This project introduces the concept of physical one-way functions. Modern public key cryptography relies extensively on one-way functions and trapdoor one-way functions. Our use of physical mechanisms for one-way functions is motivated by emerging cryptographic applications which require a means for authenticating digital information that itself cannot be copied, and adds little cost to the system. The asymmetry that we exploit is in the ease of constructing and characterizing large physical structures with fine feature sizes, and the difficulty in reproducing them.