A Novel Secure H.264 Transcoder using Selective Encryption

In digital broadcast TV systems, video data is normally encrypted before transmission. For in-home redistribution, it is often necessary to transcode the bitstream to achieve optimum utilization of available bandwidth. If a signal is decrypted before transcoding and re-encrypted, this may lead to a security loophole. This paper presents a solution in the form of a novel H.264 selective encryption algorithm that encrypts sign bits of transform coefficients and motion vectors to allow secure transcoding without decryption. The performance of this system is compared with I-frame encryption. The results show that sign encryption is more secure than I-frame encryption and has a lower complexity. A hybrid system using a modified transcoder and sign encryption is found to give an optimal compromise between security and transcoding performance.