An interactive display device comprising: a touch sensor including an electrode matrix having row electrodes and column electrodes, where the interactive display device is configured to excite one or more electrodes (202, 204) of the electrode matrix to form a capacitive coupling with an input device to capacitively send a cryptographic key via an electrostatic channel to the input device; and a radio transceiver configured to communicate with the input device via a radio channel, where the interactive display device is configured to subsequently exchange encrypted communications over the radio channel with the input device, the encrypted communications being encrypted using the cryptographic key, where: the one or more electrodes (202,203) of the electrode matrix capacitively couple with an electrode in a tip of the input device; capacitive coupling between the electrode in the tip of the input device and the one or more electrodes (202,204) of the electrode matrix is used to determine a position of the electrode in the tip of the input device relative to the electrode matrix; and the cryptographic key is sent by exciting a subset of the one or more electrodes (202,204) of the electrode matrix, the subset being selected based on the position of the electrode in the tip of the input device relative to the electrode matrix. a touch sensor including an electrode matrix having row electrodes and column electrodes, where the interactive display device is configured to excite one or more electrodes (202, 204) of the electrode matrix to form a capacitive coupling with an input device to capacitively send a cryptographic key via an electrostatic channel to the input device; and a radio transceiver configured to communicate with the input device via a radio channel, where the interactive display device is configured to subsequently exchange encrypted communications over the radio channel with the input device, the encrypted communications being encrypted using the cryptographic key, where: the one or more electrodes (202,203) of the electrode matrix capacitively couple with an electrode in a tip of the input device; capacitive coupling between the electrode in the tip of the input device and the one or more electrodes (202,204) of the electrode matrix is used to determine a position of the electrode in the tip of the input device relative to the electrode matrix; and the cryptographic key is sent by exciting a subset of the one or more electrodes (202,204) of the electrode matrix, the subset being selected based on the position of the electrode in the tip of the input device relative to the electrode matrix. the one or more electrodes (202,203) of the electrode matrix capacitively couple with an electrode in a tip of the input device; capacitive coupling between the electrode in the tip of the input device and the one or more electrodes (202,204) of the electrode matrix is used to determine a position of the electrode in the tip of the input device relative to the electrode matrix; and the cryptographic key is sent by exciting a subset of the one or more electrodes (202,204) of the electrode matrix, the subset being selected based on the position of the electrode in the tip of the input device relative to the electrode matrix.