Abstract: VoIP has become reality with spread of the network and its increasing bandwidthbut the speech is exposed to danger for the openness of network.Data encryption is usually used to ensure the safety of speech communication and chao-tic sequence is very suitable as the decryption cipher for its wide spectrum random ness,sensitiv to initial parameters.In order to avoid the danger come from using fixed chaos sequence as the cipher for long timea VoIP dynamic chao-tic encryption scheme was designed and implemented,in which Henon mapimproved Logistic map and a nonlinear map are combined to update chaotic cipher randomlyand the receiver decrypts data using the dynamic chaotic cipher obtained through the dynamic cipher exchange system .Experiment results show that this dynamic chaotic encryption system has high security and can be used for secure speech transmission.

Key words: Encryption and decryption,VoIP,Chaotic cipher,Dynamic cipher