A system has a first electronic device with optical sensor, digital radio transceiver, and processor with firmware; this device is typically portable or wearable. The system also has a computerized device with a display, a second digital radio transceiver, and a second processor with firmware. The first and computerized devices are configured to set up a digital radio link when in radio range. The second processor uses a spot on the display to optically transmit a digital message including a secret such as an encryption key or subkey and/or an authentication code adapted for authenticating an encrypting the radio link. The first device receives the digital message via its optical sensor, and uses the digital message to validate and establish encryption on the radio link. In embodiments, the system determines a location of the first device on the display and positions the transmission spot at the determined location.