問59
頻出度★★★
ディジタル署名を用いることで可能なことはどれか。
平成24年度 春期 ITパスポート試験 問59
This question's explanation and your answers so far.
解説
問 59
Incorrectア
Incorrect. A digital signature is a mechanism for confirming that a document has not been tampered with and that the sender is who they claim to be; it does not have the function of telling the recipient “which encryption method was used.” Agreeing on the encryption method and algorithm is the role of the contents of a digital certificate or of the communication protocol (such as the SSL/TLS handshake), not the purpose of the signature itself.
- イ
正解 The correct answer is イ, “Detecting tampering of the signed document.” With a digital signature, the sender first computes a hash value (message digest) over the whole document, encrypts that hash with their own private key, and attaches the result to the document as the signature. The recipient decrypts the attached signature with the sender's public key to obtain the hash value, and compares it with the hash value they recompute themselves from the document actually received. If even one character of the document was altered along the way, the hash value changes and the two no longer match, so the recipient can detect the fact that the document was tampered with. In addition, because the signature decrypts correctly with the sender's public key, it also confirms that the document really was signed by that person (authentication and non-repudiation). In other words, what a digital signature provides is confirmation of integrity and authenticity, and its concrete effect is the detection of tampering.
Incorrectウ
Incorrect. This choice confuses “detection” with “prevention.” A digital signature can expose tampering after it has happened, but it cannot physically stop a third party from actually rewriting the contents of the document in transit. The signature is, so to speak, an inspection of a seal, not a lock that blocks rewriting. This distinction between detection and prevention is the main point this question is testing.
Incorrectエ
Incorrect. A digital signature does not encrypt the body of the document, so a signed document is sent in a directly readable state. Therefore, if it is eavesdropped on, the contents leak out, and leakage cannot be prevented (confidentiality is not ensured). If you want to protect confidentiality, you must separately encrypt the document itself with symmetric-key or public-key encryption, in addition to the signature.
ここがポイント
- The effects of a digital signature are “detecting tampering” and “confirming the sender's identity (non-repudiation)” — not preventing tampering and not preventing information leakage. Do not confuse detection with prevention.
- The mechanism is: the hash value is encrypted with the sender's private key and attached, then the recipient decrypts it with the sender's public key and checks it against the hash value they computed themselves. If confidentiality is needed, the body of the document must be encrypted separately.
用語