Physically unclonable functions (PUFs) are one of the most important breakthrough for security of devices as they represent a low-cost means to provide authentication and secure storage. PUFs measure nano-scale mismatches that are inherently caused by the manufacturing process. However, the mechanisms exploited by PUF circuits depend on the working conditions, such as temperature, device aging, and current flow, making them unreliable and, hence preventing their wider employment. One of the most investigated PUF exploits pairs of ring oscillators (ROs): frequencies measured from each pair are compared for extracting one response bit. However, extracted bit-strings are not suitable for authentication purposes as they may change during time. In this paper, we propose a new identification mechanism, based on ROs, which is immune to aging and working conditions. Through a mathematical demonstration and an extensive experimental campaign, which involved real field programmable gate array devices, we demonstrate its ability to reliably accomplish identification of silicon devices.