問66
頻出度★★★
公開鍵暗号方式と共通鍵暗号方式において, 共通鍵暗号方式だけがもつ特徴として, 適切なものはどれか。
平成29年度 秋期 ITパスポート試験 問66
This question's explanation and your answers so far.
解説
問 66
Incorrectア
Being able to communicate securely even if the key used for encryption becomes known to a third party is a feature of public-key cryptography, where even a disclosed key cannot decrypt anything without the corresponding private key. In symmetric-key cryptography, once the key leaks to a third party the message can be broken, so this is not a characteristic unique to symmetric-key cryptography and is incorrect.
Incorrectイ
In symmetric-key cryptography, a separate key must be prepared for each communication partner, so the number of keys grows as the number of partners increases. “Only one key is needed even with multiple partners” is the exact opposite of a weakness of symmetric-key cryptography and cannot be called a characteristic unique to it, so it is incorrect.
Incorrectウ
Being able to confirm the owner of a key by means of a digital certificate is a feature of public-key cryptography, which is based on a public-key infrastructure (PKI). It is not a characteristic that symmetric-key cryptography has, so it is incorrect.
- エ
正解 Symmetric-key cryptography uses the same key (the shared key) for both encryption and decryption. Choice エ, “for decryption, the same key as the one used for encryption is used,” states exactly this definition of symmetric-key cryptography. Public-key cryptography, by contrast, uses different keys for encryption and decryption (a pair made up of a public key and a private key), so it does not have this feature. Therefore エ is the correct answer as the characteristic that only symmetric-key cryptography has.
ここがポイント
- Keep this distinction in mind: symmetric-key cryptography uses the same key (the shared key) for encryption and decryption (symmetric), whereas public-key cryptography uses a pair of different keys — a public key and a private key (asymmetric).
- Do not mistake features of public-key cryptography (a key can be made public; identity confirmation via digital certificates = PKI) for features of symmetric-key cryptography.
用語