Spying on the Smart Home: Privacy Attacks and Defenses on Encrypted IoT Traffic – reveals that even when data from devices is encrypted, the metadata can help identify both the device and what it is signaling.
Some devices such as the Nest indoor camera directly communicate with identifiable domain names – in this case ‘dropcam.com.’ That immediately identifies what the product is, and it is then possible to infer from that and the resulting signal what is happening: whether it has detected motion or whether it is live streaming.
Likewise the Sense sleep monitor, TP‑Link smart plug, and Amazon Echo. Even when the devices communicate with a generic DNS server – like Amazon’s AWS service – they typically have a specific IP address that can be used to identify the sensor (the Belkin WeMo switch for example communicated with the very-specific prod1-fs-xbcs-net-1101221371.us-east-1.elb.amazonaws.com address).
By digging into each device’s signal, the team was able to figure out with some certainty exactly what was happening: someone was waking up, someone was turning on a light switch, someone had walked into the kitchen, and so on.