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We prove a tight lower bound on the communication complexity of secure multicast key distribution protocols in which rekey messages are built using symmetric-key encryption, pseudorandom generators, and secret sharing schemes. Our lower bound shows that the amortized cost of updating the group key for each group membership change (as a function of the current group size) is at least log2(n) - o(1) basic rekey messages. This lower bound matches, up to a subconstant additive term, the upper bound due to Canetti et al. [Proc. INFOCOM 1999], who showed that log2(n) basic rekey messages (each time a user joins and/or leaves the group) are sufficient. Our lower bound is, thus, optimal up to a small subconstant additive term. The result of this paper considerably strengthens previous lower bounds by Canetti et al. [Proc. Eurocrypt 1999] and Snoeyink et al. [Computer Networks, 47(3):2005] , which allowed for neither the use of pseudorandom generators and secret sharing schemes nor the iterated (nested) application of the encryption function. Our model (which allows for arbitrarily nested combinations of encryption, pseudorandom generators and secret sharing schemes) is much more general and, in particular, encompasses essentially all known multicast key distribution protocols of practical interest.