Frequency Hopping Spread Spectrum (FHSS) system is often deployed to protect wireless communication from jamming or to preclude undesired reception of the signal. Such themes can only be achieved if the jammer or undesired receiver does not have the knowledge of the spreading code. For this reason, unencrypted M-sequences are a deficient choice for the spreading code when a high level of security is required. The primary objective of this paper is to analyze vulnerability of linear feedback shift register (LFSRs) codes. Then, a new method based on encryption algorithm applied over spreading codes, named hidden frequency hopping is proposed to improve the security of FHSS. The proposed encryption security algorithm is highly reliable, and can be applied to all existing data communication systems based on spread spectrum techniques. Since the multi-user detection is an inherent characteristic for FHSS, the multi-user interference must be studied carefully. Hence, a new method called optimum pair “key-input” selection is proposed which reduces interference below the desired constant threshold.
 Telecommunications Industry Association, “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” Telecommunications Industry Association, Arlington, 1998.
 M. Tafaroji and A. Falahati, “Improving Code Division Multiple Access Security by Applying Encryption Methods over the Spreading Codes,” IET Communication, Vol. 1, No. 3, 2007, pp. 398-404. doi:10.1049/iet-com:20060295
 I. Mansour, G. Ghalhoub and A. Quilliot, “Security Architecture for Wireless Sensor Networks Using Frequency Hopping and Public Key Management,” IEEE International Conference on Networking, Sensing and Control (ICNSC), 11-13 April 2011, Delft, pp. 526-531.
 J. K. Tugnait and T. Li, “Blind Detection of Asynchronous CDMA Signals in Multipath Channels Using Code-Constrained Inverse Filter Criteria,” IEEE Transactions on Signal Process, Vol. 49, No. 7, 2001, pp. 1300-1309. doi:10.1109/78.928685
 J. Colic, “Towards Fast Correlation Attacks on Irregularly Clocked Shift Registers, Information Security Research Centre,” EUROCRYPT’95 Proceedings of the 14th Annual International Conference on Theory and Application of Cryptographic Techniques, Louvain-la-Neuve, pp. 248-262.
 X. Xu, “Blind Estimation of PN Code in Multipath Fading Direct Sequence Spread Spectrum Systems,” 11th IEEE International Conference on Communication Technology Proceeding, Hangzhou, 10-12 November 2008, pp. 213-216.
 Q. Ling, T. Li and J. Ren, “Physical Layer Built-in Security Enhancement of DS-CDMA Systems Using Secure Block Interleaving,” Conference Record of the Thirty-Eighth Asilomar Conference on Signals, Systems and Computers, Pacific Grove, 7-10 November 2004, pp. 1105-1109.
 L. Gang and A. Nakansu and M. Ramkumar, “Security and Synchronization in Watermark Sequences,” 2002 IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP), Orlando, 13-17 May 2002, pp. IV-3736-IV-3739. doi:10.1109/ICASSP.2002.5745468