Abstract: Served as one of the most widely used wireless air link interfaces, code division multiple access (CDMA) has been identified as a major technique for 3G wireless communications. Here, a new method based on applying encryption algorithm over spreading codes is proposed to improve the security of CDMA. The security produced in this way is primarily associated with the complexity of the employed encryption algorithm. Because the encryption algorithm's security is highly reliable, it is suitable for any kind of data communications. Since the multi-user detection is the inherent characteristic of the CDMA, the multi-user interference must be studied carefully. For this reason, the cross-correlation between outputs of encryption algorithm, causing the multi-user interference, is studied thoroughly. Then, the combination of encrypted and unencrypted M-sequence is used as a spreading code to mitigate the system performance, and the advantage of this combination is considered from the interference level and the security point of view. A compatible algorithm for the key exchange is produced. As people rely more and more on wireless communication networks for critical information transmission, security has become an urgent issue and a bottleneck for new wireless communication services such as wireless mobile Internet and e-commerce [1] . For military communications where information transmission heavily relies on wireless networks (for instance, from aircraft to aircraft, from aircraft to ground control centre and so on), the security and reliability of the wireless communication systems is of number one priority, especially in national defence and emergency response to abrupt enemy attacks. Spread-spectrum systems are often used to protect digital communication from jamming or to preclude undesired reception of the signal. Both of these purposes can only be met if the jammer or undesired receiver does not have knowledge of the spreading code. For this reason, M-sequences are a deficient choice for the spreading code when a high level of security is required [2] . The main weakness of the wireless communication security system is the simplicity of accessing the communicating signal through the channel. Eavesdroppers can easily place an antenna in the desired field and after demodulation, the message bits can be obtained in the base-band form. If the messages are encrypted, after storing the encrypted messages with some cryptoanalysis methods, he can smear out the original message. Now, if the received radio signal from the wireless channel is spread in a form that the intruder cannot access the de-spread spectrum, and receives only a signal similar to noise; hence, the radio transmission channel is called perfectly secure. In the current commercial code division multiple access (CDMA) systems, each user's signal is first spread using a code sequence C(t) known as channelisation code spanned over just one symbol or multiple of symbols. The spreading signal is then further scrambled using a pseudo-random sequence L C (t) to randomise the interference and meanwhile makes it difficult for intruder to intercept and detect the transmitted signal. To recover the desired user's signal, one has to know both the user's channelisation code and scrambling code (as shown in Fig. 1 ). This is known as the built-in security feature of CDMA systems. However, the system is fragile to hostile security attacks [3] . More specifically, the security of the CDMA system mainly relies on the long-code generator that consists of a 42-bit long-code mask that is generated by a 42-bit linear feedback shift registers (LFSRs). The maximum complexity to recover the 42-bit long-code mask is O( 2 42 ). However, if eavesdroppers can obtain 42 bits of plaintext -ciphertext pairs, then the long-code mask can be recovered after dropping the transmission on the traffic channel for about 1 s [3, 4] . In this article, a new method called hidden direct sequence is proposed to enhance the built-in security of CDMA systems by applying the cryptographic algorithm in the channelisation code, that is, at the physical layer. Security enhancement in the CDMA system with encryption hidden within spread-spectrum section In the CDMA technique, several users spread their signals spectrums in a wide frequency band by a special code with a small chip time that is called direct sequence C(t). These codes must have a little cross-correlation because other
A compatible algorithm for the key exchange is produced REF .
Improving code division multiple access security by applying encryption methods over the spreading codes
IET Communications
IET Communications
Computer Science