This paper designs Z cipher scheme to satisfy the security requirements for mobile ends, including the identity authentication and key exchange. The design of Z cipher is dynamic in the way that the algorithm structure depends on the user's key, which is different from known ciphers with static structure and different keys for different users. This means that different users have different encryption logic which appears not only in different cipher parameters, but also in different logic layers, different operations, different components, and different data processing. This dynamic feature makes the Z cipher algorithm secure against static attacks. We call this Z's characteristics as `different user, different algorithm'. Some new security concepts are proposed, such as instance security, system security and distance security, which expands the security measures of traditional block ciphers. The users can update their keys and hence the cipher structures periodically or occasionally. Furthermore, the encryption algorithms in Z cipher scheme can be provided in the form of executable code, so it is easy to use and manage in software.