System and method for communications security protection
First Claim
1. In a communications system for interconnecting first and second sites over a two-direction line, an apparatus for preventing intelligible interception of tone encoded information signals transmitted over said line in at least one direction from said first site to said second site but allowing intelligible reception of said tone encoded information signals at said second site comprising means for sensing the amplitude of said tone encoded information signals at the second site;
- means at said second site for injecting a masking signal on said line;
means at said second site for extracting tone encoded information signals received on said line from said first site which are superimposed on said masking signal;
means for attenuating the amplitude of said masking signal received by said extracting means;
means for measuring the amplitude of said masking signal at said extraction means after attenuation by said attenuating means; and
means for adjusting the amplitude of said injected masking signal so that the level thereof at said extraction means relative to the measured level of said tone encoded information signals at the extraction means allows extraction of said tone encoded information signals in the presence of said masking signal while enhancing confusion in an eavesdropping detector which may be connected to said line.
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Accused Products
Abstract
A system and method are disclosed for preventing intelligible interception of information signals transmitted over a two-direction line. A masking signal is applied through a hybrid circuit at the receiving end of the line, and this masking signal, which appears on the line together with the information signal, prevents intelligible decoding. The masking signal includes a series of frequencies which are sequentially applied. Only at the receiving end of the line, where the hybrid circuit attenuates the masking signal which it receives at its receive port, can intelligible decoding take place. The amplitude of the information signals is sensed at the receiving end and the amplitude of the masking signals on the line is adjusted to be great enough to provide security, by confusing an eavesdropping detector, yet small enough at the receiving end so that the receiving detector is not confused.
39 Citations
16 Claims
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1. In a communications system for interconnecting first and second sites over a two-direction line, an apparatus for preventing intelligible interception of tone encoded information signals transmitted over said line in at least one direction from said first site to said second site but allowing intelligible reception of said tone encoded information signals at said second site comprising means for sensing the amplitude of said tone encoded information signals at the second site;
- means at said second site for injecting a masking signal on said line;
means at said second site for extracting tone encoded information signals received on said line from said first site which are superimposed on said masking signal;
means for attenuating the amplitude of said masking signal received by said extracting means;
means for measuring the amplitude of said masking signal at said extraction means after attenuation by said attenuating means; and
means for adjusting the amplitude of said injected masking signal so that the level thereof at said extraction means relative to the measured level of said tone encoded information signals at the extraction means allows extraction of said tone encoded information signals in the presence of said masking signal while enhancing confusion in an eavesdropping detector which may be connected to said line. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
- means at said second site for injecting a masking signal on said line;
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9. A method, for use in a communications system interconnecting first and second sites over a two-direction line, for preventing intelligible interception of tone encoded information signals transmitted over said line in at least one direction from said first site to said second site but allowing intelligible reception of said tone encoded information signals at said second site, comprising the steps of sensing the amplitude of said tone encoded information signals at the second site;
- injecting a masking signal on said line at said second site;
measuring the amplitude of said masking signal at an extraction point at said second site;
extracting at said second site tone encoded information signals received on said line from said first site which are superimposed on said masking signal; and
adjusting the amplitude of said injected masking signal so that the level thereof at said extraction point relative to the measured level of said tone encoded information signal at said extraction point allows extraction of said tone encoded information signal in the presence of said masking signal while enhancing confusion in an eavesdropping detector which may be connected to said line. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
- injecting a masking signal on said line at said second site;
Specification