The United States of America As Represented by the Director, National

The United States of America As Represented by the Director of National

Ronald M Benincasa

Richard W Marsh

Robert L Ward

An authenticator circuit and message authentication apparatus. The circuit includes a plurality of shift register stages, some of which are separated by adders having an input from a succeeding stage. Switching circuits selectively control the second adder inputs to determine the shifting sequence of information passing through the register stages. A message authentication apparatus includes a key generator and authenticator circuit converted at the output of a transmitter. The input of a receiver includes a similar authenticator circuit and key generator, plus a counter.

Authenticator circuit

A method is disclosed for encoding signals for transmission to provide signals which simultaneously have specifiable autocorrelation properties and low predictability. To enhance the accuracy of time of arrival measurements in the presence of either noise or multipath due to skywaves in systems such as LORAN, the coding method provides for low autocorrelation function values at time offsets approximately equal to the expected delay between direct waves and skywaves. This is accomplished by constructing the codeword which is used to encode the signal, from an Nth difference constant ratio codeword, where the expected time delay is equal to N-bits. The constant ratio codewords and the transmitted codewords are both pseudo-randomly generated, resulting in a transmitted signal having very low predictability to an unintended recipient, thereby considerably reducing the effectiveness of attempts at jamming or other electronic countermeasures attacks.

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Pasadena, MD, United States of America

Ronald M Benincasa