問90
頻出度★★★
PKI(公開鍵基盤)の特徴として, 適切なものはどれか。
令和8年度 ITパスポート試験 問90
This question's explanation and your answers so far.
解説
問 90
Incorrectア
PKI deals with the keys of public-key cryptography, which consist of a public key and a private key; it is not a mechanism that provides verification of the owner of a shared (symmetric) key used in symmetric-key cryptography. A shared key is one that is securely shared and kept secret between the parties involved, and the very idea of guaranteeing its owner with a certificate falls outside the scope of PKI.
Incorrectイ
Trusting a key signed by an acquaintance through a 'web of trust' is a decentralized trust model typified by PGP. PKI does not use a web of trust; instead, it guarantees the validity of public keys through a hierarchical (tree-shaped) trust structure with a certificate authority at the top. The two are different approaches, and this is not a characteristic of PKI.
- ウ
正解 PKI (Public Key Infrastructure) is a mechanism in which a certificate authority (CA), acting as a trusted third party, guarantees 'whose' a public key is. After verifying the applicant's identity, the CA issues a digital certificate that binds the public key to the owner's information and signs it with its own private key. By verifying this signature with the CA's public key, a user can confirm that the digital certificate has not been tampered with and is legitimate. Therefore option ウ, which states that 'a certificate authority guarantees the validity of a digital certificate', is correct.
Incorrectエ
A private key is a key that only its owner manages strictly and must never hand over to others or make public. If a private key is made public, impersonation and signature forgery become possible, and the premise of cryptography collapses. What PKI makes public is the public key, not the private key, so 'a method to safely make a private key public' is incorrect.
ここがポイント
- PKI (Public Key Infrastructure) is a hierarchical trust model with a certificate authority (CA) at the top, in which the CA issues and signs digital certificates and guarantees the validity of the binding between a public key and its owner.
- A private key is kept secret and managed only by its owner and is never made public; only the public key may be distributed or disclosed to third parties. The 'web of trust' is the approach used by PGP, not PKI.
用語