問93
頻出度★★★
ディジタル署名やブロックチェーンなどで利用されているハッシュ関数の特徴に関する,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。 ハッシュ関数によって,同じデータは,［ a ］ハッシュ値に変換され,変換後のハッシュ値から元のデータを復元することが［ b ］。
令和元年度 秋期 ITパスポート試験 問93
This question's explanation and your answers so far.
解説
問 93
Incorrectア
b = not possible is correct, but a = different each time is wrong. Because a hash function is deterministic, the same data always yields the same hash value every time. If the value differed each time, it could not be used for tamper detection or for verifying digital signatures, which would contradict the purpose of a hash function.
Incorrectイ
Both a = different each time and b = possible are wrong. The same data always produces the same hash value (not different each time), and the original data cannot be restored from the hash value (one-way property).
- ウ
正解 A hash function is “deterministic”: entering the same data always produces a hash value of the same length and the same value (a = always the same). Because of this property, if even a single bit of the data changes, the hash value changes, so it can be used for tamper detection. In addition, a hash function has a “one-way” property, and the original data cannot be computationally restored from the hash value alone (b = not possible). Therefore the combination a = always the same, b = not possible, namely “ウ”, is correct.
Incorrectエ
a = always the same is correct, but b = possible is wrong. A hash function has a one-way property, and the original data cannot be restored from the hash value. If it could be restored, confidentiality could not be maintained, and uses such as password protection would not hold.
ここがポイント
- Determinism of a hash function: the same input is always converted into the same fixed-length hash value (which is why it can be used for tamper detection and for verifying the integrity of digital signatures and blockchains).
- One-way property of a hash function: the original data cannot be restored from the hash value (irreversible).
用語