Cryptography – Cryptanalysis
Patent
1991-09-23
1992-10-20
Cangialosi, Salvatore
Cryptography
Cryptanalysis
380 10, H04K 100
Patent
active
051577259
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and an apparatus for preventing external detection of signal information in video signals occurring in, and being emitted from, a display unit, or a similar unit, and comprising substantially consecutive frame or field signals each consisting of substantially consecutive line signals.
Display units are widely used as components in, for example, data processing systems in which confidential information is processed and stored, and also in similar units, such as matrix printers. The video signals occurring in these units and containing components of radio frequency, are emitted therefrom and can be intercepted distantly from the display unit itself. By such detection, the information can comparatively easily be displayed on another display unit. Since it is important, both from commercial and legal standpoints, that such information does not leak, efforts have been made to either reduce the emission or alter it in such a manner that detection of the information is rendered more difficult or even impossible.
It is known to reduce the emission of video signals by encapsulating or shielding the equipment at issue, such that the signal strength of the emitted signals becomes too low to be detected. This, however, is a very expensive solution.
It is also known to emit an interfering signal in the form of white noise around the equipment, which renders the interception more difficult, but it is nevertheless possible to detect the video signal information by using advanced computer analysis. Furthermore, this technique may create more problems than it solves, since the requires noise signals may exceed the standards of radio interference other adjacent equipment is knocked out.
It is, moreover, known to emit, instead of white noise, an interfering signal with properties similar to those of the video signals. Thus, existing radio interference standards can be maintained, while the detection of video signal information is rendered very difficult.
In this methods of the type mentioned by way of the introduction, it is thus previously known to emit a phantom signal in the form of at least one pseudo-random bit signal sequence with properties similar to those of the video signal.
Apparatuses for carrying out these methods comprise a generator for generating the phantom signal, and an aerial for emitting the phantom signal from the generator.
A first object of the present invention is to further improve the methods and the apparatuses of the type mentioned by way of the introduction to prevent, in actual practice, any type of external detection of the signal information in the video signals.
According to the present invention, external detection of the signal information in the video signals may be rendered even more difficult if the phantom signal is also supplied on an external power supply line to the unit containing the video signal circuits.
Although the video signal circuits are, conventionally, separated from the power supply line by means of a low-pass filter, the video signals can nevertheless by transmitted to the power supply line, e.g. a mains connection, and the signal information in the video signals may thus be detected on, for example, external lines connected to the power supply line. For lower frequencies, the power supply line may also serve as a part of the emitting construction if the power supply filtration of the display unit is insufficient, which is extremely common in commercial data terminal equipment. Thus, the video signals may be emitted from the power supply line.
With the method according to the invention, it is no longer possible to detect the signal information in the video signals because of the cascade connection of power supply filters. Thus, a video signals are filtered twice, while the phantom signal is only filtered once, such that the effect of the video signals is reduced to a fraction of that of the phantom signal.
An apparatus suitable for carrying out the method according to the invention may be characterised in that the generator for
REFERENCES:
patent: 4225962 (1980-09-01), Meyr et al.
patent: 4563546 (1986-01-01), Glitz
Electronic Design, vol. 25, No. 9, Apr. 1977, C. R. Smiley: "Exchange data between digital systems", pp. 96-98.
The Computer Law and Security Report, vol. 1, No. 3, 1985, R. C. Marshall: "Radio-Frequency Emission From VDUS", pp. 4-5.
Cangialosi Salvatore
Cominvest Research AB
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