IPA IP 2025 Question 89
A question IPA Japan published in 令和7年度 ITパスポート試験 公開問題 問89, translated into English. ITPEC is based on the same exams.
Mr. A received the following four types of messages from Mr. B. Which of the following lists all and only the received messages that Mr. A can decrypt and read?
Explanation
(i), (ii) and (iv) — Mr. A can decrypt whatever was encrypted with a key whose decrypting partner he holds or can obtain.
- •(i) Common key: the same key encrypts and decrypts, and Mr. A shares it with Mr. B. Readable.
- •(ii) Mr. A's public key: only the matching private key decrypts it, and that is Mr. A's own. Readable.
- •(iii) Mr. B's public key: only Mr. B's private key decrypts it, which Mr. A does not have. Not readable.
- •(iv) Mr. B's private key: anyone can decrypt it with Mr. B's public key, which is published. Readable; this is the principle behind a digital signature.
Answer (a)
Why not others:
- •(b) — includes (iii), which only Mr. B can decrypt, and leaves out (ii), which Mr. A decrypts with his own private key
- •(c) — leaves out (i), though Mr. A holds the common key
- •(d) — includes (iii) and leaves out (i) and (ii)
Key rule: A public key and its private key undo each other: what the public key encrypts only the owner can read; what the private key encrypts anyone with the public key can read.
Security · AI-written, may contain errors
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.