This question's explanation and your answers so far.
問96
頻出度★★★
S 社の T 営業所では，ID カードによる入室管理システムを導入することになった。T 営業所の施設の種類と管理対象者の区分（以下，対象区分という）を表のように分類し，入室許可の有無の区分（以下，許可区分という）の設定を行う。ここで，表は施設に対して入室を許可する対象区分を“○”，入室を許可しない対象区分を“×”で示す。
表 許可区分の設定
The goal “so that entry is not allowed” is correct as a crime-prevention idea, but the means is wrong. By condition (2), ACCEPT is what is written when entry is permitted, so if the policy is set to ACCEPT, every omitted combination becomes able to enter, producing exactly the opposite of the goal. The goal and the means contradict each other.
○イ
正解The correct answer is イ. As stated in condition (3), the policy in Format 1 is the default value applied to any combination of a facility and a person category that matches none of the lines written from the 2nd line onward (Format 2). Therefore, if a Format 2 line is left out when a new facility or person category is added, that combination matches no individual rule and is necessarily judged by the permission category of the policy. From the standpoint of crime prevention, the iron rule is to let nothing through that cannot be judged or was not anticipated, so the policy should be set to DENY (entry not permitted) from condition (2). With this setting, even if a setting is omitted, an unregistered combination is automatically denied entry, and no accident of a door opening occurs. Choice イ says “If there is an omission in the settings, set it to DENY so that entry is not allowed,” and it is the only choice in which the goal (not letting people in) and the means (DENY = not permitted) correctly match.
×Incorrectウ
The statement that ACCEPT = entry is allowed is itself consistent with condition (2), but the approach is the opposite of the crime-prevention viewpoint. It becomes a default-allow scheme (a blacklist approach) that lets through every combination whose setting was omitted; for example, if blocking visitors from a newly added facility is forgotten, they are simply allowed in. Such a setting does not fall to the safe side (fail-safe), so it is inappropriate.
×Incorrectエ
By condition (2), DENY is what is written when entry is not permitted, so it is the exact opposite of the goal “so that entry is still allowed”; the statement itself is contradictory. This choice mixes up the meanings of ACCEPT and DENY.
ここがポイント
The policy in Format 1 = the default permission category applied to combinations that match no individual rule (Format 2). Understand it together with the evaluation method in condition (5): the lines from the 2nd onward are evaluated from top to bottom, and the permission category of the matching line is applied and settles the result there. If no line matches all the way to the end, the policy takes effect.
The basic principles of security are default deny (a whitelist approach) and fail-safe design. If only the items to be permitted are explicitly listed and everything else is denied, the system falls to the safe side even when a setting is omitted or something unexpected is added. The same idea is used both in entry/exit management and in firewall access control.