ITPEC FE Subject B October 2024 Question 19

Digital signature + encryption — choosing correct keys for signing and encrypting in RSA communication.

Two requirements are stated:
1. Ms. A must confirm the sender is Mr. L → digital signature is needed
2. Only Ms. A can view the file → encryption for confidentiality is needed

Signing (blank A):
- To prove identity, Mr. L signs with his own private key
- Ms. A verifies the signature using Mr. L's public key
- Only the holder of the private key can produce a valid signature

Encrypting (blank B):
- To ensure only Ms. A can read the file, encrypt with Ms. A's public key
- Only Ms. A can decrypt it using her private key

Process: sign first → encrypt second.

Answer: (d) — signed using Mr. L's private key, then encrypted using Ms. A's public key.

Why not others:
- (a) Encrypting with Mr. L's private key means anyone with his public key can decrypt — no confidentiality
- (b) Encrypting with Mr. L's public key means only Mr. L can decrypt — wrong recipient
- (c) Encrypting with Ms. A's private key means anyone with her public key can decrypt — no confidentiality
- (e–h) Signing with Mr. L's public key is meaningless — public keys are shared, so anyone could "sign" with it, proving nothing about identity

Key rule: sign with sender's private key (authentication), encrypt with recipient's public key (confidentiality).

AI-generated — may contain errors