Privacy Weaknesses in Biometric Sketches

The increasing use of biometrics has given rise to new privacy concerns. Biometric encryption systems have been proposed in order to alleviate such concerns: rather than comparing the biometric data directly, a key is derived from these data and subsequently knowledge of this key is proved. One specific application of biometric encryption is the use of biometric sketches: in this case biometric template data are protected with biometric encryption. We address the question whether one can undermine a user's privacy given access to biometrically encrypted documents, and more in particular, we examine if an attacker can determine whether two documents were encrypted using the same biometric. This is a particular concern for biometric sketches that are deployed in multiple locations: in one scenario the same biometric sketch is deployed everywhere; in a second scenario the same biometric data is protected with two different biometric sketches. We present attacks on template protection schemes that can be described as fuzzy sketches based on error-correcting codes. We demonstrate how to link and reverse protected templates produced by code-offset and bit-permutation sketches.