  To avoid delays arising from a need to decrypt a video prior to transcoding
and then re-encrypt it afterwards, this paper assesses a selective encryption
(SE) content protection scheme. The scheme is suited to both recent
standardized codecs, namely H.264/Advanced Video Coding (AVC) and High
Efficiency Video Coding (HEVC). Specifically, the paper outlines a joint
crypto-transcoding scheme for secure transrating of a video bitstream. That is
to say it generates new video bitrates, possibly as part of an HTTP Adaptive
Streaming (HAS) content delivery network. The scheme will reduce the bitrate to
one or more lower desired bit-rate without consuming time in the
encryption/decryption process, which would be the case when full encryption is
used. In addition, the decryption key no longer needs to be exposed at
intermediate middleboxes, including when transrating is performed in a cloud
datacenter. The effectiveness of the scheme is variously evaluated: by
examination of the SE generated visual distortion; by the extent of
computational and bitrate overheads; and by choice of cipher when encrypting
the selected elements within the bitstream. Results indicate that there
remains: a content; quantization level (after transrating of an encrypted
video); and codec-type dependency to any distortion introduced. A further
recommendation is that the Advanced Encryption Standard (AES) is preferred for
SE to lightweight XOR encryption, despite it being taken up elsewhere as a
real-time encryption method.
