A method of self-calibrating a physical token, which provides measurable parameters, and of deriving at least one data set from said measurable parameters, which data set is used as a cryptographic key, the method comprising the steps of: At an enrolment stage: measuring a plurality of values of a plurality of said parameters provided by the physical token; calculating, from the measured values, a measure of interclass variance, the interclass variance being a variance of values taken from different parameters; storing an interclass mean value and interclass variance along with further information which enables determination of quantization intervals; subsequently: determining, based on said measure of variance, quantization intervals into which a measured value is to be quantized, each quantization interval being associated with a possible value of a data set; and measuring a value of at least one parameter and quantizing the measured value into a determined quantization interval, measuring further values of said at least one parameter provided by the physical token and calculating an interclass mean value and an interclass variance for said further values; and comparing the interclass mean value and the interclass variance of said further values with the stored interclass mean value and interclass variance wherein if the interclass mean value and interclass variance of said further values differ considerably from the stored interclass mean value and interclass variance further action should be stopped, else the data set is derived from a value associated with the quantization interval into which the measured value is quantized. At an enrolment stage: measuring a plurality of values of a plurality of said parameters provided by the physical token; calculating, from the measured values, a measure of interclass variance, the interclass variance being a variance of values taken from different parameters; storing an interclass mean value and interclass variance along with further information which enables determination of quantization intervals; measuring a plurality of values of a plurality of said parameters provided by the physical token; calculating, from the measured values, a measure of interclass variance, the interclass variance being a variance of values taken from different parameters; storing an interclass mean value and interclass variance along with further information which enables determination of quantization intervals; subsequently: determining, based on said measure of variance, quantization intervals into which a measured value is to be quantized, each quantization interval being associated with a possible value of a data set; and measuring a value of at least one parameter and quantizing the measured value into a determined quantization interval, determining, based on said measure of variance, quantization intervals into which a measured value is to be quantized, each quantization interval being associated with a possible value of a data set; and measuring a value of at least one parameter and quantizing the measured value into a determined quantization interval, measuring further values of said at least one parameter provided by the physical token and calculating an interclass mean value and an interclass variance for said further values; and comparing the interclass mean value and the interclass variance of said further values with the stored interclass mean value and interclass variance wherein if the interclass mean value and interclass variance of said further values differ considerably from the stored interclass mean value and interclass variance further action should be stopped, else the data set is derived from a value associated with the quantization interval into which the measured value is quantized.