Question: What is the primary purpose of the f1* message authentication code (MAC) function in the context of USIM application security, as defined by 3GPP standards?
Option 1: To directly compute the Cipher Key (CK) for encryption purposes.
Option 2: To ensure no valuable information about other MAC functions (e.g., f1, f2, f5) can be inferred from its function values, and vice versa.
Option 3: To generate a response (RES) for authentication challenges, similar to f2.
Option 4: To derive the Authentication Key (AK) used in secure communication setup.
Explanation: The correct answer is Option 2 because the f1* function is specifically designed with a property that ensures the outputs of f1* do not reveal any useful information about the outputs of other MAC functions (f1, f2, ..., f5), and vice versa. This is crucial for enhancing the security of the USIM application by preventing potential reverse-engineering or inference attacks across different authentication and key generation processes.