Question: During USIM authentication in a 3G/EPS/5G network, what happens if the received sequence number (SQN) is considered valid but its value is lower than SQNMS, yet within a certain threshold of previously used numbers?
Option 1: The USIM immediately computes RES, CK, and IK using the provided RAND.
Option 2: The USIM initiates a synchronization failure procedure and responds with AUTS.
Option 3: The USIM accepts it if among the last 32 sequence numbers generated and then proceeds with computing RES, CK, and IK.
Option 4: The USIM requests a new RAND from the network to re-initiate authentication.
Explanation: According to the specified security context for 3G/EPS/5G in USIM applications, if a received SQN is unused and its value is lower than SQNMS but among the last 32 sequence numbers generated, it shall still be accepted. After acceptance, the USIM would proceed with the authentication process, which includes computing RES (result), CK (cipher key), and IK (integrity key) using the provided RAND.