Question: What is the primary security requirement for the anonymity key derivation function f5*, used for re-synchronization in telecom networks?
Option 1: It must be computationally feasible to derive the key K from RAND and AK to ensure fast reconnection.
Option 2: The function should allow for easy reversibility to enable quick authentication of User Equipment (UE).
Option 3: It shall be computationally infeasible to derive the key K from knowledge of RAND and AK, ensuring security.
Option 4: The output AK must always be a fixed length, regardless of input sizes for RAND and K.
Explanation: This is correct because the primary goal of a key derivation function in secure telecommunications, such as f5*, is to ensure that even with knowledge of some inputs (RAND) and outputs (AK), an attacker cannot feasibly compute the original secret key (K). This protects against various attacks that might exploit known or guessed values. Options 1 and 2 are incorrect because they suggest vulnerabilities rather than security enhancements. Option 4, while potentially a characteristic of some cryptographic functions, does not address the core security requirement as specified for f5*.