A system for preventing unauthorized use of a secured device, the system comprising: an activation unit that includes a first ultrasound transducer positioned to acoustically couple with an ultrasound conductive body when the activation unit is placed in contact with the ultrasound conductive body, and an ultrasound signal generator configured to drive the first ultrasound transducer, wherein the first ultrasound transducer and the ultrasound signal generator are configured such that when (a) the first ultrasound transducer is acoustically coupled with the ultrasound conductive body and (b) the ultrasound signal generator drives the first ultrasound transducer, the first ultrasound transducer will transmit an ultrasound activation signal into the ultrasound conductive body and wherein the ultrasound signal generator drives the first ultrasound transducer in a manner that embeds a code into the ultrasound activation signal; and a secured device that includes hardware that performs a function, a second ultrasound transducer positioned to acoustically couple with the ultrasound conductive body prior to use of the secured device, and to generate an output signal responsive to ultrasound energy arriving at the second ultrasound transducer, an ultrasound receiver that (a) receives the output signal generated by the second ultrasound transducer and (b) generates an enable signal when variations in the output signal generated by the second ultrasound transducer indicate that the ultrasound activation signal has arrived at the second ultrasound transducer and that the code is present in the ultrasound activation signal, and an interface circuit that receives the enable signal, wherein the interface circuit is configured to (a) allow the hardware to perform the function when the enable signal arrives and (b) prevent the hardware from performing the function when the enable signal has not arrived. an activation unit that includes a first ultrasound transducer positioned to acoustically couple with an ultrasound conductive body when the activation unit is placed in contact with the ultrasound conductive body, and an ultrasound signal generator configured to drive the first ultrasound transducer, wherein the first ultrasound transducer and the ultrasound signal generator are configured such that when (a) the first ultrasound transducer is acoustically coupled with the ultrasound conductive body and (b) the ultrasound signal generator drives the first ultrasound transducer, the first ultrasound transducer will transmit an ultrasound activation signal into the ultrasound conductive body and wherein the ultrasound signal generator drives the first ultrasound transducer in a manner that embeds a code into the ultrasound activation signal; and a first ultrasound transducer positioned to acoustically couple with an ultrasound conductive body when the activation unit is placed in contact with the ultrasound conductive body, and an ultrasound signal generator configured to drive the first ultrasound transducer, wherein the first ultrasound transducer and the ultrasound signal generator are configured such that when (a) the first ultrasound transducer is acoustically coupled with the ultrasound conductive body and (b) the ultrasound signal generator drives the first ultrasound transducer, the first ultrasound transducer will transmit an ultrasound activation signal into the ultrasound conductive body and wherein the ultrasound signal generator drives the first ultrasound transducer in a manner that embeds a code into the ultrasound activation signal; and a secured device that includes hardware that performs a function, a second ultrasound transducer positioned to acoustically couple with the ultrasound conductive body prior to use of the secured device, and to generate an output signal responsive to ultrasound energy arriving at the second ultrasound transducer, an ultrasound receiver that (a) receives the output signal generated by the second ultrasound transducer and (b) generates an enable signal when variations in the output signal generated by the second ultrasound transducer indicate that the ultrasound activation signal has arrived at the second ultrasound transducer and that the code is present in the ultrasound activation signal, and an interface circuit that receives the enable signal, wherein the interface circuit is configured to (a) allow the hardware to perform the function when the enable signal arrives and (b) prevent the hardware from performing the function when the enable signal has not arrived. hardware that performs a function, a second ultrasound transducer positioned to acoustically couple with the ultrasound conductive body prior to use of the secured device, and to generate an output signal responsive to ultrasound energy arriving at the second ultrasound transducer, an ultrasound receiver that (a) receives the output signal generated by the second ultrasound transducer and (b) generates an enable signal when variations in the output signal generated by the second ultrasound transducer indicate that the ultrasound activation signal has arrived at the second ultrasound transducer and that the code is present in the ultrasound activation signal, and an interface circuit that receives the enable signal, wherein the interface circuit is configured to (a) allow the hardware to perform the function when the enable signal arrives and (b) prevent the hardware from performing the function when the enable signal has not arrived.