The random assignment of control channel keys to system users is described as follows. For each user u 2 U
and time slot t, the subset Ktu of mt keys4 is randomly selected from Kt and assigned to u, independent of other
users in U. The choice of random key assignment is motivated by the following observations. First, without
prior assumptions on the maximum number of compromised users, choosing the parameters of a deterministic
key assignment scheme [7] may not be possible. Second, the imposed structure of deterministic key assignment
schemes may allow the adversary to learn information about the assignment of keys to users other than those
in C, as shown in [18]. In random key assignment, each user is assigned keys independently, so the adversary
cannot learn any information about the assignment of keys to users other than those in C.