問74
頻出度★★★
データの送信側は受信者の公開鍵で暗号化し，受信側は自身の秘密鍵で復号することによって実現できる対策はどれか。
平成21年度 秋期 ITパスポート試験 問74
This question's explanation and your answers so far.
解説
問 74
Incorrectア
To prevent sender impersonation (that is, to authenticate the sender), a digital signature is needed: the sender signs with their own private key and the receiver verifies with the sender's public key. This question uses the recipient's key pair, so the keys are used the other way around and the identity of the sender is not guaranteed. Since anyone can obtain the recipient's public key, a third party can also impersonate a sender and send ciphertext.
- イ
正解 In public-key cryptography, text encrypted with a public key can only be decrypted with the paired private key. Here the sender encrypts with the recipient's public key, so only the recipient who holds that private key can decrypt it. Even if a third party intercepts the ciphertext along the communication path, they cannot read its contents without the private key. In other words, what this procedure achieves is confidentiality of the communication content, that is, prevention of data eavesdropping on the communication path, so the correct answer is イ.
Incorrectウ
Preventing transmission errors from occurring is the job of line quality and data transmission techniques such as parity, CRC, error-correcting codes, and retransmission control; it has nothing to do with encryption. Even with encryption, bit errors caused by noise still occur (in fact, if errors occur, the decrypted result is corrupted).
Incorrectエ
Encryption only conceals the content; it has no function to restore parts that have been destroyed. Tampering can be detected using hash values, digital signatures, or message authentication codes, but that is detection, not restoration. If data must be restored, other means such as retransmission, backups, or error-correcting codes are required.
ここがポイント
- To obtain confidentiality (eavesdropping prevention) with public-key cryptography, the keys are used as: encrypt with the recipient's public key, then decrypt with the recipient's private key. Only the recipient holding the private key can decrypt.
- With the same public-key cryptography, when the purpose is authentication or tamper detection (digital signature), the keys are used the other way around: sign with the sender's private key, then verify with the sender's public key. This contrast is a frequently tested point.
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