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Multicast communication in dynamic environments like ad hoc networks are potentially of critical concern. As group members move in and out of the group, in order to preserve confidentiality, it becomes imperative to use cryptographic keys with rekeying mechanism each time a user enters or leaves. Therefore, in dynamic environments rekeying introduces a significant computational and communication overhead. The contribution of this paper is mainly twofold: (1) Exploiting the orthogonal principle this paper proposes a provably secure n-way cryptosystem, namely, Multicrypt. (2) Using the proposed n-way cryptosystem, this paper also proposes a new secure authenticated key establishment (SAKE) protocol. The proposed SAKE decouples membership change and rekeying. SAKE has less join and leave latency, bandwidth, work per data and control packets by then reducing the communicational and computational overheads. SAKE does not require stateful members due to the use of multiple keys for encryption. Since it uses multiple keys, a compromise of one member does not compromise the group. Multicrypt is a provably secure n-way cryptosystem which uses O(1) keys per user and O(n/m) keys per sub-controller where n is the number of users in the current membership view and m is the number of sub-groups. It supports dynamic member addition and revocation in SAKE. Multicrypt is proved to be IND-CCA and IND-CCA2 secure.