Cryptographic key update management method and apparatus

A system comprising multiple apparatuses and a key-hierarchy manager for managing said key hierarchy in dependence on the addition or removal of members and for outputting key update records reflecting changes made to the key hierarchy;

• the apparatuses being configured in a multiple-level hierarchical arrangement comprising a first-level apparatus arranged to receive the records output by the key-hierarchy manager, and one or more lower levels of apparatuses each arranged to receive the key history tree, or a subset of key history tree, produced by a said apparatus at the next level up, the apparatuses at the lowest level of the hierarchical arrangement each being arranged to provide its key history tree, or a subset of its key history tree, to a respective sub-group of members;
• the apparatuses at each level of said hierarchical arrangement, other than said first level, each being arranged to maintain its said key history tree only in respect of keys corresponding to the nodes of a respective predetermined sub-hierarchy of said key hierarchy and keys for the path from the head of this sub-hierarchy that terminates at the root of the key hierarchy.

A method of consolidating key updates provided in records that a key-hierarchy manager generates for members using a key-hierarchy, each comprising an encrypted key corresponding to a node of the key hierarchy that is used by the members and encrypted using a key that corresponds to a descendant of that node, hierarchy-node information for both the encrypted and encrypting keys, and key-version information for at least the encrypted key;

• the method comprising a tree-maintenance step of maintaining in a computer, on the basis of said records, a key history tree with tree nodes corresponding to nodes in said key hierarchy, this tree-maintenance step comprising a sub-step of storing in association with each tree node of the key history tree, for each encrypting key used in respect of the encrypted key associated with the node, the most up-to-date version of the encrypted key and its version information with any earlier versions being discarded.