Analyzed the channel structure, it is found that there is an inherent signal feature on the plaintext in N-CDMA system. A new ciphertext-only attack method is proposed to solve the initial phase of the key sequence by eavesdropping 20 ms ciphertext frame. By exploiting the linear relations between the key sequence and the state of the long code generator, an algorithm for decoding the private mask is proposed. It is proved that the voice encryption of N-CDMA system is unsecure while attacked by ciphertext-only attack. Aiming at the safety defect of IS-95 system and possible attack methods, a new enhancement scheme is proposed to improve the security framework of IS-95 system.