Methods, systems, and apparatus for accurately authenticating a document (e.g. a credit card) having a magnetic stripe are provided. The magnetic field at different points of the magnetic stripe are measured, e.g., by a checkout scanner to create a magnetic signature. Digital samples of the measurements may be used to create a representation of the magnetic signature. The representation can then be compared to reference values to produce a measure of the authenticity of the document. The reference values are updated over time to reflect changes in the magnetic stripe of the authentic document, as may occur due to physical deterioration. For example, reference values may be optimized based on recent measurements of the authenticated document to provide a more accurate determination of authenticity for future measurements.