GB1590579A - Information scrambler system - Google Patents

Description

(54) INFORMATION SCRAMBLER SYSTEM (71) We, MICRO CONSULTANTS LIMITED, a British Company of Interface House, Croydon Road, Caterham, Surrey, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a scrambler system to provide a high level of security for the transmission of video information.

A scrambler system requires that the transmitted information contains no clues as to how the information has been scrambled so that should the scrambled information be intercepted, the unauthorised decoding of the signal is a substantially impossible task.

According to the invention there is provided a video scrambler comprising picture storage means having a plurality of locations sufficient for storing picture point information from at least one video field derived from an input signal, and scrambler means for controlling the addressing of the store locations such that the information is written into respective locations in a difference sequence relative to that provided on read out.

Further according to the invention there is provided a video scrambling system comprising a scrambler including picture storage means having a plurality of locations sufficient for storing at least one field of video information derived from an input signal and scrambler means for addressing the store locations in a random manner to scramble said video information; a transmitter, a receiver; and an unscrambler including picture storage means having a plurality of locations for storing the information received and unscrambler means for addressing the store locations in the random manner to unscramble said video information.

According to a further aspect of the invention there is provided a method of scrambling video information from at least one video field comprising writing picture point information into a plurality of store locations in a first sequence and reading out said picture point information from said store locations in a second and different sequence relative to that used for writing in said information to provide scrambled picture information.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows the basic arrangement for a scrambler system, Figure 2 shows a block schematic diagram of one embodiment of the scrambler of the present invention used in a video system in the transmit mode of operation so as to form a transmitter system; Figure 3 shows a block schematic diagram of the Figure 2 system in the receive (unscramble) mode of operation so as to form a receiver system; and Figure 4 shows the construction of the scrambler addressing system in more detail.

The basic system for a scrambler arrangement is shown in Figure 1. The incoming signal is received by a scrambler 5 which scrambles the signal which is passed to a transmitter terminal 6. The scrambled transmitted signal is received by a receiver terminal 7 which passes the signal to unscrambler 8 which unscrambles the signal back to its original form.

A scrambler arrangement suitable for transmission and receipt of a scrambled video signal is shown in Figures 2 and 3.

The system enables a conventional microwave link to be used as the transmitter 6; a scrambler arrangement 5 being inserted in the video chain prior to the transmitter; an unscrambler 8 being inserted in the chain after the receiver 7 and before receipQy a display 29.

To an unauthorised intruder the transmitter picture will look like random noise. It can be shown that searching for the correct combination in order to decode the picture is a hopeless task involving more than 10380 combinations.

A television picture consists of over three million individual picture points (or pixels) normally arranged in a raster type format. Consider the effect of rearranging these picture points in a random manner. The result is pictures that look like white noise. The picture can be decoded by reconstructing the image in the same random format but unless the receiver knows the key the decoding operation will be virtually impossible since some 10380 combinations are possible. The proposed video scrambler employs this technique on a full colour picture. The colour itself is also scrambled in such a way that all sense of colour is destroyed in the encoded picture. Only within the unscrambler itself will the colour information be reconstructed.

It is obvious from the above arguments that the casual observer will be quite unable to make sense of the transmitted picture. Indeed even the determined observer, when equipped with the necessary frame store equipment, will be embarking on a seemingly impossible task should he choose to attempt to decode the transmission. The picture contains no clues as to the order in which the picture points should be rearranged and no data associated with the key for the code is transmitted. The observer is therefore left with the problem of trying each combination in turn until the picture appears. The time taken for this task approaches infinity.

In the system of Figure 2, the incoming video signal which has been generated for example by a normal camera source or tape recorder is applied to the system input 10 connected to a sync separator 11 which has one output for activating a store write address control circuit 12 and another output connected to a fast analogue-to-digital converter 13 in which the video is digitized into an 8 bit word.

The digital video from converter 13 is passed to a frame store 15.

The digital video held in the frame store 15 is read continuously to a fast digital-to-analogue converter 18 and then to a processing amplifier 19 which generates composite video for transmission by the microwave transmitter 6.

Read out from the store is effected by read address control 23 itself controlled by a sync pulse generator 24 that both creates a composite sync waveform and sets up the appropriate read address circuits.

The sync waveform is also received by the process amplifier 19. The system is asynchronous because transfer to and from the store block can be effected simultaneously at two differing store addresses.

Whilst the high speed video input port 10 accepts information that is basically raster in format, the asynchronous nature of the system allows operation over a very wide range of frame rates from standard television through slow scan systems such as electron microscopes, to line scan cameras such as IRLS (Infra-red line scan) or SLAR (sideways looking Airborne radar).

The sync separator 11 accepts conventional composite video from any normal CCTV camera, video tape recorder, or broadcast source. However, many other types of input may readily be used with the addition of an interface to indicate for example start of line or start of frame.

The maximum input sampling rate is 15MHz and the video is digitised via the converter 13 in 8 bits giving 256 possible shades of grey.

Non-raster type formats such as spiral and polar scans can be entered into the system with additional processing.

The frame store 15 is arranged as two fields of 256 by 512 words, 8 bits wide in order that 512 video lines each containing 512 picture points can be held in memory.

The frame store may comprise for example, the store disclosed in our co-pending British Patent Application 6585/76. (Serial No. 1568379).

This video frame store employs random access memory elements (RAMS) and is normally addressed serially to produce a raster type T.V. picture by means of write and read control circuits 12 and 23 respectively.

However to achieve scrambling additional circuit block 14 is provided which can be switched via switch 20 into the circuit to replace control 12. Scrambler circuit 14 contains read only memory circuits (ROMS).

The address structure will be controlled by these read only memories containing a code chosen at random by the user. The ROM and address counter architecture is chosen so that all of the picture points are scrambled so that detailed observation of the scrambled picture provides absolutely no clue as to the correct address of any picture point. Processing a colour picture will result in the colour information being apparently destroyed in the scrambled image since only when the picture points have been reordered into the correct sequence will colour information be restored: the colour encoding processing relying on the relationship of the adjacent picture points.

The microwave receiver 7 of Figure 3 receives the scrambled signal which is handled in a similar manner to that of Figure 2 with the exception that the unscrambler 25 is switched into circuit via switch 21 which has the same key as the scrambler 14 so that correct unscrambling is provided. The unscrambled picture may then be observed on the monitor 29.

The separate cards in the scrambler and unscrambler blocks 14 and 25 will contain the read only memories that set the scramble key for the system. The ROMS may be of a standard available type and are ultraviolet erasable allowing frequent reprogramming should the user feel that this is necessary,.

Changing the key for the equipment is very straight-forward. The operator would erase the existing ROMS by subjecting them to ultraviolet light and then reprogramme them with a series of random numbers from random number tables or any other suitable source. He would do this to both sets of ROMS in the scrambler and unscrambler. He will have an almost infinite number of combinations to choose from and can change the programme or key when ever he wishes. The almost infinite number of combinations suggest that the code will only need to be changed at a rate governed by an intruder gaining unauthorised access to the key. A safeguard against this occurence would be to destroy the data after programming the two sets of ROMS.

The ROMS that form the key can be programmed by proprietory programming equipment which it either controlled by a key board or paper tape. Entering the codes by key board is a formidable task since in excess of 100,000 entries must be made to define the code.

A standard mini computer could be used to generate random numbers and produce the appropriate paper tape for subsequent automatic programming of the ROMS.

As explained above the structure of the scrambler blocks 14 and 25 could simply comprise a ROM interposed between the normal address counters and the frame store. In practice this would give rise to a ROM of very large dimensions which would be equivalent to the number of locations in the video store.

To reduce the ROM size the arrangement shown in Figure 4 is a preferable alternative.

The ROM is split into a 32 word block 50 and a 4096 word block 51. The address counters 52, 53 and 54 are provided in a similar way to those in the write and read address counters 12 and 13. However the normal picture point counter of the conventional addressing arrangement is replaced by a 5 bit counter 52 and a 4 bit counter 53. The line counter 54 is an 8 bit counter as provided in the normal addressing control.

By providing two separate ROMS each can be programmed separately so that random scrambling of the addresses in ROM 50 is independent of the scrambling in ROM 51. The number of combinations possible is equal to 32! x 4056!.

Conventional addressing of the store by switching in write and read controls 12 and 23 allows easy setting up of the work prior to scrambling.

The bandwidth of the transmitted signal produces some potential difficulty with a video scrambler of the type described. The normal television picture, (e.g. PAl 1 at 625 lines 50Hz) relies on the behaviour of adjacent picture points to occupy less bandwidth than might at first be regarded as necessary. The scrambler destroys this relationship resulting in a requirement for higher bandwidth. To overcome the necessity of these high bandwidths the scrambler will slightly reduce the definition of the original scene. However a horizontal bandwidth of 2.5 MHz luminance and 650 KHz chrominance is possible which will produce an excellent picture equal in performance to a normal 'colour under' video tape recorder.

In order to further reduce the bandwidth of the transmitted signal the scrambler could have the characteristic of operating in single field mode although a fully interlaced picture will be reproduced at the output from the unscrambler. This is achieved by storing only 1 field per frame in store 15 and reading out the information to the transmitter at 1/2 normal rate. The received information is written into the store at 1/2 normal rate (i.e. over 2 fields) and read out from the store at full rate, repeating the single field twice. Thus the normal input vertical definition of 600 lines will be reduced to 300 lines and on scenes where very rapid movement take place very slight jerking may be noticed. Nevertheless on normal scenes such effects are completely unnoticable and picture quality is equal to many broadcast programmes.

The effect on the output of noise in the transmission link will be merely to reduce the signal to noise ratio of the displayed picture in proportion to the incoming noise until eventually the decoding circuits of the unscrambler will drop out and the picture appear permanently scrambled.

Multipath effects will superimpose a ghost image as is the case with normal transmission.

However with this equipment the image will be a 'scrambled ghost'. This scrambled image superimposed on the main video will suffer the normal attenuation of a conventional multipath ghost.

The following information gives an idea of the enormity of the task that an intruder would be taking on if he tries to decode the scrambled signal.

(a) Number of Combinations The internal workings of the scrambler system gives two sets of combinations possible in the picture as explained above with regard to Figure 4. First 32 picture points are randomly organised into blocks and then 4,672 blocks organised at random make up the complete field. In the arrangement of Figure 4 a slightly smaller number of blocks (i.e. 4096) was quoted. By way of illustration the number of blocks will now be assumed to be 4,672.

Therefore the total number of combinations possible is 32! x 4672! which approximately equals 1015114.

(b) The Sequential Decode Problem Given 105114 combinations clearly many will reproduce clear pictures of totally the wrong scene and many will reproduce the correct scene with some added noise. Let it be assumed that given only 1% of the picture correct the intruder could guess the remaining picture content correctly. The total number of meaningless combinations then becomes (32! x 4672! (32 x 0.99)! x (4672 x 0.99)! which reduces to 15144 10'5144 10380 1014734 = 10380 Working at 50 fields per second 10380 pictures would be displayed in 10380 50 x 60 x 60 x 24 x 365 years = 10380 109 years = Infinity Clearly such numbers are so large as to be quite uncomprehensible, suffice it to say that decoding by searching sequentially for the key is impossible.

(c) Decoding by Intuition The best laid schemes of random coding can be thwarted by the human if intuitively he is able to see patterns or trends. The degree of scrambling achieved completely destroys any patterning and even regular shapes like test cards etc appear as noise after scrambling.

Since the adjacent groups of picture points from the original scene can be spread throughout the transmitted picture the intruder cannot just explore one small part of the scrambled picture, run through all combinations to form a sensible picture, and hence deduce the overall code. Exploring all combinations for a small part of the picture will only result in other scrambled pictures.

(d) Decoding by Statistical Analysis Since the picture scrambling code is not dependant upon picture content it is thought that no statistical analysis of the scrambled image will assist in decoding the picture. In this respect the scramble is similar to an 'once only code' scheme for coding text - a scheme acknowledged to be the most difficult of all to decode.

(e) Decoding by Recovering any Residual Signal Clearly the whole scrambler system is by passed if a large residual of the input signal appears as cross-talk on the output. Typical cross-talk for this system is -46dB and since this is added to a signal on the output representing pure noise no recovery of the original picture utilising cross-talk will be possible.

Although the scrambler has been described as using a microwave link, land lines and satellite transmissions are possible as is the incorporation of sound scrambling should this be required.

Alternatively if the equipment is to be used permanently in either the transmit or receive mode, the inclusion of both scramble and unscramble circuits is not required.

Although the embodiments of the invention described have been for a scrambler using video signals converted into digital form it is possible to scramble the signal whilst still in analogue form so that storage in digital form is not required. Charge coupled devices (CCD) could be used for analogue storage.

Alternatively the video information could be sent in digital form without reconversion to analogue form prior to transmission. The receiver store would receive the digital information directly and not via the ADC.

Typical uses for a system requiring such security aspects are in the transmission of medical data (e.g. case notes x-rays), space exploration data, industrial data, infra-red imaging etc.

Although the embodiment described has related to the handling of video signals, the system could be modified to handle sound signals with the video signal.

When the scrambler and unscrambler are interconnected via a land line, for example, the transmitter and receiver can be considered as merely the respective ends of that land line.

It is to be noted that the scrambler arrangement for scrambling the addressing of the store locations may scramble the addressing when writing in the information to the store or when reading out the information from the store. The unscrambler may also unscramble the input or the output addressing.

WHAT WE CLAIM IS: 1. A video scrambler comprising: picture storage means having a plurality of locations sufficient for storing picture point information from at least one video field derived from an input signal, scrambler means for controlling the addressing of the store locations such that the information is written into respective locations in a difference sequence relative to that provided on read out.

2. A scrambler as claimed in claim 1, wherein said scrambler means includes memory means programmed to provide random addressing of said storage means.

3. A scrambler as claimed in claim 2, wherein said memory means comprises a read only memory.

4. A scrambler as claimed in claim 2 or 3, wherein addresss counters are provided connected to said memory means to access a programmed address in dependence on the address present in said address counters.

5. A scrambler as claimed in any one of claims 1 to 4, wherein the scrambler means scrambles the write addressing of the storage means.

6. A scrambler as claimed in any one of claims 1 to 4, wherein the scrambler means scrambles the read addressing of the storage means.

7. A scrambler as claimed in any one of claims 1 to 6, wherein the number of locations within said storage means are sufficient to store a frame of video information.

8. A scrambler as claimed in claim 4, wherein said address counters comprise picture point and line counters and said memory means comprise a first memory connected to receive at least part of the picture point counter address and a second memory connected to receive the line counter address and any portion of the picture point address not received by the first memory, the first and second memories being independently programmable in random manner.

9. A scrambler as claimed in claim 8, wherein said first and second memory comprise read only memories.

10. A scrambler as claimed in any one of claims 1 to 9, wherein said storage means is a digital store and analogue to digital converter means are provided prior to said storage means to convert an analogue video signal to digital form.

11. A video scrambling system comprising a scrambler including picture storage means having a plurality of locations sufficient for storing at least one field of video information derived from an input signal and scrambler means for addressing the store locations in a random manner to scramble said video information, a transmitter, a receiver, and an unscrambler including picture storage means having a plurality of locations for storing the information received and unscrambler means for addressing the store locations in the random manner to unscramble said video information.

12. A scrambling system as claimed in claim 11, wherein said scrambler and unscrambler means each include memory means programmed to provide random addressing of said storage means.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (22)

**WARNING** start of CLMS field may overlap end of DESC **. this is added to a signal on the output representing pure noise no recovery of the original picture utilising cross-talk will be possible. Although the scrambler has been described as using a microwave link, land lines and satellite transmissions are possible as is the incorporation of sound scrambling should this be required. Alternatively if the equipment is to be used permanently in either the transmit or receive mode, the inclusion of both scramble and unscramble circuits is not required. Although the embodiments of the invention described have been for a scrambler using video signals converted into digital form it is possible to scramble the signal whilst still in analogue form so that storage in digital form is not required. Charge coupled devices (CCD) could be used for analogue storage. Alternatively the video information could be sent in digital form without reconversion to analogue form prior to transmission. The receiver store would receive the digital information directly and not via the ADC. Typical uses for a system requiring such security aspects are in the transmission of medical data (e.g. case notes x-rays), space exploration data, industrial data, infra-red imaging etc. Although the embodiment described has related to the handling of video signals, the system could be modified to handle sound signals with the video signal. When the scrambler and unscrambler are interconnected via a land line, for example, the transmitter and receiver can be considered as merely the respective ends of that land line. It is to be noted that the scrambler arrangement for scrambling the addressing of the store locations may scramble the addressing when writing in the information to the store or when reading out the information from the store. The unscrambler may also unscramble the input or the output addressing. WHAT WE CLAIM IS:

1. A video scrambler comprising: picture storage means having a plurality of locations sufficient for storing picture point information from at least one video field derived from an input signal, scrambler means for controlling the addressing of the store locations such that the information is written into respective locations in a difference sequence relative to that provided on read out.

2. A scrambler as claimed in claim 1, wherein said scrambler means includes memory means programmed to provide random addressing of said storage means.

3. A scrambler as claimed in claim 2, wherein said memory means comprises a read only memory.

4. A scrambler as claimed in claim 2 or 3, wherein addresss counters are provided connected to said memory means to access a programmed address in dependence on the address present in said address counters.

5. A scrambler as claimed in any one of claims 1 to 4, wherein the scrambler means scrambles the write addressing of the storage means.

6. A scrambler as claimed in any one of claims 1 to 4, wherein the scrambler means scrambles the read addressing of the storage means.

7. A scrambler as claimed in any one of claims 1 to 6, wherein the number of locations within said storage means are sufficient to store a frame of video information.

8. A scrambler as claimed in claim 4, wherein said address counters comprise picture point and line counters and said memory means comprise a first memory connected to receive at least part of the picture point counter address and a second memory connected to receive the line counter address and any portion of the picture point address not received by the first memory, the first and second memories being independently programmable in random manner.

9. A scrambler as claimed in claim 8, wherein said first and second memory comprise read only memories.

10. A scrambler as claimed in any one of claims 1 to 9, wherein said storage means is a digital store and analogue to digital converter means are provided prior to said storage means to convert an analogue video signal to digital form.

11. A video scrambling system comprising a scrambler including picture storage means having a plurality of locations sufficient for storing at least one field of video information derived from an input signal and scrambler means for addressing the store locations in a random manner to scramble said video information, a transmitter, a receiver, and an unscrambler including picture storage means having a plurality of locations for storing the information received and unscrambler means for addressing the store locations in the random manner to unscramble said video information.

12. A scrambling system as claimed in claim 11, wherein said scrambler and unscrambler means each include memory means programmed to provide random addressing of said storage means.

13. A scrambling system as claimed in claim 12, wherein said memory means comprises

a read only memory.

14. A scrambling system as claimed in claim 12 or 13 wherein address counters are provided connected to said memory means to access a programmed address in dependence on the address present in said address counters.

15. A scrambling system as claimed in claim 12, 13 or 14, wherein said memory means are reprogrammable to allow the random addressing to be changed.

16. A scrambling system as claimed in any one of claims 11 to 15, wherein the number of locations within both said storage means are sufficient to store a frame of video information.

17. A method of scrambling video information from at least one video field comprising writing picture point information into a plurality of store locations in a first sequence and reading out said picture point information from said store locations in a second and different sequence relative to that used for writing in said information to provide scrambled picture information.

18. A method as claimed in claim 17, wherein the scrambling step is effected by randomly addressing the store locations during writing in of said information.

19. A method as claimed in claim 17, wherein the scrambling step is effected by randomly addressing the store locations during reading out of said information.

20. A video scrambler substantially as described herein and with reference to the accompanying drawings.

21. A video scrambling system substantially as described herein.

22. A method of scrambling video information substantially as described herein.