A major limitation in authenticating passive and remotely powered sensors, tags and cards (for e.g. radio-frequency identification tags or credit cards) is that these devices do not have access to a continuously running system clock. This obviates the use of SecureID type authentication techniques involving random keys and tokens that need to be periodically generated and synchronized. In this paper we present a dynamic hardwaresoftware authentication approach for passive assets using zero-power timers and synchronization circuits. The timers are shown to achieve robust temporal synchronization due to the self-powering and self-compensating physics of Fowler-Nordheim (FN) quantum transport of electrons tunneling onto a floatinggate. The output of the timers are then used to seed a pseudorandom number generator which produce random and synchronized authentication tokens. We validate the proposed approach using prototypes fabricated in a standard 0.5μm CMOS process where we demonstrate synchronization accuracy greater than 40dB. Compared to conventional static authentication methods that are currently used for passive sensors, tags and cards, the proposed dynamic approach should provide enhanced security and make it more immune to counterfeiting and data theft.