An apparatus for generating a revocable numerical representation of a biometric characteristic, comprising: a biometric feature extractor arranged to: extract data identifying predetermined features from a biometric characteristic depicted in an image; define at least one polygon, wherein the or each such polygon is determined by the position within the image of at least some of the features, each polygon having at least one geometric feature, which is invariant with respect to similarity transforms, this representation forming a first geometric part of the extracted data; a photometric feature extractor arranged to extract from the image defining data at least one predetermined photometric feature, which feature is non-geometric and invariant with respect to any rigid transform applied to said data defining the portion of the image, said photometric feature being associated with an image region containing at least one part of a polygon; and transform said data defining said at least one photometric feature by applying a decorrelating transform, this representation forming a second photometric part of the extracted data; create a combined representation by concatenating the geometric part of the extracted data and the transformed photometric part of the extracted data; a data transformer arranged to apply a numerical transform to the combined representation, wherein said data transformer is further arranged to apply a predetermined distortion to the combined representation before applying the numerical transform thereby creating a revocable numerical representation of the biometric characteristic. a biometric feature extractor arranged to: extract data identifying predetermined features from a biometric characteristic depicted in an image; define at least one polygon, wherein the or each such polygon is determined by the position within the image of at least some of the features, each polygon having at least one geometric feature, which is invariant with respect to similarity transforms, this representation forming a first geometric part of the extracted data; a photometric feature extractor arranged to extract from the image defining data at least one predetermined photometric feature, which feature is non-geometric and invariant with respect to any rigid transform applied to said data defining the portion of the image, said photometric feature being associated with an image region containing at least one part of a polygon; and transform said data defining said at least one photometric feature by applying a decorrelating transform, this representation forming a second photometric part of the extracted data; create a combined representation by concatenating the geometric part of the extracted data and the transformed photometric part of the extracted data; a data transformer arranged to apply a numerical transform to the combined representation, wherein said data transformer is further arranged to apply a predetermined distortion to the combined representation before applying the numerical transform thereby creating a revocable numerical representation of the biometric characteristic. extract data identifying predetermined features from a biometric characteristic depicted in an image; define at least one polygon, wherein the or each such polygon is determined by the position within the image of at least some of the features, each polygon having at least one geometric feature, which is invariant with respect to similarity transforms, this representation forming a first geometric part of the extracted data; a photometric feature extractor arranged to extract from the image defining data at least one predetermined photometric feature, which feature is non-geometric and invariant with respect to any rigid transform applied to said data defining the portion of the image, said photometric feature being associated with an image region containing at least one part of a polygon; and transform said data defining said at least one photometric feature by applying a decorrelating transform, this representation forming a second photometric part of the extracted data; create a combined representation by concatenating the geometric part of the extracted data and the transformed photometric part of the extracted data; a data transformer arranged to apply a numerical transform to the combined representation, wherein said data transformer is further arranged to apply a predetermined distortion to the combined representation before applying the numerical transform thereby creating a revocable numerical representation of the biometric characteristic.