問77
頻出度★★★
暗号化に関する記述のうち, 適切なものはどれか。
平成21年度 春期 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
The term is wrong. Restoring ciphertext back to the original plaintext is called 'decryption'. 'Reset' means returning a device or a system to its initial state; it is a different term and does not describe a procedure in cryptography. Note also that turning plaintext into ciphertext is called 'encryption', and decryption is the reverse operation.
Incorrectイ
In symmetric-key cryptography the same key is used for both encryption and decryption, so if the ciphertext and the shared key are sent over the same channel at the same time, a third party who eavesdrops on the way can decrypt the ciphertext with that key, and the encryption becomes meaningless. The shared key must be given only to the communication partner in advance, by a safe means separate from the ciphertext. This difficulty of key distribution is the weak point of symmetric-key cryptography.
- ウ
正解 The correct answer is ウ. In public-key cryptography, the key used for encryption and the key used for decryption (restoring the plaintext) are two different keys that form a pair, and this is the core of the method. The sender encrypts the plaintext with the 'public key' that the receiver has published, and the receiver decrypts it with the 'private key' that only the receiver keeps secret. It does not matter who knows the public key, and working out the private key from the public key is computationally extremely difficult, so there is no need to prepare a separate channel for delivering the key safely; this is a major advantage. Because the two keys are different, it is also called asymmetric-key cryptography, and it is best learned in contrast with symmetric-key cryptography (common-key cryptography), in which both sides share the same key.
Incorrectエ
Electronic signatures (digital signatures) use public-key cryptography. The sender creates the signature with their own private key, and the receiver verifies it with the sender's public key, which confirms that the person themselves created it (authentication) and that the content has not been tampered with (integrity). In symmetric-key cryptography both sides hold the same key, so it cannot prove that only the signer could have created the signature, and therefore it cannot be used for signatures.
ここがポイント
- Public-key cryptography uses a pair of different keys: a public key for encryption and a private key for decryption (asymmetric). Symmetric-key cryptography uses the same key for both encryption and decryption (symmetric), which creates the key distribution problem of how to hand the key to the other party safely.
- Organize the basic terms of encryption and its applications. Turning plaintext into ciphertext is encryption, and restoring ciphertext to plaintext is decryption (not reset). A digital signature is an application of public-key cryptography: you sign with the private key and verify with the public key.
用語