Video Encryption

 by: Alastair Taylor

Video Encryption is an extremely useful method for the stopping unwanted interception and viewing of any transmitted video or other information, for example from a law enforcement video surveillance being relayed back to a central viewing centre.

The scrambling is the easy part. It is the un-encryption that's hard, but there are several techniques that are available. However, the human eye is very good at, spotting distortions in pictures due to poor video decoding or poor choice of video scrambling hardware. Therefore, it is very important to choose the right hardware or else your video transmissions may be un-secure or your decoded video may not be watchable.

Some of the more popular techniques are detailed below:

Line Inversion:

Method: Whole or parts of the signal scan lines are inverted.

Advantages: Simple, cheap video encryption.

Disadvantages: Poor video decrypting quality, low obscurity, low security.

Sync Suppression:

Method: Hide/remove the horizontal/vertical line syncs.

Advantages: Provides a low cost solution to Encryption and provides good quality video decoding.

Disadvantages: This method is incompatible with some distribution equipment. Obscurity (i.e. how easy it is to visually decipher the image) is dependant on video content.

Line Shuffle:

Method: Each signal line is re-ordered on the screen.

Advantages: Provides a compatible video signal, a reasonable amount of obscurity, good decode quality.

Disadvantages: Requires a lot of digital storage space. There are potential issues with video stability. Less secure than the cut and rotate encryption method (see below)

Cut & Rotate:

Scrambling Method: Each scan line is cut into pieces and re-assembled in a different order.

Advantages: Provides a compatible video signal, gives an excellent amount of obscurity, as well as good decode quality and stability.

Disadvantages: Can have complex timing control and requires specialized scrambling equipment

The cut and rotate video encryption method is probably the best way of achieving reliable and good quality video encryption, an example of a good implementation of this system is in the Viewlock II

Implementing vice scrambling

The video scrambling hardware, in particular the decoder should function correctly even if there is a noisy (for example having what are commonly known as 'snow' on the screen. 'Snow' is when there are flecks on your TV screen, often seen in poor reception areas) or unstable signal. If the link to the encrypted signal should stop working then this should not be a problem. The link between the video encoder and video decoder should be regained and the decryption quickly continued.

The very nature of security camera systems is that they are often outdoors as so must be able to withstand the rigours of the weather. The video encryption hardware should be stable under or protected from the effects of rain, sunlight, extreme heat and cold. It should not be damaged if there is a power spike in the supply. In these systems the video encoder emits a wireless signal to the video decoder unit before it is viewed, it obviously must be the case that the very act of broadcasting the signal does not effect the video encoding hardware and likewise the video encoding hardware should not effect the radio transmitter.

The most important item is that the video scrambling system should be secure, else why bother? It is amazing how some encryption methods can easily be cracked. For example certain cable television stations 'encrypt' their channel broadcasts via a relatively un complex method, which can easily be decoded using a number of cheap bits of electronics from radio shack. That would obviously be illegal! The cable TV's method of encryption is very crude, they usually just dynamically alter the vertical sync signal so that your TV cannot get a proper lock on it and so it scrolls randomly.

The other extreme is to scramble the transmitted video signal too much so that it is costly both in equipment and time to the video at the receiver. Remember that this is a 'live' video scrambling broadcast followed by a 'live' video decryption display. ANY electronics can be copied, given enough money and time, but making this process as hard as possible is of benefit as it at least delays the time when illegal copies will be available.

Finally and most obviously each user must have a unique encryption key so that other users of the system cannot view the transmitted video by accident or purpose without the key owners knowledge. The total number of possible user keys must be such that it is highly unlikely for someone to guess the correct key.

By Ovation Systems