Skipping, cascade, and combined chain schemes for broadcast encryption

We develop a couple of new methods to reduce transmission overheads in broadcast encryption. The methods are based on the idea of assigning one key per each partition using one-way key chains after partitioning the users. One method adopts skipping chains on partitions containing up top revoked users and the other adopts cascade chains on partitions with layer structure. The scheme using the former has the transmission overhead r/p+1 + N-r/c, which is less than r/p if r > p² N/c. The scheme using the latter keeps the same transmission overhead with the Subset Difference (SD) scheme when r approaches 0, where r is the number of revoked users. Combining the two schemes, we propose a new broadcast encryption scheme whose transmission overhead is the same with that of the SD scheme for small r and becomes smaller than that of the SD as r grows. The scheme using skipping chains possesses an advantage that any number of new users can join any time at no cost for current users. Finally, we show that the proposed key assignment scheme satisfies key-indistinguishability assuming pseudorandom generators.