Methods of providing policy based access to master keys, enabling keys to be distributed to groups of users in a secure manner while minimizing disruptions to the user in the event of changes to group membership or changes to user attributes. User attributes are identified. Policies are rewritten in terms of user attributes. New unique user attribute keys are generated for each attribute for each user. An access tree is constructed with user attribute keys as leaf nodes and Boolean algebra operations as internal nodes. Shamir polynomials are used for AND nodes, and broadcast polynomials are used for OR nodes. Master keys are accessible by traversing the access tree from the leaf nodes to the root node constructing the polynomials attached to all the nodes along the access path.