System and method for encrypted resonant inductive power transfer

Systems and methods for ensuring that resonant inductive power transfer goes only to authorized users using encryption. Resonant inductive power transfer requires near-identical resonant frequencies in the transmitter and the receiver. The frequency of the power transfer signal changes on a schedule known only to the transmitter and receiver so a "power eavesdropper" cannot track the frequency well enough to efficiently receive power. To make the frequency transitions energetically efficient, a capacitive or inductive element is switched in or out of each circuit at moments of zero-crossing: zero charge on a capacitor or zero current in an inductor. To maintain phase alignment, either switching an inductor on the transmit side is nearly simultaneous with switching a capacitor on the receive side, or switching a capacitor on the transmit side is nearly simultaneous with switching an inductor on the receive side.