Wireless terminals, such as but not limited to wireless telephones that communicate using Code Division Multiple Access (CDMA) spread spectrum modulation techniques, communicate over the air with system infrastructure using wireless over-the-air communication protocols, e.g., the CDMA protocols known as IS-95A, IS-95B, IS-2000, TD-SCDMA, Large Area Synchronous (LAS-CDMA), and W-CDMA. The system infrastructure, which can include base stations (BTS), base station controllers (BSC), and other components, connects the wireless terminal to another communication device, such as a land line or another wireless communication system.
To provide for secure communication using spread spectrum wireless systems, encryption principles can be used. For example, U.S. Pat. No. 5,727,064 discloses a method for secure communication that encrypts frames of communication (the data symbols, or payload) using a nonlinear scrambler. The scrambler is coupled to the output of the long code generator to remove linearity from the long code sequence produced by the long code generator. Portions of the long code sequence subsequently are combined with the main data signal that is output by the bit interleaver, thereby encrypting frames of data prior to spreading the frames in accordance with spread spectrum communication principles.
According to the '064 patent, breaking a nonlinear encryption is more difficult to do than breaking an encryption using a linear scrambler. As recognized herein, however, symbol-based encryption, even if nonlinear, remains vulnerable to eavesdropping because it is a relatively small and aligned data payload and is thus less cumbersome to break down and decrypt than a larger and non-aligned payload would be. Moreover, since the frames are encrypted in the above-mentioned patent, other system control channels, such as the synchronization and pilot channels are not encrypted. With the above considerations in mind, the present invention provides the solutions disclosed below.