Question: What is the primary purpose of the COUNT-I input parameter in 3GPP security protocols, and how does its structure contribute to achieving this goal?
Option 1: To authenticate user equipment (UE), with COUNT-I consisting solely of a randomly generated 32-bit number.
Option 2: To protect against replay attacks during connections, utilizing a static 32-bit value that never changes.
Option 3: To prevent replay attacks by incorporating a dynamic value that increments with each integrity-protected message, structured as a combination of Hyperframe Number (HFN) and RRC Sequence Number.
Option 4: To encrypt data transmissions, using COUNT-I as a key derived from the UE's IMSI and the network's MCC.
Explanation: The correct answer is Option 3 because COUNT-I protects against replay attacks during connections. Its dynamic nature, incrementing with each integrity-protected message, ensures no COUNT-I value is reused with the same integrity key. The structure of combining Hyperframe Number (HFN) and RRC Sequence Number within the 32-bit COUNT-I facilitates this protection by ensuring a unique identifier for each message, preventing replay attacks in 3GPP security protocols.