A method is disclosed and apparatus is described whereby plain text is modified before it is input to a binary key generator (encryptor). According to the invention, a plain text bit may be modified by a current modifying bit, which in turn, is based on a prior modifying bit, a prior plain text bit, and a prior ciphered text bit. The only data path provided to the transmitter is the encrypted key generator cipher text output. As a result, the only feed-foward path of the plain text to the channel is through the key generator, and thus the fault-immune physical barrier between the plain text and the cipher information inherent with the key generator is maintained. The invention provides a way of utilizing conventional binary key generators to digitally encrypt for radio transmission multilevel sub-band coded speech.