Question: What is the primary security requirement for the anonymity key derivation function f5: (K; RAND) → AK in a telecommunication network's normal operation?
Option 1: It must be able to derive RAND from knowledge of K and AK.
Option 2: The output AK should always be a fixed length, regardless of input sizes.
Option 3: The function should prioritize speed over computational security for faster key exchange.
Option 4: It shall be computationally infeasible to derive the base key K from knowledge of RAND and AK.
Explanation: This question assesses understanding of cryptographic principles in telecom security, specifically key derivation functions. The correct answer reflects a fundamental security requirement: ensuring that knowing some outputs (RAND and AK) does not compromise the secrecy of the base key (K), crucial for maintaining anonymous communications security in mobile networks.