A data stream, such as a digital motion picture, is encrypted in units of one or more blocks, each block having an assigned encryption key. A plurality of encryption keys is assigned to the complete data stream, with a synchronization index provided to map each individual encryption key to its starting data block. Encryption keys and associated synchronization indices are provided separately from the data stream, using one or more additional data transfer mechanisms. An optional offset, randomly generated, allows variation in intervals between data blocks at which encryption by a specific encryption key can be performed.