E-commerce companies like Amazon have gained efficiency by delivering parcels to centrally-located hubs of lockers, rather than delivering to individual homes. Customers pick up their parcels by entering a unique code, which causes the appropriate locker to open. Hubs come in many sizes. Cathy the criminal thinks she has a slick idea; she plans to enter random codes into the hub until a locker opens, and then she'll take the parcel inside it. Amazon wants to protect their customers from criminals like Cathy. They want the passcode system to be strong enough that there will be