Biometric traits have to have some requirements to be effective. A trait that lacks of some requirement is considered less efficient.
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Universality: the trait must be owned by any person.
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Uniqueness: no equal trait between two people exists. A trait has to be unique to that person in order to be able to identify them.
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Permanence: the trait should not change over time. Some traits become and remain permanent after a certain age. For example, the face changes a lot at a younger age, then remains stable in adulthood and changes again at an elder age. Of course, if an individual enrolled at a younger age, in an adult age the face is different and should enroll in the system again.
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Collectability: the biometric trait should be measured by some sensor.
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Acceptability: Involved people should allow and be at ease with the collection or measurement of the trait.
For example, the process of retina recognition needs a sensor that has to touch the pupil in order to correctly measure the trait, and that’s something that not most people would allow to be done on themselves.
A permanent biometric trait can be a threat to privacy since if someone in some way steals the biometric trait which cannot be changed. That’s why in the systems is required not only that from a template it must not be possible to return back to the original sample; but also that if a biometric trait is compromised, the system must be able to create a new template by extracting a different template from the same sample.
Having a higher variance around the average value of a certain feature is more valuable, since a feature that’s present in a smaller quantity of people, on average, is less characteristic.
Physiological vs Behavioural vs Biological vs Mixed biometric traits
- Physiological traits: are traits that are embedded in the physiological features of an individual. Inside this category, we can find fingerprints, eye (iris and retina), face, ear, and hand geometry.
- Behavioural traits: are traits that depend on the character of an individual. We can find signature biometrics, keyboard typing (also keyboard typing on touch devices), and gait (the way the body moves during walking). These kinds of traits are the best ones to identify a person in a short period since they’re very difficult to imitate. In the short period because they’re not permanent over a long interval of time.
- Biological traits: in this category, we can find DNA. DNA recognition is very accurate but, to be efficient, requires a sample of blood and various days.
- Mixed traits: In this category, we find voice biometry. It’s considered mixed since it’s both physiological and behavioural. Physiological since the tone of the voice depends on the shape of the throat and other body parts. Behavioural since the voice is also influenced by the mood and character of the individual.
Genotypic vs Randotypic biometric traits
- Genotypic biometric traits are traits that can be inherited from relatives. An example is the face, which has some characteristics shared with other relatives.
- Randotypic biometric traits are traits that inherit only soft features. For example, eye color could be inherited but the shape and other details between relatives could be totally different.
Hard vs Soft biometric traits
- Hard biometric traits: it’s useful and sufficient alone to distinguish people in an accurate way;
- Soft biometric traits: it’s not sufficient alone to identify a person since may lack some requirements such as uniqueness. An example of a soft biometric trait is eye and hair color.
Soft and hard biometric traits can be combined together in order to have a more robust system. It’s also better to have multiple less strong traits than just one hard trait since an impostor should have to imitate multiple traits instead of just one.