  Privacy-enhancing technologies are technologies that implement fundamental
data protection principles. With respect to biometric recognition, different
types of privacy-enhancing technologies have been introduced for protecting
stored biometric data which are generally classified as sensitive. In this
regard, various taxonomies and conceptual categorizations have been proposed
and standardization activities have been carried out. However, these efforts
have mainly been devoted to certain sub-categories of privacy-enhancing
technologies and therefore lack generalization. This work provides an overview
of concepts of privacy-enhancing technologies for biometrics in a unified
framework. Key aspects and differences between existing concepts are
highlighted in detail at each processing step. Fundamental properties and
limitations of existing approaches are discussed and related to data protection
techniques and principles. Moreover, scenarios and methods for the assessment
of privacy-enhancing technologies for biometrics are presented. This paper is
meant as a point of entry to the field of biometric data protection and is
directed towards experienced researchers as well as non-experts.
