Secure presentation of media streams includes encoding the media streams into digital content, encrypting a portion of that digital content, the portion being required for presentation, in which the encrypted version is substantially unchanged in formatting parameters from the clear version of the digital content. Selecting those portions for encryption so there is no change in distribution of the media stream: packetization of the digital data, or synchronization of audio with video portions of the media stream. When encoding the media stream into MPEG-2, refraining from encrypting information by which the video block data is described, packet formatting information, and encrypting the video block data using a block-substitution cipher. A block-substitution cipher can be used to encrypt each sequence of 16 bytes of video data in each packet, possibly leaving as many as 15 bytes of video data in each packet in the clear.