問57
頻出度★★★
暗号化方式の特徴について記した表において，表中の a～d に入れる字句の適切な組合せはどれか。
| 暗号方式 | 鍵の特徴 | 鍵の安全な配布 | 暗号化／復号の相対的な処理速度 |
|---|---|---|---|
| a | 暗号化鍵と復号鍵が異なる | 容易 | c |
| b | 暗号化鍵と復号鍵が同一 | 難しい | d |
令和6年度 ITパスポート試験 問57
This question's explanation and your answers so far.
解説
問 57
Incorrectア
The methods for a and b are swapped. Row a, where the encryption key and the decryption key differ, is public-key cryptography, so treating it as symmetric-key cryptography is wrong (and the speed assignments for c and d do not match reality either).
Incorrectイ
Not only are the methods for a and b swapped, but the speeds are also reversed. Public-key cryptography (a) is slow to process and symmetric-key cryptography (b) is fast, so labeling c as fast and d as slow is wrong on two counts.
- ウ
正解 Row a in the table states that the encryption key and the decryption key differ, which is a feature of public-key cryptography, where encryption and decryption use separate keys. A public key may be given to anyone, so key distribution is easy, but the processing is heavy and relatively slow; therefore a is public-key cryptography and c is slow. Row b states that the encryption key and the decryption key are the same, which is symmetric-key cryptography, where the same secret key is shared. Because the key must be delivered secretly, distribution is difficult, but the processing is light and relatively fast; therefore b is symmetric-key cryptography and d is fast. Hence choice ウ, the combination where a is public-key cryptography, b is symmetric-key cryptography, c is slow, and d is fast, is correct.
Incorrectエ
The assignment of a as public-key and b as symmetric-key is correct, but the speeds are reversed. Public-key cryptography (a) is slow to process, so c should be slow; symmetric-key cryptography (b) is fast, so d should be fast. Labeling c as fast and d as slow is wrong.
ここがポイント
- Identify the method by whether the keys differ: if the encryption key and the decryption key differ, it is public-key cryptography; if they are the same, it is symmetric-key cryptography.
- Grasp the contrast in speed and distribution: public-key cryptography has easy key distribution but slow processing, while symmetric-key cryptography has difficult distribution but fast processing.
用語