Longitudinal Keystroke Biometric Studies on Long-Text Input

1. Longitudinal Keystroke Biometric Studies on Long-Text Input Elizabeth Wood, Julio Zelaya, Eric Saari, Kenneth King, Mike Gupta, Nicola Howard, Sadia Ismat, Mary Angela Kane, Mark Naumowicz, Daniel Varela, and Mary Villani

2. Background The Pace system has shown high identification accuracy when data are collected at around the same point in time. Preliminary studies also show high authentication accuracy using the dichotomy-model-based pattern classifier. There is little data on how keystroke patterns (and thus accuracy) change over time.

3. Current Study • Extends preliminary studies to assess accuracies over two- and four-week periods • Provides new data to assess accuracies over a two-year interval • Assesses both identification and authentication accuracies over these intervals.

4. Experimental Design Each subject was asked to complete one or more “complete data sets” consisting of five samples in each of the following four “quadrants”: • Desktop free-text • Laptop free-text • Desktop copy operation • Laptop copy operation

5. Two- and Four-Week Studies Subjects consisted of the four previously studied subjects (Buch et al.) plus nine new subjects. Each subject completed three complete data sets, spaced at approximately two-week intervals.

6. Two-Year Study Subjects were 8 of the “original 36” subjects studied by Dr. Villani in 2006 for her doctoral dissertation. Study compared one complete data set entered in 2006 to another entered in 2008.

7. Analysis Laptop Desktop Copy Free Text Identification and authentication studies were performed across different quadrants at same and different time points:

8. Analysis (continued) For each pair of quadrants (arrows in prior slide), the classifier was trained on one quadrant and tested on the other. Primary results are average accuracies across all quadrant-to-quadrant comparisons for same and different time points.

9. Measurement Two- and Four-Week Study (n=13) Two-Year Study (n=8) Weeks Years 0 2 4 0 2 Identification Acc 90 85 83 84 67 Authentication Acc 90 87 85 94 92 FAR 5.0 3.8 4.6 3.1 3.0 FRR 10.9 13.7 16.0 6.8 8.5 Results Acc: accuracy; FAR: false acceptance rate; FRR: false rejection rate

10. Accuracy as a Function of Time

11. Authentication Studies:FAR and FRR Over Four-Week Period

12. Authentication Studies:FAR and FRR Over Two-Year Period

13. Conclusions • Identification accuracy appears to decline progressively over a 4-week period, and even more over a two-year period. • Authentication results are ambiguous, although accuracies appear to decline similarly to identification accuracies over a four-week period. • Time-related declines are attributable to an increase in false rejection rate, rather than an increase in false acceptance rate.

14. Twin Studies • Three of the subjects in the 2/4-week study were identical twins and their father. • Separate analyses were performed to assess: • Ability to distinguish each twin from the other • Ability to distinguish each twin from the father • Ability to distinguish each twin from an unrelated subject

15. Twin Results

16. Conclusions Identical twins appear to have significant similarity in keystroke patterns, suggesting a biological basis for these patterns. Twins’ keystroke patterns were closer to those of their father than to those of an unrelated subject. However, the degree of similarity between the twins is much higher than their similarity to their father. Additional studies are needed to confirm biological influence on keystroke patterns.

17. Future Studies Longitudinal studies should be repeated in a larger group of subjects. Better control of experimental conditions might yield more conclusive authentication results. Future work could also focus on refining the feature extractor (perhaps adding more features) to improve both identification and authentication accuracies. Recruitment of subjects is a challenge; a structured study offering clear incentives would be desirable.

18. Questions?