This paper considers the use of selective encryption systems for compressed video data that can support transcoding of the encrypted bit-streams without the need for decryption and re-encryption. The focus is on the use of the H.264 video codec. However, the concepts can be easily extended to any video coding standard. I-frame encryption encrypts only the intra frames, making it suitable for use with modified interframe transcoders that operate only on P and B frames. These interframe transcoders are found to offer improved performance for un-encrypted data, due to the temporal propagation of the higher quality of l-frames. The operations of both interframe open loop transcoding and Fast Pixel Domain Transcoding (FPDT) are unaffected by l-frame encryption yielding identical results. However, the lack of I-frame data causes a significant drift problem when l-frame encryption is used with a more complete Cascaded Pixel Domain Transcoder (CPDT). A modified CPDT is proposed, called Intra Block Copy (IBC) transcoding, which offers improved performance at the expense of a reduced range of possible output bitrates. Sign bit encryption systems such as Real-time Video Encryption Algorithm (RVEA) offer improved security compared to l-frame encryption. However transcoding this data can result in some sign bits being omitted causing a loss of cipher synchronization. A novel sign bit encryption technique called Synchronous Video Encryption Algorithm (SVEA) is proposed that preserves the cipher synchronization. While the operation of open loop transcoding systems is unaffected, the encryption interferes with the drift compensation in the more complex transcoders, leading to reduced performance. (C) 2010 Elsevier B.V. All rights reserved.
