Question: What is the symbol "f1" used for in the context of message authentication?
Option 1: Computing RES and XRES
Option 2: Computing MAC (Message Authentication Code)
Option 3: Generating AK (Anonymity Key) in normal procedures
Option 4: Computing CK (Confidentiality Key)
Explanation: The symbol "f1" is used to denote the message authentication function that computes the MAC (Message Authentication Code). This is a critical component of secure communication, as it ensures the integrity and authenticity of messages exchanged between parties.