Question: When using a USIM for IMS (IP Multimedia Subsystem) access, which of the following options is applicable for sharing security functions and data?
Option 1: No security functions or data are shared.
Option 2: The authentication key, authentication functions, and the sequence number checking mechanism are shared.
Option 3: Only the algorithm and sequence number checking mechanism are shared.
Option 4: Only the sequence number checking mechanism is shared.
Explanation: According to the specified security protocols for IMS access via USIM, sharing security functions and data is limited to a specific set that ensures robust security across domains. Sharing the authentication key, authentication functions, and the sequence number checking mechanism (as in Option 2) is the only permitted configuration when a USIM is used for IMS access. This setup maintains the integrity of both CS/PS (Circuit Switched/Packet Switched) domains and IMS, preventing vulnerabilities from spreading across domains due to independent key agreement protocols.