IPA IP 2021 Question 76
A question IPA Japan published in 令和3年度 ITパスポート試験 公開問題 問76, translated into English. ITPEC is based on the same exams.
There is an IoT system that consists of a group of IoT devices and an IoT server that manages them. When all the IoT devices encrypt communications by using the same key, and the IoT server decrypts the communications with a key that is different from the key that the IoT devices have, which of the following is this encryption technology?
Explanation
Public key cryptography — encrypting with one key and decrypting with a different key is what defines it.
In public key cryptography each key pair has a public key and a private key. Data encrypted with the public key can be decrypted only with the matching private key. Here, the IoT server keeps the private key and gives its public key to every IoT device. All the devices therefore encrypt with the same key, the public key, and only the server, holding a different key, the private key, can decrypt. A thief who extracts the public key from one device still cannot read the other devices' communications.
Answer (b)
Why not others:
- •(a) Common key cryptography — the sender and receiver use the same secret key to encrypt and decrypt, so the server would need the devices' key, not a different one
- •(c) Hash function — produces a fixed-length digest that cannot be turned back into the original data; it does not encrypt for later decryption
- •(d) Blockchain — chains records with hashes to make tampering detectable; it is not a method of encrypting and decrypting communication
Key rule: Same key for encryption and decryption means common key cryptography; different keys, public to encrypt and private to decrypt, means public key cryptography.
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This question comes from the past questions IPA Japan publishes, translated into English by ITPEC Practice. ITPEC Practice is an independent study tool and is not affiliated with IPA or ITPEC. See the original paper (PDF) or Report an issue.