ITPEC FE Subject B April 2025 Question 18

The system uses two encryption steps:

Blank A — symmetric key encryption:
- StoreFile(A(file, KDF(password))) encrypts the file using a key derived from the password via KDF (Key Derivation Function)
- The same password-derived key is used by both sender and recipient to encrypt/decrypt the file
- This is symmetric encryption — one shared secret (the password) for both operations
- Symmetric encryption is efficient for large files

Blank B — asymmetric key encryption:
- Message(file id, B(password, Q's public key)) encrypts the password using Q's public key
- Only Q can decrypt it with their private key
- This is asymmetric (public-key) encryption — used to securely transmit the shared secret

This is the classic hybrid encryption pattern (same concept behind TLS/HTTPS):
1. Encrypt bulk data with a symmetric key (fast, efficient for large files)
2. Encrypt the symmetric key with the recipient's public key (secure key exchange)

Why not others:
- (a) A as asymmetric — file is encrypted with a password-derived key (shared secret), not a public key
- (b) B as symmetric — the password is encrypted with Q's public key, which is asymmetric
- (c) A as hash — hashing is one-way; the file must be decryptable, so encryption is needed
- (d) A as hash — same reason as (c); hashing cannot be reversed to recover the file
- (f) B as hash — hashing the password would make it unrecoverable; Q needs the actual password to decrypt the file

Key rule: When a password/key encrypts bulk data → symmetric; when a public key protects that password → asymmetric. Together = hybrid encryption.

AI-generated — may contain errors