Question: In EPS AKA, when deriving KeNB from a fresh KASME after an Attach Request, what value is used as a freshness parameter if a new EPS NAS security context is activated?
Option 1: Current downlink NAS COUNT value
Option 2: Randomly generated nonce by the UE
Option 3: Last successfully received NAS message sequence number
Option 4: Start value of the uplink NAS COUNT
Explanation: According to EPS AKA procedures, when a new EPS NAS security context is established after an Attach Request, the start value of the uplink NAS COUNT is used as a freshness parameter in deriving KeNB from the freshly obtained KASME. This ensures that the combination of KASME and NAS COUNT cannot be reused, guaranteed by the UE's first integrity-protected NAS message sent after AKA, which includes this start value.