Examples associated with user authentication are described. One example method includes authenticating a user of a device using a static authentication technique. A behavior profile associated with the user is loaded. The behavior profile describes a pattern of device usage behavior by the user in a three-dimensional space over a time slice. The behavior profile also identifies distinctive user habits. Usage of the device is monitored, and a behavior similarity index is periodically updated. The behavior similarity index describes a similarity between the usage of the device and the pattern of device usage behavior. The behavior similarity index is weighted based on the distinctive user habits. Access to the device is restricted when the behavior similarity index reaches a predefined threshold.