System and method for communications security protection

In a communication system for transmitting information signals generated by a sending device to a receiving device, said information signals being encoded as frequency shift keyed data within a predetermined transmission passband, apparatus for securing said information signals comprising:

• means for superimposing a masking signal on said information signal to generate composite communicated signals, rendering interceptions of said communicated signals unintelligible, said masking signal being a tone which is continuously varied in amplitude or frequency over the transmission passband;
• and
• means for extracting said information signal from said composite communicated signals.

In a communications system wherein information signals are generated by a sending device and communicated to a receiving device, said information signals being encoded as phase shift keyed data, apparatus for securing said information signals comprising:

• means for superimposing a masking signal on said information signals to generate composite communicated signals, rendering interceptions of said communicated signals unintelligible, said masking signal being a tone whose phase is continuously varied;
• and
• means for extracting said information signal from said composite communicated signals.

In a method wherein tone encoded information signals are generated by a sending device and transmitted to a receiving device over a communication link, wherein said tone encoded information signals are encoded as phase shift keyed data, said method comprising the steps of:

• injecting a masking signal onto said link, superimposing said masking signal on said information signals to generate composite communicated signals rendering interceptions of said communicated signals unintelligible, said masking signal comprising at least one tone used for said encoded signals whose phase is continuously varied;
• and
• means for extracting said information signal from said composite communicated signals.

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 injecting a masking signal on said line 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 sensing the level of tone encoded signals at said second site and controlling the amplitude of the injected masking signal which appears on said line in accordance with the sensed level;
• wherein said tone encoded information signals are dual tone multi-frequency digits, each digit of which is represented by one of four row frequencies and one of four column frequencies, and said injecting step includes injecting a masking signal which consists of at least two row frequencies or at least two column frequencies.

In a system for communicating information signals from a sending device to a receiving device, said information signals being dual tone multifrequency encoded, each digit represented in said information signal being represented by one of a first set of discrete frequencies and one of a second set of discrete frequencies, apparatus comprising:

• means for superimposing a masking signal on said information signal to generate composite communicated signals, rendering interceptions of said communicated signals unintelligible, said masking signal comprising at least two discrete frequencies chosen from one of said sets of frequencies;
• and
• means for extracting said information signals from said composite communicated signals.

A method for communicating dual tone multifrequency encoded information signals generated by a sending device to a receiving device, each digit of said information signals being represented by one of a first set of discrete frequencies and one of a second set of discrete frequencies, said method comprising the steps of:

• superimposing a masking signal on said information signal to generate composite communicated signals rendering interceptions of said communicated signals unintelligible, said masking signal comprising at least one discrete frequency chosen from one of said sets of four frequencies;
• and
• extracting said information signal from said composite communicated signals.