Abstract: Driver's licenses and other security documents include one or more machine-readable features, each conveying plural bits of information. These features are used in a variety of ways to increase security, and/or to enhance functionality. In one embodiment, data encoded on a driver's license is used at airport check-in, to link to a state DMV database and obtain information by which the document, and its custodian, can be authenticated. In like fashion, a license can be used to authenticate a bearer and/or his/her age prior to the sale of alcohol or tobacco products. In other embodiments, different elements of a driver's license (e.g., the substrate, photo, text data, and machine-readable data) are logically bound together (e.g., interlinked through payloads conveyed by different machine-readable features) as a deterrent against counterfeiting. Driver's licenses can be similarly logically bound to personal checks and other documents. Many other arrangements are also disclosed.