In this paper we are try to summarize cloud computing RAS Reliability, Availability and Security issues and also clarify availability solution for some of them. In this paper I try to summarize virtualization level of cloud computing security in details views. The main objective of this paper is to propose such a secure and efficient threshold-level security scheme. The paper uniquely defines the fundamental properties of threshold-level security schemes and shows that the proposed scheme satisfies these properties and eliminates the latest attacks to which other similar schemes are subject. The paper also proposes a discrete logarithm based distributed-key management infrastructure DKMI, which consists of a round optimal, publicly verifiable, distributed-key generation DKG protocol and a one round, publicly verifiable, distributed-key redistribution updating DKRU protocol.