Question: Under what circumstances are the NAS COUNTs for a particular KASME (Key Access Security Management Entity) set to their start value in a 3GPP SAE (System Architecture Evolution) security architecture?
Option 1: During handover for an already existing native EPS NAS security context, or when a UE moves back and forth between two MMEs.
Option 2: For a partial native EPS NAS security context created by a successful AKA run, or during context mapping from UTRAN/GERAN to E-UTRAN (handover or idle mode mobility).
Option 3: When the NAS COUNTs reach their maximum value and need to be reset for continuous secure communication.
Option 4: Upon every NAS Security Mode Command (SMC) message sent by the MME for a given security context.
Explanation: This is correct because, as per the 3GPP SAE security architecture specifications, resetting NAS COUNTs to their start value (0) is only allowed in specific scenarios to prevent security issues like key stream re-use. These scenarios include creating a new partial native EPS NAS security context after a successful Authentication and Key Agreement (AKA) run, or during the mapping of an EPS NAS security context from UTRAN/GERAN to E-UTRAN due to handover or idle mode mobility. This ensures that NAS COUNTs are not reused with the same NAS keys in different contexts, maintaining the integrity of the security protocol.