IPA IP 2022 Question 60
A question IPA Japan published in 令和4年度 ITパスポート試験 公開問題 問60, translated into English. ITPEC is based on the same exams.
Which of the following is the appropriate combination of words to be inserted into blanks A and B in the following description concerning the keys used in public key cryptography?
When Company A and Company B, each of which has a public key and a private key, send and receive information, it was decided that the information would be encrypted before it is sent, so that its content would not be known even if the communication were intercepted by others.
When information is sent from Company A to Company B, Company A sends Company B the information encrypted by using A. Company B decrypts the information received from Company A with B and extracts the information.
Explanation
A: Company B's public key, B: Company B's private key — to keep a message secret, the sender encrypts it with the recipient's public key, and only the recipient's private key can decrypt it.
In public key cryptography each party has a key pair. What one key of the pair encrypts, only the other key of the same pair can decrypt. The public key is given to anyone; the private key is held by its owner alone.
- •Company A wants only Company B to read the information, so it encrypts with Company B's public key, which it can obtain freely.
- •That ciphertext can be decrypted only with Company B's private key, which no one but Company B holds. An eavesdropper who intercepts it cannot read it.
Answer (d)
Why not others:
- •(a) — a public key cannot decrypt what a public key encrypted, and Company A's public key is known to everyone, so it would protect nothing
- •(b) — Company B's private key does not match Company A's public key; only Company A's private key could decrypt it, and then Company B could not read it
- •(c) — a public key cannot decrypt what was encrypted with Company B's public key, and Company A's key is from another pair
Key rule: For confidentiality, encrypt with the recipient's public key and decrypt with the recipient's private key; for a digital signature it is the reverse, the sender's private key and the sender's public key.
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.