Embodiments of systems and methods for client and/or server authentication are provided. In one embodiment, a method includes sending information from a mobile network device to a server, wherein the information comprises a seed that is used by both the mobile network device and the server to compute a series of one time passwords. The method also includes receiving, by the mobile network device, a succession of one time passwords generated by the server throughout a session. And the method further includes comparing the received one time passwords generated by the server throughout the session to corresponding one time passwords generated at the mobile network device. In this manner, the server can be authenticated. In various embodiments, the process may be reversed to facilitate client, e.g., mobile network device, authentication.