Meet-in-the-Middle Attack is done by the Attacker choosing 232 different 64-bit keys at random. He then computes the Message Authentication Code on the ‘‘Are you ready to receive a transaction?’’ message, and stores both the MAC result and the key in a table.
The Attacker then eavesdrops on each transaction and checks if the MAC of the first message appears in his table. If the MAC does appear in the table, then there is a very good chance that the authentication key for that transaction is the same key as the attacker used to compute that table entry, and that key value is stored right alongside the MAC value in the table. Now that the attacker knows the authentication key, he can insert arbitrary messages of his choosing into the transaction. (The Birthday Attack only allowed him to insert messages from an old transaction.)