Portable devices can support many multimedia communication applications (e.g., voice & video calls, movie-on-demand, etc.). For some applications, the media should be encrypted for confidentiality, privacy or access control. Since portable devices are equipped with low-end CPUs and powered by batteries, it is desirable that encryption and decryption would not use significant CPU time and battery energy. In this paper, we propose a dynamic encryption protocol for secure multimedia communication. It dynamically applies multiple cryptographic algorithms to encrypt different parts of the media contents, where some algorithms may be computation-intensive ones for stronger security protection and other algorithms may be lightweight ones for smaller CPU and energy usage. This protocol has two desirable features: i) it provides desirable and adjustable tradeoff between security and CPU/energy usage, and ii) it is applicable to any media and encoding methods.