  Recently, energy encryption for wireless power transfer has been developed
for energy safety, which is important in public places to suppress unauthorized
energy extraction. Most techniques vary the frequency so that unauthorized
receivers cannot extract energy because of non-resonance. However, this
strategy is unreliable. To stimulate the progress of energy encryption
technology and point out security holes, this paper proposes a decryption
method for the fundamental principle of encrypted frequency-varying wireless
power transfer. The paper uses an auxiliary coil to detect the frequency and a
switched-capacitor array to adaptively compensate the receiver for a wide
frequency range. The switched-capacitor array contains two capacitors and one
semi-conductor switch. One capacitor compensates the receiver all the time
while the other's active time during one wireless power transfer cycle is
regulated by the switch. Thus, the proposed hacking receiver controls the
equivalent capacitance of the compensation and steals energy. Finally, a
detailed simulation model and experimental results prove the effectiveness of
the attack on frequency-hopping energy encryption. Although any nonnegligible
energy extracted would be problematic, we achieved to steal 78% to 84% of the
energy an authorized receiver could get. When the frequency changes, the
interceptor is coarsely tuned very quickly, which can hack fast
frequency-varying encrypted system.
