  The proliferation of online biometric authentication has necessitated
security requirements of biometric templates. The existing secure biometric
authentication schemes feature a server-centric model, where a service provider
maintains a biometric database and is fully responsible for the security of the
templates. The end-users have to fully trust the server in storing, processing
and managing their private templates. As a result, the end-users' templates
could be compromised by outside attackers or even the service provider itself.
In this paper, we propose a user-centric biometric authentication scheme
(PassBio) that enables end-users to encrypt their own templates with our
proposed light-weighted encryption scheme. During authentication, all the
templates remain encrypted such that the server will never see them directly.
However, the server is able to determine whether the distance of two encrypted
templates is within a pre-defined threshold. Our security analysis shows that
no critical information of the templates can be revealed under both passive and
active attacks. PassBio follows a "compute-then-compare" computational model
over encrypted data. More specifically, our proposed Threshold Predicate
Encryption (TPE) scheme can encrypt two vectors x and y in such a manner that
the inner product of x and y can be evaluated and compared to a pre-defined
threshold. TPE guarantees that only the comparison result is revealed and no
key information about x and y can be learned. Furthermore, we show that TPE can
be utilized as a flexible building block to evaluate different distance metrics
such as Hamming distance and Euclidean distance over encrypted data. Such a
compute-then-compare computational model, enabled by TPE, can be widely applied
in many interesting applications such as searching over encrypted data while
ensuring data security and privacy.
