IPA IP 2023 Question 86
A question IPA Japan published in 令和5年度 ITパスポート試験 公開問題 問86, translated into English. ITPEC is based on the same exams.
A message is to be sent by using a hybrid encryption method. In the figure that shows the procedure for encrypting the message and the key for decryption, when the key used for encrypting the message is (1) and the key used for encrypting (1) is (2), which of the following is the appropriate combination of words to be inserted into A and B in the figure?
Explanation
A: common, B: public — hybrid encryption encrypts the message with a common key and sends that key encrypted with the recipient's public key.
Common key cryptography is fast, so it suits the message itself, but the common key has to reach the recipient safely. Public key cryptography solves the delivery: anything encrypted with the recipient's public key can be decrypted only with the recipient's private key, which never leaves the recipient. So:
- •(1), the key that encrypts the message, is a common key (blank A), generated for this message
- •(2), the key that encrypts (1), is the recipient's public key (blank B)
The recipient first decrypts the encrypted common key with their private key, then decrypts the message with that common key.
Answer (a)
Why not others:
- •(b) — the recipient's private key is secret to the recipient; the sender does not have it, and what it encrypts anyone could decrypt with the public key
- •(c) and (d) — public key encryption of the whole message is slow, which is exactly what the hybrid method avoids; and (c) would need a common key shared in advance, the problem being solved
Key rule: Hybrid encryption = message under a fast common (session) key, and the common key under the recipient's public key; the recipient's private key unlocks the session 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.