The master key already exists. It is the key used to sign builds.

Edit: downvotes of easily verifiable facts are causing hn to block me from responding. At the time Apple made the false marketing claims, no passcode was required to install a signed build. Hence, the FBI's request.

The FBI was asking for no more than what Apple could already do, and it was letting Apple control the whole process. The problem was that what Apple could already do disagreed with what Apple told its customers that it could do.