することにした。
平成24年度 春期 ITパスポート試験 問68
テクノロジ系セキュリティ › 情報セキュリティ
This question's explanation and your answers so far.
解説
問 68
Incorrectア
The “allowed” in b is correct, but a is wrong. A blacklist is a list for registering the parties you want to reject, which is the exact opposite role of the “email addresses he wished to receive” that Mr. A registered. Moreover, allowing senders registered on a blacklist, a list that gathers the targets of rejection, contradicts the very definition of a blacklist.
Incorrectイ
The combination of “rejecting the parties on the blacklist” is in itself consistent as the blacklist approach, but what is registered in a in the text is “the email addresses he wished to receive”. If you put the senders you want to receive from into a rejection list and reject them, only the necessary emails would be blocked while spam passes through freely, which is the opposite result of what the countermeasure aims for.
- ウ
正解 The correct answer is “ウ (a-Whitelist, b-Allowed)”. A whitelist is a list in which you register in advance the parties whose communication or messages you want to allow. What Mr. A registered in a is “the email addresses he wished to receive”, that is, the senders he wants to allow, so a is the whitelist. Next, for b, the text says that “only emails sent from addresses registered in a would be b”. Since emails from registered senders are exactly the ones he wants to receive, allowing them is the logical choice, so b is “allowed”. This setting behaves as “reject everything in principle, and let through only the senders on the whitelist”, so spam from unregistered senders automatically stops arriving. Therefore the combination a-Whitelist, b-Allowed, namely “ウ”, is appropriate.
Incorrectエ
The “whitelist” in a is correct, but the “rejected” in b is wrong. The whitelist is where the parties you want to allow are registered, so rejecting emails from those senders would mean that precisely the emails Mr. A wants to receive fail to arrive. That is the opposite behavior from the intent of the text, which is to receive “only those sent from” those addresses.
ここがポイント
- The whitelist approach means “reject everything by default and allow only the registered parties”, while the blacklist approach means “allow everything by default and reject only the registered parties”. Which list you register in flips the default behavior, so reading from the text whether the registered parties are the ones you want to allow or the ones you want to reject is the decisive point for answering.
- For anti-spam measures, when the parties you correspond with can be limited, the whitelist approach is more reliable and blocks spam from unknown senders all at once. On the other hand, it has the weakness that legitimate emails from unregistered senders also fail to arrive, whereas the blacklist approach struggles to keep up with spam that keeps switching to new sender addresses. Keep this trade-off in mind.
用語