A communication device comprising a communication interface unit comprising: a transmitter unit configured to transmit data using close proximity extremely high frequency, EHF, electromagnetic energy to a computing device over a first transmission path; a receiver unit configured to receive data from the computing device over a second transmission path; and an interface control circuit coupled to the transmitter unit and the receiver unit, and configured to: detect identifying information from an EHF transmission transmitted by the computing device; detect training and capability information from the EHF transmission transmitted by the computing device; evaluate one or more attributes of the detected training and capability information; determine whether to establish a communication link over the first and the second transmission paths with the computing device based on the evaluation; calculate a round-trip propagation time of training information transmitted along a communication path beginning from the communication device, through the computing device, and returning to the communication device; generate an encryption key based at least in part on the calculated round-trip propagation time of the training information; and encrypt information exchanged between the communication interface unit and the computing device based at least in part on the generated encryption key. a transmitter unit configured to transmit data using close proximity extremely high frequency, EHF, electromagnetic energy to a computing device over a first transmission path; a receiver unit configured to receive data from the computing device over a second transmission path; and an interface control circuit coupled to the transmitter unit and the receiver unit, and configured to: detect identifying information from an EHF transmission transmitted by the computing device; detect training and capability information from the EHF transmission transmitted by the computing device; evaluate one or more attributes of the detected training and capability information; determine whether to establish a communication link over the first and the second transmission paths with the computing device based on the evaluation; calculate a round-trip propagation time of training information transmitted along a communication path beginning from the communication device, through the computing device, and returning to the communication device; generate an encryption key based at least in part on the calculated round-trip propagation time of the training information; and encrypt information exchanged between the communication interface unit and the computing device based at least in part on the generated encryption key. detect identifying information from an EHF transmission transmitted by the computing device; detect training and capability information from the EHF transmission transmitted by the computing device; evaluate one or more attributes of the detected training and capability information; determine whether to establish a communication link over the first and the second transmission paths with the computing device based on the evaluation; calculate a round-trip propagation time of training information transmitted along a communication path beginning from the communication device, through the computing device, and returning to the communication device; generate an encryption key based at least in part on the calculated round-trip propagation time of the training information; and encrypt information exchanged between the communication interface unit and the computing device based at least in part on the generated encryption key.