This paper discusses encryption operations that selectively encrypt content-carrying segments of multimedia data stream. We propose and analyze three techniques that work in different domains, namely, a syntax-aware selective bitstream encryption tool with bit stuffing, a generalized index mapping encryption tool with controlled overhead and an intra-bitplane encryption tool compatible with fine granularity scalable coding. The designs of these proposed encryption operations take into consideration the inherent structure and syntax of multimedia sources and have improved friendliness to communications, compression and computation.