Content protection with bitstream watermarking at decryption stage

A new method for protecting digital video content from illegal copying is proposed. Before spreading to clients, a content is encrypted by a stream cipher with a key stream filter generator. At the same time, the latter is an encryption key of the cipher. Every client receives the content together with a decryption key which differs from the encryption key only by the filtering function. This difference is such that the decrypted content differs from the original one in several unique bits not essentially distorting the video and completely identifying the client.