A technique is presented for performing a physical unclonable function (PUF) using an array of SRAM cells. The technique can be viewed as an attempt to read multiple cells in a column at the same time, creating contention that is resolved according to process variation. An authentication challenge is issued to the array of SRAM cells by activating two or more wordlines concurrently. The response is simply the value that the SRAM produces from a read operation when the challenge condition is applied. The number of challenges that can be applied the array of SRAM cells grows exponentially with the number of SRAM rows and these challenges can be applied at any time without power cycling.