Template distortion consists of applying either invertible or non invertible distortions to a biometric template. The distortion can be performed using either an invertible or a non invertible transform. In both cases the transform is chosen on the basis of a user key, which must be known when authentication is performed. In the case when an invertible transform is chosen, the whole security of the system relies on the key. On the contrary when non invertible transforms are used, even if the key is known by an adversary, no significant information about the template can be acquired. Then, the distorted data are stored in the database. Different distorted data can be generated from the same original data, simply by changing the transform key. Moreover, even if the database is compromised, the biometric data cannot be retrieved unless user dependent keys are revealed, when dealing with invertible transforms.
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