It has become a hot topic that how to construct an efficient and secure media streaming overlay over a highly diverse and dynamic peer-to-peer networks recently. SecureCast proposed in this paper is a secure peer-to-peer media streaming system, which adopts an overlay optimization scheme. The streaming model is based on an efficient peer selection algorithm and a set of dynamic maintenance protocol. It leverages the performance information and synchronization estimation between peers to select the best subset of peers as senders. SecureCast also presents a scalable streaming protection scheme based on credentials and filters. With the dynamic peer management protocol, SecureCast is resilient to network dynamic. The valuation of the prototype system shows good performance and security.