Patent ID: 7237115

Claim:
A method of authenticating an item, the method comprising: a) acquiring an unencrypted reference signal, Y ref , of an item; where Y ref is an n-dimensional row vector {Y 1 (ref), Y 2 (ref), . . . , Y n (ref)} of unencrypted reference measurements subject to measurement error; b) applying a transformation to the unencrypted reference signal, Y ref , to generate an encrypted reference signal, U ref of the item; where U ref is an n-dimensional row vector {U 1 (ref), U 2 (ref), . . . , U n (ref)} of encrypted reference measurements; c) acquiring an unencrypted new signal, Y new , of the item, where Y new is an n-dimensional row vector {Y 1 (new), Y 2 (new), . . . , Y n (new)} of unencrypted new measurements subject to measurement error; d) applying the transformation to the unencrypted new signal, Y new , to generate an encrypted new signal, U new , of the item; where U new is an n-dimensional row vector {U 1 (new), U 2 (new), . . . , U n (new)} of encrypted new measurements; e) calculating an unencrypted Euclidean distance metric, E, between the unencrypted new and reference signals, Y new and Y ref ; f) calculating an encrypted Euclidean distance metric, D, between the encrypted new and reference measurements, U new and U ref ; g) comparing the encrypted Euclidean distance metric, D, to a critical value, D crit ,; h) if D<D crit , then deciding that the item is authentic; and i) providing the result of the decision made in step h) to an authenticator or inspector, thereby allowing the authenticator or inspector to decide if the item is authentic; wherein the transformation has the property that the unencrypted Euclidean distance metric, E, is equal to the encrypted Euclidean distance metric, D.