The scalability issues of reliable group rekeying and performance improvement are the two main problems that this paper addresses. There are two types of rekeying access control: back access control and forward access control. The traditional implemention of the latter is to distribute a new key to all of N - 1 users after one user leaves, and this process is repeated every time a user leaves. This approach has a large overhead and leads to synchronization problems when the rekeying process repeats faster than it can complete itself.
This paper addresses these problems by making the following four observations, improvements, and methods:
I give this paper a rating of 3 for modest contribution. I think this paper has some good ideas and improvements for rekeying process.
The graphs and diagrams in the paper help to make things a little clearer for me. The equations are quite complex and a bit hard to understand. The observations made in the paper are good, but I think they're still quite a ways to being implemented. I'd love to see a comparison of how much better this model works than previous models.
Limitations include that this paper focuses on interactions with only one key server. How are things different when multiple key servers are needed to handle a situation where too many users exist to fulfill all the system constraints discussed in 4.3?