Thesis - Instituto de Telecomunicações

32 CHAPTER 2. STATE OF THE ART2.5 Continuous BiometricsIn the effort to produce even more secure systems, an extension to the multimodal perspectivewith continuous biometric verification has been recently proposed [7]. The securityin a system can be broken by a non-authorized person, after a user successfully acceptedby the biometric system carelessly leaves his authenticated session opened. A continuousverification approach would continue to request or monitor biometric information from theuser to guarantee that the person accessing the system is still the same. In [221], fingerprintand face are used to feed a HMM based classifier for the transitions in the temporal domainof the traits.One other format of continuous biometrics is related to some biometric techniques thatpresent a low performance, though their recognition merits improve, if the data is acquired ina long duration period (long being more than 5-20 seconds, which is the typical acquisitionduration in conventional biometric system). This idea has been proposed for our mousebehavioral biometric system [83] and in a keystroke dynamics based work [216].A continuous verification system has to take into account that the person may be absentfrom the computer during some periods of time. Information fusion from several sourcesin time is inspired in the multibiometric work, though a temporal integration must beperformed in order to adapt to changes in characteristics of the signal. There is the need totake into account interruption periods where some, or all, of the biometrics traits are notavailable. In [221] this is performed using information from the recent history of the severalbiometric channels. A practical example is presented in [7] with a multibiometric approachintegrating face, voice, and fingerprint modalities.2.6 Discussion and ConclusionOur body is becoming a password. Our biological characteristics in all their diversity areincreasingly being used to speak about our identity, by observing our body as it is and asit behaves. The recent trend in biometrics research is focused on multibiometrics, wheremultiple traits are combined to guarantee that the system is not mistaken on acceptinggenuine individuals and rejecting potential impostors.Recent trends have shown that some of the physical characteristics are not enough