In secure video multicast systems, access control is a very important and challenging issue. A common practice is to encrypt the video content using a group key, shared only by legitimate clients. Since the clients can join and leave the multicast session frequently, the group key needs to be updated and delivered to the legitimate clients accordingly in order to provide forward/backward secrecy. Therefore, key management and distribution plays a crucial role in building a secure and scalable video multicast system. Conventional approaches usually use a dedicated secure channel independent of the video data channel, which is expensive and inefficient. In this paper, we propose a novel and scalable Media-dependent Key Management and Distribution (MKMD) solution that embeds key management messages in video data so as to save network bandwidth and enhance the security level, and most importantly, without sacrificing the quality of video content too much. We have built a prototype system of MKMD, and our extensive experimental 'results show that MKMD is a secure, scalable, reliable and efficient key management and distribution solution for video multicast system.