In this paper, the first class of low power currentbased physically unclonable functions (PUFs) is introduced. The new PUF circuit is able to convert the analog variations present in device leakage currents to a unique digital quantity at high speed and low power. Robust digital responses are achieved with the new architecture in presence of fluctuations in operational conditions such as temperature and supply voltage. The experimental results suggest 3% response error rate under extreme temperature variations from -55oC to 125oC and 20% fluctuations in supply voltage. The PUF consumes 150 micro-Watt for a duration of 250 pico-second per each response bit (37.5 femto joules of energy per response bit).