問54
頻出度★★★
ある認証システムでは虹彩認証とパスワード認証を併用しており, 認証手順は次のとおりである。この認証システムの特徴として, 適切なものはどれか。 〔認証手順〕 ① 虹彩認証に成功するとログインできる。 ② 虹彩認証に3回失敗するとパスワードの入力を求める。 ③ 正しいパスワードを入力することでログインできる。 ④ パスワード認証に3回失敗するとアカウントがロックアウトされる。
平成27年度 春期 ITパスポート試験 問54
This question's explanation and your answers so far.
解説
問 54
Incorrectア
In this mechanism a path that lets you log in with the password alone still remains (steps (2) and (3)), so the password is still a key that can get through authentication by itself. Even though iris authentication is used alongside it, if the password leaks or is guessed the attacker gets straight in, so this is no reason to skip proper password management (sufficient length, no reuse, immediate change if leaked, including periodic changes). On the contrary, assuming that it is safe because the two are used together is itself a dangerous misunderstanding.
- イ
正解 The correct answer is “イ”. If you follow authentication steps (1) to (4), this mechanism is not built so that you must satisfy both iris authentication and password authentication in order to log in (AND = multi-factor authentication). By (1), if iris authentication succeeds you can log in at that point; as stated in (2) and (3), only after iris authentication has failed 3 times does it switch to password entry, and entering the correct password lets you log in. In other words, the two methods are in an OR relationship (an alternative means, a fallback): it is enough to pass just one of them. With biometric authentication, it is unavoidable that the real person is sometimes rejected because of factors such as bloodshot eyes, injury, changes in physical condition, or environmental factors like lighting (false rejection, the phenomenon corresponding to FRR). With this mechanism, when that happens the user can still log in by the separate means of a password, so compared with running biometric authentication alone, users are less likely to be locked out. Therefore, the appropriate characteristic of this system is イ, which states that it is highly convenient. Note also that the lockout in (4) is a brake that keeps the password, the alternative means, from being broken by brute force; it is worth understanding together with the rest as a device for balancing convenience and security.
Incorrectウ
Authentication strength rises in the case of multi-factor authentication (an AND configuration) that demands both the biometric information and the password. This question presents an OR configuration in which passing either iris authentication or password authentication lets you log in, so from an attacker's point of view the number of ways in simply increases to two, and the overall strength is dragged down to that of the weaker method (here, password authentication). Note that the mere fact that biometric information is used does not raise the strength.
Incorrectエ
As stated in step (1), the moment iris authentication succeeds the login is complete, and no password entry is requested. Password authentication appears only after iris authentication has failed 3 times. Therefore, if iris authentication mistakenly accepts another person as the user (false acceptance, the phenomenon corresponding to FAR), there is no opportunity for an additional check by password. The very premise that this forms a two-stage check contradicts the procedure.
ここがポイント
- Even when several authentication methods are combined, an AND (both are required = multi-factor authentication) raises the strength, but an OR (either one is enough = an alternative means) does not raise it and instead pulls it down to the strength of the weaker method. Reading the procedure and telling whether it is AND or OR is the heart of this question.
- Biometric authentication has two kinds of errors: rejecting the genuine user (false rejection) and accepting another person as the user (false acceptance); physical condition and the environment can cause false rejection. Providing an alternative means such as a password is a design choice for handling that false rejection and securing convenience and availability, while the lockout in (4) is a countermeasure against brute-forcing that alternative means.
用語