Security of video monitors

SECURITY OF VIDEO MONITORS

0002This invention is concerned with security of a radiation-emitting display such as a video monitor, particularly employing a cathode-ray tube (CRT).

0003In a conventional video monitor, a video signal is used to modulate the beam of a cathode ray tube as the beam is raster-scanned over the tube face. This presents a security problem, because the radiation given off by the tube and its circuitry can be detected from a distance, and it is possible to reconstruct the video image.

0004It is known, in order to tackle this problem, to shield the monitor in an attempt to reduce the radiation given off, but this is expensive, cumbersome, and not entirely successful.

0005We have now discovered that a monitor and/or an associated device can be caused to give off radiation, optionally of a random nature, in addition to that caused by the display of information by a CRT or other thermionic device and its circuitry. -The dummy radiation need not be random, and it may substantially complement that emitted by the monitor such that the combined radiation is substantially

0006SUBSTITUTE SHEET uniform. If such dummy radiation were sent to a CRT, its screen would show substantially a negative of the picture to be displayed by the monitor. Random in this specification includes substantially random.

0007We have also discovered that the scanning of an electron beam can be scrambled, preferably in a random manner, while maintaining integrity of a displayed video image.

0008Thus, the present invention provides a method of operation of a monitor, wherein video or other information is displayed on the monitor intermittently, characterised in that dummy radiation is genesrated during the non-display of information.

0009The invention also provides a method of operation of a monitor wherein a stream of video or other information is received in one order, characterised in that the stream of information is displayed on the monitor in a. different order.

0010The two aspects of the invention may be combined, and the additional radiation given off during blank periods during display of a field and/or during the inter-line and inter-field periods of the raster-scanning by the electron beam. Specific embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings in which:

0011Figure 1 is a schematic illustration which especially when considered with Figure 5 is of a video monitor embodying the invention;

0012Figure 2 is a schematic diagram illustrating one form of random line generator employed in the arrangement of figure 1;

0013Figure 3 is a flow diagram illustrating the method employed by the random line generator shown in figure 2;

0014Figure 4 is an alternative form of random line generator;

0015Figure 5 is a block diagram showing how a dummy tube can be used in conjunction with a CRT to disguise radiation proudced by the CRT; and

0016Figures 6 and 7 show wave forms appropriate to the generation of a signal for the dummy tube of figure 5.

0017SUBSTITUTE SHEET Referring to the drawing, a standard composite video signal is received on input line 10. For each field of the video image, the video signal contains a series of video lines. A sync separator 12 extracts from the video signal a field sync signal on line 14 and a line sync,, signal on line 16. After sync separation, the video signal at line 18 is passed to the data inputs of two frame stores, store A 20 and store B 22 and also to a pixel sync separator 24, which produces a pixel svnc signal on line 26. Thus, the pixel sync signal ©n line 26 pulses with each input pixel; the line sync signal on line 16 pulses with each line of input pixels; and the field sync signal on line 14 pulses with each field of input lines.

0018The pixel sync pulses on line 26 are counted by a pixel counter 28, which is reset by the line sync signal on line 16, and the output count value on bus 30 is used for X address selection of store A 20 and store B 22. The pulses of the line sync signal on line 16 are counted by a line counter 32, which is reset by the field sync signal on line 14, and the output line count value on bus 34 is used for Y address selection for store A 20 and store B 22 during writing to the stores. The field sync signal on line 14 is also supplied to a bistable flip-flop 36 which supplies read/write selection signals on lines 38A,

0019SUBSTIT T SHEET 38B alternately and oppositely to store A 20 and store B 22 so that for one field period store A 20 is being written to and store B 22 is being read from, and for the next field period store B 22 is being written to, and store A 20 is being read from.

0020From the above, it is will be appreciated that the video information for successive fields of the input video signal on line 10 is successively written to store A 20 and store B 22 and that while one of the stores is being written to, the other store is enabled for reading by the signal on the appropriate line 38A, 38B. Whereas the input video lines are written into the stores in the order in which they arrive, the video lines are read out from the stores in random order, as .described in detail below.

0021A random line number generator 40, described in detail below, is responsive to the field sync pulses on line 14 and the line sync pulses on line 16 and provides a random line number on bus 52. With each line sync pulse, a different line number is output on the bus 52, until all of the permissible line numbers have been output and a field sync pulse is received. The random line number is supplied on bus 52 to the read address inputs of store A 20 and store B 22 and also to a digital-to-analogue converter 54. Therefore,

0022SUBSTITUTE SHEET data is read out on line 56A or 56B from that one of store A 20 and store B 22 which is enabled for reading by the signal on line 38A or 38B, the data corresponding to that one of the lines denoted by the random number on bus 52, and all of the pixels for that line are read out by virtue of incrementation of the value on the pixel count bus 30. The output data on line 56A or 56B is input to a gate 58 and is supplied via an amplifier 60 to the grid or cathode of a cathode ray tube 62. Therefore, as the electron beam is raster-scanned across the tube, it is modulated by the output data. In order to provide for scanning of the electron beam in the horizontal direction, the line sync signal on line 16 is supplied to a line generator circuit 64, which supplies a saw tooth driving signal to a horizontal deflection coil 66 of the cathode ray tube 62. In order to provide that the line of video data is positioned on the face of the cathode ray tube 62 at the correct position, the output of the digital-to-analogue converter 54 is supplied to a field amplifier 68, which in turn drives a vertical deflection coil 70 of the cathode ray tube 62. As shown in the drawing, a circuit 72 is provided for supplying biassing, focussing and other control voltages to the cathode ray tube 62, and a power supply 74 is included for providing EHT to the tube 62. From the above, it will be appreciated that while one field of video information is being written in order into one of the stores 20, 22, the previous field of video information is being read out of the other store line-by-line in random order, with the lines of video information being displayed in the correct position on the cathode ray tube 62. Therefore, the radiation given off by the tube 62 and its associated circuitry will be scrambled by comparison with the sequence of video data received on the input line 10.

0023The random line number generator 40 described above may be implemented by a microcomputer, as shown in figure 2, comprising a microprocessor 90 with associated ROM 92 storing programme data, RAM 94, and input-output port 96 connected to the field and line sync pulse lines 14, 16 and the line address bus 52.

0024The microprocessor 90 is programmed to perform the steps of operation illustrated by the flow diagram of figure 3. In the step 100, a field sync pulse is awaited on line 14 indicating the beginning or a new field. When the field sync pulse is received, variables L(l) L(2)....L(N) are set to ZQΓO in step 102. These variables are used line flags, and when set to zero they indicate that the respective line has not been used in the current field, and when set to

0025SUBSTITUTE SHEET one, they indicate that the line has been used in the current field. N is the number of lines in a field. In step 104, variable R is set to a random integer number between one and N the number of lines in a field. In step 106, it is determined whether the line having the number R has already been used in the current field. If not, then in step 108 the used line flag for line R is set to one, and in line 110 a line sync pulse on line 16 is awaited. When the line sync pulse is received, then in step 112 the output line address on bus 52 is set equal to the random number. In step 114, it is determined whether a field sync pulse is also present on line 14, and if so, then the routine returns to step 102. However, if there is no field sync pulse, then the routine returns to step 104. In step 106, if the used line flag for line R has already been set, then in step 116 the variable R is incremented by one. Then the routine returns to step 106, unless it is determined in step 118 that the variable R is greater than the number N of lines in a field, in which case the variable R is set to one in step 120, before returning to step 106. Thus, it will be appreciated that, after a field sync pulse has been received, then with every line sync pulse a new line number is output on the bus 52 until all of the lines in the field have been output.

0026E SHEET In a modification of the above arrangement, an increment of one and a decrement of one may be used alternately in step 116 for successive random line numbers to be generated.

0027Rather than implementing the random line address generator using a microprocessor and software, as described above with reference to figures 2 and 3, a hardware implementation of the random line generator may be used, as now described with reference to figure 4. A random number generator 130 generates a random number on a bus 132 in response to a line sync pulse on line 16, the random number having a value between one and the maximum number of lines in a field. The random number on bus 132 is added to the count in a ring counter 134. The ring counter 134 employs modular arithmetic having a base equal to the maximum number of lines in a field. The result of the addition is output on bus 136 to a latch circuit 138 which is enabled by the line sync signal on line 16 and also to a used line memory 140. The used line memory provides a flag for each line of the field, and the flag for a particular line is set when that line number appears on the bus 136. The used line memory 140 also has the function of outputting on line 142 a signal in the case where the line number on bus 136 has already been flagged, and the signal on line 142

0028SUBSTITUTE SHEET - io - is used to increment to the ring counter by the value of one. All of the flags in the used line memory 140 are reset at the beginning of each field by the field sync pulse on line 14. It will thus be appreciated that the output from the latch 138 on the address line bus 52 will not be repeated during any particular field, and that within a particular field period a series of random line numbers will appear on a bus 52.

0029With the random line address generation described above with reference to figure 2 and 3 or with reference to figure 4, it will be noted that large jumps in the line address can arise, and it is even possible for the first line of the field to be addressed and for this to be followed by the last line of the field; Such a system will therefore require large changes in the current to the vertical deflection coil 70, resulting in high energy consumption. In order to improve power economy, the software of figure 3, or the hardware of figure 4 may be modified in order to ensure that there is a limit on the maximum jump of line number between two consecutive lines, for example a maximum jump equal to half of the number of lines in a field. A further feature of the arrangement shown in the drawing to disguise inter-pixel, inter-line and inter-field spaces will now be described. A pixel generator 76 is supplied with the pixel sync signal on line 26 and provides on line 78 a series of pixel signals which are combined with the data read out from the stores 20, 22 by a pair of gates 80, 82 so that a dummy pixel signal is output from the gate 82 when there is no pixel data being output from either store 20, 22. The dummy pixel signal is then applied via an amplifier 84 to the cathode or grid of a dummy thermionic valve 86, which may be of similar construction to the gun of the cathode ray tube 62. The dummy valve 86 is supplied with anode voltage from the EHT supply 74, and may also be supplied with bias and focussing voltages from the circuit 72. Thus, whenever data is not supplied to the cathode ray tube 62, dummy data is supplied to the valve 86 and dummy radiation is created. To an eavesdropper, this disguises the radiation given off as a result of the data supplied to the cathode ray tube 62, and it also makes it difficult, if not impossible, to detect the line and field synchronisation of the cathode ray tube 62.

0030It will be appreciated that many modifications and developments may be made to the arrangement described

0031SUBSTITUTE SHEET above. For example, even if the dummy valve 86 is not used, then security will be improved. Furthermore, the dummy valve idea may be used with a conventional monitor in order to obscure the radiation given off by the monitor. Moreover, if desired, rather than producing^ dummy radiation whenever there is no radiation from the tube 62, the dummy radiation may be randomised, o some ordered pattern provided which together with the radiation from the CRT provides either ^"no Coherent information, or incorrect or different "information. Furthermore, although one particular arrangement of logical circuit devices has been^* described, it will be appreciated that many other alternative forms are possible.

0032The description below applies to Figures 5-7.

0033If every beat of a clock synchronizing the pixels were displayed as a pixel or dot on the screen, then the result would be a plain illuminated area, having no information content whatsoever, and the radiated emissions from such a display would be useless to an eavesdropper.

0034The object of this proposal is a device which, although <img file="WO9102344A1_D0001.tif" /> information in the normal way, nevertheless radiates a full screen of dots. In a modification of this idea, less than a full screen of dots is radiated, random or ordered radiation is added to that resulting from the displayed dots such that the result conveys no information or other different information from that displayed.

0035In this way, it should not be necessary to reorganise the structure of the scan as described above, although the two ideas may be used together.

0036The VDU may contain a first CRT for displayed information and a dummy or other tube (or other radiation emitter), which preferably has similar characteristics to the CRT. The dummy could take the form of a thermionic valve, built using a normal CRT gun, but optionally with a solid anode instead of the CRT deflection system and screen, hence it could be small, and easily contained within the volume of a normal VDU. Alternatively, the second tube could display a negative picture of that shown on the first CRT; a second full screen would in general not be preferred because of size etc.

0037Incoming information, in the form of coherent pixels, is detected and used to synchronise a slave pixel generator inside the display. This generator is used to provide video pulses which, if applied as video to

0038SUBSTITUTE SHEET the CRT, would result in a full screen of dots (or in a pattern not revealing the information to be displayed to the legitimate viewer) as described earlier. In this case however, the incoming video signal, information carried by which is to be displayed by the monitor, causes respective pixel pulses to be sent selectively to the monitor or to the dummy thermionic device. In general only the pulses containing valid information would be routed to the CRT.

0039Preferably all the pixel signals are generated by the same clock, and are fed to as near as possible identical thermionic devices (CRT and dummy), and therefore it should not be possible for a would-be eavesdropper to discriminate between the two, hence he could detect only a full, blank, screen.

0040A further refinement may be desirable for the following reason. The way that characters are formed and presented to the monitor by differing computer systems varies. Some represent straight horizontal lines (such as underlining or the composition of individual letters) as a series of individual pixels, and others as long, uninterrupted, video pulses. Where this latter representation occurs, such lines might be identifiable from their radiated patterns. Hence there might be a desire to break up any such solid lines into individual pixels, and make them indistinguishable from other information on the screen. The proposed system may accommodate this..

0041Figure 5 shows an overall block diagram of a preferred system. It may comprise elements of a monitor such as sync separator, video and scan circuits, together with the CRT and necessary power supplies together with a pixel generator and means to switch "scrambler" pixels into a dummy or other second tube.

0042The pixel generator provides a continuous•stream of video pulses in synchronism with incoming information; synchronising is achieved as follows (see waveform diagrams Figures 6 and 7).

0043An internal clock is used to drive a counter (in this case, a shift register). The counter is reset at the start of every scεmned line (see Figure 6) . On receipt of the first incoming video pulse the number contained in the counter at that time, is loaded into a simple number store and the counter is reset, whereupon it starts counting again. When it reaches the number set in the number store, a number

0044SUBSTITUTE SHEET comparator, which iε: connected to both counter and store emits a pulse, which again resets the counter.

0045In this way the comparator will emit pulses at regular intervals depending upon the clock rate and the number held in store.

0046The next video pulse to arrive will interrupt the count and shorten the cycle, and so on, until the number held in the store represents the minimum time interval between incoming video pulses; this number is held in store for as long as necessary. The comparator at this time will emit a steady stream of pulses synchronously with incoming video, and representing the maximum pixel frequency.

0047The probability of encountering incoming video representing two adjacent pixels early in the first scanned line is high, and hence the "sync hunting" phase will be short.

0048All incoming video pulses are valid, no matter how apparently random, since they are all synchronous with the external computer clock.

0049Figure 6 shows a simplified version of this sequence. The comparator output pulses are then delayed, as shown in Figure 7, and passed to the pixel shaper. The pixel shaper determines the length of pulses forming the pixels on the VDU screen.

0050As explained before, if all the pixels from the generator were displayed on the CRT, the nett result would be a screen completely filled with dots, which would look completely white. Incoming video however is used to divert all unwanted pixels into the dummy CRT by gating them εis shown in Figure 7.

0051In order to provide "clean" gating to switch pixels between CRT and dummy, it is necessary to introduce a slight delay when generating the pixel video waveform. The delay,, as shown in Figure 7, is introduced between the output of the number comparator and the pixel shaper. The pixel shaper is triggered by ^• the delayed pulse train and generates a series of matching pulses, each of which has a duration which will provide clear legibility when displayed upon the VDU screen.

0052At the same time, incoming video pulses are passed to a "gate shaper", which lengthens and shapes each pulse, such that it completely brackets the pulses generated by the pixel shaper. In this way the pulses

0053SUBSTITUTE SHEET generated by the pixel shaper may be switched cleanly between the CRT and the dummy, with no residual edges to betray the presence, in the emitted radiation, of a character.

0054The CRT and dummy are closely matched, in terms of electrical characteristics as possible, and are run from common supplies in order to make the radiation from each, as far as possible, indistinguishable.

0055Principal sources of radiation in general might be:

0056a) logic processing (10 MHz upwards at 5v amplitude)

0057b) video amplifier and connecting leads, (10 MHz upwards at 50v amplitude)

0058c) EHT leads (somewhat filtered video at 15KV)

0059d) CRT and dummy.

0060For best results, CRT and dummy are preferably mounted as closely together as possible, and video drive leads should be routed together.

0061All video logic circuits should be screened since they will in general radiate differently, and care should be devoted to earthing since circulating earth currents might cause differential radiation.

0062A programme was carried out with the object of demonstrating the feasibility of scrambling the radiated emissions from a VDU in the way described above.

0063A dummy tube was constructed, using a CRT electron gun and this was mounted close to the gun of the display CRT. A second video amplifier was introduced, of the same type as that used in the VDU.

0064A pixel generator a d gating circuit was constructed, run from the pixel clock in the character generator, and the whole was screened as well as practicable.

0065Arrangements were mεide to switch either valid or dummy pixels - (or both) onto the display CRT.

0066An eavesdropping device was constructed from a second VDU driven from a very broad bandwidth oscilloscope amplifier. The lower part of the bandwidth of the amplifier was restricted by means of . a simple high-pass filter, in order to limit sensitivity to line-flyback frequencies.

0067SUBSTITUTE SHEET In order to display any received coherence as clearly as possible, the eavesdropping VDU was synchronised, line and field, to the character generator. A character generator was used for these tests in preference to a PC, from the point of view of accessibility of the pixel clock.

0068The results showed that the displayed information was not detectable.