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A current-measuring technique is introduced, which promises to substantially enhance power analysis attacks against cryptographic co-processors. The proposed technique exploits an active circuit to measure the instantaneous current consumption of a device under attack while supplying, at the same time, the device with a stable voltage. Higher gain-bandwidth product, higher sensitivity and lower insertion error are the main advantages with respect to a resistor-based measurement. Experimental results when the proposed circuit is used to measure the current consumption of an FPGA are reported, and the achievable advantage in terms of sensitivity is discussed. Results of a differential power analysis attack are reported too.