a signal transmitted from a first device to a second device is encrypted using an encryption key generated from a preceding part of the signal itself . the signal is decrypted in the second device using a decryption key generated from a preceding part of the received and decrypted signal . this encryption method provides effective privacy protection , because the encryption and decryption keys are constantly changing . since the transmitted signal provides its own encryption and decryption keys , the method is inexpensive to implement , and can be used in systems such as packet transmission systems that transmit signals intermittently .