問54
頻出度★★★
X さんは, Y さんにインターネットを使って電子メールを送ろうとしている。電子メールの内容を秘密にする必要があるので, 公開鍵暗号方式を用いて暗号化して送信したい。電子メールの内容を暗号化するのに使用する鍵はどれか。
平成22年度 春期 ITパスポート試験 問54
This question's explanation and your answers so far.
解説
問 54
Incorrectア
If the message is encrypted with Mr. X's public key, the only person who can decrypt it is Mr. X himself, who holds the paired private key. It becomes ciphertext that only the sender can read, and Mr. Y, who receives the email, cannot decrypt the content, so it does not work as a way to send email.
Incorrectイ
Text encrypted with Mr. X's private key can be decrypted by anyone who has Mr. X's published public key, so it cannot be used to keep the content secret. This usage is the operation for a digital signature, which shows that the sender is really Mr. X (sender authentication and tamper detection), and it differs from what this question requires, namely confidentiality.
- ウ
正解 The correct answer is ウ, “Mr. Y's public key.” In public-key cryptography, a public key and a private key form a pair, and text encrypted with one key can be decrypted only with the other key of the pair. The purpose here is to keep the content of the communication secret (to ensure confidentiality), so a state must be created in which only Mr. Y, the recipient, can read it. To do that, the message body is encrypted with Mr. Y's public key, which anyone can obtain, and then sent. Only Mr. Y himself, who holds the paired private key, can decrypt the ciphertext, so even if the communication is intercepted on the way, the content cannot be read. The sender, Mr. X, does not need to know Mr. Y's private key; being able to encrypt just by obtaining the public key is the advantage of this method.
Incorrectエ
A private key is a key that only its owner keeps under strict control and never hands over to anyone else. Therefore Mr. X, the sender, cannot obtain Mr. Y's private key, and it is impossible to use it for encryption. Even supposing he could use it, that ciphertext could be decrypted by anyone using Mr. Y's published public key, so the secret would not be protected.
ここがポイント
- When protecting confidentiality (keeping the content secret) with public-key cryptography, you encrypt with the recipient's public key and decrypt with the recipient's private key. The owner of the keys is always the recipient side; the sender's keys do not come into play.
- Even with the same key pair, the two keys are used in opposite ways depending on the purpose. A digital signature is created with the sender's private key and verified with the sender's public key. The key to judging is to read whether the question asks for secrecy or for authentication of the sender and detection of tampering.
用語