Patent ID: 7680269

Claim:
A method for ciphering a compressed audio or video data stream that includes a plurality of data packets each having data bits to be ciphered and each delimited by two consecutive synchronization markers, the method comprising: for each data packet, producing a new pseudo-random cipher stream at each initial synchronization marker of the data packet, each pseudo-random cipher stream being different for each data packet; for each data packet, applying the cipher stream for the data packet bit by bit, with the data bits to be ciphered for the data packet, using an “exclusive OR” function, to obtain ciphered data bits; and replacing in the data stream data bits to be ciphered with the ciphered data bits at positions the data bits to be ciphered respectively occupied before ciphering; wherein each cipher stream is generated by processing non-ciphered data of the data stream and wherein the compressed audio or video data to which the method is applied are partly coded by tables of reversible codewords of variable length and of non-reversible codewords of variable length, which are signed or unsigned.