A practical evaluation of free-text keystroke dynamics

Free text keystroke dynamics is a behavioral biometric that has the strong potential to offer unobtrusive and continuous user authentication. Unfortunately, due to the limited data availability, free text keystroke dynamics have not been tested adequately. Based on a novel large dataset of free text keystrokes from our ongoing data collection using behavior in natural settings, we present the first study to evaluate keystroke dynamics while respecting the temporal order of the data. Specifically, we evaluate the performance of different ways of forming a test sample using sessions, as well as a form of continuous authentication that is based on a sliding window on the keystroke time series. Instead of accumulating a new test sample of keystrokes, we update the previous sample with keystrokes that occur in the immediate past sliding window of n minutes. We evaluate sliding windows of 1 to 5, 10, and 30 minutes. Our best performer using a sliding window of 1 minute, achieves an FAR of 1% and an FRR of 11.5%. Lastly, we evaluate the sensitivity of the keystroke dynamics algorithm to short quick insider attacks that last only several minutes, by artificially injecting different portions of impostor keystrokes into the genuine test samples. For example, the evaluated algorithm is found to be able to detect insider attacks that last 2.5 minutes or longer, with a probability of 98.4%.