Question: In a GBA security context (Bootstrapping Mode) on an ISIM, what happens if the computed XMAC does not match the previously produced MAC?
Option 1: The ISIM proceeds with the remaining AUTN* checking as in IMS security context.
Option 2: The ISIM abandons the function and does not generate a response related to key material update.
Option 3: The ISIM stores the new RAND in EFGBABP but skips updating GBA_U bootstrapped key material.
Option 4: The ISIM automatically initiates a re-sync procedure by requesting a new RAND and AUTN*.
Explanation: According to the described process for GBA security context in Bootstrapping Mode, if the computed XMAC does not match the MAC previously produced, the ISIM abandons the function. This is a security measure to prevent potential attacks or errors in the bootstrapping process, ensuring the integrity of the key material update. Abandoning the function at this stage prevents any potentially compromised key material from being stored or used.