問70
頻出度★★★
暗号方式には共通鍵暗号方式と公開鍵暗号方式がある。共通鍵暗号方式の特徴として, 適切なものはどれか。
平成27年度 秋期 ITパスポート試験 問70
This question's explanation and your answers so far.
解説
問 70
Incorrectア
If there is only one other party, symmetric-key cryptography needs just one key shared between both sides. In public-key cryptography each person holds a key pair (two keys)—a public key and a private key—so symmetric-key cryptography actually uses fewer keys. The statement that it uses “more” keys is the opposite of the truth and is incorrect.
Incorrectイ
The key in symmetric-key cryptography is a secret key that must not be known to anyone other than the parties involved. If it is sent as-is without encryption, it can be eavesdropped in transit and all the ciphertext can be decrypted. This is the key distribution problem, a weakness of symmetric-key cryptography, so “safe to send without encryption” is incorrect.
- ウ
正解 Symmetric-key cryptography uses the same single key for both encryption and decryption. It does not involve the large-number exponentiation used in public-key cryptography, and its algorithm is simple with a low computational load, so for processing the same amount of data, the time required for encryption and decryption is shorter than with public-key cryptography. Therefore ウ, which states that the processing time is shorter than with public-key cryptography, is correct. In real systems, a hybrid approach is widely used: key distribution is done with public-key cryptography, while the main data is encrypted with the faster symmetric-key cryptography.
Incorrectエ
Generating a key pair (a public key and a private key) and being able to decrypt ciphertext encrypted with one key only with the other key is precisely the mechanism of public-key cryptography itself. Symmetric-key cryptography encrypts and decrypts with the same key, so this description is not a characteristic of symmetric-key cryptography and is incorrect.
ここがポイント
- Symmetric-key cryptography uses the same key for encryption and decryption and is faster than public-key cryptography, but it has the key distribution problem of how to share the key securely.
- Public-key cryptography uses a key pair consisting of a public key and a private key, encrypting with one and decrypting with the other. It is computationally heavier but solves the key distribution problem.
用語