Combined physical unclonable function (PUFs); methods, apparatuses, systems, and computer program products for enrolling combined PUFs; and methods, apparatuses, systems, and computer program products for authenticating a device physically associated with a combined PUF are described. In an example embodiment, a combined PUF includes a plurality of PUFs and one or more logic gates. Each PUF includes a plurality of stages and an arbiter configured to generate a single PUF response based on response portions generated by the plurality of stages. The one or more logic gates are configured to combine the single PUF response for each of the plurality of PUFs in accordance with a combination function to provide a combined response.