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Mechanical locks were not quickly supplanted by electric locks. They are still being researched and improved, along with advanced electronic methods of attack. Reading pin lengths by detecting their natural frequencies (lock decoding) to forge a copy of the legitimate key can be done quickly using ultrasonic detectors, active or passive. Hence, advanced methods of defence must be further researched. One method is to make the lock’s pins out of functionally graded materials (FGM). A pin’s natural frequency (in the range 100 kHz-1 MHz) and hence its ultrasonic pulse transit/reflection time can be correlated to its length if it is made of a homogeneous material. The idea is to design pins made of functionally graded alloys, to achieve equal natural frequencies, but also desired positions of standing wave nodes regardless of the pin’s length. Mathematical models of pins vibrations must be devised first to enable calculations of FGM alloys composition. Two simple and fast mathematical models are first derived from finite-element model (FEM) of a pin. These models are used in an optimization procedure based on the Nelder-Mead simplex method to calculate optimal profiles of Young’s modulus and density along the pin’s longitudinal axis. A successful optimization procedure for 10 key pin lengths is performed, to make a pin-tumbler lock resistant to ultrasonic attacks.