Question: In a GBA security context on a USIM, what happens when the computed XMAC does not match the previously produced MAC?
Option 1: The USIM proceeds with checking the AUTN* as in UMTS security context.
Option 2: The USIM computes RES = f2K (RAND) and derives GBA_U bootstrapped key material.
Option 3: The USIM abandons the function and does not send a command response.
Option 4: The USIM stores the mismatch in EFGBABP for later reference.
Explanation: When the computed XMAC does not match the previously produced MAC in a GBA security context on a USIM, it indicates a potential security issue. To maintain security integrity, the USIM will abandon the function to prevent any potentially compromised operations from proceeding. This immediate abandonment is a security measure to protect against unauthorized access or synchronization failures, aligning with the principles of secure bootstrapping architectures like GBA.