METHOD AND DEVICE FOR DETERMINING A MEASURED VALUE INDICATING THE DEGREE TO WHICH DATA SIGNALS ARE REPRODUCIBLE OUTSIDE A DIGITAL INFORMATION PROCESSING SYSTEM

Abstract: In the neighbourhood of information processing and telecommunication systems, data signals, which are exchanged within that system, can, due to electromagnetic emission outside the relevant system, to a greater or less degree, be received - in a wireless way - and - generally after the received signal has been restored - be reproduced and displayed. The present invention relates to a measuring method and a device for determining a characteristic value indicating the degree to which data signals of a digital information processing system are reproducible outside that system. According to the invention a signal (3) received outside the information processing system to be tested is compared with a threshold value (5), and converted into a binary signal (6), of which the relatively shortest time (t₀) between two consecutive starting times of the ascending pulses is determined, said characteristic value being proportional to that number of ascending pulses for which it applies that the time (t x ) between the starting time of that ascending pulse and the starting time of the ascending pulse preceding that pulse is, within certain tolerances, equal to {(n *t₀) / 2}, in which t₀ represents said relatively shortest time and n is a random natural number from the series {3,4,5,...}.

Description: A. Background of the invention 1. Field of the invention The present invention relates to a measuring method and a device for determining a characteristic value indicating the degree to which data signals of a digital information processing system are reproducible outside that system. 2. Problems In the neighbourhood of information processing and telecommunication systems, data signals, which are exchanged within that system, can, due to spurious electromagnetic emission outside the relevant system, to a greater or less degree, be received - in a wireless way - and, generally after the received signal has been restored, be reproduced and displayed. It is obvious that the more confidential the information content of the exchanged data is the greater said problem is. The problem of receipt of data signals from spurious emission outside information processing systems is known from "Computers and Security", vol. 4, no. 4, December 1985, pages 269-276, the article entitled "Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?" by W. van Eck. Document EP-A- 0 137 948 shows a prior art arrangement whereby an input signal is converted to positive and negative pulses by comparison with upper and lower thresholds. An integrator produces a saw-tooth signal of limited maximum amplitude in synchronism with the positive pulses. B. Summary of the invention The present invention provides a measuring method as claimed in claim 1 and a device as claimed in claim 2 for determining in an unambiguous way to what degree an information processing or telecommunication system is susceptible to "tapping" information via electromagnetic emission. A preferred embodiment of the device in accordance with the invention comprises a device for converting the binary signal into a saw-tooth shaped signal with ascending flanks, linearly increasing with the time, and steep descending flanks at times corresponding to the starting times of the pulses of the first binary value of the binary signal, a device for determining the relatively smallest peak amplitude of the saw-tooth shaped signal, a device for comparing the amplitudes of the consecutive peaks of the saw-tooth shaped signal with the determined relatively smallest peak amplitude, and a device for detecting and adding to one another the number of peaks of the saw-tooth shaped signal, the amplitude of which is equal, within a certain tolerance, to {(n * u o ) / 2}, in which u o is said relatively smallest peak amplitude (u o ) and n a random natural number from the series {3,4,5,6,...}. In this preferred embodiment the respective times (t o and the respective time differences t x between the starting times of the consecutive pulses) are not compared with one another via conversion into numbers of time units, but via conversion into signal amplitudes, e.g. voltages (u x ), which can be compared with one another in a considerably simpler and quicker way. According to a further working-out of the device indicated hereinabove it comprises a device for storing and comparing the respective characteristic values over more than one consecutive period of consideration, and for representing the greatest value of them. In this way - for a certain period of measuring consisting of more than one period of consideration - the greatest of the consecutive characteristic values per period of consideration is registered and displayed as a final characteristic value. C. Embodiments 1. Figures Figure 1 shows part of a binary signal within an information processing system. Figure 2 shows a signal received outside the information processing system, which signal is compared with a threshold value. Figure 3 shows a binary signal resulting from the signal received and compared with the threshold value. Figure 4 shows the data pulses detected by the device. Figure 5 shows interference pulses and "periodic pulses" detected by the device. Figure 6 shows a saw-tooth shaped voltage signal with steep descending flanks at times corresponding to the starting times of the pulses of the first binary value of the binary signal represented in figure 3, and with linearly increasing ascending flanks between them. Figure 7 shows a diagrammatic representation of an embodiment of a device according to the invention. 2. Description Figure 1 Figure 1 shows a binary signal 1 - during a period of time t o ...t₃ - which signal is transmitted within an information processing system - e.g. a computer network. The part of the signal 1 which is shown in the figure is successively positive for one clock period 2, zero for one clock period, positive for two clock periods, zero for one clock period, positive for one clock period, zero for three clock periods, positive for one clock period and zero for four clock periods. Particularly due to the steep flanks of the signal, the signal 1 is emitted to outside the information processing system in the form of an electromagnetic field, where it can be received by means of a radio receiver. Figure 2 and figure 3 Figure 2 shows the course of a signal 3 received outside the information processing system by means of a radio receiver. Owing to among other things the shielding of the information processing system and external interference fields the received signal 3 considerably differs from the signal 1 shown in figure 1. Inter alia an interference pulse 4, which does not originate from the information processing system, is to be seen. The received signal 3 is processed in the device according to the invention in such a way that a signal is obtained which forms the best possible reflection of the original signal 1. For that purpose the received signal 3 is compared with a signal threshold 5, equal to about the average value of the received signal, due to which a distinction is achieved between parts of the received signal 3 which are greater (+) than and parts which are smaller (-) than the signal threshold 5. Figure 3 shows a binary signal 6 thus reconstructed. The object of the measuring method and the device according to the present invention is to determine the degree to which an information processing system emits data signals in such a way that they can be reconstructed. Figure 3 shows a "restored" signal 6. In this respect it should be borne in mind that the received signal 3 has not only the information processing system to be investigated as a source, but that there can be other sources which are responsible for the signal 3. One can distinguish: sources transmitting periodic pulses, sources transmitting (accidental) interference signals and - as already mentioned - sources transmitting data pulses. Consequently, the task of the present measuring method and measuring device is to distinguish between data pulses and periodic or accidental signals. Data signals can contain periodic pulse combinations (e.g. 0101) as well as non-periodic pulse combinations (e.g. 1000). Because of the fact that it is, however, impossible to derive from the received signal 3 whether a periodic pulse combination originates from a data signal source or from a clock signal source, all the periodic pulse combinations are left out of consideration for the measuring , in other words, only non-periodic pulse combinations - of which it is practically sure that they originate from a data signal source, in this case the relevant information processing system - are regarded as originating from the information processing system. In order to distinguish between non-periodic data signals on the one hand and periodic (data or clock) signals or arbitrary interference signals on the other hand, the period times t x between the consecutive ascending flanks of the constructed signal 6 are subjected to the criterion t x = {(n * t o ) / 2} (n = 3,4,5,...). Before doing this the relatively shortest time to between two consecutive ascending flanks is determined. Subsequently, it is determined whether each of the other period times t x (t₁...t 3b in figure 3) meets or does not meet said criterion. In figure 3 it is to be seen that over the period of consideration t o ...t₃ shown in this figure, the period to is the shortest period (it is obvious that this need not always be the first period). Period t₁ = 1½ * to, period t₂ = 2 * to and period t₃ = 2½ * t o ; period t₃ is, however, divided into a time t 3a and a time t 3b , where t 3a = a * t o (1½ < a < 2) and t 3b = b * t o (1½ < b < 2) . The pulses B and C (with the indication of time t₁ resp. t₂) meet the criterion t x = {(n * t o ) / 2} , viz. for n = 3 resp. for n = 4, so that these pulses B and C are considered to be data pulses. Neither pulse A (with the indication of time t o ) nor the pulses D and E (with the indication of time t 3a resp. t 3b ) meet the criterion (n = 2), for which latter two pulses it applies that 3 < n < 4. Figures 4 and 5 The figures 4 and 5 indicate how the different pulses A up to E inclusive are classified by means of the above-mentioned algorithm. As already described hereinabove the pulses B and C are classified by the algorithm as non-periodic data pulses. This is represented in figure 4 by the status pulses I. In figure 5 the other pulses A, D and E (periodic or interference pulses) are represented by the status pulses II. Figure 6 Figure 6 shows a saw-tooth shaped voltage signal with steep descending flanks at moments corresponding to the starting times of the ascending pulses of the binary signal represented in figure 3 and to linearly increasing ascending flanks between them. In this way the respective intervals of time t o , t₁, t₂, t 3a and t 3b are converted into a voltage signal, the consecutive peak values U o , U₁, U₂, U 3a and U 3b of which correspond to the length of time of those intervals. Owing to this, it is achieved that in a device for classifying pulses according to the aforesaid algorithm no times, but voltages can be compared with one another, which is feasible in a relatively simple way and works quickly. Figure 7 Figure 7 shows a diagrammatic representation of an embodiment of a device according to the invention. Signals which are received via an aerial 8 and a receiver 9 are emitted (inter alia) by an information processing system 10. In a comparator 11 the signal 3 delivered by the receiver is compared with a reference signal 5, delivered by a reference signal generator 12. The comparator 11 delivers the binary signal 6 shown in figure 3, which signal is subsequently supplied to an integrator 13, where the binary signal 6 is integrated with the saw-tooth shaped signal 7 shown in figure 6. The binary signal 6 is also supplied to a differentiator 14, which delivers a pulse at each of the ascending flanks of the binary signal 6. The integrated signal 7 is supplied to a peak value detector 15, where the peak value of the signal is determined. The peak value signal is subsequently supplied to a comparator circuit 16, which is moreover connected to a peak value buffer 17. At each moment of an ascending flank of the binary signal 6 the comparator circuit 16 compares, under the control of the pulses delivered by the differentiator 14, the peak value stored in the peak value buffer 17 with the peak value signal delivered by the peak value detector 15. If the value of this peak value signal is smaller than the peak value stored in the buffer 17, the latter will be replaced by the peak value of the peak value signal; if the value of the peak value signal is greater than the value stored in the buffer 17, the latter will remain unchanged. In this way the peak value buffer 17 always contains the relatively smallest peak value corresponding to the peak value u o in figure 6, which value is in its turn a measure for the shortest time to between consecutive ascending flanks of the binary signal 6. The peak value signal originating from the peak value detector 15 is subsequently supplied to a counter 18, comprising three voltage windows for respectively 1½, 2 and 2½ times the peak value stored in the peak value buffer 17 (which peak value is also supplied to the counter 18). The voltage window is formed by a voltage divider, from which three reference voltages are derived, viz. 1½, 2 and 2½ times the peak value stored in the buffer, as well as by three comparators to which on the one hand said reference voltages are applied and on the other hand the peak value signal originating from the peak value detector 15. The output terminals of the comparators are connected via a network of logic gates to a 2-decade counter for counting the number of data pulses and to a 1-decade counter for counting the number of periodic or interference pulses. If a peak value corresponds to one of the three window values, the signal peak value is considered to represent a data pulse, and the 2-decade counter is increased by one counting unit. If the signal peak value does not correspond to one of the window values, the signal peak value is considered to represent a periodic (clock or data) signal or an interference signal, and the 1-decade counter is increased by one counting unit. The 2-decade counter is always reset after the 1-decade counter has counted ten non-data pulses. The highest state of the counter of the 2-decade counter is - prior to the resetting - transmitted to a maximum buffer 19. In this maximum buffer 19 the transmitted state of the counter is compared with the state of the counter already stored in this maximum buffer and the highest state of those two states is then stored in the maximum buffer 19. Finally this state of the counter stored in the maximum buffer 19 is transmitted on to a display 20, which displays the contents of the maximum buffer during the measuring and after it.

Claims: A measuring method for determining a characteristic value indicating the degree to which data signals of a digital information processing system are reproducible outside that system, characterized in that a signal (3) received outside the information processing system to be tested is compared with a threshold value (5) and is converted into a binary signal (6) with pulses of a first binary value as long as the received signal is greater than the threshold value, and pulses of a second binary value as long as the received signal is smaller than the threshold value, of which binary signal the relatively shortest time (t o ) between two consecutive starting times of the pulses of the first binary value is determined, said characteristic value to be determined being dependent upon a ratio of a number of data pulses and a number of non-data pulses, where data pulses are those pulses of the first binary value for which the time (t x ) between the starting time of that pulse of the first binary value and the starting time of the preceding pulse of the first binary value is, within certain tolerances, equal to {(n * t o ) / 2} , in which t o represents said relatively shortest time and n is a random natural number from the series {3,4,5,6,...}, and where non-data pulses are those pulses of the first binary for which this equality does not apply. A device for carrying out the measuring method in accordance with claim 1, characterized by means (8,9) for receiving signals outside a digital information processing system (10) to be tested, means (11) for comparing a received signal (3) with a threshold value (5), and converting it into a binary signal (6), consisting of pulses of a first binary value as long as the received signal is greater than the threshold value, and pulses of a second binary value as long as the received signal is smaller than the threshold value, means (13...17) for determining the relatively shortest time (t o ) between two consecutive starting times of the pulses of the first binary value, means (18) for determining the number of data pulses, these being pulses of the first binary value for which it applies that the time (t x ) between the starting time of that pulse of the first binary value and the starting time of the preceding pulse of the first binary value is, within certain tolerances, equal to {(n * t o ) / 2} , in which t o represents said relatively shortest time and n is a random natural number from the series {3,4,5,6,...}, and for determining the number of non-data pulses, these being pulses of the first binary value for which this equality does not apply, and means (18...20) for determining and representing the characteristic value referred to, which characteristic value depends upon a ratio between a number of data pulses and a number of non-data pulses. A device in accordance with claim 2, characterized by a converting device (13) for converting the binary signal (6) into a saw-tooth shaped signal (7) with ascending flanks linearly increasing with the time and steep descending flanks at times corresponding to the starting times of the pulses of the first binary value of the binary signal, a device (16, 17) for determining the relatively smallest peak amplitude (U o ) of the saw-tooth shaped signal (4), a device (18) for detecting and counting the number of peaks of the saw-tooth shaped signal, the amplitude of which is, within a certain tolerance, equal to {(n * u o ) / 2} , in which u o is said relatively smallest peak amplitude (u o ) and n a random natural number from the series {3,4,5,6,...}. A device in accordance with claim 3, characterized by a device (19) for comparing the respective characteristic values over more than one consecutive period of consideration, and for determining the greatest value of them.

Inventors: SMULDERS, PETRUS FRANCISCUS MARIA
Applicants: KONINKLIJKE PTT NEDERLAND N.V.
Application year: 1989
Main classification: G01R29
IPC codes: H04L25, G01R29