The invention provides a method, a system and an equipment for key distribution. During the course of computing a shared key (Kab) between a client node and a third party equipment, the shared key (Kab) is computed by introducing variable parameter into keying material rather than using constant parameter. Once the shared key (Kab) is compromised, it may be updated simply and in time by updating the variable parameter. Then the transmission security of the message is improved. A method for security protection and security verification for message is also used to effectively prevent from the threat to the message security caused by message forgery or replay attack. Thus the transmission security of the message is improved further.