A server may perform server side authentication of a user device. The user device may generate a first authentication string by performing a hash function on a username, a password, and a first salt. The first authentication string may be registered with the server for subsequent login attempts. At login, the user device generates the first authentication string and transmits the first authentication string to the server. When the authentication strings match, the user device is authenticated. The user device may also update the first authentication string. The server may provide the first salt and a second salt to the user device. The user device may generate a first authentication string and a second authentication string from the first salt and the second salt, respectively. When the first authentication strings match, the server may update the user device's authentication string by replacing it with the second authentication string.