We study the problem of multicasting encryption to some subsets of the privileged users and ensuring that only non-revoked users can decrypt the contents in the context of a single source multicast. We concentrate on large dynamic multicast group case with low-state users or stateless users. We propose a Practical Clumped-tree Multicast Encryption scheme (PCME) based on the idea of two-layer tree (container-tree and clumped-tree) and digital label method. The important feature of this scheme is the separation between the static container-tree and the dynamic clumped-tree. We regard a clumped-tree as an autonomous unit, and do not spread the information about revoked users in a clumped-tree to the container-tree. The separation also provides secure multicast channel for distinct GC to multicast completely different content and for any user to multicast encryption to any collection of intended subtrees. Let n be the number of privileged users. Group center, clumped-tree center and user each stores only n/29 –1, (212-1)/3+log(n/210)+1 and 6 keys independently with revocation cost being only 15 to revoke a user. Digital label method accelerates the collection of privileged subsets and the multicast of encryption, and any subtree or user can determine its size and relative position in the whole tree immediately from its digital label. The PCME scheme is truly realistic: even for an astronomical number of 256,000,000 stateless users, GC storage is less than 4 Mbytes, CC storage is less than 10K bytes, and user storage is less than 100 bytes, while for 512 revocations, message length is only 512. Except for efficiency, PCME scheme is fully scalable and it is resistant to adversarial coalitions of various sizes.
['Ling Dong', 'Kefei Chen']
A practical clumped-tree multicast encryption scheme