We use padlocks, deadbolts, and safes to protect unattended things. We arrange it so that only authorized users may open these locks. We either restrict copies of keys to the trusted people, or we share lock combinations with only those trusted few.
Bob shares his computer with his employees and with those of Alice’s Arts. Sharing causes no problems as long as neither Alice nor her employees meddle with Bob’s bookkeeping files. In FIGURE 6.1, we break the problem down into its components. On the left we have Bob, the person who wants to use the computer. He types in a user identity he wishes to use, and provides a credential to prove the identity belongs to him.