The new video coding standard H.264/AVC offers major improvements in the coding efficiency and flexible mapping to transport layers. It consists of a video coding layer (VCL) and a network abstraction layer(NAL). The VCL carries out the coding, and the NAL encapsulates data from the VCL in a manner where transmission over a broad variety of transport layers is readily enabled. Since no security features are offered, an authentication scheme to authenticate the sender and data integrity is needed. In this paper we propose SANAL, a stream authentication scheme for H.264/AVC. Unlike existing schemes that carry out authentication procedures at the packet level, authentication procedures in SANAL are carried out at the NAL level. This makes it possible to set priorities to H.264/AVC-specific data without interfering with the H.264/AVC features. We implemented a SANAL prototype and carried out comparative evaluations on playout rate, communication overhead, and process load. The evaluation results show that the playout rate is improved by 40% compared to existing schemes.