The present invention provides for validating an association between computing devices using a succession of human-perceptible stimuli such as sounds, lights colors or shapes. Commands are sent from the initiating device to the responding device in encrypted messages. Human-perceptible stimuli are formed at the responding device in response to at least some of the commands. The responder searches for messages that the responder is unable to decrypt and that are received in a time interval before messages that the responder is able to decrypt. The succession of human-perceptible stimuli may be harmonized, in which case, an association between the initiating device and the responding device is validated when the human-perceptible stimuli formed by the initiating device and the human-perceptible stimuli formed by the responding device are harmonized together.