問73
頻出度★★★
共通鍵暗号方式の説明として，適切なものはどれか。
平成27年度 春期 ITパスポート試験 問73
This question's explanation and your answers so far.
解説
問 73
Incorrectア
It is public-key cryptography that is used for digital signatures. The signer creates a signature with a private key that only the signer holds, and anyone can verify it with the publicly available public key, which proves to a third party that the signer really created the data. With symmetric-key cryptography both sides hold the same key, so you cannot tell which side created the data, and it cannot be used for signatures.
Incorrectイ
The speed relationship is the other way around. Symmetric-key cryptography involves simpler processing, so both encryption and decryption are generally faster than with public-key cryptography. In real communication (such as SSL/TLS), a hybrid approach is used: the slow public-key cryptography is used only to deliver the shared key, while the bulk of the data itself is encrypted with the fast symmetric-key cryptography.
Incorrectウ
RSA is a representative method of public-key cryptography, not of symmetric-key cryptography. Representative examples of symmetric-key cryptography are AES, DES, and 3DES.
- エ
正解 Symmetric-key cryptography (shared-key cryptography) uses the same key for both encryption and decryption, and that key is shared as a secret known only to the sender and the receiver. If the key leaks to a third party, the contents of the communication can be read, so the key may be shared only with that one communication partner. Therefore, every time a new communication partner is added, a separate shared key must be prepared for each partner, which makes choice エ appropriate. When n people communicate with one another, the total number of keys needed is n×(n−1)÷2, so key management becomes dramatically harder as the number of people grows (the key distribution problem). Public-key cryptography was devised to solve this weakness.
ここがポイント
- Symmetric-key cryptography = a method that uses the same key for encryption and decryption. A separate key must be prepared for each communication partner, so n people need n×(n−1)÷2 keys (the key distribution and key management problem).
- Memorize the contrast between symmetric-key and public-key cryptography as a set: representative methods (AES / DES ⇔ RSA), speed (fast ⇔ slow), and whether they can be used for digital signatures (no ⇔ yes).
用語