問76
頻出度★★★
IoT デバイス群とそれを管理する IoT サーバで構成される IoT システムがある。 全ての IoT デバイスは同一の鍵を用いて通信の暗号化を行い, IoT サーバでは IoT デバイスがもつ鍵とは異なる鍵で通信の復号を行うとき, この暗号技術はどれか。
令和3年度 ITパスポート試験 問76
This question's explanation and your answers so far.
解説
問 76
Incorrectア
Symmetric-key cryptography uses the same key for both encryption and decryption. This question explicitly states that the server decrypts with a key different from the device's key, which contradicts the premise that the encryption and decryption keys are the same, so it is incorrect.
- イ
正解 In public-key cryptography, the key used for encryption and the key used for decryption are different (asymmetric). In this question, all the IoT devices encrypt their communications with the same key (the public key), while the IoT server decrypts them with a different key (the private key). The condition that the encryption key and the decryption key differ is exactly the definition of public-key cryptography, so the correct answer is イ. Another advantage is that the public key may be handed out to every device, while only the server holds the private key that can decrypt; even if a device-side encryption key leaks, the contents of the communication are hard to decode.
Incorrectウ
A hash function is a one-way function that derives a fixed-length digest (hash value) from an input, and in principle it cannot be decrypted back to the original. It cannot be used for the purpose of encrypting a communication and later decrypting it, so it is incorrect. It is a technique used for tamper detection and password storage.
Incorrectエ
Blockchain is a ledger technology that manages transaction records in a distributed manner to make tampering difficult; it is not itself a cryptographic method that encrypts and decrypts communications. It also does not match this question's description of encrypting and decrypting with different keys, so it is incorrect.
ここがポイント
- In public-key cryptography the encryption key and the decryption key differ (asymmetric); in symmetric-key cryptography they are the same. Tell the two apart by whether the keys are symmetric or asymmetric.
- A hash function is a one-way function that cannot be decrypted, and blockchain is a distributed ledger technology; distinguish that neither is a cryptographic method that encrypts and decrypts with keys.
用語