This is Donnie. He is my friend.
The main mechanism is to trust each other.
Another metaphor that is used is the sticker that they give you when you enter a guarded building. In the morning, you show your valid identifier (credentials) to the security person, and for the rest of the day, the sticker acts as a pass (token). When you return the next day, you must again show your ID.
- , - . , Kerberos .