問97
頻出度★★★
公開鍵暗号方式と比べた場合の, 共通鍵暗号方式の特徴として適切なものはどれか。
平成28年度 秋期 ITパスポート試験 問97
This question's explanation and your answers so far.
解説
問 97
Incorrectア
Using different keys for encryption and decryption (a public-key and private-key pair) is a characteristic of public-key cryptography. In symmetric-key cryptography the sender and receiver share and use the same key, so this option is wrong.
- イ
正解 The correct answer is “イ”. Symmetric-key cryptography uses the same single key for both encryption and decryption, and its operations (such as bit-level permutation and mixing) are simple with a small computational cost. Therefore, compared with public-key cryptography, which performs heavy mathematical processing such as modular exponentiation, it can perform encryption and decryption at high speed — this is its greatest characteristic. This is precisely why symmetric-key methods are used to encrypt large volumes of data.
Incorrectウ
Being able to distribute keys more securely is an advantage of public-key cryptography. Symmetric-key cryptography faces the “key distribution problem” of safely handing the secret key itself to the other party, so its distribution is actually more difficult.
Incorrectエ
Symmetric-key cryptography needs a different key for each communication partner, so when n people communicate with one another, as many as n(n−1)/2 keys must be managed. The more partners there are, the harder management becomes, so this option is wrong. It is public-key cryptography whose key management stays easy even with many partners.
ここがポイント
- Symmetric-key cryptography = uses the same single key for encryption and decryption. Processing is fast, but it has two weaknesses: the “key distribution problem” and the growing number of keys (n(n−1)/2).
- Public-key cryptography = uses a public-key and private-key pair. Processing is slow, but key distribution is secure and key management stays easy even with many partners. The strengths and weaknesses of the two are mirror images of each other.
用語