A memory device includes a memory block that includes a plurality of memory bits, wherein each bit is configured to present a first logical state; and an authentication circuit, coupled to the plurality of memory bits, wherein the authentication circuit is configured to access a first bit under either a reduced read margin or a reduced write margin condition to determine a stability of the first bit by detecting whether the first logical state flips to a second logical state, and based on the determined stability of at least the first bit, to generate a physically unclonable function (PUF) signature.