1. A method of generating authentication data for authenticating a subject, the method comprising a computer to execute the steps of: receiving physical data representing a physical characteristic of the subject; processing the physical data by performing a predetermined processing operation to generate i) a reference number whose value is representative of the subject, and ii) variability correction data for improving the repeatability of the generation of the reference number; generating identification data comprising at least part of the reference number and at least part of the variability correction data; encrypting the generated identification data; and generating authentication data comprising said variability correction data and said encrypted data, wherein said encrypting step comprises applying a one-way encryption algorithm to said identification data to generate a hash value, and applying a two-way encryption algorithm to encrypt the generated hash value.