DNP Secure Authentication MAC


I'm new to DNP3. But reading the IEEE1815 Clause 7spec, it talks about some random data that are generated by the Challenger in a message authentication process. If both sides are using a pre-shared key, and use that to run the MAC algorithm to compare results, but the challenger ('initiator') has added some random data to the initial message, how does the far side know how to run the MAC algorithm to get the same resulting MAC?