問75
頻出度★★★
電子掲示板やブログに投稿するとき，図のようなゆがんだ文字の画像が表示され，それを読み取って入力するよう求められることがある。その目的はどれか。
平成22年度 春期 ITパスポート試験 問75
This question's explanation and your answers so far.
解説
問 75
Incorrectア
This is not a judgment about the browser's display capabilities. What is displayed is just an ordinary image file, and it requires no special display capability. Furthermore, whether a browser has a particular capability can be determined automatically on the system side using scripts or the User-Agent information, without making the user read and type characters. This option cannot explain why manual input by a human is deliberately demanded.
Incorrectイ
This is not user authentication. What a CAPTCHA distinguishes is “human or program”, not “who the person is”. The character string shown is generated on the spot and is single-use; it is not a secret distributed in advance only to specific users, so anyone who sees the screen can read it and type it in, and there is no way to tell whether that person is really the one authorized to post. Confirming prior authorization requires separate authentication, such as a user ID and password.
Incorrectウ
This is not a check that the display is working. To examine whether a display device has a problem, one would normally use a dedicated image such as a test pattern laying out colors and gray levels; there is no need to distort characters on purpose so that they become hard to read. Nor can an inspection of display capability explain why a different character string is generated and has to be entered for every single post.
- エ
正解 The correct answer is エ. Displaying a distorted character image like the one in the figure — one that a human can read but a machine finds hard to decipher — and asking the user to type in the characters they read is a mechanism called CAPTCHA. The characters are deliberately deformed and rotated, or overlaid with lines and background noise, so that automatic character-recognition programs such as OCR cannot read them correctly. A human, by contrast, can read them without difficulty, so whoever types them in correctly can be judged to be “a human at the keyboard”. The purpose of placing this on the posting screen of a bulletin board or a blog is to stop programs (bots) that write advertisements and spam from submitting large volumes of posts automatically through the posting form, which is exactly “To prevent automated posting by programs”.
ここがポイント
- A CAPTCHA is a mechanism that poses a task machines are poor at processing automatically (such as reading a distorted character image) in order to tell whether the other party is a human or a program (a bot). Its purpose is to prevent bot abuse such as spam posting, automated account creation, and brute-force login attempts.
- What a CAPTCHA verifies is “whether you are a human”, and that goal differs from user authentication (ID and password, biometrics, digital certificates, and the like), which verifies “who you are”. Not confusing the two is what decides this question.