Question: In IPsec ESP for SIP signaling between UE (User Equipment) and P-CSCF (Proxy-Call/Session Control Function), how is the integrity key IKESP typically derived for establishing Security Associations (SAs)?
Option 1: Directly from the user's IMSI (International Mobile Subscriber Identity)
Option 2: Through a random number generator within the UE
Option 3: From a pre-shared static key between the UE and P-CSCF
Option 4: Using a key expansion function from the key IKIM established during the AKA (Authentication and Key Agreement) procedure
Explanation: The correct derivation of the integrity key IKESP is crucial for securing SIP signaling. According to telecom security standards, leveraging an existing authenticated key (like IKIM from AKA) and expanding it through a specified function ensures both security and efficiency. This approach avoids weaknesses associated with static keys, random generators without a secure base, or direct use of identifiable information like IMSI. Thus, the integrity key IKESP is securely derived using a key expansion function from the IKIM key established during the AKA procedure, ensuring robust protection for IPsec ESP between UE and P-CSCF.