Abstract: A method for encrypting signals utilizes the receiver serial number to generate an index number which is used in turn to select an encryption table from among a plurality of such tables. The index number used in conjuction with any given signal is modified for greater security by adding the units digit of the signal length. Further modification of the index number can be made by adding the units digit of the time of transmission. The index number is further modified by adding a shift number by which all data characters are shifted during encryption. In signals based on a defined protocol, the shift number and the units digit of the transmission time are hidden in unused protocol positions within the signal.