Question: What is the primary purpose of RES (and XRES) in the context of 3GPP security specifications, and what is the valid range for their lengths?
Option 1: Confidentiality of user data; length can be any multiple of 4 bits between 16 and 256 bits.
Option 2: Integrity protection of signaling; fixed length of exactly 64 bits.
Option 3: Encryption key exchange between UE and RAN; minimum of 16 bits, maximum of 256 bits, with no specific increment requirement.
Option 4: Entity authentication of the user to the network; length shall be a multiple of 8 bits, at least 32 bits, and at most 128 bits.
Explanation: The correct answer is Option 4 because RES (and XRES) are primarily used for entity authentication of the user to the network. Their lengths must be a multiple of 8 bits, at least 32 bits, and at most 128 bits, ensuring alignment with standard byte boundaries crucial for efficient processing in digital systems.