An article is authenticated by providing a magnetic security mark in the form of an optically-passive randomly-generated nanoscale magnetic pattern. The pattern is pre-imaged and this reference image is uploaded to a secure database along with an identifier for the article such as a serial number. A user of the article verifies its authenticity by scanning it magnetically to obtain a scanned image of the magnetic pattern. The serial number is used to retrieve the previously uploaded reference image which is compared to the scanned image. If the images match, the article's authenticity is confirmed. A single article may have multiple magnetic security marks, each unique, placed at predetermined, non-uniform locations. The magnetic patterns are generated using thin film deposition of yttrium iron garnet. In one embodiment the article is a physical key having additional security features, such as mechanical features and a radio-frequency identification chip.