問77
頻出度★★★
暗号化通信で使用する鍵 a～c のうち，セキュリティ上，第三者に知られてはいけないものだけを全て挙げたものはどれか。 a 共通鍵暗号方式の共通鍵 b 公開鍵暗号方式の公開鍵 c 公開鍵暗号方式の秘密鍵
平成26年度 秋期 ITパスポート試験 問77
This question's explanation and your answers so far.
解説
問 77
Incorrectア
Key a (the shared key) does indeed need to be kept secret, but key b, the public key, is meant to be distributed and published, and knowing it does not undermine security. On the contrary, unless it is given to the other party, encrypted communication cannot even begin. This option is also wrong because it omits key c, the private key, which does need to be kept secret.
- イ
正解 The key point is to sort each key into one that is meant to be handed out and used, versus one that only you hold. Key a, the shared key in symmetric-key cryptography, uses one and the same key for both encryption and decryption, so if it reaches a third party the communication can be read directly; it must be kept secret. Key c, the private key in public-key cryptography, is used to decrypt messages encrypted with the paired public key and to generate digital signatures. Only because the owner alone holds it do impersonation prevention and confidentiality hold, so it too must be kept secret. Key b, the public key, is by design meant to be distributed to the other party or published widely, and the scheme ensures that the private key cannot be deduced even when the public key is made public. Therefore the only keys that must not be known by a third party are a and c, and the correct answer is “イ a, c”.
Incorrectウ
This option correctly includes key c (the private key), but it is wrong to list key b, the public key, among the keys that must not be known. As its name says, the public key may be made public, and the scheme rests on the premise that deriving the private key from the public key is computationally difficult. Moreover, this option omits key a, the shared key, which does need to be kept secret.
Incorrectエ
It is correct that key c (the private key) must be kept secret, but this option overlooks key a, the shared key. In symmetric-key cryptography the sender and receiver share one and the same key, and that single key performs both encryption and decryption, so if it reaches a third party the ciphertext is left unprotected. Since the question asks for all such keys to be listed, an option that does not include a is incomplete.
ここがポイント
- Symmetric-key cryptography uses the same key (the shared key) for both encryption and decryption, so that key must be strictly managed as a secret shared only by the communicating parties. This is also why handing the key over (key distribution) becomes a problem.
- The essence of public-key cryptography is the paired relationship that the public key may be published while the private key is held only by its owner. What is encrypted with the public key can be decrypted only with the corresponding private key, and if the private key leaks, both confidentiality and proof of identity (the trustworthiness of signatures) are lost.
用語