問78
頻出度★★★
受け取ったデータが改ざんされていることを検知するのに使われる技術はどれか。
平成27年度 春期 ITパスポート試験 問78
This question's explanation and your answers so far.
解説
問 78
Incorrectア
Compression is a technology that removes redundancy to reduce data size, saving storage space and shortening transfer time. It is about whether the original can be restored (lossless or lossy); it has no mechanism for checking whether received data was altered along the way. Compressed data can be tampered with just the same.
- イ
正解 The correct answer is イ (Digital signature). The sender passes the data through a hash function to obtain a fixed-length hash value (message digest), encrypts that value with their own private key, and attaches it to the data as a signature. The receiver decrypts the attached signature with the sender's public key to recover the hash value, then compares it with a hash value they recompute themselves from the data actually received. If the two match, the data was not altered in transit (integrity is preserved); if even a single bit was tampered with, the hash value changes, the two no longer match, and the tampering is detected. At the same time, because the signature decrypts correctly with the sender's public key, it also confirms that this is not an impersonation (authentication) and that the sender cannot later deny having sent it (non-repudiation).
Incorrectウ
Password authentication is a technology for verifying that a user is who they claim to be. It can establish who is accessing the system, but it cannot detect whether data flowing over the network after authentication has been tampered with. What it protects is not integrity but identity (access control), which is the difference.
Incorrectエ
Filtering is a technology that lets through only the communications or content matching preset conditions and blocks inappropriate ones (URL filtering, packet filtering, and so on). It decides whether to allow or block, and has no function for verifying whether the content of data that passed through is still the original.
ここがポイント
- Among the three elements of information security, integrity — guaranteeing that data has not been tampered with — is provided by the digital signature. Remember the flow: a hash value is produced by a hash function, encrypted by the sender with their private key, and verified by the receiver with the public key.
- A digital signature satisfies three things at once: tamper detection (integrity), confirmation of the sender (authentication), and non-repudiation. By contrast, password authentication verifies identity, compression reduces size, and filtering screens communications — each has a different purpose, so learn them by comparison.
用語