In these protocols, the focus is on how to manage keys within a group in a way that minimizes the cost of key distribution when the membership of the group changes
In a system that secures data by encryption, users are given one or more keys that will enable them to decrypt the data that they are authorized to access. When someone in the group is denied access (e.g., when he or she leaves the organization), the security system must make sure that person's key is no longer valid. This entails generating a new key, re-encrypting the data with the new key, and distributing the new key to the people who still have authorization. The cost of those steps is the time and computing power it takes to effect the change.