VI. CONCLUSION
Privacy preservation in biometric based verification systems
is a growing area of research with many challenges.
This paper first outlined the basic framework and performance
measures of PPBSs. Then it provided a comprehensive
review of the existing PPBSs, including biometric encryption
based schemes, cancelable biometric based schemes, multimodal
and hybrid based schemes, and SC based schemes.
The problems associated with the existing PPBSs were also
summarized and discussed. Furthermore, we highlighted the
challenges and future directions of PPBSs. It is hoped that
the review and analysis presented in this paper can help and
motivate researchers to develop more effective and efficient
PPBSs in the future.

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