On the reconstruction of biometric raw data from template data

Biometric
Raw Data, Templates, and Privacy
Damn! Here I thought hashes were the answer to everything!

The experience with passwords suggests us to store only the hashed biometric template data, making a reconstruction of original template data and thus of the raw data extremely difficult. Unfortunately, this method only makes sense in those trivial cases where the matcher is a simple comparator based on mathematical subtraction. But even if this should be the case, template data normally show variations due to unavoidable variations in the raw data. As a result, the hash values of biometric data will never be the same even if the raw data are coming from multiple samples of the same biometric feature. That is, hashing cannot be used for biometric templates. It is really one-way, unfortunately not only for attackers. (If two raw data sets are really equal, it must be assumed that one template is the digital copy of the other, e.g., originating from a replay attack. In biometric systems such equalities should be used to trigger an alarm rather than to enable a successful authentication!)