1. A method for authenticating a document that includes a magnetic stripe having a distinct magnetic characteristic, wherein the magnetic characteristic exists over one or more predetermined segments of the magnetic stripe, the method comprising: receiving, at an authentication device, a set of expansion coefficients corresponding a set of continuous basis functions, wherein the expansion coefficients are based on digital samples of an analog signal representative of the magnetic characteristic; comparing the received expansion coefficients to reference expansion coefficients; and based on the comparison, providing a measure of an authenticity of the document.