ITPEC FE Subject B October 2025 Question 19

Hybrid Encryption — one-line summary: choosing the correct encryption scheme for secure, efficient file transfer across multiple offices.

The scenario requires:
1. Large files → symmetric encryption is needed for speed
2. Confidentiality during transmission → encryption is mandatory
3. Key must be protected from MITM → symmetric key cannot be sent in plaintext
4. Many branch offices, no in-person key distribution → public key infrastructure needed

This is a classic hybrid encryption setup:
- The survey data is encrypted with a symmetric key (fast, suitable for large files)
- The symmetric key itself is encrypted with the public key of the recipient

The recipient is Ms. T at the head office, because:
- Branch offices send data → they encrypt
- Ms. T receives data → she must decrypt
- Branch offices encrypt the symmetric key using Ms. T's public key
- Only Ms. T can decrypt it with her private key

Why not others:
- (b) hybrid encryption + branch office staff — wrong recipient; encrypting with the sender's public key means only the sender could decrypt, not the intended receiver
- (c) public key encryption only + Ms. T — public key encryption alone is too slow for large files, violating the efficiency constraint
- (d) public key encryption only + branch office staff — both wrong: slow method and wrong key owner
- (e) symmetric key encryption only + Ms. T — no secure way to distribute the symmetric key without public key infrastructure
- (f) symmetric key encryption only + branch office staff — same key distribution problem, and wrong key owner

Key rule: Hybrid encryption = symmetric key encrypts data (speed) + public key encrypts the symmetric key (secure key exchange). The public key used belongs to the recipient, not the sender.

AI-generated — may contain errors