A method and system for authenticating an item by using a security marking. The security marking is provided on the item with an OVD ink capable of absorbing light in a visible wavelength range to appear visibly black and producing a red fluorescent emission under ultraviolet excitation. Under visible light illumination and ultraviolet excitation, a visible image and a fluorescent image are obtained from the security marking using image scanners. The images are compared to find a substantial match with each other. The security marking can be a postage indicium, a barcode, a symbol, a message or an image. The item to be authenticated can be a mailpiece, a banknote, a tag, a ticket, a document, an identification card, or the like.